SouthWest Victoria Irrigation guide
Contents
1. Making the most of irrigation water in south west Victoria A guide to improving irrigation water use efficiency on dairy farms Our Water Our Future Victorian Government ini itiative Making the most of irrigation water in south west Victoria A guide to improving irrigation water use efficiency on dairy farms By Graeme Ward Joe Jacobs amp Frank McKenzie Published by the Victorian Government Department of Primary Industries 78 Henna Street Warrnambool Victoria 3280 Australia January 2006 The State of Victoria Department of Primary Industries 2006 This publication is copyright No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968 Authorised by the Victorian Government 1 Treasury Place East Melbourne Victoria 3002 Australia Printed by XXXXXXXXXXXXX Name amp address of commercial printer or if printed in house the name of DPI Site ISSN 1 74146 621 0 Further information regarding Making the most of irrigation water in south west Victoria contact Graeme Ward Telephone 03 5561 9946 Facsimile 03 5561 9988 Email graeme ward dpi vic gov au Find more information about DPI on the internet at http www dpi vic gov au or contact the DPI Customer Service Centre on 136 186 Disclaimer This publication may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw o2. Yield Lost kg DM ha 0 674 1154 Yield Lost over recovery period 0 35 60 Recovery Period days 0 45 55 When irrigation does commence late it is important that sufficient water is applied to bring the soil in the root zone up to near field capacity or else further losses in plant growth will occur Visual estimation of soil moisture content Judging when the soil moisture content has dropped to around the refill point and hence when irrigation should commence can be difficult Our experience is that simple observation of the pasture appearance can be quite inaccurate By the time visual plant symptoms are apparent in the pasture the soil refill point has been well passed and the pasture growth rates have already dropped leading to significant pasture losses Testing the soil from the root zone of the pasture for moisture content by the feel in the hand can in some cases be a useful and practical guide Table 4 This feel varies with different soil types over different moisture contents and considerable experience is needed in interpreting the results For pasture it is essential that the soil water content does not drop below 50 of available soil water content if reductions in yields and water use efficiency are to be avoided Table 4 Assessing the water content of soils by feel On squeezing free Soil waterlogged No air eles water is expressed AE
3. levels are exceeded Other soil moisture measuring instruments such as neutron probes can be used to determine optimum irrigation startup times When to stop irrigating for the season One detrimental effect of irrigation can be an increase in the incidence and severity of winter waterlogging In particular pastures on heavier textured soils or soils with impeded drainage can suffer significant reductions in winter growth and other problems associated with the earlier onset of waterlogging conditions Under normal dryland conditions the pasture extracts large quantities of water from the soil profile during summer creating a buffer for the absorption of the autumn and winter rainfall In contrast the soil profile under an irrigated pasture is kept moist and requires much less rain to become saturated Unless the soil is free draining irrigation should cease early enough in the autumn to allow the soil profile to at least partially dry out Evaporation rates and hence pasture water use usually start to drop significantly from mid March onwards Local experience will be the best guide to when irrigation of particular soil types should stop in autumn As a general rule any irrigations after the end of March should be applied with caution 6 Irrigation scheduling Irrigation scheduling is the process of deciding when to apply irrigation water and how much water to apply Good irrigation scheduling is critical to maximizing the feed produc
4. This is a combination of evaporation E directly from the soil surface and transpiration T from the foliage of the plants Water lost to the atmosphere by evapotranspiration comes from the water stored in the soil and must be replaced by either irrigation or rainfall if plant growth is not to be restricted Weather conditions including temperature wind relative humidity and solar radiation have major effects on evapotranspiration rates of crops and pastures Across Victoria variation in potential ET and rainfall mean that there are large differences in annual irrigation water requirements with the cooler and wetter south west having lower requirements than northern Victoria In addition irrigation water requirements at a location will vary from year to year depending on rainfall and other weather conditions Estimated irrigation water requirements for perennial pasture a Annual requirements Based on long term weather records the maximum annual water requirement to fully irrigate perennial pasture in south west Victoria is estimated to be in the order of 6 7 ML ha in the southern parts and 7 8 ML ha in the northern parts of the region As 1 megalitre ML when applied to 1 hectare ha of land gives an application of 100 mm this is the equivalent of 700 and 800 mm respectively As lucerne is known to have a higher water requirement than perennial pasture fully irrigated lucerne is likely to require an additional 1 2 ML ha annually
5. and considerations Table 15 A general comparison of four different spray irrigation systems against a number of key elements and considerations for a 20 ha and a 40 ha scheme Prepared with the assistance of Frank Mahony Irrigation Engineer Aquaflow Solutions Pty Ltd Rotation length Good Heavy Soil Excellent Excellent Good Fair Good Good Excellent Fair Good Poor Rotation length f Good Light Soil Excellent Excellent Fair Good Fair Good Excellent Fair Poor Good Good Good Good Gnittorraity Excellent Excellent Reece Fair Excellent Excellent Paican For Capital Cost Medium High Low Medium Medium Low High Low Medium Medium Operating Pressure Low Medium Medium High Low Medium Medium High we Medium Medium Medium Medium f Pump Efficiency High High Medium High Low High High Medium High Low Running Costs Low Medium Medium High Low Medium Medium High Labour Medium Requirements Low Low Medium High Low Low High High Maintenance Low Medium Low Medium Low Medium High Low Medium Low Medium Low Medium High Flexibility Crops Excellent Excellent Fair Excellent Excellent Excellent Fair Excellent cies ci Poor Poor Poor Excellent Poor Poor Poor Excellent Site Constraints Poor Fair Good Excellent Fair Good Poor Fair Good Excellent Fair Good Pasture Growth Excellent Excellent Good Fair Good Excellent Excellent Fair Good Fair Heavy Soi
6. the tensiometer will need to be removed pre soaked again and re installed in the soil Tensiometer maintenance Tensiometers are vulnerable to damage and need to be protected from the grazing animal and other hazards In grazed pastures it is usual to place a small weldmesh or similar pasture cage over the tensiometer to prevent it being knocked or trodden on Pasture inside the cage should be cut just before cows are put into the paddock or any other harvesting takes place This reduces the chance of cows pushing the cage over and keeps the pasture under the cage as the same growth stage as the rest of the paddock Tensiometers should be checked at least twice per week If there is an air gap of more than 2 cm in the top of the water column it should be topped up The drier the soil the more often the column will need topping up Rapid drops in the water level are often caused by leaking rubber seals that need to be replaced When not in use over the winter period tensiometers should be removed from the field and put into storage Tensiometers are vulnerable to damage if the water in the tube freezes during frosts Further information Department of Primary Industries Information Note Series Note number AGO298 How to use tensiometers Appendix B The use of water budgets for irrigation scheduling Accurate irrigation scheduling requires a reliable estimate of the water losses evapotranspiration from the pasture
7. 1989 For some aquifers requiring more intensive management WSPAs are established and a water management plan is developed An application for a new Groundwater Licence in a WSPA will be deferred until the Minister for Water has determined the Management Plan WSPAs in the south west include the Nullawarre Yangery and Warrion groundwater areas When a licence is issued it is subject to a number of conditions including licenced total annual and daily volume available for use the installation of a meter to measure and monitor water usage the need to comply with restrictions rosters and bans in extreme circumstances and development clauses these make the issue of all or part of your licence conditional on the completion of particular increments stages or finalisation of the whole project If an application for a Groundwater Licence is refused due to the resource being fully allocated irrigation water can sometimes be obtained by a temporary or permanent transfer of water entitlement from an existing licence holder see Section c b Surface water licences As with groundwater sources quantities of irrigation water available from waterways are regulated to ensure sustainable use of the resource Since the release of the White Paper Our Water Our Future in June 2004 the Victorian Government has placed upper limits on water extractions on all Victorian river basins For river basins that are fully or over allocated no new licences will
