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EVALUATION OF THE PERFORMANCE OF SOlL MOISTURE

1. 20 0 50 100 150 200 capillary pressure kPa Fig 11 Soil moisture characteristic curve for Silt Loam with p 1250 kg m 49 4 3 3 Aqua Tel A calibration equation was developed to convert data collected with the Aqua Tel capacitance sensors to volumetric water content The equation was established from a statistical analysis of the numerical values derived from soil specific curves published by Automata Inc We found from the statistical analysis of the calibration curves presented in the user s manual that the volumetric water content value 8 was dependent only on the meter readings R and the proportion of sand S with a significance of 97 R 0 976 This relationship is expressed by the following equation 0 2 698 66 237R 170 003R 154 197R 4 257x10 S 19 4 3 4 VIRRIB Data obtained with the VIRRIB phase transmission sensors did not require conversion readings were directiy displayed in volume fraction units 4 3 5 ThetaProbe The voltage standing wave ratio VSWR measurements obtained with the ThetaProbe were converted to volumetric water contents by first correlating the voltage output read on the voltmeter V to the apparent dielectric permittivity of the bulk soil e which was then correlated to 6 The square root of the apparent dielectric permittivity Ve was correlated to V using the following third degree polynomial equation provided by Delta T Devices Ltd Je 1 07 6 4V 6 4V
2. 10 with f a 11 EA and C C C 12 where f resonance frequency Hz fo resonance frequency for C and G equal zero Hz AC change in capacitance due to C and G F Co total capacitance of the oscillator circuit F Cu capacitance in the oscillator circuit F C capacitance in the oscillator circuit due to L F L inductance in the oscillator circuit H The frequency shift method was also used by Dean et al 1987 and Gardner et al 1998 For greater fraquencias measurement techniques such as the slott Taw ee eee A seen e e we ur y aso 17 To Amplifier Soil Equivalent T Coupting Oscillator Resonence Circuit Network Circuit Fig 3 Equivalent circuit of a capacitance sensor The effect of the soil capacitance C and conductance G on the oscillator resonance frequency is shown The connection is made via the T network Wobschall 1978 18 vector voltmeter or the swept frequency method are required The slotted line measures the ratio of the maximum voltage to the minimum voltage which is termed the voltage standing wave ratio VSWR The VSWR is in fact the ratio of the maximum amplitude to the minimum amplitude on the transmission line The vector voltmeter used with a dual directional coupler measures the voltage amplitude and the phase difference between two points on the transmission line The swept frequency method developed by Hewlett Packard and considered an i
3. were taken with an impedance analyser for laboratory use from which and the electric conductivity were derived Dielectric mixing models range in complexity from Wagner s dielectric spheres model which consists of oniy two phases to the semidisperse model which is a complex multi phase system Wobschall 1977 described the semidisperse model as well as the chronological development of the Hanai Bruggelman Wagner HBW theory The semi disperse model proposed that the water and particles are mutually inter 15 dispersed It can be simplified to a moist particle phase consisting of a solid particle within which water filled micropores are dispersed and around which is a water layer In soils the moist particles and air are dispersed in the remaining water This multi phase model used the two phase HBW theory to partially solve each portion of the more complex model Three phase solids air and water and four phase solids air and bound and free water models have been widely used to correlate bulk soil complex dielectric permittivity to the complex dielectric permittivity and the volume fraction of each of the soil s constituents Heimovaara et al 1994 Gardner et al 1998 Various characteristics of the electromagnetic signal sent through the soil can be observed to obtain dielectric permittivity measurements The amplitude and phase of the reflected signal or the combination of the incident and reflected signals the propagation ti
4. gypsum blocks Watermark granular matrix sensors GMS and Agwatronics heat dissipation blocks under controlled soil moisture conditions Their study was conducted in loam and sandy loam using 0 61 m deep soil columns equipped with a tension controlled drainage system and weighed with load cells Their results showed that in order of decreasing performance Aqua Tel sensors the Sentry probe the Troxler neutron gauge and Watermark sensors performed best when considering accuracy reliability durability and ease of installation Ripley et al 1998 evaluated the performance of various types of soil moisture sensors including time domain reflectometry probes the ThetaProbe Model ML 1 capacitance sensors and neutron probes at three locations They reported that the TDR probes and the ThetaProbe performed satisfactorily and were well suited for unattended operation as opposed to neutron probes that require constant assistance They recommended the ThetaProbe over TDR probes because it was insensitive to temperature bulk density and soil texture Hanson 1999 evaluated the performance of tensiometers Watermark GMS gypsum blocks and the dielectric instruments Sentry 200 AP TRIME TRACE Enviroscan Aquaterr and ThetaProbe The investigator reported that both tensiometers and GMS showed good response to changes in moisture content except in loamy sand where they failed to respond The response of the GMS however tended to lag that o
5. we decided that the field study was inconclusive and that further testing under controlled soil and water conditions was required An example of the field results is shown in Fig 12 52 Tansiometer Watermark Aquaterr Virrib across Virrit parailel Aqua Tel side AA A ToS Er a 2 oe A e O Measured volumetric water content Actual volumetric water content Fig 12 Results from the field study of 1998 in Loam in the rain fed experimental plot Horizontal error bars illustrate the 95 confidence interval calculated on the average of three 6 measurements obtained with gravimetric sampling Vertical error bars show the 95 confidence interval calculated on the average of three 6 measurements obtained with the sensors when no error bar only one 9 is represented The thin lines offset from the 1 1 line shows the maximum standard error calculated for the gravimetric method 53 5 2 Lysimeter study 5 2 1 Experimental design in the 2000 test all sensors responded to variations in soil moisture content within their specified operational limits except for the VIRRIB sensors If the method or in other words the principie of operation alone had been the focus of the experiment we could have concluded that all those sensing methods were adequate to measure soil moisture variations However the objective of this experiment was to evaluate the performance of various soil moisture sensors with regard to accu
6. 4 74 20 where V VSWR mV The volumetric water content 6 was correlated to Ye as follows a 8 v 21 50 where a a Soil specific constants dimensionless To determine a and a the probe was inserted into dry soil a VSWR measurement was taken V and the volumetric water content of the dry soil O was determined by gravimetric sampling followed by conversion using Eq 1 A first Ve value was obtained from Eq 20 which was then substituted in Eq 21 for which 8 was equal to 8 The probe was then inserted in wet soil a second VSWR measurement V was taken and the volumetric water content of the wet soil 6 was determined A new V value was calculated with Eq 20 and substituted in Eq 21 for which O was replaced by O The values used to calculate the constants a and a are shown in Table II Finally the two equations were solved to obtain values of a and a for each soil Table III Table Il Measured values used to calculate a and a for loam and silt loam Vo V 9 VW V 8 Loam 0 1277 4 372 0 8747 39 304 Silt Loam 0 1373 4 031 0 8820 41 506 Table ll Calculated constants a and a of Eq 21 for loam and silt loam Loam Silt Loam a 1 4016 1 5047 a 0 0894 0 0833 51 5 RESULTS and DISCUSSION 5 1 Field study For both field trials the standard error of the volumetric water content measurements obtained by gravimetric sampling exceeded 8 which was attributed to th
7. 61 Tensiometers results in loam of p 1250 kg m modified 62 Watermark results in loam of p 1190 kg m 63 Watermark GMS results in loam of 0 1150 kg m 64 Watermark GMS results in loam of p 1150 kg m modified 65 Watermark GMS results in silt loam of p 1250 kg m 66 vit 18b 19 20 21a 21b 22 23 24a 24b 25 26 27 28 Watermark GMS results in silt loam of p 1250 kg m modified 67 Aquaterr results in loam of p 1190 kg m 68 Aquaterr results in loam of p 1150 kg m 2 2 eee 69 Aquaterr results in silt loam of p 1250 kg m ooo ee 70 Aquaterr results in silt loam of p 1250 kg m modified 71 Aqua Tel sensors results in loam of p 1190 kg m Ea 72 Aqua Tel sensors results in loam of p 1150 kg m 73 VIRRIB sensors results in foam of p 1190 kg m 74 VIRRIB sensors results in toam of p 1190 kg m modified 75 VIRRIB sensors results in loam of p 1150 kg M 76 VIRRIB sensors results in silt foam of p 1250 kg m 77 ThetaProbe sensor results in loam of p 1150 kg m 78 ThetaProbe sensor resuits in silt loam of p 1250 kg m 79 vili LIST OF TABLES R values for the Aquaterr at different moisture condi
8. Devices Ltd in the United Kingdom 128 Low Road Burwell Cambridge CB5 OEJ England and is distributed in Canada by Lakewood Systems Ltd 8709 50 Avenue Edmonton AB T6E 5H4 It operates at a fixed frequency of 100 MHz The sensor consists of a coaxial transmission line that extends into the soil via the sensing probes which is formed of four 0 050 m long stainless steel rods three of which positioned in the periphery of the rod transmitting the electromagnetic signal are at ground potential Even if the effective volume of the probe is relatively small a cylinder of 0 025 m in diameter and 0 060 m in length accounts for 90 of the influence on the measurements holding the probe while taking measurements may affect the results Although the ThetaProbe operates at a frequency for which conductivity losses are minimal soil specific calibration is recommended by the manufacturer In the user s manual an accuracy of 2 of 8 is specified after soil specific calibration whereas 5 of O is specified if the supplied calibration factors are used 30 4 MATERIALS AND METHODS 4 1 Field study During the growing season of 1998 we conducted a field experiment to determine the most suitable methods to measure soil moisture in potato fields under Manitoba conditions Moreover this first trial was intended to help develop a protocol for proper installation calibration and use of tensiometers Watermark granular matrix sensors the Aquaterr soi
9. EEE Transactions on Geoscience Electronics GE 15 1 49 58 Wobschall D 1978 A frequency shift dielectric soil moisture sensor EEE Transactions on Geoscience Electronics GE 16 2 112 118 Yoder R E D L Johnson J B Wilkerson and D C Yoder 1998 Soil water sensor performance Applied Engineering in Agriculture 14 2 121 133 104
10. domain reflectometry waveforms 1 A four component complex dielectric mixing model for soils Water Resources Research 30 2 201 209 Hillel D 1971 Soil and Water Physical Principles and Processes NewYork NY Academic Press Howell T A A D Scheinder and M E Jensen 1991 History of lysimeter design and use for evapotranspiration measurements In Proceedings of the international 101 Symposium on Lysimetry 1 9 Honolulu Hawaii Irrigation and Drainage Division of the American Society of Civil Engineers irrometer Co Watermark Operation and Installation Soil Moisture Measurement Irrometer Co Riverside CA Klocke N L D F Heermann and H R Duke 1985 Measurement of evaporation and transpiration with lysimeter Transactions of the ASAE 28 1 183 189 and 192 Kutilek M and D R Nielsen 1994 Soil Hydrology Cremlingen Destedt Germany Catena Verlab Litschmann T 1991 VIRRIB A soil moisture sensor and its application in agriculture Communications in Soil Science and Plant Analysis 22 5 amp 6 409 418 McCann I R D C Kincaid and D Wang 1992 Operational characteristics of the Watermark model 200 soil water potential sensor for irrigation management Applied Engineering in Agriculture 8 5 603 609 Morgan M T R K Wood and R G Holmes 1993 Dielectric moisture measurement of soil cores Transactions of the ASAE 36 1 17 22 Perdok U D B Kroesbergen and M A Hilhorst 1996 Influence of
11. in silt loam of p 1250 kg m Volumetric water content measured with the probe in percentage as a function of 6 obtained with the lysimeter as percentage Each data point represents the mean of the 8 measurements taken at three different locations with the portable device The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the finear regression 79 range of 6 of 6 or less and as poor when they corresponded to a range of 8 greater than 6 During the first lysimeter trial tensiometers Watermark sensors the Aquaterr soil moisture meter Aqua Tel sensors and VIRRIB sensors were tested in loam with a Pe of 1190 kg m over one drying cycle for moisture contents ranging from 34 0 to 17 0 Figs 13a 13b 16 19 22 24a and 24b The standard errors calculated on the lysimeter 8 i e the volumetric water content obtained by gravimetric sampling used to calibrate the lysimeter readings did not exceed 0 6 During the lysimeter trial of 2000 the performance of tensiometers Watermark sensors the Aquaterr soil moisture meter Aqua Tel sensors VIRRIB sensors and the ThetaProbe FDR sensor was evaluated in loam with a P of 1150 kg m over a range of moisture contents from 43 1 to 20 0 Figs 14a 14b 17a 17b 18 23 25 and 28 The standard errors calculated on the lysimeter 6 did n
12. it can also be installed permanently In both cases care should be taken to avoid disturbances and air pockets in the effective volume of the instrument which can greatly affect the measurements 91 6 CONCLUSIONS 6 1 Experimental considerations As discussed in the previous section the use of volumetric water content as a unit of reference for the purpose of comparing sensors was unfavourable to the tensiometers Watermark GMS and Aquaterr probe since an additional conversion had to be performed The conversion from soil matric potentials to volumetric water contents required soil specific information that introduced a new source of error Furthermore in order to evaluate the performance of the various sensors tested with regard to the principle of operation used the experiment would have required several distinctive stages First to observe the unprocessed output signal sent from the sensing device to the meter Then to assess the conversion or processing of the data integrated in the electronic module if applicable And finally to interpret the displayed value for the purpose of comparison 6 2 Performance of the sensors 6 2 1 Accuracy and precision Tensiometers granular matrix sensors capacitance probes phase transmission sensors and the frequency domain reflectometry tested however did not measure soil moisture with the same accuracy and precision in the entire range The assessment of accuracy was based on 1 1 line falling wi
13. it was observed during the calibration of the lysimeter also affected the pressure read on the manometer Prior to the trial a 72 h period was allowed for equilibration of the soil moisture conditions and thus this time period was assumed sufficient for the membrane to stabilize Furthermore the calibration relationship was compared periodically with gravimetric sampling of the soil from the lysimeter 55 5 2 3 Accuracy and precision of the sensors The term accuracy means the conformity of an indicated value to an accepted standard or true value CSA 1979 The term precision means the quality of being sharply defined or stated CSA 1979 it can also be defined as the repeatability of the measurement Sensors readings were compared with soil moisture contents determined with the lysimeters The volumetric water content measurements obtained with the various sensors expressed as a percentage and labelled sensors 8 were plotted against the volumetric water contents derived from the hydraulic weighing system of the lysimeters also expressed as a percentage and labelled lysimeter 6 Figs 13a to 28 In the present study the accuracy of an instrument is an evaluation of the conformity of the 8 obtained with the sensor sensors 9 to the soil moisture content measured with the lysimeter iysimeter 6 The assessment of accuracy was based on the 95 confidence interval of the linear regression performed on the 8 measured with the sensors
