189 8 Modeling Phosphorus with Hydrologic Simulation Program-Fortran David E. Radcliffe University of Georgia, Athens, GA Zhulu Lin University of Georgia, Athens, GA CONTENTS 8.1 Brief History of Model Development 189 8.2 Modeling of Hydrology 190 8.3 Modeling of Phosphorus 195 8.4 Modeling of Sediment 199 8.5 Calibration 202 8.6 Case Study: Upper Etowah River Watershed 203 8.7 Comparing HSPF and SWAT 210 8.8 Conclusions 211 Acknowledgments 212 References 212 8.1 BRIEF HISTORY OF MODEL DEVELOPMENT The Hydrologic Simulation Program-Fortran (HSPF) is a watershed-scale, semi- distributed model developed from the original Stanford model (Bicknell et al. 2001). It is one of the two dynamic models intended for modeling watersheds dominated by nonpoint sources in the U.S. Environmental Protection Agency (EPA) Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) package (U.S. Environmental Protection Agency 2004a). The other model is the Soil Water Assessment Tool (SWAT) described by Arnold et al. (1998) and Neitsch et al. (2002). The functions and processes included in HSPF were derived primarily from the following group of predecessor models: © 2007 by Taylor & Francis Group, LLC 190 Modeling Phosphorus in the Environment Hydrocomp Simulation Programming (HSP) (Hydrocomp, Inc. 1976, 1977) Nonpoint Source (NPS) Model (Donigian and Crawford 1976a) Agricultural Runoff Management (ARM) Model (Donigian and Crawford, 1976b; Donigian et al. 1977) Sediment and Radionuclides Transport (SERATRA) (Onishi and Wise 1979) The original development of HSPF was sponsored by the EPA Environmental Research Laboratory in Athens, Georgia, during the 1970s. HSP was a descendant of the Stanford Watershed model (Crawford and Linsley 1966). It was first released in 1980 as Release 5. Later development was sponsored by the U.S. Geological Survey (USGS) Water Resources Division in Reston, Virginia. Probably the best-known application of the HSPF model is its use as part of the Chesapeake Bay Model (Linker et al. 2002). HSPF is one of three linked model components: a watershed model (HSPF), an airshed model, and a bay model. The model was used to establish a goal in 1987 of a 40% nutrient — nitrogen (N) and phosphorus (P) — reduction to the bay by the year 2000. In the current version, the bay watershed is divided into 94 sub-basins with an average area of 194,000 ha. A bibliography of articles describing HSPF applications is available at hspf.com/hspfbib.html (confirmed February 25, 2006). Aside from the Chesapeake Bay Model, applications modeling phosphorus include Donigian et al. (1996), Donigian and Love (2002), and Hummel et al. (2003). 8.2 MODELING OF HYDROLOGY Most of the material in this chapter is taken from the HSPF User’s Manual for Release 12 (Bicknell et al. 2001), which can be downloaded from the EPA Web site, www.epa.gov/waterscience/basins/bsnsdocs.html (confirmed February 25, 2006). A mixture of English and metric units are used in the user’s manual, and this chapter has aimed for consistency with the manual. HSPF uses elements, zones, and nodes. One type of element is the land segment, which can be a pervious land segment (PLS) or an impervious land segment (ILS). Within the PLSs, there are snow, surface, upper soil, lower soil, and groundwater zones. A segment is a portion of the land assumed to have uniform (lumped) properties. Another type of element is a reach element. Within a reach, water moves through a single zone from an upstream node to a downstream node. A simulation might consist of a single watershed (completely lumped) or multiple subwatersheds connected together (partially distributed). U.S. Environmental Protection Agency (2004b) describes which processes need to be simulated for PLSs, ILSs, and reaches to simulate hydrology. Basic hydrology must include PWATER in the PLSs, IWATER in the ILSs, and HYDR in the reaches elements — there is a process for simulating what happens in the snow layer if that is applicable. PWATER is the portion of the model that simulates the water budget for PLSs. Potential evapotranspiration (PET) is based on U.S. Weather Bureau Class A pan evaporation times a crop factor, further adjusted by the vegetative cover percentage in each PLS. Actual ET is calculated by trying to meet the PET from © 2007 by Taylor & Francis Group, LLC