8. Hunter typically have comparable to better herbage quality than irrigated pasture Table 12 Values can vary depending on factors such as nitrogen fertiliser use and stage of maturity These C3 temperate species forage crops are therefore capable of producing feed of high nutritional value and can replace other high quality feeds such as bought in concentrates However the C4 tropical species crops such as millet produce only moderate quality herbage The lower metabolizable energy content and higher neutral detergent fibre content of the millet herbage will limit its potential for lactating dairy animals unless it is used to replace other high fibre feeds such as hay or silage Table 12 Typical feed quality values for irrigated turnip Hunter and millet forage crops compared to a perennial ryegrass white clover pasture Ecklin and Wollaston trial sites 2000 03 Turnip 13 7 18 24 Hunter Brassica 12 0 20 28 Millet 9 5 15 62 Pasture 11 5 20 47 9 Coping with reduced irrigation water supplies South west Victorian irrigators who are reliant on water supplies from streams and other surface diversions regularly face the problem of reduced and unreliable irrigation water supplies A combination of low stream flows during dry seasons together with the requirements of stream flow management plans will result in water volumes of less than licence allocation for the season A more difficult problem to manage is the
9. In years of higher summer rainfall or lower temperatures leading to reduced evaporation the requirement will be less Similarly above average spring rainfall will often result in later optimum irrigation startup times reducing water requirements for the season as will early autumn breaks b Weekly requirements An understanding of the average and range of weekly irrigation water requirements is important when designing an irrigation system An irrigation system must be capable of applying the required average quantity of water within normal operating hours plus have the capacity to apply additional water in weeks of high evaporation The volume of water needed to fully irrigate perennial pasture was estimated at three sites in the south west during the 1997 98 irrigation season Evaporation from a Class A evaporation pan was measured from mid October through until late March The estimated weekly irrigation water requirement was calculated Table 1 using a crop factor of 0 8 see Section 6 October 20 15 14 6 27 17 25 24 November 3 0 0 8 10 24 25 24 17 15 0 0 24 45 28 31 December 1 33 37 34 8 23 24 22 15 35 31 33 22 27 38 30 29 43 40 37 January 5 35 45 37 12 30 41 38 19 30 30 48 26 36 28 29 February 2 32 39 32 9 19 28 25 16 27 27 29 23 36 42 42 March 2 37 46 41 9 26 32 31 16 32 41 38 23 29 38 33 Over the peak December to
10. The accepted standard pan for measuring evaporation throughout Australia and overseas is known as the Class A pan This is a circular galvanized tank 1 2 m in diameter containing water 0 25 m deep The pan sits 15 cm above the ground on a slatted wooden frame and is usually covered with netting to prevent birds or animals affecting the water level The pan is filled with water up to a standard mark usually being 5 10 cm from the top This mark usually being the top of a steel rod placed vertically in the middle of the tank A stilling well a circular sheet metal cylinder 15 20 cm in diameter is usually placed around the steel rod to reduce the effect of wind when reading the pan Every day at the same time the pan is refilled with water up to this standard mark By recording the volume of water that had to be replaced and knowing the surface area of the pan the depth of the water replaced in millimeters can be calculated This depth of water is the millimeters of evaporation less any rainfall that has occurred over that time period Mini pans or Evaporimeters While the Class A pan will give the most accurate results evaporation can also be measured on farm using smaller home made Mini pans or Evaporimeters These Mini pans can be made from either the bottom of a 200 litre steel drum or a plastic Cherry Barrel Various designs have been produced but all operate on the same principle We
11. and the contribution that any rainfall has made Such estimates can be made by keeping a water budget The first requirement of a water budget is to be able to collect actual evaporation E results Some weather recording stations measure evaporation and in some districts the daily evaporation rates are published in the press If this information is not available then evaporation needs to be measured by some means Some automatic weather stations are able to calculate evaporation but their accuracy needs to be checked periodically A more simple and practical method is to measure the evaporation from an evaporation pan see Appendix C The evapotranspiration ET or the amount of water lost by the pasture can be estimated by Evapotranspiration Evaporation x Crop Factor We have found a crop factor of 0 8 to be suitable for irrigated perennial pasture over the summer period In other words perennial pasture loses water by evapotranspiration at a rate of approximately 80 that of the evaporation from an evaporation pan The estimated pasture water loss or evapotranspiration sometimes has to be adjusted for any rainfall R that has occurred over the same period Not all the rain that falls will be effective as a proportion of it will evaporate and not add to the soil moisture reserves We disregard the first 3 mm of each rainfall event Therefore Effective Rainfall Actual Rainfall 3 mm Table 14 An example of a water budget for u
12. be issued In the south west these include the Hopkins Glenelg Barwon and Moorabool river basins For river basins that are not fully allocated new winterfill licences can be allocated in some cases A winterfill licence allows you to take and use water from the waterway during the winter months July to October inclusive to fill appropriate storages or dams for use during the summer months Where surface water applications in southern Victoria are refused in accordance with Ministerial Guidelines irrigation developments may still occur provided water can be traded within the river basin Irrigation water licences may be transferred on a temporary or permanent basis As there may be restrictions on these transfers depending on the volume of water and the location of both buyer and seller any applicant should check any proposed transfer with Southern Rural Water early in the planning process c Water trading Water trading is often the only option for obtaining a new licence or to increase your existing licenced volume in some areas This can be done on a temporary or permanent basis Water trading can only take place within the same GMA WSPA or river basin A temporary transfer is when you lease all or part of your licenced volume for a season A season finishes on 30 June each year Although you are transferring your licenced volume to another landowner for an agreed period of time and price you continue to hold ownership of the lic
13. crops such as sorghum sudax and maize are attractive options because of their high water use efficiency and the ability to produce high yields of forage However with some of these crops such as the sorghums there is a risk of stock poisoning from prussic acid Potentially dangerous levels of prussic acid can develop when these crops are water stressed Some other crops in this group such as maize are particularly sensitive to water stress at certain growth stages e Carrying water over for an autumn start irrigation In some circumstances it may be possible to stop irrigating during the summer and carry over the saved water to the autumn Such water can be used as an autumn start irrigation onto pasture that is to create an early autumn break However if the water is stored in a dam or other surface storage the high evaporation losses and the subsequent reduction in the volume of water available in the autumn need to be considered Appendix A Use and care of tensiometers What is a tensiometer A tensiometer is an instrument that can be used to measure the availability of water in the soil for plant use They are simple easy to use and relatively inexpensive In irrigated pasture management they are a particularly useful aid for a Determining when irrigation should commence at the start of the season and throughout the season and b Monitoring how effective the irrigation is in meeting plant requirements It is