14. least one of the tensiometers had reached its limit of operation and had released water from the ceramic cup thereby increasing the moisture content of the soil in the vicinity For the 1999 trial tensiometers measured 6 accurately over the 23 to 30 range for which the measurements were qualified of very accurate in the 26 to 30 range based on the 95 confidence interval of the linear regression Fig 13b The tensiometers tended to overestimate 8 above 30 and they did not respond to moisture change below 23 which corresponded to their operational limit of 85 kPa In the 2000 trial tensiometers mainly overestimated 8 in both lysimeters i e in both soils In Fig 14b the regression line shows that the difference between the tensiometers 6 measurements and the lysimeter 8 increased with soil moisture Accurate measurements were obtained in the narrow 25 28 range whereas a divergence of as much as 18 was observed for lysimeter 6 of 38 In silt loam the regression line is offset from the 1 1 line by less than 3 at lysimeter 8 of 27 and by 7 at 45 The variability of the tensiometers 8 measurements was generally small for all trials illustrated by error bars covering on average a 8 range of less 4 and thus we found the tensiometers to have very good precision A few data points however show large error bars in the 2000 trial two data points for loam Fig 14b and one for silt loam Fig 15b The 6 measurements obtaine
15. module was used to adjust the signal gain by setting the probe s reading to the actual volumetric water content W Finally the OFFSET and AMPLIFICATION buttons were filled with caulking paste and covered with electrical tape to prevent moisture from entering the electronic module 4 2 3 Data collection Data were collected 24 times over a period of 34 d in 1999 and 17 times over a period of 51 d in 2000 The location where each reading was taken with the portable probes i e Aquaterr and ThetaProbe as well as each sampling location was randomly chosen and documented to ensure that measurements and samples be taken in undisturbed soil Watermark GMS and the Aquaterr were calibrated as described in section 4 1 3 To follow the recommendations made in conclusion of the field study a period of 90 s was allowed for temperature equilibration before each Aquaterr reading Furthermore the temperature of the water used for calibration was monitored and kept within 3 C of the soil temperature A voltmeter and a power supply were used with the ThetaProbe instead of a data logger Once the probe had been inserted into the soil an input valtage of 10 V DC at about 19 mA was provided to the probe and the VSWR read by the probe was displayed on the voltmeter and manually recorded Although Delta T Devices Ltd stated that for complete stability a warm up time of 5 s is sufficient a period of 90 s was allowed for stabilization for each measurem
16. of the medium surrounding the sensing device that affects the measurements The range of operating frequencies of the Aquaterr is also not specified Thus it can only be hypothesised from the fact that the readings require calibration against soil texture that the operating frequencies are in the range where conductivity losses are not negligible i e below 50 MHz 3 2 4 Aqua Tel The Aqua Tel Automata Inc 10551 E Bennett Road Grass Valley CA 95945 7806 USA is a capacitance sensor The Aqua Tel model Aqua Tel94 29 consists of two parallel electrodes attached to a small electronic module The 0 83 m 29 in long electrodes made of stainless steel are flat which leads to the hypothesis that the dielectric properties are derived from measurements of fringe capacitance The effective volume of the Aqua Tel sensors is not specified in the literature As for the Aquaterr readings made with the Aqua Tel require calibration against soil texture which 28 can demonstrate the effect of conductivity losses and thus leads to the assumption that the operating frequencies are below 50 MHz 3 2 5 VIRRIB The VIRRIB is a phase transmission sensor that was manufactured in the Czech Republic and distributed in North America by Environmental Sensors Inc PO Box 720698 San Diego CA 92172 0698 USA who seems to have bought the rights to the technology but are not distributing it anymore Environmental Sensors Inc ESI is now manufacturing a TDT sen
17. onsite calibration against moisture content The probe was submerged into a bucket of water and the meter set to 100 between each set of readings The Watermark sensors required that soil temperature be measured at the three depths at which the sensors are installed which was done by inserting a stainless steel thermometer at proximity of the sensors Then the soil temperatures measured at each depth were entered in the meter accordingly so that it automatically calibrated the sensor against temperature The other moisture sensors did not require any on site calibration as stated by their respective manufacturers 4 2 Laboratory study 4 2 1 Lysimeters 4 2 1 1 Lysimeter design The lysimeters were designed with hydraulic weighing systems to facilitate continuous monitoring of the soii moisture content This design was preferred to the other weighing systems because of its simplicity and low cost The soil container was an open box consisting of a 1 10 m long 1 10 m wide and 0 27 m deep wooden box with an open bottom The required dimensions of the container were selected based on the effective volume of each soil moisture sensor i e the soil region that affects the measurements To maintain uniformity of moisture within the soil blocks the maximum depth of soil in the lysimeters was limited to 0 20 m This was well below the displacement pressure of the soils selected for testing and thus suction controlled drainage systems were not req
18. sensor They found the ideal location of the sensors to be 0 15 m offset from the centre of the hill and buried at a depth of 0 1 to 0 2 m The sensors responded within four hours of wetting and within 12 hours of drying Sensors placed deeper than 0 3 m responded slowly and inconsistently 6 McCann et al 1992 evaluated the static and dynamic response characteristics of the Watermark model 200 using the pressure plate method and experiments conducted in greenhouses They found the sensor s resistance to respond nearly linearly to water potential over the range from 0 to 200 kPa They also concluded that the Watermark model 200 did not fully respond to rapid drying or partial re wetting of the soil Lischmann 1991 evaluated the performance of the VIRRIB phase transmission sensor in the laboratory by installing the sensor and a gypsum resistance block in an isolated block of soil placed on a scale The VIRRIB readings were monitored over several drying cycles and compared to the total mass on the scale and to the gypsum block measurements Vinegar and sulfuric acid were added to the soil block to evaluate the effect of the soil solution s composition on the VIRRIB performance A field study was also conducted to determine the performance of the sensor under field conditions At completion of both studies it was concluded that VIRRIB sensors can be used to monitor irrigation needs Tensiometers and granular matrix sensors have been widely used a
19. soil container with loose soil taken from the natural environment where the lysimeter will be placed This technique disturbs the soil properties and gives therefore results that are not representative of the surrounding soil The monolith method solves that problem by encasing an undisturbed block of soii into the container This method is the most cumbersome of the reconstitution methods and can be very costly Bhardwaj and Sastry 1979 A variant from the monolith 20 method called the Ebermeyer method also uses an undisturbed soil block that is not isolated laterally by vertical walls of a container For all types of lysimeters a system is required to allow the water to flow through the lysimeter Two basic types of drainage systems were reported in the literature based on the driving force causing the flow of water Aboukhaled et al 1982 In a free drainage system the excess water is drained by gravity Whereas in a suction controlled system constant suction also called tension is artificially maintained to control drainage Porous collectors generally made of ceramic are placed at the bottom of the soil block and connected to a vacuum system Suction controlled systems should be used for deep lysimeters to overcome variations of moisture content within the soil profile Lysimeters equipped with tension systems are often used to simulate dry conditions or to maintain a high watertable in water stress studies Yoder et al 1998 ut
20. the dimensions of the lysimeter accordingly 7 2 Application of research results The following recommendations are based on the evaluation of the performance of tensiometers Soilmoisture Equipment Corp model 2725AR Watermark granular matrix sensors Irrometer Co the Aquaterr electrical capacitance probe Aquaterr Instruments Inc model 200 Aqua Tel electrical capacitance sensors Automata inc model Aqua Tel94 29 VIRRIB phase transmission sensors Environmental Sensors Inc and the ThetaProbe FDR sensor Delta T Devices Ltd type ML2 These recommendations are addressed to both producers and researchers for the selection of a sensing device assuming that there are no cost limitations First l recommend the ThetaProbe be used as a portable device for application requiring very accurate measurements of soif moisture content and several sampling locations Assuming the soil moisture characteristics specific to the field are known also recommend the use of tensiometers and Watermark granular matrix sensors because they are easy to install and operate and they are very precise Moreover recommend 98 the use of the Watermark sensors assuming that soil temperature can be monitored as required for the calibration As for the tensiometers their use is recommended with the assumption that measurements will be performed for moisture contents within their operational limits of 0 to 85 kPa Finally recommend the Aquaterr for
21. to come back to the transmitter as a reflection In other words the two way propagation time is measured When applied to soil moisture measurement devices the TDR sensor consists of a discontinued transmission line extending into the soil for which the soil is part of the dielectric thereby creating the impedance discontinuity that causes the reflection of the signal The signal sent is a pulse and the operating frequency is fixed and generally greater than 250 MHz in cable fault detection Sinnema 1988 Another time domain technique referred to as time domain transmissometry 24 TDT can be used to measure soil moisture content In the TDT technique the signal is observed at the end of the transmission line The time measured is thus the one way propagation time and the reflection is not involved in the measurements htto www envsens com April 2000 3 1 2 3 Frequency domain sensors The most elementary design of frequency domain sensors is also referred to as a capacitance sensor It consists essentially of a pair of electrodes which form a capacitor for which the soil acts as dielectric A free running oscillator generates an alternative current AC field and adjusts to the capacitor to form a tuned circuit Eqs 9 10 and 11 The resonance frequency is then related to the dielectric permittivity and in turn to the soil moisture content by calibration A more sophisticated type of frequency domain sensor uses the reflectometry t
22. tre imprim s ou autrement reproduits sans son autorisation 0 612 62826 4 Canad THE UNIVERSITY OF MANITOBA FACULTY OF GRADUATE STUDIES keakk COPYRIGHT PERMISSION EVALUATION OF THE PERFORMANCE OF SOIL MOISTURE SENSORS IN LABORATORY SCALE LYSEMETERS BY SOPHIE PROULX A Thesis Practicum submitted to the Faculty of Graduate Studies of The University of Manitoba in partial fulfillment of the requirement of the degree of MASTER OF SCIENCE SOPHIE PROULX 2001 Permission has been granted to the Library of the University of Manitoba to lend or sell copies of this thesis practicum to the National Library of Canada to microfilm this thesis and to lend or sell copies of the film and to University Microfilms Inc to publish an abstract of this thesis practicum This reproduction or copy of this thesis has been made available by authority of the copyright owner solely for the purpose of private study and research and may only be reproduced and copied as permitted by copyright laws or with express written authorization from the copyright owner ABSTRACT Soil moisture sensors were evaluated in laboratory scale lysimeters The performances of tensiometers granular matrix sensors GMS capacitance sensors phase transmission sensors and of a portable capacitance probe and a frequency domain reflectometry FDR sensor was observed in loam 31 5 sand 45 2 silt and 23 1 clay and silt loam 20 sand 54 silt and 26 cla
23. 2000 Although the electronic module was resealed after the adjustment of the resistive trimmer capacitors water entered all sensors despite the caulking and electrical tape used The water intrusion caused the malfunction of the sensors for the first part of the drying cycle down to 9 of 30 Five of the sensors responded to variations of soil moisture below 30 whereas one sensor did not respond over the entire test A single sensor however gave accurate measurements in the 20 to 30 range which led to conclude that the calibration performed was inadequate for five sensors out of six 5 2 3 5 ThetaProbe FDR sensor The 0 measurements obtained with the ThetaProbe FDR sensor are compared to the lysimeters 6 measurements in Figs 27 and 28 In foam the ThetaProbe measured accurately 8 over the entire soil moisture range Fig 27 The regression line shows that the probe responded finearly to moisture changes with an offset from the 1 1 line of less than 3 In silt loam the probe gave very accurate measurements The linear regression follows the 1 1 line perfectly Fig 28 Greater variability was observed for measurements made in loam than for those made in silt loam Based on the 95 confidence interval of the mean of the three measurements taken on each occasion the ThetaProbe had good precision in loam and very good precision in silt eam However a few measurements showed very high 87 variability in loam three data points have an err
24. 5 and 0 45 m below the soil surface Thus in 1999 all sensors were tested in loam only To install tensiometers an auger hole with a smaller diameter than the tensiometers was made to the required depth The resulting hole was then partially filled with water and the tensiometer inserted into it The soil surface was lightly packed around the tensiometer to prevent preferential flow along the stem 32 4 1 2 2 Watermark GMS In 1998 18 Watermark sensors were tested whereas only 12 were used during the following season Before instaliation the sensors were soaked in water then allowed to dry and soaked again The user s manual published by Irrometer Co recommends that Watermark sensors be installed wet The sensors were installed following the same procedure and pattern of sets as for the tensiometers 4 1 2 3 Aquaterr The Aquaterr is a portable probe therefore a single sensor was required To take water content readings the probe was pushed into the soil to the desired depth A soil auger was used to drill an access hole for measurements at depths greater than 0 30 m A measurement immediately followed the drilling of the required access hole to prevent soil moisture loss by evaporation Readings were taken on each plot following the pattern of sets described for the tensiometers 4 1 2 4 Aqua Tel For the 1998 experiment only three Aqua Tel sensors were used They were all installed at a depth of 0 20 m In the irrigated plot two probes w
25. 54 5 2 1 Experimental design o0o oooooooooooommooo 54 5 2 2 Calibration of the weighing system 55 5 2 3 Accuracy and precision of the sensors 56 5 2 3 1 Tensiometers and Watermark GMS 81 5 2 3 2 Aquaterr soil moisture meter 84 5 2 3 3 Aqua Tel capacitance probes 85 5 2 3 4 VIRRIB phase transmission sensors 86 5 2 3 5 TNELarTobe FDR SEMSOF ooooooooooooo or 5 3 Functional considerations 0 0000 00 88 5 3 1 T nsiomet rs ico oa oe ties eRe ceo kee ae Pee eka is 89 5 3 2 Watermark GMS 222 222 ee eee eee 89 5 3 3 Aquaterr portable capacitance probe nnd tos ae 89 5 3 4 Aqua Tel capacitance sensors oooooooooooo 90 5 3 5 VIRRIB phase transmission sensors 90 5 3 6 ThetaProbe FOR sensor tii Ra 90 6 CONCEUSION Sd oa 92 6 1 Experimental considerations 0 0 002 92 6 2 Performance of the sensors 000 0 0 2 92 6 2 1 Accuracy and precision 0 0 02 eee eee ee eee 92 6 2 2 Ease OF Use Las oy eke tls eh e ss 94 6 2 3 Overall performance o oooooooooroomomoo 96 7 RECOMMENDATIONS 013 te clyde ye a ey i et 98 7 1 Recommended future research o ooooooooooooooo 98 7 2 Application of research results o oooooomocoooomooo 98 S REFERENCES sui