Modeling Phosphorus with Hydrologic Simulation Program-Fortran 191 five different sources in the following order until PET is satisfied (ET = PET) or until sources are exhausted (ET < PET): from the groundwater zone as seepage, from vegetation interception, from the upper soil zone, from the groundwater zone directly, and from the lower soil zone. The model recognizes that within a land segment there is variability in ET due to differences in, for example, rooting density. Rainfall is distributed in PWATER in the following manner. Some of the rainfall first goes to interception by vegetation (i.e., grass, leaves, stems, and trunks). This vegetation has a storage capacity that accepts water until it is filled. The intercepted water is lost through evaporation. The water that remains may infiltrate to upper zone storage or interflow storage, may enter surface detention storage, or may run off. None of the conventional methods (e.g., curve number, Green-Ampt) or soil parameters (e.g., saturated hydraulic conductivity, field capacity) are used to calcu- late infiltration and interflow. IBAR is the average infiltration rate over the land segment (in. hr −1 or in. day −1 ), depending on the time step in use (Figure 8.1). The model recognizes that within a land segment there is variability in infiltration rates and that the actual infiltration rate can be less than or greater than IBAR, as this chapter shows. IBAR is calculated as (8.1) where INFILT is a parameter (in. hr −1 ), LZS is the lower zone water storage (in.), LZSN is the nominal (or average) lower zone water zone storage (in.), and INFEXP is a parameter (unitless). FIGURE 8.1 Determination of infiltration and interflow. (Redrawn from B.R. Bicknell, J.C. Imhoff, J.L. Kittle, Jr., T.H. Jobs, and A.S. Donigian, Jr., Hydrological Simulation Program- Fortran: HSPF Version 12 User’s Manual, Mountain View, CA, Aqua Terra Consultants, 2001. With permission.) IBAR INFILT LZS LZSN INFEXP =       − potential direct potential surface runoff potential Interflow Inches of water/interval IMAX IIMAX IBAR IIBAR line I (infiltration capacity) line II (interflow + infiltration capacity) MSUPY Inches of water/interval IIMIN IMIN 050100 % of area © 2007 by Taylor & Francis Group, LLC 192 Modeling Phosphorus in the Environment This represents infiltration into the lower soil zone; infiltration into the upper soil zone (a relatively thin layer) is described later. In the case that frozen ground occurs for a significant period of time, INFFAC accounts for this effect, but it has been left out of Equation 8.1 for the sake of simplicity. Equation 8.1 shows that as water content in the lower zone water storage increases during a storm, the infiltration rate decreases. Once this zone is saturated (LZS = LZSN), the infiltration rate reaches the minimum asymptotic rate of INFILT. This implies that INFILT is equal to the field saturated hydraulic conductivity. The parameter IMAX in Figure 8.1 is the product of INFILD and IBAR. INFILD is a unitless parameter with a recommended value of 2, so IMAX is twice IBAR. The water that is available in a time step for infiltration, interflow, or runoff is moisture supply in inches, MSUPY (Figure 8.1). The total water that infiltrates the lower soil zone is the area beneath line I and the MSUPY line (clear area). (IBAR is the average infiltration rate, and this is used in determining the total) As IBAR increases, so does the amount of water that infiltrates. The water that goes into potential interflow is the area beneath line II and the MSUPY line and above line I (lightly shaded area in Figure 8.1). Interflow is water that moves laterally to a stream due to a restrictive layer in the unsaturated zone (Fetter 1988). Potential interflow water can become actual interflow or inflow into the upper soil zone. As IBAR increases, so does the amount of water that goes to potential interflow. IIMIN and IIMAX are calculated as follows: (8.2) where INTFW is a parameter (unitless). It is apparent from Equation 8.2 that as INTFW increases, line II in Figure 8.1 rises, and the amount of potential interflow increases. Also, as the water content in the lower soil zone increases, so does the amount of potential interflow. The water that is available for potential upper soil zone infiltration, surface detention, or