14. eee eke can get to roots Growth pacity from the ball of soil q i retarded Soil sticky No free water Field No free water appears appears on soil when ball is Capacity on the soil when the squeezed but wet outline of 100 ball is squeezed buta ball is left on hand Possible ge ois i d available wet outline is left on to roll long thin rods 2 5 mm p i water the hand diameter between finger and thumb 75 Slightly coherent Will Soil coherent Soil has a slick available form a weak ball under feeling and ribbons easily Will Adequate air and water water pressure but breaks not roll into long thin rods 2 5 Plant grows well easily mm diameter 50 Appears dryish Tends Soil coherent Forms ball under Close to refill point of the available to ball under pressure pressure Will just ribbon when soil Moisture stress will but seldom holds pressed between finger and soon start to reduce growth water together thumb rates 25 Somewhat crumbly but will available Appears dry Will not form a ball under pressure Will Well past refill point Very ball under pressure not ribbon between finger and low growth rates water thumb a Crumbly powdery Small lumps Wilting Point rey oe and break into powder Will not ball a aC ee SIESS flows through fingers Plants begin to die under pressure Use of tensiometers to determine startup time A much more objective and accurate method of determining optimum start up
15. have found the plastic 200 litre Cherry Barrel design to be the most reliable To make one Cut the drum exactly in half and clean up any rough edges on the bottom section to make the pan Cut a V notch about 5 cm down from the lip of the pan Fix a length of tape with millimetre markings either below the notch or on a central rod Alternatively the volume of water to refill the drum to the notch or mark can be measured To set up the pan Place the pan on cement blocks or pieces of timber e g A wooden pallet to allow air circulation under the pan Place wire mesh over the pan to prevent birds or animals drinking from it Locate the pan in a typical open pasture situation preferably in an irrigation paddock and protect it from animals or other interference Note Mini pans overestimate the amount of evaporation when compared to a standard Class A pan In our experience a Mini pan made from a 200 litre plastic Cherry Barrel will overestimate evaporation by approximately 15 and one made from the bottom of a steel 200 litre drum will over estimate evaporation by up to 40 If possible a Mini pan should be calibrated against a standard Class A evaporation pan Further information Department of Primary Industries Information Note Series Note number AG0293 Construction of an evaporation pan for irrigation scheduling Appendix D Irrigation methods general comparison against key elements
16. important to note that tensiometers are only an aid to making irrigation management decisions The irrigator shouldn t rely on tensiometer readings alone but should also use his own experience and other aids such as weather data to assist in decision making How does a tensiometer work Tensiometers consist of a rigid plastic tube with a porous ceramic tip at one end The tube is filled with water that usually has some colouring dye and methylated spirits 5 10 added Some tensiometers have a built in vacuum gauge while others have an electronic vacuum gauge that can be transferred from one tensiometer to another When the tensiometer tube is filled with the water solution and installed in the soil water can move in and out through the ceramic tip As the soil dries water moves out of the tensiometer and creates a vacuum in the tube that is then measured on the vacuum gauge If the soil continues to dry out the vacuum reading increases When water is added to the soil water will move back into the tube through the tip and the vacuum reading will decrease The vacuum reading on the tensiometer is in fact a direct measure of the force or suction that a plant root has to exert to extract water from the soil It does not measure the actual water content of the soil but rather the difficulty the plant has in extracting water from the soil As a result the reading is comparable between different soil types Where to place tensiometers A n
17. in an average to drier south west Victorian summer Table 10 In this trial two regrowth brassica forage crops Hunter and Graza and millet were irrigated at 100 50 and 25 of the estimated perennial pasture irrigation water requirement In cooler and wetter summers the crop responses to irrigation are likely to be lower with correspondingly lower WUEs Hunter 1 6 1 8 1 8 Graza 1 4 1 3 1 4 Millet 1 5 2 9 3 3 iii Responses to restricted irrigation regimes There is good evidence that summer forage crops with the exception of maize are more tolerant than perennial pasture of underwatering or the deliberate application of quantities of irrigation water less than full plant requirements Perennial pasture is quite sensitive to underwatering Growth rates and WUE falls off quite quickly as the pasture plants go into partial moisture stress when only part of their full water requirement is applied However fodder crops in some cases Table 10 show an improvement in WUE when only part of the estimated full irrigation water requirement is applied e g 50 of estimated full requirement Part of the better water use efficiency may also be a result of the deeper root system of forage crops allowing more of the water stored in the soil profile to be used Total yields from the crops will be lower than if they are fully irrigated but more additional herbage is grown per megalitre of water applied iv Lower t
18. time is by the use of tensiometers installed in the soil under the pasture or crop A tensiometer is a simple instrument that measures how difficult it is for a plant root to extract water from the soil rather than the actual soil moisture content see Appendix A for details on the operation and use of tensiometers Ideally a number preferably 4 or more tensiometers should be installed in typical parts of the farm s irrigated pasture in the early spring and the readings monitored regularly Findings from our trials across the south west are that irrigation of perennial pasture should commence by the time readings have risen to 35 kPa and 30 kPa for tensiometers installed at 20 cm and 30 cm respectively Table 5 These critical tensiometer readings have been found to apply across a range of soil types and textures If irrigation is commenced after these critical values are exceeded reductions in pasture growth will occur 20 cm 35 kPa 30 cm 30 kPa When these critical readings are reached pasture growth rates will be just starting to decline In practice we should aim to water all irrigated areas on the farm by the time these critical levels are reached As the tensiometer readings often rise quite quickly with the onset of dry conditions the tensiometer readings need to be monitored regularly and experience gained on when irrigation should actually commence so that paddocks are irrigated before the critical
19. to start irrigating pasture at the optimum time in spring irrigation water is likely to be more efficiently used on summer forage crops midway through their growing period see Section 8 Fully irrigate or partially irrigate pasture A common response to reduced irrigation water supplies is to continue to irrigate the same area of perennial pasture but apply less water per hectare Our trials in the south west together with results from other parts of the state clearly indicate that it is far more efficient to fully irrigate a smaller area of pasture with its full requirement of water see Section 6 than to under irrigate a larger area of pasture Shallow rooted perennial pasture is particularly sensitive to drought stress Where less water is applied growth rates drop quickly and less feed is grown per megalitre of water applied Irrigated summer forage crops appear to behave differently Our results indicate that the best water use efficiency and hence the most additional feed grown per megalitre of irrigation water on forage crops is where the crop is partially irrigated see Section 8 e Use the water for autumn startup In some circumstances a late summer early autumn start to irrigation could be an option for irrigators with limited water No trials on this strategy have been conducted in the south west but work in north east Victoria is relevant This work suggests that the timing of the first irrigation is important Depending on t
20. week Optimum Irrigation Schedules and System Capabilities Guidelines for the optimum irrigation schedule The optimum irrigation schedule that will maximize the yield of pasture and give the best water use efficiency will vary from week to week depending on the pasture water use Our findings outlined below indicate what a spray irrigation system on pasture should be designed and operated to deliver as part of its normal operation Water application per week The system should be designed to be capable of delivering 40 mm of water to the pasture per week in its standard hours of operation On average during the December through to March period around 35 mm per week is required Additional capacity to apply a further 20 is required to meet peak demands and to cover leaching uniformity requirements Irrigation interval If possible the system should be designed to enable an irrigation interval i e days between irrigations of between three and six days Ideally the system should have the capability of covering the area in three to four days Intervals of greater than six days lead to excessive reductions in pasture growth rates Water application per pass Water applications per pass or shift ideally should be in the range of 15 to 25 mm Applications of greater than 25 mm per shift on pasture are likely to be inefficient as a Pasture growth rates are likely to have slowed already if these quantities are requ
