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HSPF Modeling Guidance

1. The following sections of this memorandum discuss how to obtain HSPF software the major data entry components of an HSPF runoff model and the model parameters used to develop the IMP sizing factors The memo is intended as a guide to building an HSPF model in Contra Costa County but it is not a general HSPF user manual The technical level of the discussion assumes the user is an experienced hydrologic modeler and has some familiarity with HSPF 1 The simplified IMP sizing approach uses a spreadsheet tool to select the necessary sizes for hydromodification facilities based on the user s description of a project site s drainage characteristics The IMP sizes were computed through an extensive HSPF modeling process The simplified IMP sizing approach is summarized in a technical memorandum Contra Costa County Clean Water Program Hydrograph Modification Program Integrated Management Practices Modeling Methods and Results dated April 29 2005 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE The remainder of this memorandum is arranged as follows e The Obtaining HSPF Software section describes where to download the software and identifies some valuable tools for model creation and data management for new HSPF users e The Building and Running an HSPF Model section describes the major data requirements of HSPF and describes the components of the model with particular emphasis on the model elements that are used for h
2. In fact the length of the time series data usually determines the length of the model simulation period Por Contra Costa County precipitation and evapotranspiration data are available from the National Oceanic and Atmospheric Administration NOAA and the Contra Costa Flood Control District Table 2 These datasets were used to develop the sizing factors used in the simplified IMP sizing approach Attachment 3 Page 3 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE Table 2 Time Series Input Data Sources Station Name Location patna conan Elev ft Mean Annual Rain Hourly Precipitation Data Sources Martinez City of Martinez se An oe 70 1 20 2 in Flood Control a Flood Control H thru AG aa 160 16 4 in St Mary s St Mary s College ae ui re aw 620 24 8 in Orinda Fire Orinda Fire Station 3 203 hm 37554 N 700 25 1 in Los Medanos Seen ly tru er Aa 130 8 4 in Dublin Fire a aa Ramon i mrg H pee 355 12 5 in Hourly Evaporation Data Sources Source Location Data Type Period of Record Los Alamitos Los Alamitos Recharge Basin San Jose Pan Evaporation 1960 to 1996 SFO San Francisco Airport Pan Evaporation 1948 to 2004 A Our examination of the Martinez Gauge record showed several questionable records where an entire storm s depth was recorded in a single hour For these questionable storms the recorded rainfall depth at
3. Monthly values for LZETP for evapotranspiration from the lower zone can be developed using an expected maximum value from the PWAT PARM4 LZETP discussion and the range of values presented in the Summary Tables Monthly variable values should be used to reflect the seasonality of evapotranspiration in response to changes in density of vegetation depth of root zone and stage of plant growth PWAT STATE1 Table CEPS SURS IFWS UZS LZS AGWS are initial values for storage of water in interception surface ponding interflow the upper zone lower zone and active groundwater respectively and GWVS is the initial index to groundwater slope All these storages pertain to the first interval of the simulation period The surface related storages i e CEPS SURS IFWS are highly dynamic and will reach a dynamic equilibrium within a few days at most These state variables can be left blank or set to 0 0 unless an individual storm is being simulated The soil storages i e UZS LZS and AGWS and the GWVS are much less dynamic so their beginning values can impact the simulation for a period of months to a few years If possible users should allow as long a startup time period as possible i e set the simulation period to begin prior to the period you ll use for comparison against monitoring data or other use as noted each of these storages should reach a dynamic equilibrium within a few years of simulation UZS and LZS should be set equal t
4. ranges are shown in the Summary Table If significant riparian vegetation is present in the watershed then non zero values of BASETP should be used Adjustments to BASETP will be visible in changes in the low flow simulation and will effect the annual water balance If riparian vegetation is significant start with a BASETP value of 0 03 and adjust to obtain a reasonable low flow simulation in conjunction with a satisfactory annual water balance AGWETP Fraction of model segment i e pervious land segment that is subject to direct evaporation from groundwater storage e g wetlands or marsh areas where the groundwater surface is at or near the land surface or in areas with phreatophytic vegetation drawing directly from groundwater This is represented in the model as the fraction of remaining potential ET i e after base ET interception ET and upper zone ET are satisfied that can be met from active groundwater storage estimate then calibrate If wetlands are represented as a separate PLS pervious land segment then AGWETP should be 0 0 for all other land uses and a high value 0 3 to 0 7 should be used for the wetlands PLS If wetlands are not separated out as a PLS identify the fraction of the model segment that meets the conditions of wetlands marshes or phreatophytic vegetation and use that fraction for an initial value of AGWETP Like BASETP adjustments to AGWETP will be visible in changes in the low flow simulation and will effect t
5. typical range of 0 01 to 0 25 in hr in the Summary Table Fontaine and Jacomino 1997 show sediment and sediment associated transport to be sensitive to the INFILT parameter since it controls the amount of direct overland flow transporting the sediment Since INFILT is not a maximum rate nor an infiltration capacity term it s values are normally much less than published infiltration rates percolation rates from soil percolation tests or permeability rates from the literature In any case initial values are adjusted in the calibration process INFILT is primarily a function of soil characteristics and value ranges have been related to SCS hydrologic soil groups Donigian and Davis 1978 p 61 variable INFIL as follows NRCS Hydrologic INFILT Estimate Soil Group in hr mm hr Runoff Potential Table Al Recommended INFILT Parameter Range for Initial Model Setup NRCS Hydrologic Initial Model Setup Runoff Potential Soil Group INFILT range in hr A 0 4 to 1 0 Low B 0 1 to 0 4 Moderate C 0 05 to 0 1 Moderate to High D 0 01 to 0 05 High An alternate estimation method that has not been validated is derived from the premise that the combination of infiltration and interflow in HSPF represents the infiltration commonly modeled in the literature e g Viessman et al 1989 Chapter 4 With this assumption the value of 2 0 INFILT INTFW should approximate the average measured soil infiltration rate at saturation