26. 93105 USA and distributed in Canada by Hoskin Scientific 239 East 6 Ave Vancouver B C The only difference between the two models is the reservoir We used the 2710AR without a reservoir whereas the 2725AR was equipped with the Jet Fill reservoir cap The Jet Fill reservoir features a mechanism that allows for the refill of the stem of the tensiometer and the removal of accumulated air Without removing the cap the water contained in the reservoir is injected in the stem by pushing the button of the Jet Fill The flexible reservoirs cover fits tightly around the button to prevent air from entering the reservoir Both models of tensiometers can measure soil matric potential from saturation to approximately 785 kPa Soilmoisture Equipment Corp 1984 3 2 2 Watermark granular matrix sensors Watermark granular matrix sensors give indirect readings of the soil water potential based on measurement of electrical resistance within a granular matrix in contact with the moist soil The GMS sends an electrical resistance measurements ranging from 0 5 to 30 0 KQ to the meter which then converts the value using a programmed calibration function that relates the signal output to soil matric potential This moisture sensor consists of two concentric electrodes embedded in a porous matrix The matrix is composed of loose granular material held 26 in place with a permeable membrane covered by a perforated metal case A solid gypsum wafer divides the gran
27. EVALUATION OF THE PERFORMANCE OF SOIL MOISTURE SENSORS IN LABORATORY SCALE LYSIMETERS by SOPHIE PROULX A Thesis Submitted to the Faculty of Graduate Studies in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Department of Biosystems Engineering University of Manitoba Winnipeg Manitoba August 2001 ivi National Library of Canada du Canada Acquisitions and Bibliographic Services 395 Wellington Street Ottawa ON KIA ONS Canada Acquisitions et Canada The author has granted a non exclusive licence allowing the National Library of Canada to reproduce loan distribute or sell copies of this thesis in microform paper or electronic formats The author retains ownership of the copyright in this thesis Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author s permission Biblioth que nationale services bibliographiques 395 rue Wellington Ottawa ON K1A ONG Your file Votre r idrence Our fle Notre rdf rence L auteur a accord une licence non exclusive permettant a la Biblioth que nationale du Canada de reproduire pr ter distribuer ou vendre des copies de cette th se sous la forme de microfiche film de reproduction sur papier ou sur format lectronique L auteur conserve la propri t du droit d auteur qui prot ge cette th se Ni la th se ni des extraits substantiels de celle ci ne doivent
28. Figs 13a to 24b 27 and 28 Measurements were qualified as very accurate in a given 8 range when the 1 1 line was contained within the dashed lines delimiting the 95 confidence interval of the linear regression For cases where the 1 1 line did not fall within the 95 confidence interval of the linear regression but the data points or the error bars were touching the 1 1 line the measurements were qualified as accurate Finally the measurements were qualified as inaccurate in a given 8 range when these conditions were not met The precision of a sensor was assessed based on the 95 confidence interval calculated for each data point illustrated by the error bars in s 13a to 24b 27 and 28 The precision was qualified as excellent when the error bars corresponded to a range of 8 of less that 2 for most data points over the entire drying cycle as very good when the error bars covered a range of 8 of less that 4 as good when they represented a 56 Sensors Oy Fig 13a Actual 6y Tensiometers results in loam of p 1190 kg m Volumetric water content in percentage measured with tensiometers as a function of 6 in percentage obtained with the lysimeter Each data point represents the mean of the 6 measurements from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence inte
29. applications that do not need high accuracy and precision and that require numerous sampling locations e g to determine the uniformity of soil moisture within a garden or the infiltration depth after applying water because it is relatively easy to use and portable 99 8 REFERENCES Aboukhaled A A Alfaro and M Smith 1982 Lysimeters FAO Irrigation and Drainage Paper No 39 Rome Italy FAO Aquaterr Instuments Aquaterr Instruments User Manual Aquaterr Instruments Fremont CA Automata Inc 1996 Remotely Measure Soil Moisture with Aqua Tel 94 Automata Inc Grass Valley CA Bhardwaj S P and G Sastry 1979 Development and installation of a simple mechanical weighing type lysimeter Transactions of the ASAE 22 4 797 802 Campbell M D G W Gee R R Kirkham S J Phillips and N R Wing 1991 Water balance lysimetry at nuclear waste site in Proceedings of the International Symposium on Lysimetry 125 132 Honolulu Hawaii Irrigation and Drainage Division of the American Society of Civil Engineers Canadian Standards Association 1979 Canadian Metric Practice Guide CSA Dean T J J P Bell and A J B Baty 1987 Soil moisture measurements by an improved capacitance technique part Sensor design and performance Journal of Hydrology 93 67 78 Delta T Devices Ltd 1998 ThetaProbe Soil Moisture Sensor Type ML2 User Manual Delta T Devices Ltd Cambridge UK Eldredge E P C C Schock and T D Stiebe
30. asurements using TDR and other techniques in Kwazulu Natal South Africa http marathon usask ca eripley theta html April 2000 Shock C C and J M Barnum 1993 Calibration of soil sensors for potato irrigation Abstract American Potato Journal 70 840 Sinnema W 1988 Electronic Transmission Technology Lines Waves and Antennas 2 ed Englewood Cliffs NJ Prentice Hall Smedema L K and D W Rycroft 1983 Land Drainage Planning and Design of Agricultural Drainage Systems Ithaca NY Cornell University Press Smit J 1996 TDR vs FDR vs capacitance what exactly are these http www sowacs com sensors whatistdrfdt ntmi April 2000 Soilmoisture Equipment Corp 1984 2725 Series Jet Fill Tensiometers Soilmoisture Equipment Corp Santa Barbara CA 103 Starr G The VIRRIB sensor http Awww sowacs com sensors virrib html April 2000 Stieber T D and C C Schock 1995 Placement of soil moisture sensors in sprinkler irrigated potatoes American Potato Journal 72 533 543 Thomas A M 1966 In situ measurements of moisture in soil and similar substances by fringe capacitance Journal of Scientific Instruments 43 21 27 Topp G C J L Davis and A P Anna 1980 Electromagnetic determination of soil water content measurements in coaxial transmission line Water Resources Research 16 574 582 Wobschall D 1977 A theory of the complex dielectric permittivity of soil containing water the semidisperse model
31. asy to install do not require calibration and the measurements are continuously displayed The measured quantity however is the soil matric potential Furthermore tensiometers need to be refilled periodically and because of the fragility of the sensing tip they must be handled with care Watermark GMS are also simple to install and the measurements are instantaneously displayed at the press of a button Nevertheless because they require a calibration against temperature the operator must measure soil temperature independently and enter it onto the meter Watermark sensors display values of W that need to be converted to volumetric water content The Aquaterr capacitance probe is a portable device and thus can easily measure soil moisture in several locations It is labourious however to insert the probe at depths greater than 0 30 m without previously digging an access hole On the other hand it is difficult to establish a good soil electrodes contact when inserting the sensing tip shallowly in soils with low bulk density The Aquaterr also needs onsite calibrations against moisture content which requires submerging the sensing tip in water The temperature of the calibration water should be within 3 C of that of the soil as 94 temperature was found to have an effect on the measurements The readings are interpreted based on a colour code specific to three general soil textures i e sand loam and clay provided on the meter To achieve a
32. brated during the manufacturing process a soil specific calibration would likely improve their performance After the sensors had been installed in dry soil a first reading was taken with each probe and the actual volumetric water content of the soil was determined by gravimetric sampling and converted using Eq 1 Then the soil was saturated and a second reading was taken as well as the actual volumetric water content The setting value SV was then calculated as follows SV F F K 14 with W K 15 w T W where F sensor s reading in dry soil m m F sensor s reading in wet soil m m W volumetric water content of the dry soil m m W volumetric water content of the wet soil m3 m K constant dimensionless The output signal of each sensor is then adjusted to its respective setting value by 43 Fig 8 Top view of the half filled lysimeter The installation position of the VIRRIB and Aqua Tel sensors are shown turning the resistive trimmer capacitor labelled OFFSET located on the back of the sensing probe To access the OFFSET button a trench was dug to uncover the electronic module without uncovering the rods of the probe The protective seal was open and the setting value was set by turning the button using a screw driver The second resistive trimmer capacitor labelled AMPLIFICATION which is also located at the back of the sensing probe on the electronic
33. cannot be expressed with a single relationship over the entire moisture range from 0 by volume to saturation The f function increases slower at low moisture content where a large portion of the water is tightly bound to the soil particles than at higher moisture content As the soil moisture content increases the layer of bound water at the surface of soil particles becomes larger and the binding forces decrease Thus as the moisture content increases the proportion of water molecules that are free to get polarized also increases thereby causing a rapid increase of f Since the adhesive force varies with soil texture amp f of bulk soil is dependent on the particle size distribution or soil texture at low moisture contents Empirical relationships and mixing models are commonly used to determine soil moisture content from f measurements Thomas 1966 established two empirical relationships between soil moisture 9 and the real relative dielectric permittivity of bulk soil A linear equation described the f 9 correlation for 8 below 10 0 by volume whereas a semi logarithmic relationship was selected for 8 from 4 5 to 45 0 by volume He concluded that soil texture did not have a significant effect on fringe capacitance measurements made with a dielectric sensor operating at a frequency of 30 MHz In 1980 Topp et al developed a noniinear equation that has been widely 14 accepted as reference for time domain
34. certain level of accuracy though the operator needs to correlate the displayed value to soii moisture content which requires soil specific information such as texture and moisture characteristics The Aqua Tel capacitance sensors do not need onsite calibrations and the measurements are obtained instantaneously by pressing a button The installation of the 0 83 m long electrodes requires that a trench be dug The readings must be converted using texture specific calibration curves published by the manufacturer or with the equation developed by the experimenters for which texture only has minimal influence on the conversion The VIRRIB phase transmission sensors display a reading of 8 directly at the trigger of a button Due to the size and configuration of the sensing device the installation requires digging trenches as for the Aqua Tel sensors The VIRRIB sensors also need a calibration which is time consuming and labour intensive due to the location of the adjustment knobs The ThetaProbe FDR sensor was used as a portable probe in this experiment but can also be permanently installed An access hole is required to insert the sensing device in the soil without compacting it excessively The user s manual specifies an accuracy of 15 of 8 when calibration constants provided by the manufacturer are used However a specified accuracy of 2 of 8 can be achieved when a soil specific calibration is performed The ThetaProbe can be used with a hand held
35. ch they travel is based on the propagation of electromagnetic waves in transmission lines The velocity of propagation is described by the following equation y 4 Hr for which H 4 5 Ho and E Ers 6 r E where v velocity of propagation m s H relative permeability constant decimal fraction H absolute permeability of the medium H m Mo permeability of free space 1 257x107 H m E relative permittivity or dielectric constant decimal fraction E absolute permittivity of the medium F m absolute permittivity of free space 8 854x10 F m c velocity of light in free space 3x10 m s The velocity of wave propagation in a transmission line can also be expressed by 10 la v 7 where d distance of travel m t propagation time to travel the distance d s Although the physical configuration of sensing instruments can vary greatly they all require that the medium be placed somewhere within the electric field produced either by simple electrodes or by a transmission line In the most simple configuration the porous medium is positioned between two electrades which form the plates of a capacitor In more complex systems the medium forms either a portion or the whole dielectrics of a transmission tine which can itself vary in its configuration The electrical properties of the surrounding medium therefore directly affect the propagation of the electr
36. d for all trials A few data points however show error bars of about 5 in magnitude in loam three data points in the 1999 trial Fig 16 and four in the 2000 trial Fig 17b Soil matric potential is a quantity of significance in agriculture and thus measurements obtained with tensiometers and Watermark GMS would not require conversion for an experienced user For the purpose of comparison however the W values were converted to 8 using the soil moisture characteristic curve respective to each soil Figs 9 10 and 11 The soil moisture characteristic curves specific to loam and silt loam of the Ramada series were not available thus they were determined in the laboratory with the pressure plate method Moreover the deviation of the converted 0 values from the lysimeter 6 obtained with the lysimeter followed the same trend for both tensiometers and GMS 83 greatly overestimating 8 in both soils for the trial of 2000 This can indicate that the W to 8 conversion or more specifically the soil moisture characteristic curves estimated for the soil conditions of 2000 may have been prone to error 5 2 3 2 Aquaterr soil moisture meter The 0 measurements obtained with the Aquaterr soil moisture meter are compared with the lysimeters 8 measurements in Figs 19 to 21b Figure 21b is a revised version of 21a for which Aquaterr readings exceeding 100 were rejected Because a reading of 100 is set by immersing the probe in water values exce
37. d with the Watermark GMS are compared with the lysimeters 6 measurements in Figs 16 to 18b Figures 17b and 18b are modified versions of Figs 17a and 18a for which measurements of soil matric potential W below 10 kPa were rejected The first three data points were discarded for both trials in 2000 i e in loam and siit loam because it had been observed that Watermark sensors 82 do not respond linearly to changes in soil moisture in the 0 10 kPa range Phene et al 1989 which correspond to 8 values above 55 in loam and above 50 in silt loam Granular matrix sensors gave accurate readings in loam for the 1999 trial in the 20 26 range and slightly overestimated 8 below and above that range The regression line is offset from the 1 1 line by less that 3 Fig 16 In 2000 the 8 measurements obtained with the GMS in both soils followed the same trend as observed for tensiometers mainly overestimating 8 In Fig 17b the regression line shows that the difference between the sensors 6 measurements and the lysimeter 8 increased with soil moisture Accurate measurements were obtained in the narrow 20 23 range whereas a divergence of as much as 20 was observed at lysimeter 0 of 34 tn silt loam the difference between the sensors 9 measurements and the lysimeter 8 also increased with soil moisture the regression line is offset from the 1 1 line by 5 at 8 of 20 and by 9 at 45 The precision of the Watermark sensors was very goo
38. e heterogeneity of the soil properties such as bulk density P soil structure and moisture conditions within each experimental plot We observed that soil moisture contents also varied within small sampling areas due to variation in soil moisture intake by plants and variations in evaporation from the soil surface due to the irregular coverage by potato plant canopy Since the gravimetric method was used as a reference to evaluate the performance of the sensors this wide variability in soit moisture contents led us to question the validity of the field study results Furthermore we questioned the validity of the field study with regards to the testing conditions required for the Aquaterr probe Aqua Tel sensors and VIRRIB sensors These sensors required uniform soil conditions within their effective volume in order to give measurements representative of the surrounding soil Consequently the presence of roots potato tubers and air pockets in the proximity of these probes caused erroneous measurements The erosion resulting from an intense rain or water application also contributed to the disturbance of the soil along the slopes of the growing hills creating air gaps and sometimes exposing the soil moisture probes Finally due to the high amount of precipitation received during the growing seasons of 1998 and 1999 the performance of the sensors could not be evaluated over a wide range of soil moisture contents in the field study Consequently