21. March period the average weekly irrigation water requirements were Allansford 31 mm week Alvie 36 mm week Myamyn 34 mm week In weeks with extreme evaporation conditions this requirement can increase to around 50 mm week A well designed irrigation system should have the capability of delivering these quantities of water when required It must be stressed that the above figures are MINIMUM REQUIREMENTS to meet perennial pasture needs For lucerne an additional 5 10 mm per week over and above the perennial pasture figures is often required to fully meet the plants requirements It is also necessary to allow for distribution losses e g 10 30 of water from a travelling irrigator can be lost to evaporation and poor distribution uniformity Where highly saline water is used additional water should be applied to leach accumulating salts down below the root zones the leaching factor Requirements for Summer Forage Crops Forage crops have a lower total water requirement for the irrigation season than fully irrigated perennial pasture However forage crops usually have greater evapotranspiration and therefore uptake of water from the soil than pasture during their peak growth period Reasons for this lower annual water requirement include e Time of sowing Forage crops are generally sown after the start of the pasture irrigation season e Shorter growing period Forage crops are generally only actively growing for part of the pastur
22. Neutron probes require a licence and specialist knowledge to install and operate In some districts private agricultural consultants offer an irrigation scheduling service using neutron probe measurements Penalties of not applying the required quantities of water Applying either too little or too much water to an irrigation pasture results in losses and inefficiencies An example of these is provided by experimental results from Myamyn in February 1998 Table 6 Pasture water requirements were estimated using evaporation rates from a Class A pan and a crop factor of 0 8 see weather based scheduling below Yield of Pasture t DM ha 1 5 0 9 1 5 21 days regrowth Water Use Efficiency t DM ML 1 7 1 5 1 3 Irrigation water applied mm over 21 days a9 po 120 Clearly under summer conditions applying less water than the irrigation requirement leads to a a major reduction in the yield of pasture per hectare a less pasture grown for every megalitre applied Applying more than the predicted irrigation requirement leads to a no more pasture grown per hectare b less pasture grown for every megalitre applied Optimum Irrigation Intervals The optimum irrigation interval is the number of days a pasture can go between irrigations without suffering reductions in yield From trial work in the region the optimum irrigation interval for pasture in the south west is between three and six days for most o
23. al Water SRW SRW is responsible for monitoring and regulating rivers streams waterways and aquifers across southern Victoria including the use of water from existing and new farm dams the construction of new bores and the management of permanent and temporary water transfers If you plan to take and use any groundwater streamwater or dam water for commercial irrigation dairy or industrial purposes you need a licence from SRW Before planning any irrigation scheme advice should be sought from SRW on the likely availability of an irrigation licence a Groundwater licences Before any groundwater bore is constructed you must apply for a Bore Construction Licence If you plan to take and use the groundwater for any purpose other than domestic and or stock use you must also apply for a Groundwater Licence In a number of the groundwater irrigation districts of the south west no new Groundwater Licences for irrigation are being issued as the aquifers are already fully allocated however water trading is an option Most of the south west groundwater irrigation districts are in either Groundwater Management Areas GMAs or Water Supply Protection Areas WSPA GMAs usually have a large number of existing bores and need to be closely managed to ensure that existing licenced use is sustainable If you apply for a new Groundwater Licence within a GMA the application is assessed against the current licenced volume and the provisions of the Water Act
24. ce early seedling growth flowering climatic conditions temperature humidity light and soil type 0 750 uS cm 0 500 ppm Sensitive plants Subterranean clover white clover Suitable for use with all crops Moderately sensitive plants Balansa clover Persian clover strawberry clover lucerne maize millet sorghum turnip rape Sensitive plants have increasingly reduced growth Moderately sensitive plants suffer little or no yield decline 750 1 500 uS cm 500 1 000 ppm Moderately sensitive plants will suffer increasing yield loss Moderately tolerant plants should suffer little yield loss with good management at the lower end of this range At the upper end some yield loss will occur Moderately tolerant plants Perennial ryegrass tall fescue berseem clover paspalum barley oats wheat 1 500 3 000 uS cm 1 000 2 000 ppm Moderately tolerant plants will suffer increasing yield decline Ideally only tolerant plants should be grown with sustained use of this water 3 000 5 000 uS cm 2 000 3 300 ppm Tolerant plants Tall wheat grass barley couch grass General recommendations regarding the salinity of water used for irrigation are e If salinity is less than 800 uS cm the water is suitable for most crops and pastures on moderately to well drained soils e If salinity is between 800 2 300 uS cm care is needed It is unlikel
25. decisions on any changes are made early in the season to allow time for any changes to be effective Such decisions will often have to be made with incomplete information try to assess at the start of the season the amounts and reliability of irrigation water supplies Also consider e A balance between irrigating perennial pasture and summer forage crops One of the biggest limitations of irrigating perennial pasture is that it is particularly sensitive to missed irrigations see Section 6 When scheduled irrigations are delayed or missed the plants go into partial water stress and growth rates decline As aresult total pasture yields decline and the water use efficiency of the applied irrigation water is reduced If the stressed pasture is then fully irrigated there is still a lag period before pasture growth rates are back to normal In comparison summer forage crops appear to be more tolerant of missed irrigations In addition our research suggests that summer forage crops can respond favourably and profitably to once off irrigations in the mid to latter phases of their growth period see Section 8 A possible strategy may therefore be to irrigate a core area of perennial pasture with the irrigation water supply that is reasonably secure Additional less secure water may be better applied to summer forage crops in an opportunistic manner e Be careful of poisoning risks with some of the tropical C4 forage crops The tropical grass forage
26. e irrigation season e Conserved soil water Some spring rainfall is conserved in the soil profile as part of the seedbed preparation process This is utilised by the growing crop e Evapotranspiration is less than for pasture for much of the early growth period of the crop when the plants are seedlings and have small leaf areas e Our research experience shows that in the southern areas of the south west a summer forage crop requires 300 to 400 mm 3 4 ML F method frequency intensity and waterlogging e stage of plant growth germination emergence early seedling growth flowering e climatic conditions temperature humidity light and e soil type 4 Water quality considerations It is important that any potential source of water be tested for its suitability for irrigation use Irrigation water from groundwater bores and the summer flows of some unregulated streams can have water quality problems This is certainly the case in south west Victoria with the majority of the region s irrigation water supplies having higher salinity levels than water supplied in regulated systems in other parts of the state Plants differ in their tolerance of salt in irrigation water and or soil General differences in salt tolerance between some forage species are listed in Table 2 Salt tolerance of plants also varies with water management irrigation method frequency intensity and waterlogging stage of plant growth germination emergen
27. eased wastage of feed In addition there is increased shading of the pasture leading to reduced ryegrass tillering and white clover growth resulting in a more open pasture iii Grazing duration or how long to graze Cows should not be allowed to graze an area of growing ryegrass pasture for more than 2 days at a time If the paddock is being strip or block grazed for more than 2 days a backfence should be used to prevent the cows from regrazing the previously grazed area Grazing the new leaves and shoots that are regrowing from the stored energy reserves of the plant will further weaken the ryegrass plant reducing future regrowth and threatening the survival of tillers b Nitrogen Fertiliser Use Recent research conducted in the south west has found that the growth of irrigated perennial ryegrass white clover pasture is restricted by a lack of nitrogen Regular applications of nitrogen fertiliser to the pasture over the irrigation season have been found to be an effective and cost efficient method of increasing pasture yields and in improving water use efficiency of irrigation systems How much nitrogen fertiliser should I apply Results from trials conducted at Nullawarre and Mepunga indicate that on balance the optimum rate of nitrogen to apply to perennial pasture during the irrigation season is 50 kg N ha after each grazing This is provided that optimum irrigation management and grazing management including at least a 21 day rotation