runoff is the area below the MSUPY line and above line II (darkly shaded area in Figure 8.1). The remaining water is potential runoff. The fraction of this water that goes into the upper soil zone (FRAC) is a function of the upper soil zone water content (UZRAT), which is the upper soil zone water storage in inches (UZS) divided by the upper soil zone nominal water storage in inches (UZSN). FRAC decreases as the upper zone water content increases. Note that UZRAT is allowed to be greater than unity. This is a recognition that UZSN varies from the average (nominal) value over the pervious land segment. Once infiltration into the upper soil zone is satisfied, the remaining water goes into surface storage, runoff, and interflow. Surface storage depends in part on Manning’s n for roughness (increases with roughness and n), slope length (increases with slope length), and slope angle (decreases with slope angle). The interflow component assumes a certain storage capacity for interflow water. The rate at which IIMIN IMIN INTFW IIMAX IMAX INTFW LZS LZSN =⋅ ⋅ =⋅ ⋅ 2 2 LLZS LZSN © 2007 by Taylor & Francis Group, LLC Modeling Phosphorus with Hydrologic Simulation Program-Fortran 193 water can enter interflow storage depends on the current storage and the rate at which water is discharging from interflow storage to the stream. The rate of discharge to the stream is a function of the interflow recession parameter IRC. Upper soil zone water can percolate into the lower soil zone. Percolation only occurs when the upper soil zone water content is greater than the lower soil zone relative water content according to the following empirical relationship (units are not consistent on both sides of equation): (8.3) where PERC is the percolation rate (in. hr -1 ). The fact that PERC is proportional to INFILT reinforces the idea that INFILT is related to field-saturated hydraulic conductivity. Water that percolates into the lower zone from the upper soil zone, plus infil- trating water, can increase the lower zone soil water storage or pass on through to groundwater. The fraction that goes to increasing the lower soil zone storage depends on the lower zone relative water content, LZRAT, which is LZS divided by LZSN. As the water content increases, less percolation water is retained, and more water passes through to groundwater. Infiltrating and percolating water that reaches groundwater storage can discharge into the stream or can go to deep groundwater storage; this water is essentially lost from the watershed system. DEEPFR is the parameter (unitless) that determines the fraction that is lost. Outflow to the stream from the remaining groundwater depends on the slope of the water table (gradient), the groundwater storage, and two param- eters, AGWRC and KVARY: (8.4) where AGWO is the outflow rate (in. hr −1 ), GWVS is an index to the water table slope, AGWS is the current groundwater storage (in.), AGWRC is a groundwater outflow recession parameter (day −1 ), and KVARY is a recession parameter — nonzero values cause recession to vary as a function of groundwater levels and will produce seasonal variability in hydrographs. U.S. Environmental Protection Agency (2004c) provides guidance on how to choose hydrological parameters. The primary hydrological parameters are as follows. • FOREST: only used in snow processes. • LZSN: lower zone nominal soil water storage (in.); recommends an initial estimate of 1/8 mean annual rainfall + 4 in. (humid regions); min-max values are 3 to 8 in.; lower values of LZSN will cause more stream flow (less water lost to ET); default = 4 to 6.5 in. depending on land use. • INFILT: index to mean soil infiltration rate (in. hr −1 ); provides a range related to soil hydrologic groups (A: 0.4 to 1.0; B: 0.1 to 0.4; C: 0.05 to PERC INFILT UZSN USZ USZN LSZ LSZN =⋅ ⋅ ⋅ −       01 3 . AGWO AGWRC KVARY GWVS AGWS=⋅+⋅⋅()1 © 2007 by Taylor & Francis Group, LLC 194 Modeling Phosphorus in the Environment 0.1; D: 0.01 to 0.05); default is 0.16 in. hr −1 ; suggests that Z *INFILT* INTFN should approximate the long-term infiltration rate, or permeability, in the soil survey database (untested approach); higher values cause less runoff and less storm flow in streams. • LSUR: length of overland flow path (ft) for the pervious land segment; average length of travel for water