6. 4 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE The recommended parameters may be modified if appropriate technical justification is provided Consult the EPA publication EPA BASINs Technical Note 6 Estimating Hydrologic and Hydraulic Parameters for HSPF July 2000 for recommended ranges of HSPF parameter values Examples of appropriate technical justification for modifying the parameters listed below include 1 Local field measurements that differ from the recommended parameters 2 Local land cover may differ from the cover types provided For example heavy forest cover could be represented by increasing the interception storage CEPSC and evapotranspiration fractions Table 3 HSPF PERLND Parameters for use in Contra Costa County PERLND Parameter Value Units Description CSNO 0 None Flag to determine whether snow data are used in simulation RTOP 1 None Flag to select overland flow routing method see Appendix A UZFG 1 None Flag to select upper zone inflow computation method VCS 1 None Flag to select constant or monthly variable interception storage capacity Flag to select constant or monthly variable upper zone nominal soil VUZ 0 None moisture storage VNN 0 None Flag to select constant or monthly variable Manning s n parameter VIFW 0 None Flag to select constant or monthly variable interflow parameter VIRC 0 None Flag to select constant or monthly varied inte
7. Martinez was distributed according to the storm timing recorded at the nearest gauge Flood Control District Gauge 11 A similar procedure should be used for simulations that use the Martinez gauge data B The two data sources were combined because the higher quality dataset from Los Alamitos did not cover the entire modeling period HSPF Land Segment Parameters The project site should be divided into separate drainage management areas DMAs based on project drainage design e g location of grade breaks direction of roof drainage and routing of surface and piped drainage and preliminary location of the hydrograph modification management facilities DMAs should be configured to minimize the amount of undeveloped or landscaped area draining to the hydrograph modification management facilities Each drainage management area should be represented by a combination of PERLND and IMPLND land segments in HSPF The hydrograph modification management facilities should be located to capture runoff from all impervious areas while minimizing capture of runoff from pervious areas PERLNDs represent pervious land surfaces and IMPLNDs represent impervious surfaces Table 3 and Table 4 below contain a set of recommended PERLND and IMPLND parameters respectively for Contra Costa County Appendix A contains a more detailed description of the PERLND and IMPLND parameters below These parameters values were used in the IMP sizing analysis Attachment 3 Page
8. allow the basin outflow to match the requirements of limiting post project peak flows and durations to pre project levels from one half the pre project flow with an average recurrence interval of two years 0 5Q2 to the pre project Attachment 3 Page 8 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE flow with an average recurrence interval of 10 years Q10 For example a detention basin with two flow control orifices could have its lower orifice sized to pass 0 5Q2 when the water in the basin is just below the height of the upper orifice The upper orifice could pass flows up to Q10 when the water surface reaches the height of an overflow relief weir If the basin volume is sized to trigger the overflow relief an average of once per 10 years this setup should come close to approximating the flow and duration control standard and reduce the number of modeling simulations needed in the iterative facility sizing process HSPF Modeling Analysis of the Project Site After compiling the required input dataset defining model parameters and specifying the stormwater control scheme for the project area the next step involves running the HSPF model to determine if the post project flows are controlled to the pre project levels The program requires that projects subject to hydrograph modification control must meet a specific peak flow and duration standard Partial duration series statistics should be used to 1 parse the HSPF o
9. loads separately prepared HSPF input files i e UCI files and launches the HSPF executable WDM Utility WDM Utility is a useful tool for managing WDM watershed data management files which are the binary formatted files used by HSPF to store time series data WDM Utility can create time series datasets and perform basic statistical graphical and data aggregation functions Building and Running an HSPF Model for the Project Site Building an HSPF model to simulate stormwater runoff and evaluate the performance of stormwater control facilities involves examining the drainage patterns of the site computing the pervious and impervious areas examining the local soil types collecting time series input data and expressing the site hydrology using a collection of model parameters Building the HSPF model and analyzing the stormwater runoff are parts of the overall site development process The procedure may be summarized as follows 1 The developer s team develops a site plan that includes existing and proposed grading new impervious areas changes in land cover and soil depth and other site characteristics that affect stormwater runoff Attachment 3 Page 2 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE 2 The developer s team divides the project areas into separate drainage areas referred to as Drainage Management Areas in the simplified IMP sizing approach and determines where stormwater cont
10. nominal soil moisture storage UZSN Monthly values are commonly used for agricultural areas to reflect the timing of cropping and tillage practices VMNEG Flag to select constant or monthly variable Manning s n for overland flow plane NSUR Monthly values are commonly used for agricultural and sometimes deciduous forest land areas VIFWEG Flag to select constant or monthly variable interflow inflow parameter INTFW Monthly values are not often used VIRCFG Flag to select constant or monthly varied interflow recession parameter IRC Monthly values are not often used VLEFG Flag to select constant or monthly varied lower zone ET parameter LZETP Monthly values are commonly used for agricultural and sometimes deciduous forest land areas PWAT PARM2 Table FOREST Fraction of land covered by forest unitless measure estimate FOREST is the fraction of the land segment which is covered by forest which will continue to transpire in winter i e coniferous This is only relevant if snow is being considered i e CONOFG 1 in PWATER PARM1 Attachment 3 Page 12 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE LZSN Lower zone nominal soil moisture storage inches estimate then calibrate LZSN is related to both precipitation patterns and soil characteristics in the region The ARM Model User Manual Donigian and Davis 1978 p 56 LZSN variable includes a mapping of calibrated LZSN values across the count