39. e different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression Sensors Oy Fig 17b Watermark 2000 Loam 15 20 25 30 35 40 45 50 55 Actual 6 Watermark GMS resuits in loam of p 1150 kg m modified Modified version of Fig 17a for which Y measurements below 10 kPa were rejected 65 Sensors Oy Fig 18a Watermark 2000 Silt Loam 15 20 25 30 35 40 45 50 55 Actual 9y Watermark GMS results in silt loam of p 1250 kg m Volumetric water content measured with the sensors in percentage as a function of 8 obtained with the lysimeter in percentage Each data point represents the mean of the g measurements from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 66 Sensors Oy Fig 18b 15 20 25 30 35 40 45 50 55 Actual Oy Watermark GMS results in silt loam of p 1250 kg m modified Modified version of Fig 18a for which Y measurements below 10 kPa were rejected 67 Sensors 6y Fig 19 15 20 25 30 35 40 45 Actual 6y Aquaterr results in loam of p 1190 kg m Volumetric water content measured with the probe i
40. e first field trial the sensors were installed on two plots exposed to different water treatments Plot A was irrigated when the soil moisture content monitored with neutron gauges was less than 35 of the available water which represented a volumetric water content of approximately 25 whereas plot B was rainfed The available water content is the water that can be used by the plant and it is equal to the difference between the moisture content at field capacity FC and at the permanent wilting point PWP Field capacity is the water content that a given soil attains after it has been fully saturated and allowed to drain for 2 d The permanent wilting point is the water content that a soil reaches at the time water extraction by plants has ceased Hanks and Ashcroft 1980 For the second field trial the sensors were installed on a single plot which was irrigated when needed and did not follow any pre established conditions 4 1 2 Sensors installation 4 1 2 1 Tensiometers in 1998 a total of 17 tensiometers was used for the experiment 9 on plot A and 8 on plot B The tensiometers were grouped in sets of three sensors installed at three different depths 0 20 0 30 and 0 50 m below the soil surface This way each set covered the whole root zone A middle length tensiometer was broken during the installation Thus one set on plot B lacked a meter at the 0 3 m depth In 1999 12 tensiometers were used They were grouped in pairs installed at 0 2
41. e to problems with data conversion However they could serve as triggering devices for initiating an automatic irrigation system for soils in the high moisture range The evaluation of the VIRRIB sensors performance remains inconclusive due to the malfunction of the sensors resulting from calibration ACKNOWLEDGMENTS thank the Keystone Vegetable Producers Association and the Association of Irrigators of Manitoba for their financial support would like to thank Mr P Haluschak for his assistance in research site selection and for providing soil analysis data Thanks to the staff at the Manitoba Crop Diversification Centre in Carberry Manitoba and to the Wiebes for allowing me to conduct research on their property am grateful to my advisor Dr R Sri Ranjan for his guidance and patience needed during this research also thank Dr W E Muir and Dr M Entz for serving on my advisory committee and for their valuable suggestions in reviewing this thesis wish to thank Mr D Bourne Mr M McDonald and Mr J Putnam for their technical help as well as for their constant encouragement also wish to thank Mr G Klassen for his valued guidance and for his enthusiasm that helped me throughout this research Thanks are due to my family friends and colleagues and all staff of the department of Biosystems Engineering for their friendship and help Special thanks to Genny who always knew how to be a good friend Finally wi
42. easured soil moisture accurately only in narrow ranges in the trial of 1999 whereas they greatly overestimated 8 in both soils in 2000 Nevertheless tensiometers and Watermark GMS were the most precise instruments for all the tests The repeatability of the measurements indicated that the data conversion might be the source of inaccuracy Only one of the VIRRIB phase transmission sensors used to measure 8 in loam in 1999 was accurate Furthermore those accurate measurements were obtained only over a narrow range of 6 from 18 to 23 The other two sensors responded to variations of soil moisture content in the same manner with an offset of about 5 of 6 For the trial of 2000 the VIRRIB sensors did not respond to changes in 8 due to water 93 leaking into the electronics resulting from the calibration process Nonetheless one sensor gave accurate measurements of 8 in loam in the 20 to 30 range after the moisture inside the module had dried out 6 2 2 Ease of use In addition to their accuracy and precision the sensors were evaluated with regard to their ease of use That is the various soil moisture sensors were assessed in relation to the tasks to accomplish the installation calibration and operation of the sensing device as well as for the interpretation of the readings obtained The assessment was mostly qualitative and was based on time consumption labour intensiveness and knowledge required to perform the tasks Tensiometers are e
43. echnique to measure dielectric properties of media This frequency domain reflectometry FDR sensor operates at a fixed frequency and the signal used is a sinusoidal wave FDR sensors generally have the same extended transmission line configuration as TDR probes for which the bulk soil act as dielectric The frequency of the transmitted signal is swept under contro and the reflection caused by the impedance discontinuity is added to the transmitted signal The voltage standing wave ratio VSWR which is the quantity of interest is then obtained by plotting the reflection as a function of the swept frequency http Avww sowacs com sensors whatistdrfdt htmi Smit 1996 April 2000 3 1 2 4 Phase domain sensors The phase shift of a sinusoidal wave relative to its original phase depends on the length of travel the frequency and the velocity When operating at fixed frequency over a known distance the phase shift depends only on the velocity which is a function of the dielectric properties and thus of the moisture content 25 of the surrounding medium http Awww sowacs com sensors virrib htm Starr April 2000 No information concerning techniques used to measure phase shift could be found in the literature 3 2 Sensors tested in this research 3 2 1 Tensiometers Two models of tensiometers were included in the studies the 2710AR and the 2725AR which are both manufactured by Soilmoisture Equipment Corp PO Box 30025 Santa Barbara CA
44. ed on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 70 Sensors Oy Aquaterr 2000 Silt Loam 10 15 20 25 30 35 40 45 50 55 Actual Oy Fig 21b Aquaterr results in silt loam of p 1250 kg m modified Revised version of Fig 21a for which readings exceeding 100 were rejected 71 Sensors 0y Fig 22 Aqua Tel 1999 Loam 10 15 20 25 30 35 40 45 Actual Oy Aqua Tel sensors results in loam of p 1190 kg m Volumetric water content measured with the sensors in percentage as a function of 8 obtained with the lysimeter in percentage Each data point represents the mean of the 8 measurements from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 72 Sensors 6y Fig 23 40 35 30 25 20 15 10 if 2 e A o f Ad A i PAM B io YA i ys I l i Aqua Tel 2000 Loam 10 15 20 25 30 35 40 4 Actual 6 Aqua Tel sensors results in loam of p 1150 kg m Volumetric water content measured with the sensors in percentage as a function of 8 obtained with the lysimeter in percentage Each data point represents the mean of the 8 measurements
45. eding that were therefore assumed erroneous The first two data points were discarded because all three readings in the first test and two of the three readings in the second test were higher than 100 In the 1999 trial the 8 measurements obtained with the Aquaterr in loam matched the lysimeter 8 accurately over the 23 31 range for which the measurements were qualified of very accurate in the narrow 27 to 30 range based on the statistical evaluation of accuracy described in section 5 2 3 Fig 19 Although the error bar of the data points in the 8 range of 19 to 31 touch the 1 1 line the regression line shows that the Aquaterr probe overestimated 8 below 24 whereas it tended to underestimate it above 31 In 2000 the Aquaterr gave very accurate measurements of 6 in loam over the entire drying cycle Fig 20 In silt loam accurate 8 measurements were obtained for 6 below 28 from which measurements in the 20 to 24 range were very accurate Fig 21b However the Aquaterr showed poor precision illustrated by the large error bars for all trials especially in the 1999 trial As for the Watermark GMS the data conversion for the Aquaterr probe was performed in two stages Firstly the electrical quantity measured with the capacitance sensor was converted to a scale of 0 to 100 that was displayed on the meter Then the displayed value R was compared with a colour coded legend that interpreted the 84 readings in terms of soil mois
46. ent 45 4 3 Data conversion The readings obtained with the sensors required conversion to a unit of reference which in this case was volumetric water content 4 3 1 Tensiometers and Watermark granular matrix sensors Tensiometers and Watermark GMS gave soil matric potential readings in cBar and kPa respectively Those soil moisture tension measurements were converted to volumetric water contents using the soil moisture characteristic curve for the particular soil at a given soil bulk density Figs 9 10 and 11 4 3 2 Aquaterr The Aquaterr s readings also had to be converted to volumetric water content 6 The relationship between the meter s readings R and 8 was established for each soil at different bulk densities based on the soil specific moisture characteristic curves previously determined and the data provided by Aquaterr Instruments Inc The R values at different moisture conditions estimated from the information provided in the user s manual are presented in Table Table R values for the Aquaterr at different moisture conditions Loam Sitloam R saturation 10 O R field capacity 93 95 R permanent wilting point 25 30 The R values were then correlated to the corresponding volumetric water contents using the soil moisture characteristic curves of each soil at each soil bulk density Assuming a linear relationship readings obtained with the Aquaterr were converted to volumetric water content bv interpolation a
47. ere installed one with the blades flat and the other with the blades on the edge or side position The last sensor was installed in the side position in the rainfed plot In 1999 four probes were used and they were all installed in the side position at a depth of 0 25 m To install the Aqua Tel sensor a trench was dug to the installation depth The sensor was placed at the bottom of the trench in the desired position Then the opening was filled with soil and packed to ensure good contact between the blades and the soil 4 1 2 5 VIRRIB In 1998 four VIRRIB moisture sensors were used The two VIRRIB sensors used on each plot were installed in different orientations buried in the hill either across or along the rows In 1999 six sensors were used all installed along the rows 33 To cover the whole sensor a trench 0 30 m deep was dug the sensor positioned and the trench filled 4 1 3 Data collection In 1998 data were collected over a period of 8 wk on five occasions for the irrigated plot and on four occasions for the rainfed plot In 1999 data were collected on seven occasions over a period of 25 d Soil samples for the gravimetric method were taken following the same pattern of sets described for the tensiometers i e a total of 9 samples were taken triplicates at each of the three depths The location of each set was randomly determined and documented on each occasion to ensure the sampling of undisturbed soil The Aquaterr required
48. es a ae Be thee ee 26 3 2 2 Watermark granular matrix sensors 26 32 3 AQUI rd ria Rea DRE trig dc 27 eee MU NSE oh ce he eee aus eee toon ota late Wes 22 S25 VIRRIB dara eins Sue acer en tase es 29 3 2 6 Theta Probe ece cenae ea EA ee esas eee 29 4 MATERIALS AND METHODS 0 20 2 22 cee eee eee tenes 31 A A A eh race ee eae AY 31 4 1 1 Experimental site description o 31 4 1 2 Sensors installation 0oo oooooooommmcmr roo 32 4 1 2 1 Tensiometers 2 02 2 ce eee ee 32 4 1 2 2 Watermark GMS E 33 4 1 23 AQUEST rada a 33 SA LAAQUETO ns E eee ales 33 Fy 67 oe aida ist 33 4 1 3 Data CoulectOn vc 25 och 06504 bone ob 34 4 2 Laboratory study nc ccc Se eae e a ae 34 A 2A E A ade O EAA 34 4 2 1 1 Lysimeter design o ooo o o o 34 4 2 1 2 Lysimeter calibration 39 4213 SO DOCKS e e Oe Seed ee 40 4 2 2 Sensors installation and calibration 41 42 3 Uata ColecuOn esla rra toes widows sh dees 45 4 3 Data cOnversion oo A CORES 46 4 3 1 Tensiometers and Watermark granular matrix sensors 46 4 32 AQUI 2500 gore eae ee ne ea CEU hee aan E 46 6 Se AQUI A der eae ae kee ey 50 SAWP Re Dd eg EY eke a as 50 4 3 5 TBR ODE sous ts peel eee ae kee hd ca aes Se 50 5 RESULTS and DISCUSSION oooooooocororccnreascrcrnca no 52 5A Pie AE O A E A Ss 52 A A A ce Nie we
49. f tensiometers The ThetaProbe performed very accurately and consistently over a wide range of soil textures and was the only dielectric sensor to perform weil Eldredge et al 1993 compared GMS readings to soil water measurements obtained with tensiometers a neutron probe and gravimetric sampling The experiment was conducted in a potato field in a silt loam soil They reported that the Watermark GMS measurements were closely related to those obtained with the other techniques They also established that when calibrated against tensiometers the Watermark meter model 30KTC had a linear response over the range from 0 to 80 kJ kg represented by the following relationship with a significance of 89 W 6 44 0 738 R where Y is the soil matric potential in J kg and R is the meter s readings in dimensionless units However because GMS measure soil matric potential rather than soil moisture content they were found to be more suited for automatic irrigation triggering based on preset soil matric values Phene et al 1989 reported the use of a soil matric potential sensor SMPS for automatically initiating irrigation when the soil matric potential has reached a preset level They found a linear response between 10 3 3 and 300 5 5 J kg J kg kPa cBars Research conducted by Stieber and Shock 1995 established that the performance of Watermark sensors in silt loam in a potato field was largely dependent on the depth of installation of the
50. f the weighing system Those factors however could not be controlled and thus were considered part of the experimental error The calibration was also verified by measuring the actual soil water content by gravimetric sampling at three locations within the soil block on several occasions during the course of the study Gravimetric sampling was done on five occasions during the lysimeter study of 1999 and on four occasions during the trial of 2000 4 2 1 3 Soil blocks The lysimeters were filled with soil brought from the field site i e loam and silt loam from the Ramada series that had been air dried and sieved through a 0 01 m mesh The filled in method was used for reconstituting the soil blocks Uniform bulk density was achieved by packing the soil in the container in small equal sized lifts thereby providing homogenous hydrological conditions for testing of the soil moisture sensors Layers of loose soil approximately 0 05 m deep were packed consecutively using a 0 75 m long piece of lumber 0 038 m thick and 0 178 m wide on which the experimenter applied pressure Pressure was applied uniformly by systematically placing the lumber on an area of loose soil by stepping on it 50 times and by repeating this process unti the soil of the entire surface area of the lysimeter was packed For the first laboratory trial conducted in 1999 only one lysimeter was used The lysimeter was filled with loam packed to a dry bulk density of 1190 kg m Bot