28. ence A permanent transfer is when you sell your ownership of all or part of your licenced volume for an agreed price Depending on the location of the property and or the volume of the water transferred Southern Rural Water may request you advertise your intention to permanently transfer water to another landowner in newspapers that are distributed within the area surrounding the buyer s property notify your neighbours and request submissions from interested parties If a proposed permanent transfer is within a declared WSPA it cannot be considered until the Minister for Water has determined the Water Management Plan developed by a local Committee All applications to transfer water must be submitted to Southern Rural Water so that the impact of any proposed change to existing extraction or diversion points on other users and the environment can be assessed Even though both the buyer and seller have agreed to the transfer it is possible the transfer application may not be approved Southern Rural Water Further details and advice is available from Southern Rural Water 132 Fairy Street Warrnambool 3280 Phone 1300 139 510 or 5564 1700 Web site www srw com au About one third of the regions irrigation water supplies come from surface diversions such as the Merri River pictured 3 How much irrigation water do I require Growing pasture and crop plants lose water to the atmosphere in a process known as evapotranspiration ET
29. f any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error loss or other consequence which may arise from you relying on any information in this publication Contents 1 2 8 9 Introduction Irrigation water in south west Victoria How much irrigation water do I require Water quality considerations The irrigation season when to start and finish irrigation each year Irrigation scheduling Management of irrigated pasture Irrigation of summer forage crops Coping with reduced irrigation water supplies Appendix A Use and care of tensiometers Appendix B Use of water budgets for irrigation scheduling Appendix C Using evaporation pans Appendix D Irrigation methods Appendix E WISDAM WestVic dairy irrigation scenario decision support model 10 12 16 20 23 28 30 33 34 35 36 1 Introduction This booklet Making the most of irrigation water in south west Victoria is not intended to be a comprehensive manual dealing with every aspect of irrigation management and practice Rather the booklet draws together the results of some eight years research into improved irrigation practices by the dairy research group of the Department of Primary Industries at Warrnambool This research has been aimed at how the dairy industry the largest user of irrigation water in the region can make more efficient and profitab
30. f the irrigation season With intervals longer than this marked reductions in pasture yield and water use efficiency can occur even though the same total quantity of water is applied This is illustrated by the results of a frequency of irrigation experiment at Allansford Table 7 which showed a 35 reduction in both yield and water use efficiency for a 12 day irrigation interval compared to a 3 day interval Pasture Yield t DM ha January March 4 1 3 8 3 1 2 7 Water Use Efficiency t DM ML 1 6 1 5 1 2 1 1 In practice this optimum interval will vary from week to week depending upon the amount of evaporation and hence the crop water use Our experience from measurements of daily pasture growth rates on a range of soil types in the south west following a full irrigation are that the maximum pasture growth rates will be maintained up until and then start to decline when Evaporation E less effective rainfall 25 30 mm Evapotranspiration ET less effective rainfall 20 25 mm evaporation measured using a Class A evaporation pan see Appendix C and effective rainfall being the total rainfall for a rain event less 3mm see Appendix B Therefore on extreme days with evaporation E rates of over 12 mm per day the optimum interval can be as short as every two days Correspondingly in periods of cool weather with effective falls of rain this optimum interval can increase to more than a
31. he autumn break irrigating at the start of March may not produce a lot more feed than not irrigating at all The best responses and water use efficiencies were found for an early February startup time If irrigating from a dam high summer evaporation rates will significantly reduce the volume of water available for late summer autumn startups Typically around 600 mm can be lost from dams over the summer by evaporation e Use best pasture management techniques Good pasture management has been found to substantially improve the yield and water use efficiency of irrigated pasture Two of the key pasture management areas Section 7 are a Grazing Management Avoid excessively short grazing rotation lengths Ryegrass should be at least at the 2 leaf stage Avoid excessively hard grazing by making sure residues are at least 4 cm height If strip grazing back fence previously grazed areas e Fertiliser use Ensure that the irrigated pasture is adequately supplied with phosphorus potassium and sulphur fertiliser In trials nitrogen fertiliser application rates of between 25 and 50 kg N ha after each grazing were found to optimize improvements in yield and water use efficiencies b Where supply is also unreliable Where the volume of irrigation water available per week may vary from full entitlement to a total irrigation ban e g streams with irrigation rosters efficient use of water becomes more difficult In these situations it is important that
32. hich are typically more efficient converters of water into dry matter growth than the C3 or temperate plants However the lower feed quality of the herbage produced must be considered see section B iii and the susceptibility of these crops to lower yields in the sometimes cool south west summers 2000 01 2001 02 2002 03 2003 04 2002 03 2003 04 Turnips Ecklin Wollaston Hunter Brassica Wollaston Hunter Brassica Millet Millet Wollaston Wollaston ii Water Use Efficiency WUE of irrigated summer forage crops The water use efficiency WUE of a crop or pasture is defined as the amount of herbage produced for every megalitre of irrigation water applied This is often expressed as the tonnes of herbage dry matter produced per megalitre of water applied t DM ML Where irrigation water supplies are limited high WUE systems can be important Summer forage crops can have higher WUEs than perennial pasture and therefore are often more efficient converters of irrigation water into feed Typically a commercial irrigated perennial ryegrass white clover pasture will have a WUE in the range of 0 5 1 5 t DM ML New well managed pasture with high nitrogen fertiliser appplications will have higher WUE s than poorly managed old pasture Results from the Wollaston irrigation trial over the summer of 2003 04 indicate the type of WUE results from irrigated summer forages
33. hould also be noted that yields of irrigated maize will be affected at least as severely as perennial pasture if water supply is unreliable Our district trial work shows that responses to and the benefits of irrigating a crop will vary from year to year depending on seasonal conditions In addition irrigators need to consider the additional costs and time involved in establishing and managing crops the effect on timing of feed availability and the quality of the feed produced Over the last seven years DPI Warrnambool has conducted a number of district trials on the irrigation of summer forage crops These crops have included turnip cultivars regrowth brassica species such as Hunter and the C4 tropical grass millet Some of the relevant observations and conclusions from these trials are presented in the following pages A Benefits of irrigating summer forage crops i Yields of irrigated summer forage crops The response of a summer forage crop to irrigation varies from year to year In an average to drier summer irrigation can double the yield of a summer forage crop compared to a non irrigated crop Table 9 In other years smaller but still quite useful responses to irrigation occur However in a wet summer such as at Ecklin in 2001 02 turnip responses to irrigation were small and were unlikely to be the most efficient use of the irrigation water The high yields of the millet crops are a characteristic of the C4 tropical grass crops w