11. previous day Lower zone evapotranspiration coefficient defines portion of the the ET LZETP 0 None opportunity that occurs in the lower soil zone i e rooting zone CEPS 0 Inch Interception storage initial value SURS 0 Inch Surface ponding storage initial value UZS 0 15 Inch Upper zone storage initial value IFWS 0 Inch Interflow storage initial value LZS 4 Inch Lower zone storage initial value AGWS 0 05 Inch Active groundwater storage initial value GWVS 0 None Initial groundwater storage slope Attachment 3 Page 6 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE Table 4 HSPF IMPLND Parameters for use in Contra Costa County IMPLND Parameter Value Unit Description CSNO 0 None Flag to determine whether snow data are used in simulation RTOP 0 None Flag to select overland flow routing method see Appendix A VRS 0 None Flag to select constant or monthly variable retention storage capacity VNN 0 None Flag to select constant or monthly variable Manning s n parameter Flag to determine if lateral surface inflow to the impervious land segment RTLI 1 None will be subject to retention storage Length of assumed overland flow plane Value provided for generic 1 acre basin For specific projects the value should be calculated from the LSUR 100 None site plan Average slope of assumed overland flow path For specific project sites SLSUR 0 035 None the value may be computed drafting o
12. surface inflow is nonexistent This feature is not commonly used in most HSPF applications IWAT PARM2 Table LSUR Length of assumed overland flow plane feet measure estimate See PWATPARM2 LSUR discussion For impervious areas LSUR reflects the overland flow length on directly connected or effective impervious area EIA and is usually in the range of 50 to 150 feet although longer lengths may apply in commercial or industrial regions of large metropolitan areas Impervious surfaces that drain to pervious land rather than to a reach ate considered part of the pervious land segment and not part of the EIA SLSUR Average slope of the assumed overland flow path unitless measure estimate See PWAT PARM2 SLSUR discussion NSUR Manning s n for overland flow plane estimate See PWAT PARM4 NSUR discussion Recommendation set NSUR within the range of 0 05 to 0 10 for paved roads and parking lots RETSC Retention interception storage of the impervious surface inches estimate RETSC is the impervious equivalent to the interception storage variable CEPSC used for pervious land segments RETSC is the depth of water that collects on the impervious surface before any runoff occurs A study of five urban watersheds in the Puget Sound region conducted by the U S Geological Survey Dinicola 1990 found that a value of 0 10 for RETSC was appropriate If parking lots and rooftops are designed for detention storage larger values up
13. A COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE Table 3 HSPF PERLND Parameters for use in Contra Costa County Cont PERLND Parameter Value Units Description Exponent that determines how much a deviation from nominal lower zone INFEXP 2 None storage affects the infiltration rate INFILD 2 None Ratio of maximum and mean soil infiltration capacities The fraction of infiltrating water which is lost to deep aquifers i e inactive 0 45 groundwater DEEPFR 0 45 was used for Group A soils DEEPFR DEEPFR 0 10 None 0 1 was used for Group D soils Exponent that determines how much a deviation from nominal lower zone INFEXP 2 None storage affects the infiltration rate Fraction of PERLND that is subject to direct evaporation from AGWETP 0 None groundwater storage e g wetlands or marsh areas Amount of rainfall that is retained by vegetation never reaches the land 0 02 to surface and is eventually evaporated CEPSC 0 10 for Live Oak cover CEPSC 0 10 Inch CEPSC 0 02 for Range cover UZSN 0 5 Inch Nominal upper zone soil moisture storage NSUR 0 3 None Manning s friction coefficient n for overland flow plane The fraction of water in surface detention that becomes interflow as INTFW 0 4 None opposed to direct overland flow or upper zone storage The interflow recession coefficient is the ratio of the current daily interflow IRC 0 3 None discharge to the interflow discharge on the
14. Attachment 3 BROWN AND CALDWELL Memorandum Date May 4 2005 To Tom Dalziel Contra Costa Clean Water Program CC Christie Beeman Philip Williams Associates Jeff Haltiner Philip Williams Associates From Tony Dubin BC Seattle Steve Anderson BC Seattle Subject Contra Costa County Clean Water Program Hydrograph Modification Program HSPF Modeling Guidance Introduction This memorandum provides technical guidance on how to build an HSPF Hydrologic Simulation Program Fortran model to evaluate the performance of hydrograph modification facilities within Contra Costa County As an alternative to the simplified IMP sizing approach an HSPF model may be used to ensure site specific stormwater facilities are designed to achieve the Contra Costa County Clean Water Program s standard for runoff peak flows and durations Building an HSPF model for a project may be a better alternative than using simplified IMP sizing e When it is proposed to control runoff peaks and durations by routing runoff through detention basins constructed wetlands or other facilities for which a simplified sizing procedure has not been developed e Por large drainage areas with complex drainage where the simplified approach cannot adequately represent project and pre project conditions e To design facilities that serve more than one project site e For sites with steep slopes dense vegetation thin top soil or other atypical hydrologic conditions
15. elationship for a facility Figure 1 shows an example FTABLE that could be used to model a gravel filled detention device that allows percolation through the bottom and a flow control release to the local stormwater conveyance The first three columns define the stage area volume relationship The final two columns define stage discharge relationships for this facility FTABLE 2 rows cols RRIS 11 5 Depth Area Volume Q Perc Q Outlet ERN ft acres acre ft cfs cfs RRN 0 00 003 0 0000 0 0000 0 000 0 10 0 03 0 0012 0 0001 0 000 0 20 0 03 0 0025 0 0007 0 001 0 30 0 03 0 0037 0 0007 0 005 0 40 0 03 0 0050 0 0007 0 018 0 50 0 03 0 0062 0 0007 0 047 0 60 0 03 0 0075 0 0007 0 104 0 70 0 03 0 0087 0 0007 0 133 0 80 0 03 0 0100 0 0007 0 142 0 90 0 03 0 0112 0 0007 0 151 1 00 0 03 0 0125 0 0007 0 159 END FTABLE2 Figure 1 Sample FTABLE for Stormwater Detention Facility While the layout of the FTABLE is straightforward the values in each column and the number of outflow columns depend on the design of the facility First the model developer must select the type of facility to model including its geometry its detention and infiltration characteristics and the height and size of any flow control orifices or weirs For detention basins the careful selection of initial orifice sizes and heights can help streamline the process of sizing the facility The height and diameter of any flow control orifices should be sized to