51. frequency limit of the reat dielectric permittivity 4 22 w Static value of the real dielectric permittivity 80 10 few relaxation frequency of water 17 113 GHz f measurement frequency Hz i square root of 1 7 parameter accountina for a spread in relaxation frequency 0 013 12 Ow conductivity of free water S m Those values are given at 25 C Heimovaara et al 1994 Values of the relative complex dielectric permittivity of free water as a function of frequency are given by Thomas 1966 The real part of e f is nearly independent of the measurement frequency over the range from about 50 to 1000 MHz and it varies merely from 81 to 78 over the temperature range of 15 to 25 C The imaginary part of e f for free water without dissolved impurities has a value of about 20 at 10 MHz which decreases as frequency increases to reach 2 at 100 MHz Energy losses in that frequency range are mostly due to conductivity losses Conductivity losses can be substantial at measurement frequencies below 50 MHz in soils due to conductive solids and dissolved impurities such as soluble salts The cut off frequency value below which conductivity losses can be important is not well defined and values ranging from 20 to 50 MHz were reported in the literature reviewed Thomas 1966 Heimovaara et al 1994 Topp et al 1980 As the frequency increases to the GHz range relaxation losses become important reaching a value
52. from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 73 Sensors Oy Fig 24a 35 30 25 20 4 a FC A l VIRRIB 1999 Loam S 35 40 45 Actual Oy VIRRIB sensors results in loam of p 1190 kg m Volumetric water content measured with the sensors in percentage as a function of obtained with the lysimeter in percentage Each data point represents the mean of the 9 measurements from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 74 Sensors Oy Fig 24b 40 35 30 25 20 15 10 10 15 20 VIRRIB sensors 25 Actual 6 results 30 in FC VIRRIB 1999 Loam 35 40 loam of p 1190 kg m modified Modified version of Fig 24a each data point represents a single 6 measurement taken with a sensor 75 Sensors 6y Fig 25 10 16 20 25 30 35 40 45 50 Actual 6 VIRRIB sensors results in loam of p 1150 kg m Volumetric water content measured with a sensor in percentage as a function of 8 obtained with the lysimeter in percentage Each data p
53. gravimetric water content and bulk density on dielectric properties of soil European Journal of Soil Science 47 367 371 Phene C J C P Allee and J D Pierro 1989 Soil matric potential sensor measurements in real time irrigation scheduling Agricultural Water Management 16 173 185 Phene C J G J Hoffman T A Howell D A Clark R M Mead R S Johnson and L E Williams 1991 Automated lysimeter for irrigation and drainage control In Proceedings of the International Symposium on Lysimetry 28 36 Honolulu Hawaii Irrigation and Drainage Division of the American Society of Civil 102 Engineers Phillips S J J F Relyea C J Kemp N R Wing M D Campbell G W Gee M J Graham R R Kirkham M S Ruben 1991 Development of Hanford site lysimeter facilities in Proceedings of the international Symposium on Lysimetry 19 27 Honolulu Hawaii Irrigation and Drainage Division of the American Society of Civil Engineers Proulx S R Sri Ranjan and G Klassen 1998 Evaluation of soil moisture sensors in potato fields ASAE Paper No SD98 118 St Joseph MI ASAE Pruitt W O F J Lourence C B Tanner and P Lal 1991 The U C D drag plate lysimeter momentum and mass transfers In Proceedings of the International Symposium on Lysimetry 46 60 Honolulu Hawaii Irrigation and Drainage Division of the American Society of Civil Engineers Ripley E M Savage G Lukangu and C Everson 1998 Comparison of soil water me
54. h lysimeters were used for the laboratory trial of 2000 Because the soil contained in the lysimeter used in the trial of 1999 had been disturbed by the removal of the sensors the loam was packed once again The dry bulk density reached was 1150 kg m The second lysimeter was filled with silt loam packed to a dry bulk density of 1250 kg m 40 The soil bulk density was determined by monitory the mass of soil used to fill the soil box and measuring the depth of soil in the box once it was filled Then knowing the surface area of the soil container 0 was calculated using the following equation Po Aeh 13 where m mass of dry soil in the soil container kg A surface area of the soil container 1 21 m h depth of dry soil packed in the soil container m Prior to each trial the soil blocks were slowly saturated and allowed to equilibrate for 72 h During the trials between testing air was blown through the bottom of the soil container to increase the drying rate After each fan drying period which never exceeded 2 h in duration the soil was left to dry at ambient conditions i e without forced aeration Each drying period was followed by an equilibration period of at least 12 h during which the lysimeter was covered with a polyethylene sheet to prevent evaporation 4 2 2 Sensors installation and calibration Three sensors of the tensiometers Watermark GMS and VIRRIB probes were installed in each of the iysime
55. he air entry pressure of the porous ceramic cup causing air to break through the largest pores in the cup the tensiometer has reached its limit of operation At this point the gauge will indicate zero and the 23 tensiometer reading is no longer useful At this stage the tensiometer has to be refilled and a vacuum re established prior to further use in a wetter soil environment 3 1 2 Electromagnetic methods 3 1 2 1 Resistance sensors Electrical resistance can be used indirectly to determine the moisture content of a medium The resistance measured between electrodes placed directly in the medium or in a material in hydraulic equilibrium with the medium is directly related to the medium s moisture content Because moist soil resistivity is also a function of temperature soil salinity and solids conductivity a soil specific calibration is required 3 1 2 2 Time domain sensors The propagation time of a signal can be used to determine the dielectric properties of the medium Systems operating on time measurements are referred to as time domain sensors Time domain reflectometry TDR is a well known and widely used technique in which a signal travelling along a transmission line is reflected when meeting an impedance discontinuity and its reflection is then superimposed on the transmitted signal at the transmitter The time measured is the time required for the signal to travel from the transmitter to the end of the transmission line and
56. ilized such a drainage system to control the soil moisture content in a lysimeter used to evaluate soil moisture sensors performance Lysimeter history covers a period of about 300 yr and their application in various fields of research has been largely documented Aboukhaled et al 1982 Aboukhaled et al 1982 presented the evolution of lysimetry techniques through a detailed review of literature Whereas non weighing lysimeters are mostly used for characterization of soil solution percolates weighing lysimeters cover a wider field of utilizations ranging from evapotranspiration measurements of different canopies Fritschen 1991 Klocke et al 1985 to irrigation scheduling Phene et al 1991 Pruitt et al 1991 and Kutilek and Nielsen 1994 reported the use of floating lysimeters to measure the shear stress on the crop canopy caused by wind In addition to those agricultural applications weighing lysimeters have been used in environmental impact assessment studies either to quantify the factors affecting migration of pollutants in soil Phillip et al 1991 Campbell 21 et al 1991 or to evaluate remediation technologies http www pharm arizona edu centers tox_center superfund projects core_ci html February 1999 22 3 PRINCIPLES OF SOIL WATER CONTENT MEASUREMENT 3 1 Commonly used techniques 3 1 1 Tensiometric method Tensiometers give direct measurements of the soil water potential which can also be called soil water tens
57. iometers and Watermark sensors although very precise were mainly inaccurate due to problems with data conversion The evaluation of the VIRRIB sensors performance remains inconclusive due to the malfunction of the sensors resulting from calibration 96 Table V Summary of the evaluation of the sensors ease of use Sensors Installation Onsite calibration Operation Tensiometers easy quick no easy quick Watermark easy quick yes temperature easy quick Aquaterr easy yes moisture easy Aqua Tel labour intensive no easy quick VIRRIB labour intensive no easy quick ThetaProbe easy no easy Finally all the sensors tested in the present study responded to soil moisture variations and thus they could be used as good triggering devices for turning on irrigation systems However a clear understanding of the calibration and data conversion associated with the various instruments coupled with a soil specific calibration are required to establish a fully automated irrigation system 97 7 RECOMMENDATIONS 7 1 Recommended future research The following are recommendations addressed to researchers who are interested in evaluating the performance of soil moisture sensors more effectively 1 To evaluate the performance of the soil moisture sensors over subsequent drying cycles to observe their sensitivity to partial rewetting and their response time 2 To determine systematically the effective volume measured by each sensor and to adjust
58. ion or soil matric potential The tensiometer consists of a ceramic cup connected to a vacuum gauge by a rigid tube The tube can vary in length allowing for measurement at various depths in the soil Prior to installation the tube is filled with de aired water and the water is allowed to saturate the porous tip of the tensiometer A suction pump is then attached to the tube to remove the dissolved air from the porous tip as well as the water column within the tensiometer to insure that most of the dissolved air is evacuated from the system De aired water is poured into the stem to fill it up to the O ring seal The cap is screwed into place carefully without trapping any air in the tensiometer The tensiometer is then sealed and ready to be installed Once buried in the soii the water contained in the porous cup reaches equilibrium with the soil water in the pore space Thus in a completely saturated soil the gauge of the tensiometer would indicate zero since the free soil water would be in equilibrium with the water contained in the tip Under unsaturated conditions the soil contains less water and consequently has a higher capillary tension This creates a tension on the water contained within the porous ceramic cup which releases water until the tension within the cup is equal te the tension existing in the surrounding soil The reading on the gauge of the tensiometer thus indicates the soil water tension When the soil water potential exceeds t
59. isture content The soil matric potential which can also be called matric suction or tension is the resultant of the combination of capillary and adsorptive forces occurring in porous media Capillary forces are mainiy surface tension forces caused by ihe adhesion of water and the soii 8 and by the narrowness of the pores Smedema and Rycroft 1983 The capillary pressure is a function of the size of the pores and can be approximated using 3000 ap p 3 where Pap capillary pressure m D diameter of the pore um Adsorption forces comprise van der Waals and electrostatic forces exerted on the water by the negatively charged colloidal surfaces of the soil particles Tensiometers are used to measure the matric potential of soils 2 2 3 Electromagnetic methods Techniques that rely on the propagation of electromagnetic waves through a medium to measure its electrical properties fall into the category of electromagnetic methods Those methods often referred to as dielectric techniques comprise various type of instruments probes or sensors that differ from each other mainly by the characteristics of the signal sent and the measurement quantities For example voltage steps are commonly used to measure travel time in a medium whereas sine waves may be used to measure frequency shift or amplitude variation The principle behind the use of electromagnetic waves to measure the electrical characteristics of the medium through whi
60. l moisture meter Aqua Tel capacitance sensors and VIRRIB phase transmission sensors At the completion of the field trial of 1998 it was recommended that a laboratory study under controlled soil moisture conditions be conducted A first laboratory trial was therefore carried out during the winter of 1999 following the protocol for installation calibration and use of the sensors established from the first field study During the growing season of 1999 another field trial was conducted hoping to obtain more information concerning the performance of the soil moisture sensors previously tested The results from the second field trial as those from the 1998 trial were not conclusive due to soil heterogeneity and high soil moisture variability within the experimental plot and thus laboratory testing was resumed in the winter of 2000 4 1 1 Experimental site description For both seasons the experiment was carried out at the Manitoba Crop Diversification Centre in Carberry Manitoba The soil in that area is from the Ramada series For the experimental plots of 1998 the first 0 4 m of the soil profile was composed of loam 31 5 sand 45 2 silt and 23 1 clay whereas the underlying layer from 0 4 to 0 6 m consisted of silt loam 20 sand 54 silt and 26 clay For the 1999 trial the plot was located in an area where the loam layer was on average 0 50 m thick Potatoes Solanum tuberosum L CV Russet Burbank were grown on hills 31 For th
61. lains why there is no error bar Moreover in Fig 24b the linear regression was done for the 9 measurements taken with each sensor giving three linear regression lines Ne linear regression was performed in Figs 25 and 26 since the sensors response was not linear over the entire range of soil moisture contents As mentioned earlier the challenge of evaluating the performance of soil moisture sensors lied mostly in the task of understanding the readings obtained from the various instruments which in many cases required conversion to more meaningful soil water information The conversion can require several steps that are for some instruments performed in part by the device but mostly requires that the user be familiar with the quantity displayed by the meter which in itself might require conversion All these conversions may induce error that has to be considered when evaluating the performance of the various soil moisture sensors 5 2 3 1 Tensiometers and Watermark GMS The 8 measurements obtained with the tensiometers are compared with the lysimeter 6 measurements in Figs 13a to 15b Figures 13a 14a and 15a present the complete data collected during the lysimeter studies whereas Figs 13b 14b and 15b are the respective modified versions for which some data points were excluded A data point was rejected when at least one of the three water tension readings did not show a decrease of soil moisture content because 81 it meant that at
62. ltage at the same location on the output signal with regard to the phase of the electromagnetic signal 5 3 Functional considerations Several functional aspects of the various instruments tested should be considered when evaluating their suitability for field applications The following considerations are based on observations made in both the field and the laboratory trials of the present study 88 5 3 1 Tensiometers Tensiometers require continuous monitoring to ensure that they are adequately filled with water and air entry has not taken place They also need to be handled with care since the porous cup made of ceramic can be easily damaged and any material coming in contact with the cup could block its pores Furthermore the Operational limit of the tensiometers 85 cBar is not sufficient for certain crops or applications 3 2 Watermark GMS Granular matrix sensors are not as brittle as tensiometers but they can wear out The membrane covering the granular matrix may become clogged and impede water flow through the sensing device The gypsum wafer used to buffer the effect of soil solution s salinity will dissolve over time These factors contribute to the loss of sensitivity to moisture changes Watermark sensors are affected by temperature and thus soil temperature must be independently monitored and entered onto the meter for calibration purposes When soil moisture is measured deep below the soil surface soil temperature measurement
63. me of a signal and of its reflection and the frequency shift can be observed to measure dielectric properties of a medium Soil moisture sensors are made of a wide range of electronic devices used with various configurations of sensing probes At frequencies below 200 MHz vector impedance meters and radio frequency bridges can be used Thomas 1966 used a Wayne Kerr very high frequency V H F admittance bridge which measured capacitance and conductance to determine soil moisture content by fringe capacitance at 30 MHz Fringe capacitance can be defined as the capacitance measured by the fringing field generated by two electrodes In the case of two flat electrodes facing each other the fringing field is essentially the part of the field not included between the two plates For coplanar electrodes the fringing field is the entire field generated Thomas 1966 Eller and Denoth 1996 determined soil moisture content by measuring the impedance at 32 MHz using a twin T bridge which had been modified to cover a large range of permittivities Wobschall 1978 described a frequency shift dielectric soii moisture sensor operating at 31 MHz that used a 16 capacitor T network to connect the electrodes to the frequency determining resonance LC network Fig 3 The capacitor forms part of the feedback loop of the oscillator The resonance frequency can be expressed as a function of the change in capacitance due to C and G Eq 10 AC fx f 1