to reach a stream; typical values range from 200 ft for slopes of 15% to 500 ft for slopes of 1%; default = 300 ft; higher values should cause storm hydrograph to spread out (lower peak value). • SLSUR: slope of pervious land segment; recommends using digital elevation data to get this — check change in elevation of pixels in a transect perpen- dicular to stream, divide by distance between centers of pixels — make multiple measurements and average; probably has little effect on hydrology but may affect erosion; default = 0.036 to 0.55, depending on land use. • KVARY: nonzero values cause seasonal variation in groundwater flow; increasing the value should cause faster recession during wet months; default is 0; recommends starting with 0 and adjusting if necessary. • AGWRC: groundwater recession rate; default = 0.98; recommends finding this through calibration; higher value causes slower recession; suggests using higher values for forests. • PETMAX: used only in snow processes. • PETMIN: used only in snow processes. • INFEXP: exponent in the infiltration equation; default = 2.0 and recom- mends using the default value. • INFILD: ratio of maximum infiltration rate in a pervious land segment, IMAX, to average infiltration rate, IBAR; default = 2.0 and recommends using the default value. • DEEPFR: fraction of infiltrating water that goes into deep groundwater storage and is lost from the watershed; default = 0.10; recommends finding value through calibration; higher value causes less stream flow overall. • BASETP: the fraction of a pervious land segment area that has vegetation able to transpire water directly from groundwater (i.e., riparian or marsh land vegetation); default = 0.02; recommends calculating this based on area that is riparian or marsh land vegetation. • CEPSC: rainfall (in.) intercepted by vegetation; default = 0.10; recom- mends different values depending on land cover. • UZSN: upper zone nominal soil water storage (in.); recommends different values depending on slope, vegetation, and depression storage; overall rule of thumb is 0.10 LZSN; default = 1.128 in. • NSUR: n in Manning’s equation for overland flow; larger values of n indicate a rougher surface and slower flow; default is 0.20; probably has little effect on water flow but may affect erosion. • INTFW: interflow parameter; increasing interflow value delays water get- ting to the stream (otherwise it would become overland flow), so it lowers the hydrograph peak and spreads the curve out; default is 0.75; recom- mends using calibration to find value. © 2007 by Taylor & Francis Group, LLC Modeling Phosphorus with Hydrologic Simulation Program-Fortran 195 • IRC: interflow recession rate, analogous to groundwater recession rate; increasing value causes the hydrograph to spread out and decreases peak value; default = 0.50. • LZETP: index to lower zone ET related to root distribution; varies between 0 and 1 with 1 representing maximum potential for plant uptake; gives typical ranges for different types of vegetation; default is monthly varying values from 0.2 in winter months to 0.4 in summer months. There are a few parameters associated with the ILSs. Unless the impervious land area is a large portion of the modeled watershed, these parameters will not have much effect on model predictions. The most important factor is what percentage of urban areas is assumed to be impervious; the default is 50%. A few parameters are also associated with the reaches, which will have little effect on stream flow, although they may be important for sediment and P transport. Another source of information on hydrological as well as water-quality param- eters is HSPF Parameter (HSPFParm) (Donigian et al. 1999). This is a database of parameter values that have been used by experienced users in 45 HSPF model runs in 14 states (available at http://hspf.com/hspfprms.html). 