16. elow UZSN Nominal upper zone soil moisture storage inches estimate then calibrate UZSN is related to land surface characteristics topography and LZSN For agricultural conditions tillage and other practices UZSN may change over the course of the growing season Increasing UZSN value increases the amount of water retained in the upper zone and available for ET and thereby decreases the dynamic behavior of the surface and reduces direct overland flow decreasing UZSN has the opposite effect Donigian and Davis 1978 p 54 provide initial estimates for UZSN as 0 06 of LZSN for steep slopes limited vegetation low depression storage 0 08 LZSN for moderate slopes moderate vegetation and moderate depression storage 0 14 LZSN for heavy vegetal or forest cover soils subject to cracking high depression storage very mild slopes Donigian et al 1983 include detailed guidance for UZSN for agricultural conditions LaRoche shows values ranging from 0 016 in to 0 75 in Fontaine and Jacomino showed average daily stream flow was relatively insensitive to this value but sediment and sediment associated contaminant outflow was sensitive this is consistent with experience with UZSN having an impact on direct overland flow but little impact on the annual water balance except for extremely small watersheds with no baseflow Typical and possible value ranges ate shown in the Summary Table NSUR Manning s n for overland flow plane estimate Man
17. flow data such as plotting on a logarithmic scale as noted above estimated values will likely need to be adjusted through calibration Value ranges are shown in the Summary Table LaRoche et al 1996 reported an optimized value of 0 99 Chen et al 1995 reported values that varied with land use type ranging from 0 971 for grassland and clearings to 0 996 for high density forest Fontaine and Jacomino 1997 reported a calibrated value of 0 99 This experience reflects normal practice of using higher values for forests than open grassland cropland and urban areas PWAT PARM3 Table PETMAX Temperature below which ET will be reduced to 50 of that in the input time series deg F unless it s been reduced to a lesser value from adjustments made in the SNOW routine where ET is reduced based on the percent areal snow coverage and fraction of coniferous forest PETMAX represents a temperature threshold where plant transpiration which is part of ET is reduced due to low temperatures initialize with reported values then calibrate as needed It is only used if SNOW is being simulated because it requires air temperature as input also a requirement of the SNOW module and the required low temperatures will usually only occur in areas of frequent snowfall Use the default of 400F as an initial value which can be adjusted a few degrees if required PETMIN Temperature at and below which ET will be zero deg F PETMIN represents the temperature
18. for applying the HSPF model to compute pre project and post project flows and assess the performance of hydromodification facilities 1 Conduct long term HSPF simulations to compute hourly runoff hydrographs for the following conditions a Pre project site conditions b Proposed post project site conditions c Mitigated post project site conditions with hydromodification facilities included 2 Calculate peak flow frequencies using partial duration series statistics which may be produced using available data analysis software packages 3 Calculate flow duration statistics using database queries or data analysis software Attachment 3 Page 9 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE 4 Produce summary peak flow and flow duration graphics to assess the performance of the hydromodification approach see Figure 2 and Figure 3 The example shown in the figures meets the peak flow and flow duration standards because the mitigated post project peak flow and flow duration curves are below the corresponding pre project curves in the range from 0 5Q2 to Q10 If the post project flows do not meet the peak flow and flow duration standards the hydrograph modification management facilities or site design components should be revised and the HSPF modeling process repeated e Mitigated Post Project Site 0 70 a Pre Project Site 0 502 0 60 0 50 0 40 Peak Flo
19. he annual water balance Follow above guidance for an initial value of AGWETP and then adjust to obtain a reasonable low flow simulation in conjunction with a satisfactory annual water balance PWAT PARM4 Table CEPSC Amount of rainfall in inches which is retained by vegetation never reaches the land surface and is eventually evaporated estimate then calibrate Typical guidance for CEPSC for selected land surfaces is provided in Donigian and Davis 1978 p 54 variable EP XM as follows Table A2 Recommended CEPSC Parameter Range for Initial Model Setup Land Cover Maximum Interception in Grassland 0 1 Cropland 0 1 to 0 25 Forest Cover light 0 15 Forest Cover heavy 0 20 Attachment 3 Page 15 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE Donigian et al 1983 provide more detail guidance for agricultural conditions including residue cover for agricultural BMPs As part of an annual water balance Viessman et al 1989 note that 10 20 of precipitation during growing season is intercepted and as much as 25 of total annual precipitation is intercepted under dense closed forest stands crops and grasses exhibit a wide range of interception rates between 7 and 60 of total rainfall Users should compare the annual interception evaporation CEPE with the total rainfall available PREC in the WDM file and then adjust the CEPSC values accordingly See Monthly Input Values b
20. hin the normal ranges shown above Also calibration requires data on just overland flow from very small watersheds which is not normally available except at research plots and possibly urban sites INTFW Coefficient that determines the amount of water which enters the ground from surface detention storage and becomes interflow as opposed to direct overland flow and upper zone storage estimate then calibrate Interflow can have an important influence on storm hydrographs particularly when vertical percolation is retarded by a shallow less permeable soil layer INTFW affects the timing of runoff by effecting the division of water between interflow and surface processes Increasing INTFW increases the amount of interflow and decreases direct overland flow thereby reducing peak flows while maintaining the same volume Thus it affects the shape of the hydrograph by shifting and delaying the flow to later in time Likewise decreasing INTFW has the opposite effect Base flow is not affected by INTFW Rather once total storm volumes are calibrated INTFW can be used to raise or lower the peaks to better match the observed hydrograph Typical and possible value ranges are shown in the Summary Table IRC Interflow recession coefficient estimate then calibrate IRC is analogous to the groundwater recession parameter AGWRC Le it is the ratio of the current daily interflow discharge to the interflow discharge on the previous day Whereas INTFW affects