34. ired and b With heavy applications some of the water will be lost beyond the root zone Applications of less than 15 mm are likely not to infiltrate far into the soil leading to the development of a shallow root system Such shallow root systems make the pasture more vulnerable to drought stress Water application per hour The optimum rate will vary depending upon soil type As a guide it should not be more than 10 15 mm per hour At higher application rates the soil infiltration rate can be exceeded and water will be wasted by runoff Other factors that also need to be considered in the design of an irrigation system include the running costs evenness of watering and the labour requirements of operating the system The less even the water application the more water will need to be applied to ensure that the minimum amount of water is applied over most of the area Systems with reduced or minimum labour requirements will usually be preferred but the generally higher capital costs of these types of systems needs to be considered 7 Management of irrigated pasture a Grazing Management The yield persistence water use efficiency and ultimately the profitability of irrigated pasture in the south west are commonly reduced by poor grazing management As a result the irrigated pastures do not perform to their potential and irrigation water is not used as efficiently as it could be A common fault is that the period between grazings o
35. is practiced Responses to and economics of nitrogen fertiliser In these trials nitrogen as urea was applied after each grazing at 0 25 50 75 and 100 kg N ha during the irrigation season which extended from mid October to the end of April As illustrated in Table 8 e Lowrates of nitrogen 25 kg N ha per application gave the most efficient nitrogen response with 1 kg of nitrogen growing on average 10 kg DM 10 kg N kg N of additional pasture consumed by the cows note this is pasture consumed which will be less than what was actually grown e This low rate of 25 kg N ha per application also gave the cheapest cost of extra pasture grown at 90 t DM of additional pasture consumed based on a cost of 400 t of urea e As the rate of applied N increased the relative rate of response to N declined The high 75 amp 100 kg N ha rates gave the highest extra pasture consumed but the efficiency of nitrogen response declined At these high rates the cost of the additional pasture grown t DM increased substantially and may not be economic e The 50 kg N ha per application gave a good nitrogen response efficiency of 8 kg DM kg N of additional pasture consumed at an attractive cost of 120 t DM Given this and the practical considerations of spreading different rates of N fertiliser it is suggested that the 50 kg N ha per application is appropriate Table 8 Effect of a range of rates of nitrogen applied after each grazing on additional
36. l Pasture Growth 5 Light Soil Excellent Excellent Good Fair Good Excellent Excellent Fair Fair Appendix E WISDAM WestVic Dairy Irrigation Scenario Decision Assistance Model WISDAM or the WestVic Dairy Irrigation Scenario Decision Assistance Model is a computer based decision support model developed to assist dairy farmers in south west Victoria make decisions regarding investment in irrigation WISDAM is a discounted cash flow model used to assess the economic and financial viability of various irrigation scenarios on a dairy farm A discounted cash flow model discounts the value of money to be received or spent in the future back to what it would be worth in current dollar terms WISDAM models a period of twenty years with year one being the year of initial investment The model assesses the economic and financial viability of investment in irrigation by generating a range of measures including net present value a benefit cost ratio a payback period and a peak debt level The model allows for the comparison of a range of irrigation scenarios for any one farm It allows the user to choose from four different irrigation systems travelling gun irrigator fixed sprinkler centre pivot and long lateral van den Bosch The user can assess just one system or up to all four if required The user chooses the area to be irrigated and the anticipated improvement in productivity on this area The user can specif
37. le use of the limited and sometimes unreliable supplies of irrigation water in the region The research has been funded by the Government of Victoria through the Department of Primary Industries WestVic Dairy and Dairy Australia previously the Dairy Research and Development Corporation It is hoped that this booklet will provide useful and practical information for both existing irrigators and to those considering irrigation in the future Water Authorities Figure 1 Rural Water Authority boundaries in south west Victoria 2 Irrigation water in south west Victoria Sources of irrigation water Irrigation water for on farm use in south west Victoria comes from groundwater bore surface diversions from streams and some private dams With the exception of several regulated systems in the Werribee and Bacchus Marsh irrigation districts the remaining irrigation in the region is by private schemes Ground or bore water is the predominant source with an estimated three quarters of the irrigation water for the private schemes in the region coming from a series of aquifers predominantly in the southern parts of the region Surface diversions are largely from unregulated streams in the Hopkins Merri River systems and from streams in the Otway river basin Licencing and regulation of irrigation water The licencing and regulation of both groundwater and surface water diversions for irrigation in the region is under the control of Southern Rur
38. n effective rooting depth of 15 cm would only be able to access around one half of the soil water of a pasture with a 30 cm effective rooting depth This will require a shorter irrigation interval and result in less efficient use of water In south west research the leaf appearance rate or the rate at which new ryegrass leaves emerge has been found to be as low as 5 to 7 days in the peak of spring However under full irrigation during January and February ryegrass leaf appearance times were at least 10 days As a result grazing rotation lengths less than 20 days are likely to be detrimental to irrigated pasture yield and persistence Under these conditions a rotation length of between 20 and 30 days would be more suitable li Grazing intensity or how hard to graze Grazing irrigated perennial ryegrass pasture too hard and too low is a real risk and temptation when green feed is limited on the farm The ideal grazing residue for ryegrass when the cows come off the paddock should be 4 to 6 cm of stubble Regularly grazing to less than 4 cm stubble height will run down ryegrass plant energy reserves and weaken the plant leading to lower growth rates a retardation of the root system an increase in tiller death decreased ryegrass plant survival and invasion of less desirable species into the pasture Leaving a residue of greater than 6 cm might lead to faster regrowth in the short term but can also lead to a reduction in herbage quality and incr
39. n give an indication of approximate soil water content It is not accurate and does not predict how much water is required Plant signs are even less reliable our experience is that reductions in pasture growth rates occur long before wilting is apparent c Weather based scheduling There are several methods that predict how much water the pasture crop has lost by evapotranspiration over a period of time and therefore how much irrigation water should be applied to replace it In its simplest form evaporation is measured using an evaporation pan see Appendix C and together with rainfall data is used in a simple daily water budget Appendix B More detailed weather based scheduling techniques involve the use of data from automatic weather stations to enable the calculation of a more accurate estimation of evapotranspiration from the pasture This equipment can be expensive and requires good operator skills and knowledge d Soil based scheduling This involves monitoring the soil water content of the rootzone under the pasture or crop These readings give the best indication of how much water is available to the plant especially after rain Some instruments such as tensiometers see Section 5 and Appendix A will indicate when it is time to irrigate but not how much water to apply Other instruments such as the neutron probe measure the actual soil water content and can also be used to calculate the amount of irrigation water to apply
40. nd crop Table 11 This response was equivalent to a WUE of 2 4 t DM ha ML for the applied water One off irrigations early in the growth of the crop gave much lower responses These results are consistent with Tasmanian findings that the best responses occur if the irrigation is applied as the turnip bulbs are starting to fill Table 11 The effect on yield t DM ha and water use efficiency t DM ha ML of a once off 50 mm application of irrigation water to a dryland turnip crop at a range of growth stages Ecklin 2000 01 0 6 0 1 0 2 6 8 0 5 1 0 8 10 1 2 2 4 10 12 1 1 2 1 The response of dryland perennial pasture to such one off applications will in our experience be considerably less Therefore these results suggest that where irrigation water supplies are very limited or unreliable and where summer feed is required such applications to summer crops would be an effective use of the water A Downsides of irrigating summer forage crops Although there can be advantages in using irrigation water on summer forage crops rather than on perennial pasture there are downsides that need to be considered i Costs and time required to establish forage crops Costs of establishing turnip crops in our recent studies using current contractor fertiliser seed and chemical costs was an average of 576 ha The cost of re establishing a perennial pasture after the crop was an additional 250 ha Clearl