64. meter or a data logger both manufactured specially for the use with the sensor In this experiment measurements were made using a power supply and a voltmeter The data logger is programmable so that the readings can be converted to 6 automatically However 95 when the meter is used the output voltage must be converted by the user with the aid of a calibration equation 6 2 3 Overall performance A summary of the quantitative evaluation of the performance with regards to the accuracy and the precision of the sensors based on the criteria described in section 5 2 3 is presented in Table IV Table IV Summary of accuracy and precision of the soil moisture sensors Sensors Accuracy Precision Tensiometers inaccurate very good Watermark inaccurate very good Aquaterr accurate poor Aqua Tel accurate good VIRRIB inaccurate poor ThetaProbe very accurate good loam very good silt foam A summary of the qualitative evaluation of the performance with regards to the ease of use of the sensors based on the criteria described in section 6 2 2 is presented in Table V From both aspects of performance evaluation i e quantitative and qualitative we conclude that the ThetaProbe FDR sensor is most suitable to monitor irrigation needs with accuracy precision and ease of use Both capacitance sensors the Aquaterr probe and the Aqua Tei sensors also performed well measuring soil moisture content accurately although with less precision Tens
65. mprovement of the two techniques previously described uses a network analyser to measure the load reflection coefficient i e the ratio of the reflected voltage to the incident voltage as a magnitude and a phase quantity as a function of frequency Sinnema 1988 Morgan et al 1993 used a reflectometer to measure the amplitude and phase differences at a frequency of 1 25 GHz The values of complex permittivity thereby measured were processed to determine the real permittivity of soil 2 3 Literature review of lysimetry When studying parameters of the soil solution a lysimeter can generally be defined as a large soil block with a bare or vegetated surface in a container opened to the atmosphere located in a natural environment Lysimeters provide information related to the water balance of the system e g precipitation infiltration water storage capacity evapotranspiration and percolation Furthermore the percolates can be collected and analyzed for chemical composition Other devices also called lysimeters are used for sampling the soil solution by means of a vacuum applied to a porous cup introduced into the soil This type of lysimeter will not be further discussed nor referred to in this paper Lysimeters can be divided into two major categories based on whether a weighing system is used to determine the water balance Non weighing lysimeters 19 which are also referred to as volumetric drainage or compensation lysimeters rely
66. n percentage as a function of 8 obtained with the lysimeter in percentage Each data point represents the mean of the 8 measurements taken at three different locations with the portable device The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 68 Sensors Oy Fig 20 15 20 25 30 35 40 45 50 55 Actual 6 Aquaterr results in loam of p 1150 kg m Volumetric water content measured with the probe in percentage as a function of 6 obtained with the lysimeter in percentage Each data point represents the mean of the 8 measurements taken at three different locations with the portable device The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 69 Sensors Oy Fig 21a Aquaterr 2000 Silt Loam 10 15 20 25 30 35 40 45 50 55 Actual Oy Aquaterr results in silt loam of p 1250 kg m Volumetric water content measured with the probe in percentage as a function of 8 obtained with the lysimeter in percentage Each data point represents the mean of the 8 measurements taken at three different locations with the portable device The error bars represent the 95 confidence interval bas
67. nd several research projects evaluated their performance as a tool for triggering events Phene et al 1989 McCann et al 1992 Shock and Barnum 1993 Stieber and Shock 1995 Information reporting the performance of the Aquaterr soil moisture meter Aqua Tel capacitance sensors VIRRIB phase transmission sensors and the ThetaProbe however remains scarce The present study provides more information on these sensors 2 2 Background theory 2 2 1 Gravimetric method The gravimetric method first requires sampling of the soit and immediate weighing of the moist sample The sample is then dried at 105 C for 24 h in a convection oven and weighed again after cooling in a desiccator The mass basis water content of the sample is calculated using m ba 1 m m where 8 gravimetric water content mass fraction m mass of water contained in the sample kg m mass of dry soil in the sample kg To determine the volumetric water content of the soil the gravimetric water content is multiplied by the apparent specific gravity ASG of the soil The ASG is defined as the ratio of the soil bulk density to the density of water The relationship is simplified as follows 6 9 ASG 2 where 8 volumetric water content volume fraction ASG apparent specific gravity decimal fraction 2 2 2 Tensiometric method The tensiometric method relies on the relationship that exists between the soil matric potential and its mo
68. ned to a sheet of plywood which acted as a base The base provided more stability to the structure and uniformly 35 Plan view econ AAN H 4 i A i 4 RL Angle steel i frame 4 i Flat steel A ELE NE TT i Fig 4 Top view and elevation view of the steel grate The top view also illustrates the dimensions of the soil block 36 Fig 5 Top view of the lysimeter The empty soil container is shown with the steel grate and the acrylic grid The drainage collector is seen through the open bottom of the soil container 37 Acrylic grid RT Rm ee Fig 6 Elevation view of the Iysimeter The empty soil container and the drainage collector with the opening are illustrated 38 distributed the weight of the lysimeter on the hydraulic load cell The weighing system consisted of a hydraulic load cell connected to a manometer The manometer was inclined to increase the precision of the weighing system An angle of 15 from the horizontal was selected because it increased the precision of the lysimeter by a factor of four An inner tube filled with water was used as the hydraulic load cell The inner tube was made of butyl rubber and had an outside diameter of 0 915 m and an inside diameter of 0 485 m when totally deflated 4 2 1 2 Lysimeter calibration A calibration of the weighing system was required to establish a relationship between the mass of the lysimeter and the fluid pressure inside the h
69. netic techniques have been used for soil moisture determination Electromagnetic techniques also called dielectric methods include capacitive approaches such as time domain and frequency domain methods as well as resistive techniques Numerous sensing instruments based on those techniques have been commercialised and are available to producers However very little information concerning their performance is available Thus the most suitable methods to monitor the need for irrigation are yet to be determined The best sensing instruments should measure soil water accurately precisely quickly and should be easy to use The use of a sensor implies installing and operating the instrument as well as interpreting the readings obtained which can require calibration or conversion of the measured quantity to soil moisture units During the growing seasons of 1998 and 1999 we conducted a field study to evaluate the performance of tensiometers Soilmoisture Equipment Corp models 2710AR and 2725AR Watermark granular matric sensors irrometer Co Aqua Tel electrical capacitance sensors Automata inc model Aqua Tel94 29 the Aquaterr electrical capacitance probe Aquaterr instruments Inc model 200 and VIRRIB phase transmission sensors Environmental Sensors Inc Fig 1 Different installation patterns i e orientation of the sensing probes were also included as treatments Soil moisture data obtained with the different instruments were c
70. nsor gave very accurate measurements of 8 in the 18 to 23 range In 2000 only one of the six probes tested gave accurate measurements of 0 over the 20 to 30 range In loam all three VIRRIB sensors gave measurements following the same trend at volumetric water content below 30 Two of the three sensors however greatly overestimated 8 one by about 15 and the other by close to 30 in siit loam one sensor did not respond to changes in soil moisture content giving 9 readings of about 43 over the entire trial Another sensor did not respond to soil moisture variations above 29 and overestimated 6 by as much as 50 in that range The third sensor tested in silt loam gave accurate measurements of 8 in the 20 30 range and overestimated O by about 5 above that range As for the Aqua Tel sensors the data conversion for the VIRRIB sensors was performed in a single step In the case of the VIRRIB sensors the electronic module which is then sent to the meter to be displayed as volumetric water content The conversion equation that is used can be calibrated by first calculating the amplification and offset required and then adjusting the sensor by turning the specific resistive trimmer capacitor located on the electronic module for each sensing device In 1999 the manufacturer s setting was used and it was concluded from the results obtained that a calibration specific to each soil was required The calibration was done prior to the trial of
71. of about 30 at 10 GHz Consequently because both the real dielectric permittivity and the conductivity losses are functions of frequency and do not vary at the same rate nor reach the same maxima and minima a measurement frequency can be chosen depending on the application to obtain significant measurement quantities For example to measure soil salinity which is determined by measuring the soil conductivity a frequency of 10 MHz would be more appropriate than one of 100 MHz Moist bulk soil is a mixture of solids air and bound and free water The relative complex permittivity of the bulk soil is therefore a function of f of each constituent and their respective volume fraction For non conductive solids low salts concentration temperature ranging from 15 to 25 C and at frequencies between 50 MHz and 1 GHz 13 E f of solids is between 2 and 5 that of air is assumed to be equal to that of free space i e 1 that of tightly bound water is close to that of ice i e approximately 3 whereas E f of free water ranges from 78 to 81 The free water volume fraction has thus a great effect on the bulk soil relative complex permittivity Besides the frequency effect other parameters such as soil texture and range of moisture contents have an influence on the bulk soil s dielectric properties As it was observed by Eller and Denoth 1996 and Thomas 1966 the real dielectric permittivity of moist soil as a function of soil moisture content
72. oint represents a single 9 measurement taken with a sensor 76 Sensors Oy VIRRIB 2000 Silt Loam 15 20 25 30 35 40 45 50 Actual Oy Fig 26 VIRRIB sensors results in silt loam of p 1250 kg m Volumetric water content measured with a sensor in percentage as a function of 8 obtained with the lysimeter in percentage Each data point represents a single 6 measurement taken with a sensor 17 Sensors 0y Fig 27 50 45 40 35 30 25 20 15 10 is 9 a l Yo Ai Y K f j A i Pe i ThetaProbe 2000 Loam 10 15 20 25 30 35 40 45 50 Actual y ThetaProbe sensor results in loam of p 1150 kg m Volumetric water content measured with the probe in percentage as a function of 8 obtained with the lysimeter as percentage Each data point represents the mean of the 8 measurements taken at three different locations with the portable device The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 78 Sensors by Fig 28 50 45 40 35 30 25 20 15 10 PWP FC Sy gt ANG A NS WS ThetaProbe i l l l 1 i l l 2000 Silt Loam I l l t l 1 l I 10 15 20 25 30 35 40 45 50 Actual Oy ThetaProbe sensor results
73. omagnetic signal through the media In this case the medium consists of moist soil Electrical properties of moist soil can be described by the relative complex dielectric permittivity function a f e f eos gt 6 where e f relative complex dielectric permittivity of the moist bulk soil dimensionless f real part of e f dimensionless e f relaxation losses dimensionless 11 f measurement frequency Hz i square root of 1 O conductivity of the moist buik soil S m The real part of the relative complex permittivity function is a measure of the energy stored by the dipoles aligned in the applied electromagnetic field In other words f is a measure of the polarization and in turn a measure of the capacitance of the media In soils because most of the dipoles that are free to respond to polarization are water molecules f is closely correlated to the water volume fraction The imaginary part of f is a measure of the energy losses caused by relaxation losses f and conductivity losses O 2mfe The dependence of the relative complex dielectric permittivity of bulk soil on frequency is due to the frequency dependence of free water s dielectric properties The Cole Cole function describes the frequency dependent relative complex dielectric permittivity of free water f as follows Esw T Faw io w 1 fa 2 feo s Enf f z bow where amp high
74. ompared to the gravimetric sampling method which was used as the standard for comparison However the high standard error of the volumetric water content of the samples collected from the field using the gravimetric method indicated a wide variability in water content of the soil under field conditions Based on the results of the fieid experiments further testing under controlled conditions was recommended Proutx et al 1998 Consequently a laboratory study was carried out to test the sensors in more homogeneous soil water conditions 1 2 Objectives The objective of the laboratory study was to evaluate the performance of soil moisture sensors under controlled soil moisture conditions in laboratory scale lysimeters In addition to the five types of sensors previously tested in the field trials and in the laboratory trial of 1999 another dielectric probe was included in the laboratory trial of Tensiometer haiek granular Aquaterr matrix sensor eae VIRRIB phase capacitance een transmission sensor probe l Aqua Te capacitance sensor Fig 1 Soil moisture sensors tested in the field and laboratory trials Tensiometers Watermark GMS VIRRIB sensors Aqua Tel sensors and the Aquaterr probe were tested Fig 2 View of the ThetaProbe sensor tested the laboratory trial of 2000 2000 The ThetaProbe Delta T Devices Ltd type ML2 shown in Fig 2 which can also be buried permanently in the soil was used as a po
75. on the collection of the percolate to establish the hydrological properties of the soil block Thus the water balance is indirectly determined by subtracting the drainage water collected from the total water input The other type of lysimeter called the weighing lysimeter determines the water balance by measuring the change of mass of the soil block Here the drainage component and the soil water conditions can be measured independently and simultaneously Hillel 1971 This means that small variations of moisture content can be measured even in dry soil when no percolation has occurred This explains why most of the lysimeters used in evapotranspiration studies are of the weighing type Howell et al 1991 Grebet and Cuenca 1991 Weighing lysimeters can be classified in four subcategories that group the lysimeters based on the weighing principles or devices utilized Lysimeters equipped with mechanical scales are referred to as mechanical weighing lysimeters The same way electronic weighing lysimeters use strain gauge load cells that output electronic signals Hydraulic weighing lysimeters are based on measurements of pressure changes in a hydraulic load cell Finally floating lysimeters are based on measurements of changes in buoyancy or flotation The soil black contained within the lysimeter boundaries can be reconstituted by two methods The simplest one is called the filled in method As the term indicates the method consists of filling the