21. ion of this assumption However there has been very little research to support using a value other than 2 0 Use the default value of 2 0 and adjust only if supported by local data and conditions DEEPER The fraction of infiltrating water which is lost to deep aquifers i e inactive groundwater with the remaining fraction i e 1 DEEPFR assigned to active groundwater storage that contributes baseflow to the stream estimate then calibrate It is also used to represent any other losses that may not be measured at the flow gage used for calibration such as flow around or under the gage site This accounts for one of only three major losses from the PWATER water balance i e in addition to ET and lateral and stream outflows Watershed areas at high elevations or in the upland portion of the watershed are likely to lose more water to deep groundwater i e groundwater that does not discharge within the area of the watershed than areas at lower elevations or closer to the gage see discussion and figures in Freeze and Cherry 1979 section 6 1 DEEPER should be set to 0 0 initially or estimated based on groundwater studies and then calibrated in conjunction with adjustments to ET parameters to achieve a satisfactory annual water balance BASETP ET by riparian vegetation as active groundwater enters streambed specified as a fraction of potential ET which is fulfilled only as outflow exists estimate then calibrate Typical and possible value
22. ning s n values for overland flow are considerably higher than the more common published values for flow through a channel where values range from a low of about 0 011 for smooth concrete to as high as 0 050 0 1 for flow through unmaintained channels Hwang and Hita 1987 Donigian and Davis 1978 p 61 variable NN and Donigian et al 1983 have tabulated the following values for different land surface conditions Table A3 Recommended NSUR Parameter Range for Initial Model Setup Overland Flow Surface Manning s n Value NSUR Smooth packed surface 0 05 Normal roads and parking lots 0 10 Disturbed land surfaces 0 15 to 0 25 Moderate turf pasture 0 20 to 0 30 Heavy turf forest litter 0 30 to 0 45 Conventional Tillage 0 15 to 0 25 Smooth fallow 0 15 to 0 20 Rough fallow cultivated 0 20 to 0 30 Crop residues 0 25 to 0 35 Meadow heavy turf 0 30 to 0 40 Por agricultural conditions monthly values are often used to reflect the seasonal changes in land surfaces conditions depending on cropping and tillage practices Additional tabulations of Manning s n values for Attachment 3 Page 16 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE different types of surface cover can be found in Weltz et al 1992 Engman 1986 and Mays 1999 Manning s n values are not often calibrated since they have a relatively small impact on both peak flows and volumes as long as they are wit
23. nthly variability versus constant values for selected parameters Where flags indicate monthly variability the corresponding monthly values must be provided in Monthly Input Parameters see below following the PWAT_PARM4 Table section That section also provides guidance on which parameters are normally specified as monthly values CSNOFG Flag to use snow simulation data must be checked CSNOFG 1 if SNOW is simulated RTOPFG Flag to select overland flow routing method choose either the method used in predecessor models HSPX ARM and NPS or the alternative method as described in the HSPF User Manual Recommendation Set RTOPFG 1 This method used in the predecessor models is more commonly used and has been subjected to more widespread application UZFG Flag to select upper zone inflow computation method choose either the method used in predecessor models HSP X ARM and NPS or the more exact numerical solution to the integral of inflow to upper zone i e the alternative method Recommendation Set UZFG 1 This method used in the predecessor models is more commonly used and has been subjected to more widespread application VCSFG Flag to select constant or monthly variable interception storage capacity CEPSC Monthly value can be varied to represent seasonal changes in foliage cover monthly values are commonly used for agricultural and sometimes deciduous forest land areas VUZFG Flag to select constant or monthly variable upper zone
24. o UZSN and LZSN respectively unless it is known that the starting date is during a particularly wet or dry period starting values can be increased or decreased if wet or dry conditions were evident prior to the simulation period AGWS is a bit more problematic If far too high or too low baseflow will be excessive or skewed low for several months or years depending on AGWRC and KVARY Improper values of GWVS can also cause simulation accuracy problems again for lengths of time depending on values of AGWRC and KVARY However since when KVARY is set to 0 0 seasonal recession is not represented and GWVS is not calculated To avoid problems then AGWS should be set to 1 0 inch and GWVS to 0 0 for initial simulation runs If the simulation period is limited in duration you can check and reset these state variables to values observed for the same period in subsequent years with similar climatic conditions However if major calibration changes are made to the parameters controlling these storages e g UZSN LZSN INFILT then the initial conditions should be checked and adjusted during the calibration process The values for AGWS and GWVS should be checked and adjusted as noted above which assuming a yearly cycle of groundwater Attachment 3 Page 18 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE storage can be compared to values during similar seasons in the simulation period If the initial simulated baseflow before the firs