41. nsiometer in to the required depth A cover should be placed over the above ground parts of the tensiometer to protect it from sunlight Reading tensiometers Tensiometers should if possible be read at the same time each day preferably before the heat of the day Tensiometer gauges have a scale reading from O to 100 kilopascals kPa A tensiometer can operate effectively between 0 and 80 kPa For a tensiometer installed at 20 cm depth in irrigated perennial ryegrass white pasture the optimum levels of soil moisture for pasture growth are between 10 and 30 kPa Table 13 At 35 kPa and above the plant starts to experience increasing difficulty in extracting water from the soil and growth rate starts to decline Prolonged periods with readings less than 5 8 kPa indicate that the pasture is waterlogged 0 5 Saturated soil Plants suffering from lack of oxygen in the root zone Soil at field capacity The free water has drained away leaving a good SSA proportion of air to water in the soil 10 30 Optimum soil moisture range of the unrestricted growth of pasture 35 The refill point a the point where pasture plants start to suffer from reduced soil water availability 40 80 The plant experiences increasing difficulty in extracting soil water leading to reduced growth rates The tensiometer is at the limit of its working range It will suck air and will 80 100 need to be refilled with water In some cases
42. otal water requirements for the season Irrigated summer forage crops require less total water for the irrigation season than an equivalent perennial pasture see section 3 Our experience is that a fully irrigated summer forage crop in the southern part of south west Victoria requires between 3 and 4 ML ha of irrigation water for the season Fully irrigated perennial pasture requires approximately 7 ML ha Reasons for the lower irrigation water requirements include e ashorter period of time when irrigation is required e irrigation of crops does not commence until a number of weeks after pasture startup time e the conservation of some spring rainfall in the soil that can be used by the crop over the summer e crop water usage is lower in the seedling and early growth stages v Opportunities for once off or occasional irrigations Trial results indicate that summer forage crops such as turnips can in some years effectively utilise once off applications of irrigation water or other occasional irrigations during their growing season to boost the final yield of the crop At Ecklin in 2000 01 one off 50 mm applications of irrigation water were applied to a turnip crop cv Barkant at a range of stages in the growth of the crop Best responses to the once off irrigations occurred at 8 10 weeks after sowing when the turnip bulbs were developing rapidly A 50 mm irrigation at this stage lifted crop yield by 1 2 t DM ha over the dryla
43. pasture grown the efficiency of the nitrogen response and the cost of the extra pasture grown at Nullawarre mid October 2003 end April 2004 0 0 5 9 0 E 25 225 8 2 2 3 10 90 50 450 9 6 3 1 8 120 75 675 10 4 4 5 7 170 100 900 10 3 4 4 5 220 Other considerations Best responses to N fertiliser are obtained on pastures with a high proportion of sown improved grasses such as perennial ryegrass Suitable pastures should also be well supplied with other nutrients including phosphorus P potassium K and sulphur S N applications to pastures with a low improved grass content or on soils of low fertility are less likely to be economic Although N fertiliser will usually increase pasture growth rates it does not speed up the rate of leaf appearance of the grasses Therefore the length of the grazing rotation should not be reduced when N fertiliser is used and should still be based on the 2 3 leaf stage of perennial ryegrass If irrigation management is poor nitrates can be carried below the root zone of the pasture or crop This will lead to inefficient use of the fertiliser and can cause pollution of ground water 8 Irrigation of summer forage crops The irrigation of summer forage crops instead of perennial pasture can be a more efficient and productive use of irrigation water in some situations This is particularly so when water supplies are limited or when the availability is unreliable It s
44. r the grazing interval is too short This has the effect of weakening plants and reducing growth rates Other poor practices include grazing pastures too low and for too long Grazing irrigated perennial pastures over the summer period should be based on the same principles that apply to actively growing dryland pastures at other times of the year These principles have been well established for perennial ryegrass and should be applied to obtain the best from an irrigated pasture The three most important aspects of grazing management are grazing interval grazing intensity and grazing duration i Grazing interval or rotation length A grazing rotation length coinciding with the regrowth of 2 to 3 leaves per tiller is optimal for ryegrass persistence productivity utilisation and quality Repeated grazings at less than 2 leaves will not allow the plant to restock its energy reserves As a result growth rates and yields decline the plants become weaker and are likely to have persistence problems particularly during stress periods Grazing at more than the 3 leaf stage may result in a higher growth rate but will waste feed with an increase in dead leaves herbage will be of lower quality and the pasture may thin out Regular grazing at less than the optimal 2 3 leaf stage also has the effect of reducing the plant rooting depth Shallower roots reduce the amount of water that the pasture can extract from the soil profile An overgrazed pasture with a
45. se on a centre pivot irrigator on a five day irrigation interval 1 Pasture Irrigated 2 8 0 6 4 6 4 3 5 5 4 4 6 0 3 0 7 8 4 6 0 4 8 12 6 5 4 5 3 6 16 2 6 6 0 4 8 21 0 Total 30 24 6 3 21 0 Evapotranspiration is calculated by multiplying the evaporation by the crop factor of 0 8 Effective rainfall is estimated by subtracting the first 3 mm from each rain event 3 Soil water balance is kept by keeping a cumulative total of the evapotranspiration since the last irrigation less the effective rainfall Therefore the estimated irrigation requirement for this period is 21 mm Note In some situations where high salinity irrigation water is being used or where the system does not apply the water uniformly an additional 10 15 of water should be applied Appendix C Using evaporation pans If evaporation figures for the district are not available from newspapers or some other source then it is worthwhile installing your own evaporation pan If installed and used correctly an on farm evaporation pan will provide accurate and up to date figures of the evaporation on your farm This information when used together with the rainfall in a water budget Appendix B will enable accurate irrigation requirements to be calculated Measuring evaporation requires a regular but small time commitment The benefits however are well worthwhile Types of evaporation pans Class A Pan
46. tability for pasture and forage crop irrigation water supplies should also be tested and assessed for e Sodicity or the Sodium Adsorption Ratio SAR i e the ratio of sodium to calcium and magnesium in the water Medium and high sodicity water can cause a breakdown in soil structure reducing water infiltration rates aeration and root growth Further concentration of salt in the root zone is likely to occur Periodic applications of gypsum may be required if soils turn sodic and lose structure e pH Water with a pH of 6 0 to 8 5 is generally suitable for irrigation Alkaline water with a pH greater than 8 5 can result in plant nutrition problems by making some nutrients and trace elements less available Fouling problems in pumps pipes and other irrigation equipment can also occur Routine monitoring of soil pH is recommended if high pH water is used 5 The irrigation season when to start and finish irrigation each year Startup time Irrigation of pasture in south west Victoria often does not commence until well after the water content of the soil in the rootzone has dropped to a point where plant growth is reduced Although often not obvious the plants do become moisture stressed and growth rates decline resulting in reduced pasture yields In addition when irrigation does finally commence the pasture takes time to recover and can suffer reduced growth rates for a number of weeks The reason for these late startups is often beca