8.3 MODELING OF PHOSPHORUS HSPF has a specific routine for modeling P. The module matrix in U.S. Environ- mental Protection Agency (2004b) shows which modules need to be activated in the pervious land, impervious land, and reach segments to model P: • PERLND: activate PWATER, SEDMNT, MSTLAY, and PHOS • IMPLND: none • RCHRES: for inorganic P activate HYDR, ADCALC, SEDTRN, OXRX, and NUTRX for organic P add PLANK A surface zone, as well as the upper soil, lower soil, and groundwater zones, is considered. A flow diagram for the pervious land portion of the P routine is shown in Figure 8.2. Soil P is in organic, soluble, and adsorbed pools. Phosphate is adsorbed and desorbed using either first-order kinetics (i.e., subroutine FIRORD) or instan- taneous adsorption using a Freundlich isotherm (i.e., subroutine SV). This chapter covers only the instantaneous approach. A Freundlich isotherm from the user’s manual is shown in Figure 8.3. On the y axis, X is the P adsorbed in parts per million of soil (mg of P per kg of soil), and on the x axis, C is the P in solution in parts per million of solution (mg of P per L of solution). The y axis intercept of curve 1 and curve 2 is XFIX, the amount of P permanently adsorbed (mg of P per kg of soil). CMAX is the maximum equilibrium concentration of P in soil solution, and XMAX is the corresponding maximum adsorbed concentration of P. Adsorbed P is described by the following equation: (8.5)XKC XFIX N =⋅ +1 1 1 © 2007 by Taylor & Francis Group, LLC 196 Modeling Phosphorus in the Environment FIGURE 8.2 Flow diagram for P reactions. (Redrawn from B.R. Bicknell, J.C. Imhoff, J.L. Kittle, Jr., T.H. Jobs, and A.S. Donigian, Jr., Hydrological Simulation Program-Fortran: HSPF Version 12 User’s Manual, Mountain View, CA, Aqua Terra Consultants, 2001. With permission.) FIGURE 8.3 Freundlich adsorption isotherm. (Redrawn from B.R. Bicknell, J.C. Imhoff, J.L. Kittle, Jr., T.H. Jobs, and A.S. Donigian, Jr., Hydrological Simulation Program-Fortran: HSPF Version 12 User’s Manual, Mountain View, CA, Aqua Terra Consultants, 2001. With permission.) Atmospheric deposition Atmospheric deposition ORGP Organic phosphorus Organic phosphorus mineral- ization Adsorp- tion of phosphate P4AD Phosphorus adsorbed Desorp- tion of phosphate P4SU Phosphate in solution PLTP Plant phosphorus Plant uptake of phosphorus Phosphate immobili- zation C, ppm CMAX Curve 1 Curve 2 XMAX XJCT X, ppm XDIF XFIX © 2007 by Taylor & Francis Group, LLC Modeling Phosphorus with Hydrologic Simulation Program-Fortran 197 where N1 is the Fruendlich exponent (N1 = 1 is a linear isotherm) and K1 is the Fruendlich distribution coefficient (units of L per kg when N1 = 1). These parameters — XFIX, N1, K1, and CMAX — must be supplied for surface, upper, lower, and groundwater zones. P mineralization and immobilization are modeled using first-order kinetics with temperature corrections. The routine requires values for the mineralization rate, KMP, and immobilization rate, KIMP, at 35°C in units of inverse days or hours, depending on the time step. These parameters must also be specified for each zone. The temperature correction equation for P mineralization takes the form (8.6) where TMP is the temperature (°C) in the zone, and TH is the correction coefficient (typically 1.06). There is a similar equation for temperature correction of the immo- bilization rate. Soil temperature is modeled by HSPF. Plant uptake is based on a first-order rate or a yield approach. In the first-order rate approach, for each zone the plant uptake rate parameter (in units of inverse time) is _KPLP where the underlined space is S, U, L, or K, representing the surface, upper, lower, and active groundwater zones. After correction for temperature, the uptake rate takes the form _KPLPK (in units of inverse time). Plant uptake occurs from the soluble P pool (Figure 8.2). The amount of plant uptake each day is calculated as the rate times the mass of P in the soluble pool in each zone. The temperature correction equation takes the same form as Equation 8.6. The yield approach to plant uptake of P is designed to be less sensitive to soil nutrient levels and nutrient application rates than the first-order rate option. It allows crop needs to be satisfied, subject to nutrient and moisture availability, without being affected by soil nutrient level. In this method, a total annual target is specified by the user and is then divided into monthly targets during the crop growing season. The target is further divided into the four soil layers. Soluble P can percolate down through the soil zones, which requires use of the MSTLAY module. In the PWATER module, which is used for general hydrology, some moisture that infiltrates can reach the groundwater in a single time step — that is, a day or an hour. This has little effect on hydrology, but it is not realistic for P in many cases. The MSTLAY module takes the fluxes and storages computed in PWATER and adapts them for runoff, interflow, and percolation through the soil layers. The revised storages, in inches of water, are also expressed in units of mass per area units for use in the adsorption and desorption calculations. Percolation occurs from the surface layer through each of the underlying layers. Percolation of P from the surface layer to the upper soil zone is described by the following equation: (8.7) where SDOWN is the amount of water percolating down (in.), SMST is the amount of water stored in the surface layer (in.), SLMPF is an arbitrary reduction factor (< 1), and FSP is the fraction of the soluble P in the surface zone that percolates (between 0 and 1). KMPK KMP TH TMP =⋅ −35 FSP SLMPF SDOWN SMST =⋅ © 2007 by Taylor & Francis Group, LLC 198 Modeling Phosphorus in the Environment Percolation of P from the upper zone to the lower soil zone is described by the following equation: (8.8) where ULPF is the factor for retarding percolation (since this variable is in the denominator, it must be > 1 to cause retardation), UDOWN is the amount of water percolating down (in.), UMST is the moisture storage (in.), and FUP is the fraction of the soluble P in the upper zone that percolates (between 0 and 1). There is a similar equation for percolation from the lower zone to ground water storages. The surface layer can lose P in surface runoff. Soluble P enters runoff directly and is adsorbed, and organic P can be removed with sediment. The concentration of soluble P in runoff is assumed to be the same as the concentration in the surface layer. Particulate P is removed from the surface layer in proportion to the fraction of the surface soil layer removed by erosion, although the mass of soil in the surface layer is a parameter value that does not vary even when material is removed. As such, an enrichment ratio accounting for the fact that most of the P lost in erosion is adsorbed to the clay-size fraction (Sharpley 1985) is not employed. Phosphorus can be added to the system as organic or adsorbed P through atmospheric deposition or through the special actions block where fertilizer and manure applications are described. The special actions block is a table of annual or monthly inputs. Many processes can be modeled for P in reaches. Most of these occur in the NUTRX module. They include longitudinal advection of dissolved P, benthal release of dissolved P, adsorption and desorption of P to suspended sediment in the water column using a linear adsorption coefficient ADPM(J), which varies for different sediment size fractions J, and desorption and scour and longitudinal advection of adsorbed P. In the PLANK module, sources and sinks of P include uptake by phytoplankton or benthic algae and respiration and inorganic excretion by zooplank- ton. Atmospheric deposition is also considered. No guidance document exists — such as the one for selecting values for hydro- logical parameters — for selecting values for P parameters. The primary parameters for modeling P are as follows: • SKPLP, UKPLP: P plant uptake parameter for surface zone, upper zone • THPLP, THDSP: temperature correction factor for plant uptake, desorption • KIMP: first-order immobilization rate constant (day −1 ) • KMP: first-order mineralization rate constant (day −1 ) • CMAX: maximum equilibrium concentration of P in soil solution (mg L −1 ) • XFIX: concentration of P permanently adsorbed to soil (mg L −1 ) • K1: Freundlich distribution coefficient • N1: Freundlich exponent • ORGP: initial P storage in each layer for organic P • P4AD: initial P storage in each layer for adsorbed P • P4SU: initial P storage in each layer for solution P FUP UZS UZSN ULPF UDOWN UMST = ⋅ ⋅ © 2007 by Taylor & Francis Group, LLC [...]... in other states are listed in Schroeder et al (2004, Table 1) When DRP (mg L−1) was plotted as a function of Mehlich III P (M3 in mg kg−1) in the soil from the 0- to 2-cm © 2007 by Taylor & Francis Group, LLC 206 Modeling Phosphorus in the Environment Total Suspended Solids (mg/L), Turbidity (NTU) 1000 Simulated TSS Observed TSS Observed Turbidity 80 0 600 400 200 0 1 983 1 984 1 985 1 986 1 987 Date 1 988 ... distribution coefficient, K1, in Equation 8. 