25. ol facilities that collect both treated and untreated flows This circumstance was listed in the introduction as an example that requires an HSPF model particularly if the IMPs have underdrains In areas with Group D soils the IMP underdrains will discharge to the local stormwater conveyance system so downstream hydromodification facilities may need to be sized to manage all flows if flows from upstream IMPs cannot be segregated Two methods are proposed for modeling these combination sites Attachment 3 Page 7 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE e One method is to include the IMPs in an HSPF model of the entire project site The IMP outflows could be routed to the stormwater conveyance system and to any downstream control facilities In the IMP sizing analysis the IMPs were modeled with two layer FTABLEs in HSPF that characterized the geometry and soil moisture holding characteristics of each IMP type The Low Impact Design Technical Guidance Manual for Puget Sound released in January 2005 provides a survey of various analysis methods used to size IMPs in Western Washington e As an alternative the DMAs that contain IMPs could be modeled as the pre project soil cover type This method is conservative for the range of flows controlled by the IMPs Modeling Downstream Hydromodification Facilities HSPF models storage based facilities with the FTABLE element which defines the stage storage discharge r
26. or mean permeability LSUR Length of assumed overland flow plane ft estimate measure LSUR approximates the average length of travel for water to reach the stream reach or any drainage path such as small streams swales ditches etc that quickly deliver the water to the stream or waterbody LSUR is often assumed to vary with slope such that flat slopes have larger LSUR values and vice versa typical values range from 200 feet to 500 feet for slopes ranging from 15 to 1 It is also often estimated from topographic data by dividing the watershed area by twice the length of all streams gullies ditches etc that move the water to the stream That is a representative straight line reach with length L bisecting a representative square areal segment of the watershed will produce two overland flow planes of width 1 2 L However LSUR values derived from topographic data are often too large i e overestimated when the data is of insufficient resolution to display Attachment 3 Page 13 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE the many small streams and drainage ways Users should make sure that values calculated from GIS or topographic data are consistent with the ranges shown in the Summary Table SLSUR Average slope of assumed overland flow path unitless estimate measure Average SLSUR values for each land use being simulated can often be estimated directly with GIS capabilities Graphical techniques incl
27. r GIS software NSUR 0 05 None Manning s friction coefficient n for overland flow plane RETSC 0 1 Inch Retention interception storage of the impervious surface Temperature below which ET will be reduced to 50 of that in the input PETMAX 40 deg F time series PETMIN 35 deg F Temperature threshold below which evaporation is set to zero RETS 1 00E 03 Inch Retention storage initial value SURS 1 00E 03 Inch Surface ponding storage initial value Linking Land Segments HSPF includes two general schemes for routing water from land segments PERLNDs and IMPLNDs through a watershed Either all outflows are moved from one land segment to the next land segment or facility at each time step or a specific routing algorithm is used to weight the distribution of outflows over multiple time steps based on the travel time between model elements in the watershed Linking separate land segments with routing algorithms becomes more important in larger analysis areas As a general rule if the overland flow timing is similar to or longer than the model time step then explicit routing algorithms should be considered Flow routing is managed using RCHRES elements within HSPF Otherwise flow from adjacent land segments may be routed directly without weighting algorithms using either the NETWORK or MASSLINK element Representing DMAs That Have IMPs A special case exists for sites that include a mixture of IMPs and traditional downstream stormwater contr
28. rflow recession parameter VLE 1 None Flag to select constant or monthly varied lower zone ET parameter FOREST 0 None Fraction of forest covered area that will continue to transpire in winter LZSN 7 Inch Nominal lower zone soil moisture storage Mean soil infiltration rate Ranges of values for NRCS Hydrologic Group B 0 7 and C soils are in Appendix A The upper value of INFILT 0 7 was used INFILT 0 03 inch hour for Group A soils INFILT 0 03 was used for Group D soils Length of assumed overland flow plane Value provided for generic 1 acre basin For specific projects the value should be calculated from the LSUR 660 Feet site plan Average slope of assumed overland flow path For specific project sites SLSUR 0 1 None the value may be computed drafting or GIS software Groundwater recession flow parameter used to describe non linear groundwater recession rate This parameter affects groundwater flow rates and is relevant to larger watershed studies that track groundwater KVARY 0 per inch influence on local streams Groundwater recession rate or ratio of current groundwater discharge to AGWRC 0 95 per day that from 24 hours earlier when KVARY 0 Temperature below which ET will be reduced to 50 of that in the input PETMAX 40 deg F time series Temperature threshold where plant transpiration is effectively suspended PETMIN 35 deg F i e set to zero due to temperatures approaching freezing Attachment 3 Page 5 CONTRA COST
29. rol facilities such as integrated management practices IMPs and detention ponds will be located These first two steps should be completed before attempting to model the site runoff 3 Once a proposed site plan is in place an HSPF model should be built to reflect the site conditions linking stormwater runoff from different parts of the project site with proposed IMPs and other stormwater capture devices Building the model involves time series data collection estimating appropriate model parameter values and adding any necessary flow routing and stormwater control facilities to the model HPSF Input File Components and Data Requirements HSPF requires extensive input information to define the hydrology of the project site Time series data are compiled in a WDM file hydrologic parameters stormwater control facilities flow routing and data output controls are all defined in the UCI input file The following section lists recommended sources for time series data model parameter values and instructions on building the stage storage discharge relationships that define how hydromodification facilities perform Time Series Data Sources HSPF requires at a minimum two time series datasets precipitation and pan evaporation Including a temperature time series improves HSPF s representation of evapotranspiration The time series should have uniform time steps no greater than one hour All time series should cover the entire simulation period