76. on average a range of 6 before 4 to 6 Figs 22 and 23 However poor precision was observed for a few readings in the 2000 trial illustrated by large error bars covering in some cases a range of 8 of about 9 Fig 23 The data conversion for the Aqua Tel was performed in a single step The electrical quantity sent to the meter from the electronic module connected to the electrodes ranging from O to 1 mA was amplified to be displayed on a scale of O to 100 85 on the meter The calibration equation Eq 19 used to convert the reading displayed was developed with a statistical analysis of texture specific calibration curves published by Automata inc Equation 19 represents the data points of all the curves with a R squared value of 97 6 When plotted against readings from 0 0 to 100 0 however the maximum volumetric water content returned was 47 8 which does not match 0 at saturation of the soil tested for loam in 1999 55 1 and in 2000 56 6 This was due to the fact that only 9 of the data points from the manufacturer s calibration curves represented moisture contents above 35 2 3 4 VIRRIB phase transmission sensors The VIRRIB readings are compared with the lysimeters 9 measurements in Figs 24a to 26 In 1999 the VIRRIB sensors gave average readings that were about 3 to 7 less than the lysimeter 6 Fig 24a Figure 24b shows the readings of each of the three probes as a function of the lysimeter 8 One VIRRIB se
77. or bar covering a range of 8 larger than 14 whereas only one data points has an error bar larger than 8 in silt loam The data conversion for the ThetaProbe frequency domain reflectometry sensor was performed in a single step Because a voltmeter and a power supply were used instead of the programmable data logger manufactured by Delta Devices Ltd the texture specific calibration equation was used by the experimenter to convert the value displayed on the voltmeter to volumetric water content The calibration equation was developed by first correlating the voltage output read to the apparent permittivity of the soil which was then correlated to the soil moisture content Two distinct sources of error are associated with developing the calibration equation One is related to gravimetric sampling and the other to the electronic circuitry made of the power supply the voltmeter the probe the soil and all the cables and wires The maximum standard error calculated for gravimetric sampling performed to determine the calibration equation was 0 44 in loam and 0 65 in silt loam Because the output voltage kept oscillating even after a period of 120 s was allowed for stabilization we assumed that the signal would not stabilize thus the readings were taken precisely 90 s after the transmission of the signal had begun for all tests By doing so the experimenter tried to counter the error associated with the electronic circuitry by measuring the output vo
78. ot exceed 1 2 except for the first sampling event where it was 5 5 The performance of tensiometers Watermark sensors the Aquaterr soil moisture meter VIRRIB sensors and the ThetaProbe FDR sensor was also assessed in silt loam with a p of 1250 kg m for moisture contents ranging from 45 8 to 19 5 Figs 15a 15b 18a 18b 2ta 21b 26 and 28 The standard errors calculated for the lysimeter 0 did not exceed 0 3 Each of the data point shown in Figs 13a 13b 14a 14b 15a 15b 16 17a 17b 18a 18b 22 23 and 24a represents the average of the 9 measurements from three different sensors In Figs 19 20 21a 21b 27 and 28 each data point represents the mean of 0 measurements taken with one of the portable probes i e Aquaterr or ThetaProbe at three different locations within a lysimeter on a given occasion The error bars represent the 95 confidence interval based on the three measurements for each type of sensor Whenever the error bar is not visible in Figs 80 13a 13b 15a 15b 16 17a 18a 18b 19 20 and 28 the symbol is larger than the error bar itself The tinear regression represented by the thick line was done on the average of the three 8 measurements taken on each occasion for each type of sensor The dashed line represents the 95 confidence interval for the linear regression In Figs 24b 25 and 26 each data point represents a single 6 measurement taken with a sensor on a given occasion which exp
79. presents the mean of the 6 measurements from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 61 Sensors 0y Tensiometer 2000 Silt Loam 15 20 25 30 35 40 45 50 55 Actual 6 Fig 15b Tensiometers resuits in silt loam of p 1250 kg m modified Modified version of Fig 15a for which data points were excluded when at least one of the three tensiometers had reached its operational limit or had failed 62 Sensors 0y Fig 16 Watermark results in loam of p 1190 kg m Volumetric water content measured with Watermark GMS in percentage as a function of 6 obtained with the lysimeter in percentage Each data point represents the mean of the 6 measurements from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 63 Sensors 0y Fig 17a 15 20 25 30 35 40 45 50 55 Actual 6y Watermark GMS results in loam of p 1150 kg m Volumetric water content measured with the sensors in percentage as a function of 8 obtained with the lysimeter in percentage Each data point represents the mean of the 8 measurements from thre
80. r 1993 Calibration of granular matrix sensors for irrigation management Agronomy Journal 85 1228 1232 Eller H and A Denoth 1996 A capacitive soil moisture sensor Journal of Hydrology 185 137 146 100 Environmental Sensors Inc VIRRIB Soil Moisture Sensor Operating Manual for Irrigation Management Environmental Sensors Inc San Diego CA Environmental Sensors Inc Moisture measurement system VIRRIB http Awww envsens com April 2000 Fritschen L J 1991 Transpiration by Douglas fir using lysimeters In Proceedings of the International Symposium on Lysimetry 246 253 Honolulu Hawaii Irrigation and Drainage Division of the American Society of Civil Engineers Gardner C M K T J Dean and J D Cooper 1998 Soil water content measurement with a high frequency capacitance sensor Agricultural Engineering Research 71 395 403 Grebet P and R H Cuenca 1991 History of lysimeter design and effects of environmental disturbances In Proceedings of the Intemational Symposium on Lysimetry 10 18 Honolulu Hawaii Irrigation and Drainage Division of the American Society of Civil Engineers Hanks R J and G L Ashcroft 1980 Applied Soil Physics Soil Water and Temperature Applications Logan UT Springer Veriag Hanson B 1999 Soil moisture instruments http www greenindustry com ii 1999 0499 499soil asp April 2000 Heimovaara T J W Bouten and J M Verstraten 1994 Frequency domain analysis of time
81. racy precision quickness of the response to moisture variation and ease of use which encompasses installing and operating the instrument as well as interpreting the readings The main difficulty of evaluating the performance of the sensors was in understanding the readings obtained with the instruments In most cases the readings had to be converted to a meaningful soil water value that can be used for calculating the irrigation water requirements Furthermore the assessments of an instrument s accuracy and precision were based on the converted quantities i e soil moisture data In some instances the conversion required other soil properties that were difficult to measure reliably thereby adding the error caused by the conversion process to the errors attributed to the sensor Over the entire course of the experiment tensiometers Watermark GMS VIRRIB sensors and the Aquaterr were tested over two drying cycles in loam in 1999 volumetric water contents 8 ranging from 34 0 to 17 0 in 2000 8 ranging from 43 1 to 20 0 and over one drying cycle in silt loam in 2000 8 from 45 8 to 19 5 The Aqua Tel probes were installed in loam only for both years whereas the ThetaProbe was tested in both lysimeters but only in the trial of 2000 Because the response time i e the time required by an instrument to respond to a moisture variation is essential in field applications such as irrigation scheduling an experiment designed over shor
82. reflectometry TDR determination of soil moisture content as a function of the apparent dielectric permittivity The apparent dielectric permittivity referred to by Topp et al is the measured dielectric permittivity which is in fact the relative complex dielectric permittivity of the medium Because their measurements were made in low loss nearly homogenous material the apparent dielectric conductivity was assumed equal to the real dielectric permittivity Topp et al 1980 compared their findings to several other studies including Thomas 1966 work and concluded that bulk soil dielectric permittivity was only weakly dependent on soil texture bulk density temperature and frequency between 20 MHz and 1 GHz Although they did not conclude on the effect of soluble salt content on the measurements they noted that an increase in the conductivity of the medium also increased the attenuation of the transmitted signa while not affecting its propagation time More recently Eller and Denoth 1996 developed a nonlinear relationship between 8 and e measurements for four soils taken with a capacitive probe operating at 35 MHz Their second degree polynomial equation which held for O greater or equal to 3 by volume showed no influence of soil types on Perdok et al 1996 used a frequency domain sensor operating at 20 MHz to correlate to gravimetric water content and bulk density Measurements of the complex dielectric permittivity
83. rtable sensor The performance of the sensors was evaluated in loam and silt loam soils of the Ramada Series taken from the experimental plots located at the Manitoba Crop Diversification Centre Carberry Manitoba 1 3 Scope This thesis presents only a brief overview of the field testing conducted in 1998 and 1999 Because the main focus of the research was the evaluation of soil moisture sensors under controlled moisture conditions a detailed description of the lysimeter study is given The literature review describes the basic theory behind various soil moisture measurement techniques the principles of operation of the instruments tested as well as the performance of similar soil moisture measurement equipment reported in the literature The experimental methods of the field and laboratory study are presented followed by results The results from the lysimeter study are then discussed and conclusions are presented Finally recommendations addressed to both producers and researchers are stated 2 LITERATURE REVIEW 2 1 Review of sensors performance Despite the wide array of soil moisture measuring devices available in the market very few studies comparing the performance of the sensors investigated in the present study have been reported in the literature Yoder et al 1998 compared the performance of the Troxler neutron gage Troxler Sentry 200 AP capacitance probe Aqua Tel Capacitance sensors time domain reflectometry TDR probes
84. rval for the linear regression 57 Sensors 0y Actual 9y Fig 13b Tensiometers results in loam of p 1190 kg m modified Modified version of Fig 13a for which data points were excluded when at least one of the three tensiometers had reached its operational limit or had failed 58 Sensors 0y Fig 14a 15 20 25 30 35 40 45 50 55 Actual 6y Tensiometers results in loam of p 1150 kg m Volumetric water content in percentage measured with tensiometers as a function of 6 obtained in percentage with the lysimeter Each data point represents the mean of the 8 measurements from three different sensors The error bars represent the 95 confidence interval based on the three measurements The thick line illustrates the linear regression and the dashed line represents the 95 confidence interval for the linear regression 59 Sensors Oy 15 20 25 30 35 40 45 50 55 Actual 6 Fig 14b Tensiometers results in loam of p 1150 kg m modified Modified version of Fig 14a for which data points were excluded when at least one of the three tensiometers had reached its operational limit or had failed 60 Sensors 0y Fig 15a Tensiometer 2000 Silt Loam 15 20 25 30 35 40 45 50 55 Actual 6 Tensiometers results in silt loam of p 1250 kg m Volumetric water content measured with tensiometers in percentage as a function of 6 obtained with the lysimeter in percentage Each data point re
85. s can be difficult Watermark GMS do not fully respond to rapid drying or partial rewetting of the soil McCann et al 1992 This can result in underestimating the actual soil moisture content and thus to increasing the cost of irrigation due to over application 5 3 3 Aquaterr portable capacitance probe The Aquaterr capacitance probe was found to be affected by temperature variations between the water used for the calibration and the soil To lessen this effect the temperature of the calibration water should be kept within 3 C of that of the soil which can become a difficult task in the field on a warm summer day Measurements at shallow depths in loose structured soil are difficult due to the configuration of the instrument The electrodes are at the tip of the long stem attached to a meter and two handles The mass of the meter coupled with the length of the stem 89 makes it difficult to maintain a good contact between the electrodes and the soil However for deeper measurements the probe is more stable and a firm soil sensor contact is easy to establish and sustain Although the Aquaterr is built to be pushed into the soil to the desired depth it was found that for most soils an access hole was required to insert the probe to depths greater than 0 30 m 5 3 4 Aqua Tel capacitance sensors The Aqua Tel sensors consist of electrodes of 0 83 m in length and therefore can sample a large area Although this can be an advantage for certain applica
86. s ces e a de deci 100 vi 10 11 12 13a 13b 14a 14b 15a 15b 16 17a 17b 18a LIST OF FIGURES Soil moisture sensors tested in the field and laboratory trials 3 View of the ThetaProbe sensor tested in the laboratory trial of 2000 3 Equivalent circuit of a capacitance sensor 22 ee eee ees 18 Top view and elevation view of the steel grate 36 Top view of the lysimeter 2 eee ee te eee 37 Elevation view of the lySiMeter oooooooocooorroomocooo 38 Spatial arrangement of the soil moisture sensors inside the soil container of the lysimeter top view 2 2020 cee eens 42 Top view of the half filled lysimeter 44 Soil moisture characteristic curve for Loam with p 1190 kg m 48 Soil moisture characteristic curve for Loam with p 1150 kg m 49 Soil moisture characteristic curve for Silt Loam with p 1250 kg m 49 Results fram the field study of 1998 in Loam in the rain fed experimental plot 53 Tensiometers results in foam of p 1190 kg m 57 Tensiometers results in loam of p 1190 kg m modified 58 Tensiometers results in loam of 0 1150 kg m 59 Tensiometers results in loam of p 1150 kg m modified 60 Tensiometers results in loam of p 1250 kg m
87. s follows 46 for R gt R FC 6 FC 9 8 FC R R FC s 2 9 8 FC 16 R 4 R FC for R PWP lt R lt R FC 9 FC 6 PWP PWP R R PWP _ ___ 17 y R FC R PWP da and for R lt R PWP 6 PWP Re 18 4 Re BWP S where R R saturation referred to in Table meter s reading at saturation dimensionless R FC R field capacity referred to in Table 1 meter s reading at field capacity dimensionless R PWP R permanent wilting point referred to in Table meter s reading at permanent wilting point dimensionless 6 d volumetric water content of the soil at saturation m m 8 FC volumetric water content of the soil at field capacity m m 9 PWP volumetric water content of the soil at permanent wilting point m m 47 52 _ 48 A A A A A ee Si nn 44 i EN AR SS 40 ___ __ __ i fa gt O 36 ea gt 32 24 20 AA 0 50 100 150 200 capillary pressure kPa Fig 9 Soil moisture characteristic curve for Loam with p 1190 kg m 56 52 2 ee M MiM 4 amp 8 A AR gu ps AAA R S40 a Sete a ii o 236 RASS Sees 32 ose Fea E i ES See ee E 24 ___ _ io Mii BE Seat 0 50 100 150 200 capillary pressure kPa Fig 10 Soil moisture characteristic curve for Loam with p 1150 kg m
88. sh to express my sincere appreciation to Jacques for his love understanding and support TABLE OF CONTENTS ABSTRAE Los a E e ait haat e i ACKNOWLEDGMENTS i bees ES AA A aa til TABELE OF CONTENTS oia a da da E iia iv ESTORPGURES 0d A o dE Deo vil EIST OF TABLES ices s a da De a o de eee ee ix INTRODUCTION a alba 1 1 1 Problem statement o oooooocoooconoorooomoonocoo 1 es A A aioe ae eee 2 sl A TOS 4 ZETERATURE REVIEW tato A AA ea 5 2 1 Review of sensors performance oo ooocor cee eee 5 2 2 Background theory 2 20 eee eee eee 7 2 2 1 Gravimetric method 0 0 ccc cee cee teen eens 8 2 2 2 Tensiometric method oooooooooooooooo 8 2 2 3 Electromagnetic methods o ooooocooo oooo oo 9 2 3 Literature review of lysimetry 0 0 0c cc ce eee eee eee 19 3 PRINCIPLES OF SOIL WATER CONTENT MEASUREMENT 23 3 1 Commonly used techniques oooooooooooooomoo 23 3 1 1 Tensiometric method o oo oooooooooo 23 3 1 2 Electromagnetic methods o o 24 3 1 2 1 Resistance sensors o oooooooooo 24 3 1 2 2 Time domain sensors os 24 3 1 2 3 Frequency domain sensors 25 3 1 2 4 Phase domain sensors 0 25 3 2 Sensors tested in this research o o ooooooooooooo 26 3 2 1 Tensiometers occ idee