5, the inverse was used to obtain the value of 588 L kg−1 Since the relationship was linear, the assumption was made that the Freundlich exponent, N1, was unity and was not calibrated this parameter Estimates of the initial store of adsorbed P in the surface soil zone, SP4AD, were obtained using data from the University of Georgia Agricultural and Environmental Services... Observed Total Phosphorus (P mg/L) 1.6 1.2 0 .8 0.4 0 1 983 1 984 1 985 1 986 1 987 1 988 1 989 1990 1991 Date FIGURE 8. 7 Simulated daily total P and observed total P at Canton, Georgia, for 1 983 through 1991 © 2007 by Taylor & Francis Group, LLC Modeling Phosphorus with Hydrologic Simulation Program-Fortran 209 TABLE 8. 2 Initial Values, Bounds, and Final Values of HSPF Parameters Calibrated in the Upper Etowah... coefficient, but in HSPF the concentration in runoff is the same as the concentration of solution P in the surface layer However, the relationship between P in solution and sorbed P in the surface layer (Equation 8. 5) used in HSPF can be interpreted as a relationship between runoff P concentration and labile P in the topsoil, in which case the Freundlich partitioning coefficient is similar to the phosphorus. .. watershed into sub-basins Within each sub-basin it models water, sediment, and P movement in land segments representing the dominant land uses and in a stream-reach segment The equations describing in ltration and interflow in HSPF are unique to the model, and as a result the parameters for these processes are not readily measured U.S Environmental Protection Agency (2004c) is a good guide on selecting parameter... of 16 68 m3 day−1) and the Pilgrim’s Pride Poultry Processing Plant (average measured effluent discharge of 3335 m3 day−1) There were 13 minor permitted point sources in the watershed with average measured effluent discharge ranging from 9.5 to 7 58 m3 day−1 Phosphorus loads were reported in the DMR for the Pilgrim’s Pride Plant and for one of the minor point sources but not for any of the other point sources... and then the USGS turbidity observations were converted into SSC From the Canton USGS station, the Upper Etowah River watershed was delineated into 10 0 10 20 Kilometers N Urban Barren Transitional Cropland Pasture Forest Grass Land Water FIGURE 8. 4 Upper Etowah River basin with the solid dot indicating the watershed outlet at Canton, Georgia The gray lines indicate the stream network, and the dark lines... dark lines indicate the watershed and sub-basin boundaries © 2007 by Taylor & Francis Group, LLC 204 Modeling Phosphorus in the Environment nine sub-basins The area of watershed was 161,557 ha The HSPF land-use classes and area percentages were as follows: forest (88 .6%), pasture (7.9%), urban (2.2%), row crop (0.9%), barren land (0.07%), water (0.04%), and wetland (0.006%) A threshold for land-use categories... Group, LLC 212 Modeling Phosphorus in the Environment modeling water movement in HSPF Donigian and Love (2003) and U.S Environmental Protection Agency (2006) are helpful in finding parameters for modeling sediment HSPFParm is a database of parameter values that has been used by experienced users in 45 HSPF model runs A technical note is needed to guide users in finding parameters for modeling P with HSPF... runoff P (i.e., lower initial adsorbed P in pasture land use) and decreased runoff P (i.e., decrease in the Freundlich partition coefficient) The predicted average P loads from point and nonpoint sources for the period of calibration are shown in Table 8. 3 Point sources accounted for only 3% of © 2007 by Taylor & Francis Group, LLC 210 Modeling Phosphorus in the Environment TABLE 8. 3 Simulated Average . Development 189 8. 2 Modeling of Hydrology 190 8. 3 Modeling of Phosphorus 195 8. 4 Modeling of Sediment 199 8. 5 Calibration 202 8. 6 Case Study: Upper Etowah River Watershed 203 8. 7 Comparing HSPF and. LLC 192 Modeling Phosphorus in the Environment This represents in ltration into the lower soil zone; in ltration into the upper soil zone (a relatively thin layer) is described later. In the case. Group, LLC 204 Modeling Phosphorus in the Environment nine sub-basins. The area of watershed was 161,557 ha. The HSPF land-use classes and area percentages were as follows: forest (88 .6%), pasture