30. ry based on almost 60 applications of earlier models derived from the Stanford based hydrology algorithms LaRoche et al 1996 shows values of 5 inches to 14 inches which is consistent with the possible range of 2 inches to 15 inches shown in the Summary Table Viessman et al 1989 provide initial estimates for LZSN in the Stanford Watershed Model SWM IV predecessor model to HSPF as one quarter of the mean annual rainfall plus four inches for arid and semiarid regions or one eighth annual mean rainfall plus 4 inches for coastal humid or subhumid climates These formulae tend to give values somewhat higher than are typically seen as final calibrated values since LZSN will be adjusted through calibration initial estimates obtained through these formulae may be reasonable starting values INFILT Index to mean soil infiltration rate in hr estimate then calibrate In HSPF INFILT is the parameter that effectively controls the overall division of the available moisture from precipitation after interception into surface and subsurface flow and storage components Thus high values of INFILT will produce more water in the lower zone and groundwater and result in higher baseflow to the stream low values of INFILT will produce more upper zone and interflow storage water and thus result in greater direct overland flow and interflow LaRoche et al 1996 shows a range of INFILT values used from 0 004 in hr to 0 23 in hr consistent with the
31. t significant rainfall is much different from the initial observed streamflow then further adjustments can be made to raise or lower the flow rates IMPLND Parameters IWAT PARM1 Table The IWAT PARM1 table includes a number of flag variables to indicate either the selection of a simulation algorithm option or whether the parameter will be treated as a constant or be varied monthly As with PWAT PARM1 where flags indicate monthly variability corresponding monthly values must be provided in Monthly Input Parameter tables see below following IWATPARM3 section CSNOEFG Flag to use snow simulation data must be checked CSNOFG 1 if SNOW module is run RTOPFG Flag to select overland flow routing method If RTOPFG 0 a new routing algorithm is used RTOPFG 1 results in the use of the method used by predecessor models HSPX ARM and NPS Recommendation set RTOPFG 1 this method is more commonly used and has been subjected to more widespread application VRSFG Flag to select constant or monthly variable retention storage capacity RETSC Monthly values are not often used VNNIG Flag to select constant or monthly variable Manning s n for overland flow plane NSUR Monthly values are not often used RTLIFG Flag to determine if lateral surface inflow to the impervious land segment will be subject to retention storage RTLIPFG 1 This flag only has an impact if the another land segment drains to the impervious land segment otherwise lateral
32. ted Attachment 3 Page 20
33. the volume of interflow IRC affects the rate at which interflow is discharged from storage Thus it also affects the hydrograph shape in the falling or recession region of the curve between the peak storm flow and baseflow The maximum value range is 0 3 0 85 with lower values on steeper slopes values near the high end of the range will make interflow behave more like baseflow while low values will make interflow behave more like overland flow IRC should be adjusted based on whether simulated storm peaks recede faster slower than measured once AGWRC has been calibrated Typical and possible value ranges are shown in the Summary Table LZETP Index to lower zone evapotranspiration unitless estimate then calibrate LZETP is a coefficient to define the ET opportunity it affects evapotranspiration from the lower zone which represents the primary soil moisture storage and root zone of the soil profile LZETP behaves much like a crop coefficient with values mostly in the range of 0 2 to 0 7 as such it is primarily a function of vegetation Typical and possible value ranges are shown in the Summary Table and the following ranges for different vegetation are expected for the maximum value during the year Table A4 Recommended LZETP Parameter Range for Initial Model Setup Vegetation Crop Type Lower Zone ET Potential LZETP Forest 0 6 Grassland 0 4 Row crops 0 5 Barren 0 1 Wetlands 0 6 Mon
34. thly Input Parameter Tables In general monthly variation in selected parameters such as CEPSC and LZETP should be included with the initial parameter estimates However adjustments to the monthly values should be addressed only after annual flow volumes are matched well with monitored data All monthly values can be adjusted to calibrate for seasonal variations Attachment 3 Page 17 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE MON INTERCEP Table Monthly values for interception storage Monthly values can be developed based on the data presented in the discussion in PWAT PARM4 CEPSC and the Summary Tables MON UZSN Table Monthly values for upper zone storage For agricultural areas under conventional tillage lower values are used to reflect seedbed preparation in the spring with values increasing during the growing season until harvest and fall tillage See PWAT PARM4 UZSN discussion and Summary Tables for guidance MON MANNING Table Monthly values for Manning s n for the overland flow plane Monthly values can be used to represent seasonal variability in ground cover including crop and litter residue See discussion in PWAT PARM4 NSUR for Manning s n as a function of agricultural conditions MON INTERFLW Table Monthly values for interflow parameter INTFW are not often used MON IRC Table Monthly values for interflow recession parameter are not often used MON LZETPARM Table
35. threshold where plant transpiration is effectively suspended i e set to zero due to temperatures approaching freezing initialize with reported values then calibrate as needed Like PETMAX this parameter is used only if SNOW is being simulated because it requires air temperature as input also a requirement of the SNOW module and the required low temperatures will usually only occur in areas of frequent snowfall Use the default of 350F as an initial value which can be adjusted a few degrees if required INFEXP Exponent that determines how much a deviation from nominal lower zone storage affects the infiltration rate HSPF Manual p 60 initialize with reported values then calibrate as needed Variations of the Stanford approach have used a POWER variable for this parameter various values of POWER are included in Donigian and Davis 1978 p 58 However the vast majority of HSPF applications have used the default value of 2 0 for this exponent Use the default value of 2 0 and adjust only if supported by local data and conditions Attachment 3 Page 14 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE INFILD Ratio of maximum and mean soil infiltration capacities initialize with reported value In the Stanford approach this parameter has always been set to 2 0 so that the maximum infiltration rate is twice the mean i e input value when HSPF was developed the INFILD parameter was included to alow investigat