47. tion water use efficiency and economic viability of an irrigation system Sufficient water at the right time should be applied to the crop or pasture to match plant requirements and prevent it going into moisture stress and hence suffering reduced growth rates Similarly excessive water applications should be avoided except if leaching salts from the soil see Section 4 Many irrigation systems in the south west are still producing well below their potential due to poor irrigation scheduling In some cases this is due to the irrigation system being under engineered the system is not capable of applying enough water to the area being irrigated within the required time frame In other cases the farmer does not know or have suitable aids in place to estimate irrigation water requirements Irrigation scheduling methods There are a number of irrigation scheduling methods available to the farmer They range from the fairly simple e g time based through more complex e g evaporation measurement to detailed assessments e g measurement of soil water content Generally the more accurate methods are the more difficult or time consuming a Time based Irrigations are scheduled on a fixed number of days that can be varied from month to month based on experience This can be quite inefficient as it is difficult to take into account variation in evaporation and rainfall b Observational Physical inspection of the soil Table 5 Section 5 ca
48. umber of tensiometers preferably at least three or four should be installed across the irrigated area It is essential that the sites chosen be representative of the soils and the pasture of most of the area and that they receive the same irrigation treatment When located in grazed pasture the tensiometer will need to be protected from damage by stock If a protective cage is placed over the tensiometer ensure that the pasture around the tensiometer is kept at the same height as the rest of the paddock What depth For irrigated pasture in south west Victoria we have found that a depth of 20 cm was the most suitable for determining startup time Tensiometers can also be installed at 30 cm depth in pasture but the readings are slower to respond It is recommended that tensiometers not be installed at depths of less than 15 cm How to install a tensiometer Prior to installing the ceramic tip must be pre soaked in water for at least 24 hours to fill all the pores with water In some cases the tensiometer may also have to be pumped according to the manufacturer s directions to remove any further air bubbles The aim when installing a tensiometer is to maximize contact between the ceramic tip and the soil On most soils this is best done by augering a hole to the required depth using a T bar screw auger of the same or slightly narrower diameter than the tip of the tensiometer Pour some water down the hole to lubricate the sides and slide the te
49. unreliability of supply of this water Changes in the volume of irrigation water available from week to week can play havoc with irrigation scheduling and lead to major reductions in yields and water use efficiency of irrigated pastures and crops Some factors to consider and possible strategies for coping with reduced irrigation water supplies are a Where less irrigation water is available but its supply is reliable Pasture or forage crops Irrigated summer forage crops can have a higher water use efficiency i e produce more feed per megalitre of water than an established irrigated perennial pasture However as outlined in Section 8 it is usually not economic to plough up a pasture and sow a forage crop unless the pasture is degraded and in need of renovation Other factors such as the period of feed availability the herbage quality and the method of feeding the crop also need to be taken into account before moving water from pasture to forage crops Don t delay startup in spring Perennial pasture is very sensitive to late irrigation startup Once the pasture plants are water stressed at the start of the irrigation season growth rates drop rapidly and it takes a number of weeks for the pasture to recover and get back to its normal growth rate see Section 5 From a water use efficiency viewpoint it is preferable to start irrigating pasture at the optimum time and finish irrigating earlier in the summer autumn If it is not possible
50. use they coincide with the peak spring workload on the farm In other cases the irrigator may not be aware of how to determine the optimum startup time Late irrigation startup does lead to reduced pasture yields that can be avoided What is the optimum startup time The optimum irrigation startup time for pasture is not a specific calendar date Rather it is the time at which the moisture content of the soil in the root zone drops to the refill point i e the critical soil moisture content below which plant growth starts to decline due to moisture stress As aresult this optimum startup time will vary from year to year due to seasonal variation The date can range from September on light textured soils in a dry spring through until December or later on heavier soils in wet years Effects of late startup times In a trial at Allansford during the 1996 97 season Table 3 delaying the irrigation startup time on a perennial ryegrass white clover pasture by 10 days past the optimum time resulted in a 674 kg DM ha 35 reduction in pasture yield compared to a pasture first irrigated at the optimum startup time Where the startup time was delayed by 20 days a 1154 kg DM ha 60 reduction in pasture yield was recorded In addition the 10 day delay treatment had lower daily pasture growth rates than the optimum treatment for 45 days after the optimum startup time The 20 day delay treatment had reduced growth rates for 55 days
51. y ploughing in a good irrigated perennial pasture to grow an irrigated summer forage crop will not be economical even though there might be some advantages in WUE However if a pasture has deteriorated and requires renovation the sowing and irrigation of a summer forage crop as part of the renovation process can be cost effective and could give some WUE advantages ii Feed availability over the irrigation season An irrigated perennial pasture will grow and produce forage all year with a relatively constant supply over the irrigation season if properly managed Summer forage crops on the other hand do not have feed available for grazing for extended periods during their establishment and growing periods While it is possible to stagger the sowing and grazing times and use species or varieties with different maturity and grazing times feed supply will often be more variable and more difficult to manage with irrigated forage crops than irrigated pasture iii Quality of the herbage grown Summer forage crops can vary greatly in the nutritive quality of their herbage Any decision to grow irrigated summer forage crops should consider their nutritive quality as well as the WUE and other characteristics Irrigated perennial ryegrass white clover pasture produces a high quality forage with good metabolizable energy ME crude protein CP and fibre NDF levels making it ideal for lactating or growing stock The brassica forage crops such as turnips and
52. y productivity improvements as increased production per head increased cow numbers or a reduction in the amount of supplements fed Changes in any one or more of these variables can be recorded for each farm The WISDAM model consists of four spreadsheets in an Excel file for the inputting of data and presentation of results 1 Current situation feedbase This calculates an annual feed budget for the current farm situation without irrigation 2 Feedbase with irrigation Calculates a new feed budget incorporating the irrigation pasture 3 Investment costs Requires the user to specify capital as well as annual running costs associated with investment in the irrigation system 4 Analysis results Presents the results of the investment analysis for each irrigation system under investigation The WISDAM program and user manual are available for downloading from the WestVic Dairy website www westvicdairy com au Our Water Our Future Ad A Victorian Government initiative ihe Place tobe
53. y to be suitable for use on salt sensitive species or for continuous use on soils that are not well drained e If salinity is greater than 2 300 uS cm problems with continuous use are likely to occur Fortunately relatively few problems with using this high salinity irrigation water have occurred in the region This is mainly a result of a combination of the majority of irrigated soils having good natural drainage the plant species grown being relatively salt tolerant and the leaching effects of the high winter rainfall of the region However care still needs to be exercised On the poorer drained heavier textured soils there is evidence of salt accumulation in the topsoil resulting from irrigation In these cases the pasture yields are lower than expected and there can be a change in the species growing in the pasture such as the more tolerant strawberry clover replacing white clover The installation of sub surface drainage has been successfully used in the region to reduce such problems on these soils Even on well drained soils it is good practice when using high salinity water to include a leaching factor or an additional amount of water over and above the pasture requirements which will assist in moving the salts down past the root zone The additional irrigation water that needs to be applied as a leaching factor when saline water is used varies with the salinity level but is commonly in the order of 10 In assessing their sui
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