89. sor called the GroPoint that has the same transmission line configuration as the VIRRIB but is presented as a new product For that reason the principle of operation of the GroPoint given by ES cannot with certainty be applied to the VIRRIB Furthermore descriptions of the operating principles of the VIRRIB found in the literature differ from the information concerning the GroPoint The main disagreement between the TDT and phase domain PD technique lies in the signal sent The TDT method uses a pulse whereas the description of the PD methods implies that only a sinusoidal wave can be used In addition the description given by Litschmann 1991 does not provide technical information related to the electromagnetic technique employed It states that the sensor uses DC current with a voltage of 12 to 20 Volts from an external source and that the output data is measured by means of a current loop for which the intensity of the output is directly proportional to the moisture content of the surrounding medium It reads within a range of 0 to 55 of water by volume Environmental Sensors Inc The sensor consists of two rad electrodes shaped as concentric circles with an outer ring of 0 28 m diameter and an inner ring of 0 20 m diameter The volume of influence of the sensor extends to a distance of 0 06 m from the rings in all directions 3 2 6 ThetaProbe The ThetaProbe is a frequency domain reflectometry FDR sensor 29 manufactured by Delta T
90. t hydrologic cycles would be required to assess the adequacy of the sensors for real time field applications As stated in the literature for the Watermark sensors some instruments might not respond to partial rewetting McCann et al 1992 Because this experiment was designed to test the sensors under uniform soil moisture conditions it was assumed that a 12 h period between drying periods was sufficient for the system to reach equilibrium thereby assuming that all instruments had sufficient time to respond to the moisture depletion 5 2 2 Calibration of the weighing system Because the calibration relationship is a direct conversion of the height of the water column on the manometer to the volumetric water content its accuracy is affected by variations of the ambient conditions i e temperature and atmospheric pressure fluctuation of the load cell volume and soil moisture variation within the lysimeter To lessen the effect of the fluctuation of ambient temperature the readings were taken around the same time of day on each occasion at the proximity of the lysimeters using a mercury thermometer The ambient temperature varied from 18 to 25 C for the trial of 1999 and from 19 to 25 C for the trial of 2000 Fluctuations of atmospheric pressure have a direct effect on the height of the water column due to the surface area of the lysimeter compared to the surface area of the water column Stretching and shrinking of the load cell membrane as
91. ters used in all the trials Three Aqua Tel probes were installed in the loam lysimeter for both trials The spatial arrangement of the sensors in the lysimeters is shown in Fig 7 The Aquaterr which is a portable probe was used for all trials and the grey area shown in Fig 7 was assigned for its testing The ThetaProbe which was also used as a portable probe was introduced in the trial of 2000 and was tested in the same area assigned to the testina of the Aquaterr The VIRRIB and the Aqua Tel probes were installed at a 41 A Loam B Siit Loam 0 10 2 230 40 s 60 70 a Q w N0 0 10 2 30 40 so A 20 8 W Iw n 0 ula Fig 7 Spatial arrangement of the soil moisture sensors inside the soil container of the lysimeter top view The area reserved to tensiometers T Watermark GMS W and VIRRIB sensors V is identified with the darker grey whereas the area allocated for testing the portable probes Aquaterr and ThetaProbe is in lighter grey The Aqua Tel sensors Al were installed only in loam 42 depth of 0 10 m while filling the soil container as presented in Fig 8 The VIRRIB sensors were installed horizontally and the Aqua Tel sensors were installed on the side The tensiometers and GMS were installed at a depth of 0 13 m in saturated soil following the procedure described in section 4 1 2 1 Prior to the lysimeter study of 2000 the VIRRIB probes were calibrated in each soil Although the VIRRIB probes had been cali
92. then displayed on the meter No information concerning the principle of operation of the Aquaterr was found in the literature An on site calibration against moisture content requires that the reading be set to 100 while the sensing tip is submerged in water It can therefore be assumed from the calibration procedure and the colour coded legend shown on the meter that the C to R conversion 27 is not based on a linear relationship The conversion function seems to resembles the calibration curves illustrated in the Aqua Tel user s manual Those calibration curves show that the measured quantity does not vary a lot for moisture contents approaching or above soil saturation which is due to the high value of the dielectric constant of water compared to that of solids and air Because the geometry of the electrodes is not revealed by the configuration of the probe it can only be assumed that the dielectric properties of the medium are derived from fringe capacitance measurements The flat shaped sensing tip does not have any opening or gap that would allow for the medium to be positioned between the electrodes Furthermore the exposed portion of the sensing tip seems to be made of two coplanar plate electrodes covered by a protective polymer which closely match the probe designed by Thomas 1966 The effective volume of the Aquaterr is not specified in the literature The effective volume of the probe also called the volume of influence is the volume
93. thin the 95 confidence interval of the linear regression performed on the 8 measured with the sensors The precision of a sensor was assessed based on the length of the error bars The detailed criteria for the assessment of both accuracy and precision are presented in section 5 2 3 92 The ThetaProbe frequency domain reflectometry sensor was the most accurate instrument to measure soil moisture content in both soils over the entire drying cycle That is in loam for 8 ranging from 20 0 to 43 1 and in silt loam from 19 5 to 45 8 The precision of the measurements was very good in silt loam and good in loam The Aqua Tel capacitance sensors gave accurate measurements of 8 in loam over the 17 to 30 range from which measurements in the 20 to 27 range were very accurate in the trial of 1999 and from 22 to 38 in the trial of 2000 The sensors showed good precision in both trials The Aquaterr portable capacitance probe was accurate in loam in 1999 over the 23 31 range for which the measurements were qualified of very accurate in the narrow 27 to 30 range based on the statistical evaluation of accuracy In 2000 the Aquaterr gave very accurate measurements of 8 in loam over the entire drying cycle In silt loam accurate 9 measurements were obtained for 8 below 28 from which measurements in the 20 to 24 range were very accurate The precision of the Aquaterr was poor for all trials Tensiometers and Watermark granular matrix sensors m
94. tions 46 Measured values used to calculate a and a for loam and silt loam 51 Calculated constants a and a of Eq 21 for loam and silt loam 51 Summary of accuracy and precision of the soil moisture sensors 96 Summary of the evaluation of the sensors ease of use 1 INTRODUCTION 1 1 Problem statement Good irrigation water management requires soil moisture content data to ensure higher water use efficiencies and to achieve better yields Properly scheduled irrigation can conserve water and energy for pumping and also minimize the potential for groundwater contamination due to deep percolation losses Several sources of information are available to help growers make irrigation decisions Both measured and forecasted meteorological data including precipitation temperature relative humidity radiation and wind velocity soil water status in the form of soil moisture content or soil matric potential and plant based measurements provide producers with useful data Combinations of these are often used with soil moisture status being the most common reliable and direct indicator of irrigation needs Various approaches such as remote sensing hydrological models and in situ sensing can be used to determine soil moisture conditions Many in situ methods including lysimetry tensiometry nuclear techniques such as neutron scattering and gamma ray attenuation heat dissipation approaches and electromag
95. tions large sensing devices can also be a disadvantage when used for crops which have tubers such as potatoes Disturbances and air pockets in the effective volume of the sensor will greatly affect the measurements 5 3 5 VIRRIB phase transmission sensors As for the Aqua Tel sensors the dimensions of the VIRRIB sensors can be too large for crops like potatoes In addition the configuration of the probe with regard to the calibration procedure is awkward The calibration requires the adjustment of the resistive trimmer capacitors located on the electronic module which has to be buried in the soil while the adjustment is carried out The VIRRIB sensor is affected by interferences from transmission lines The body of the operator can also interfere with the measurements It was observed that the distance from the body of the operator to which the meter was held influenced the readings Furthermore the distance between the operator and the sensor s electrodes also affected the readings For this reason the operator should always hold the meter in the same manner to sustain the same distance between his body and the meter each time measurements are taken The operator should also position himself at the same distance from the sensor s electrodes 5 3 6 ThetaProbe FDR sensor As for the VIRRIB sensors the ThetaProbe measurements were affected by interferences caused by the experimenter s body 90 Although the ThetaProbe was used as a portable sensor
96. ture conditions saturation field capacity and permanent wilting point for three general soil textures The correlation between R and the moisture conditions specific to the soils used in the experiment was estimated by interpolation Table I Finally the readings were converted to 8 using the linear relationships developed from both the correlation between R and the moisture conditions and the 8 and the moisture conditions Once again the soil moisture characteristic curves were used in the conversion As it was discussed for the tensiometers and the Watermark GMS the final conversion was performed for the purpose of comparing the Aquaterr s readings to the 6 obtained with the lysimeter 5 2 3 3 Aqua Tel capacitance probes The 8 measurements obtained with the Aqua Tel capacitance sensors are compared with the lysimeters B measurements in Figs 22 and 23 In the 1999 trial the Aqua Tel sensors measured 6 accurately below 31 from which measurements in the 20 to 27 range were very accurate Fig 22 The regression line shows that the capacitance probes underestimated 8 above 30 In the 2000 trial although the regression line follows closely the 1 1 line the 8 measurements obtained with these capacitance sensors were accurate for 0 ranging from 22 to 38 and 6 was underestimated below and above that range Fig 23 For both trials the precision of the Aqua Tel sensors was qualified as good which is illustrated by error bars covering
97. uired Each wall of the soil container was made of three pieces of wood S P F No 2 0 038 m thick and 0 089 m wide fastened together by means of 0 050 m long screws The interior of the wooden frame was lined with a polyethylene sheet to prevent the wood from absorbing the water from the soil A steel grate shown in Fig 4 was attached to the bottom of the box to support the weight of the soil A 0 013 m mesh acrylic grid 0 013 m thick was placed on the steel grate as a support for the permeable fibreglass mesh The fibreglass mesh prevented the soil from passing through the acrylic grid while allowing for good drainage and air exchange through the bottom The soil block could dry both from the top and bottom surfaces Another wood box on which the soil container described above rests constituted the drainage collector One side of the box had an 0 14 m high opening along its entire length to provide an exit for the drained water as well as a channel for air to circulate through the soil A polyethylene sheet collector stapled to the bottom of the soil bin intercepted the water draining from the soil and led it outside the lysimeter This was done to ensure that all water within the lysimeter remained within the soil Figure 5 shows the lysimeter from the top looking through the empty soil container into the drainage collector An elevation view showing the opening of the drainage collector is also shown in Fig 6 The bottom wooden frame was faste
98. ular material in two sections In the lower section of the sensor the soil solution is allowed to move freely in and out of the sensor In the upper section however the granular material is isolated from the surrounding soil This way the soil solution has to travel through the lower section and the gypsum wafer which will buffer the effect of salinity before entering the section that contains the electrodes The granular matrix sensor operates on the principles that the movement of water in a porous medium is a function of the pore sizes of the material and the electrical resistance of that medium is a function of its water content which can also be expressed as water potential Therefore the movement of water in a granular matrix with pore sizes still remaining wet similar to that of the surrounding soil closely represents the soil water potential of the soil The electrical resistance of the sensor is therefore an indirect measurement of the soil water potential Watermark sensors cover a range of water potentials from 0 to 200 kPa Irrometer Co Box 2424 Riverside CA 92516 USA 3 2 3 Aquaterr The Aquaterr Aquaterr Instruments Inc 3459 Edison Way Fremont CA 94538 USA is a portable electrical capacitance probe The electrodes are encapsulated in the tip of a rigid metal stem attached to the meter The electrical capacitance C measured at the tip of the instrument is converted in the meter to a value R ranging from O to 100 that is
99. y of the Ramada Series The experiment was conducted over two drying cycles in loam for moisture contents decreasing from 34 0 to 17 0 by volume for the 1999 trial and decreasing from 43 1 to 20 0 by volume for the 2000 trial and over a single drying cycle in silt loam for moisture decreasing from 45 8 to 19 5 by volume The lysimeters were designed with hydraulic weighing systems to facilitate continuous monitoring of the soil moisture content For the purpose of comparison the readings obtained with the sensors were converted to volumetric water contents Soil matric potentials obtained with the tensiometers and GMS were converted using soil specific moisture characteristic curves A conversion equation was developed based on texture specific calibration curves published by the manufacturer to calibrate the readings of the Aqua Tel sensors The procedure followed for converting the readings of the Aquaterr probe was also partially developed by the experimenter to obtain more accuracy The FOR sensor was the most accurate instrument to measure soil moisture content in both soils over the entire drying cycle and thus it is most suitable to monitor irrigation needs with accuracy precision and ease of use Both capacitance sensors the Aquaterr probe and the Aqua Tel sensors also performed well measuring soil moisture content accurately although with less precision Tensiometers and Watermark sensors although very precise were mainly inaccurate du
100. ydraulic load cell First of all the weighing system was calibrated by placing weights of known mass incrementally on the lysimeter up to the estimated soil mass at saturation to verify the linearity of the relationship We found that an increase in mass of the lysimeter of 5 kg caused an increase of pressure inside the load cell corresponding to 52 mm in height of water column on the manometer for both lysimeters The precision of the weighing system was then determined in terms of variation of soil moisture content A8 in percentage for a change of 1 mm in height of water column by first calculating the AB corresponding to adding 5 kg of water to the lysimeter and then by dividing A9 by the change in height of water column caused by such a variation in mass i e 52 mm The precision thereby calculated for both lysimeters was a AG of 0 4 per mm of water column height To assess the long term stability of the weighing system maximum loading of the lysimeter was maintained over a period of 96 h followed by a 96 h period without any load The stability test showed that the fluid pressure inside the hydraulic load cell tended to decrease over time under maximum loading whereas it tended to increase over time under no loading This 39 drifting effect was attributed to the stretching and shrinking of the load cell membrane Variations of the ambient conditions such as temperature and atmospheric pressure also had an effect on the precision o

EVALUATION OF THE PERFORMANCE OF SOlL MOISTURE

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