36. to 0 5 inches may be reasonable IWAT PARM3 Table The following two parameters are used only if SNOW is being simulated Attachment 3 Page 19 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE PETMAX Temperature below which ET will be reduced by 50 of that in the input time series degree F estimate then calibrate See PWAT PARM3 PETMAX discussion PETMIN Temperature at and below which ET will be set to zero degree F estimate then calibrate See PWAT PARM3 PETMIN discussion Monthly Input Parameter Tables MON RETN Table Monthly values for retention storage Monthly values can be varied to represent seasonal changes in surface retention storage due to litter accumulation or sediment deposition on the impervious surface Monthly values are not often used MON MANNING Table Monthly values for Manning s n for the overland flow plane As described above for MONRETN monthly values can be changed to represent seasonal changes on the surface of the impervious area Monthly values are not often used IWAT STATE1 Table RETS and SURS are initial values for storage of water in retention and surface ponding respectively Both of these storages pertain to the first day of the simulation period RETS and SURS are highly dynamic and are only non zero if the simulation starts during or just following a storm event They can be left blank or set to zero unless an individual storm is being simula
37. ude imposing a grid pattern on the watershed and calculating slope values for each grid point for each land use KVARY Groundwater recession flow parameter used to describe non linear groundwater recession rate inches initialize with reported values then calibrate as needed KVARY is usually one of the last PWATER parameters to be adjusted it is used when the observed groundwater recession demonstrates a seasonal variability with a faster recession i e higher slope and lower AGWRC values during wet periods and the opposite during dry periods LaRoche et al 1996 reported an extremely high optimized value of 0 66 mm 1 or 17 in 1 much higher than any other applications while Chen et al 1995 reported a calibrated value of 0 14 mm 1 or 3 6 in 1 Value ranges are shown in the Summary Table Users should start with a value of 0 0 for KVARY and then adjust i e increase if seasonal variations are evident Plotting daily flows with a logarithmic scale helps to elucidate the slope of the flow recession AGWRC Groundwater recession rate or ratio of current groundwater discharge to that from 24 hours earlier when KVARY is zero day estimate then calibrate The overall watershed recession rate is a complex function of watershed conditions including climate topography soils and land use Hydrograph separation techniques see any hydrology or water resources textbook can be used to estimate the recession rate from observed daily
38. utput time series into discrete flow events and 2 compute the recurrence interval and peak flow for each flow event The peak flow and duration control standard is summarized as follows Peak Flow Control e From 0 5Q2 to Q2 inclusive the post project peak flows should not exceed pre project peak flows e Por recurrence intervals from Q2 to Q10 the post project peak flows may exceed pre project peak flows by up to 10 percent for a 1 year band within the 2 to 10 year recurrence interval range For example the post project flows could exceed the pre project flows by up to 10 percent between Q9 and Q10 or from Q5 5 to Q6 5 but not from Q8 to Q10 Flow Duration Control e From 0 5Q2 to Q2 inclusive the post project flow durations i e the aggregate time for which the site discharge exceeds a specific flow rate should not exceed the pre project flow durations This recognizes the impact of these relatively frequent events on the stream channel stability e For flow rates above Q2 post project flow durations should not exceed pre project flow durations by more than 10 percent at any flow rate e The post project durations should not exceed pre project durations for more than 50 percent of the flow levels from 0 5Q2 to Q10 Sizing facilities to meet the peak flow and duration control standard is often an iterative process that involves several HSPF simulations and statistical analyses The following steps outline a general procedure
39. w cfs 0 30 0 20 0 00 li li li li li fi li li li 0 1 2 3 4 5 6 7 8 9 10 Recurrence Interval years Figure 2 Example Peak Flow Frequency Plot for Post Project Flows that Meet Control Standard Attachment 3 Page 10 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE e Mitigated Post Project Site s Pre Project Site 0 502 Flow cfs 0 00 T T T i T T T T T 0 0000 0 0001 0 0002 0 0003 0 0004 0 0005 0 0006 0 0007 0 0008 0 0009 0 0010 Time Exceeded Figure 3 Example Flow Duration Plot for Post Project Flows that Meet Control Standard Attachment 3 Page 11 CONTRA COSTA COUNTY CLEAN WATER PROGRAM HMP HSPF MODELING GUIDANCE APPEND XA HSPF PARAMETER DESCRIPTIONS This section provides a list of descriptions for the pervious and impervious land surface parameters PERLND and IMPLND respectively used in the HSPF model for Contra Costa The values for these parameters were derived from numerous sources the USGS regional calibration on Calabazas Creek in Santa Clara County the WWHM and the EPA publication EPA Basins Technical Note 6 Estimating Hydrologic and Hydraulic Parameters for HSPF July 2000 from which the parameter descriptions below are reproduced PERLND Parameters PWAT PARM1 Table Sets PERLND Flags The PWAT PARM1 table includes flags to indicate the selected simulation algorithm option other selection of mo
40. ydromodification simulations e The HSPF Modeling Analysis section describes the iterative procedure for sizing hydromodification facilities with HSPF simulations e Appendix A provides detailed descriptions of the pervious and impervious land segment model parameters included in HSPF Obtaining HSPF Software HSPF is publicly available software maintained and distributed by the US Environmental Protection Agency EPA HSPF is distributed as part of the EPA BASINs software suite which includes HSPF Soil Water Assessment Tool SWAT and PLOAD which is a GIS based model for estimating non point source pollutant loads and other GIS based watershed analysis tools BASINSs also includes three utilities for building and running HSPF models and for managing time series data Table 1 that may be very helpful to the novice HSPF user who is getting started building a model The BASINs software suite may be downloaded from the EPA s web site http www epa gov OST BASINS Table 1 EPA BASINs Utilities for HSPF Utility Description WinHSPF WinHSPF provides a Windows based graphical user interface with menus and input forms for building HPSF models This tool may be particularly valuable to new HSPF users who would prefer to develop models interactively rather than using a text editor to create a user control input UCI file from scratch WinHSPF Lite WinHSPF Lite is a convenient tool for running already built HSPF models This utility

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