Modeling phosphorus in the environment - Chapter 9 pptx

The conversion factor converts nutrient concentration in soil to mass in kilograms using Equation 9.1: 9.1 where conv is the intensive unit-to-extensive-unit conversion factor kg, ρb is

in the Annualized

Agricultural Nonpoint Source Pollution

(AnnAGNPS) Model

Yongping Yuan

U.S Department of Agriculture-Agricultural

Research Service, Oxford, MS

Ronald L Bingner

U.S Department of Agriculture-Agricultural

Research Service, Oxford, MS

Indrajeet Chaubey

University of Arkansas, Fayetteville, AR

CONTENTS

9.1 Model Introduction 216

9.2 Watershed Processes Considered in AnnAGNPS 216

9.3 Model Inputs and Outputs 217

9.4 AnnAGNPS Model of Phosphorus Processes 219

9.4.1 Soil Initial Phosphorus Content 220

9.4.2 Organic P Simulation Processes 221

9.4.3 Inorganic P Simulation Processes 222

9.4.3.1 Calculation of Inorganic P Additions to a Cell 222

9.4.3.2 Calculation of Intermediate Inorganic P Mass Balance 223

9.4.3.3 Calculation of Inorganic P Losses from the Soil Profile 224

9.4.4 Total Runoff Losses 226

9.5 Model Application 226

9.5.1 Study Watershed and Monitoring Information 226

9.5.2 Input Data Preparation 227

9.5.3 Sensitivity Analysis 229

9.5.4 Model Calibration and Validation 232

9.6 Model Limitations 238

9.7 Conclusions 238

References 238

9.1 MODEL INTRODUCTION

The Annualized Agricultural Nonpoint Source Pollution (AnnAGNPS) model is an advanced technological watershed evaluation tool that has been developed through a partnership between two U.S Department of Agriculture (USDA) agencies — the Agriculture Research Service (ARS) and the Natural Resources Conservation Service (NRCS) — to aid in the evaluation of watershed responses to agricultural management practices (Bingner and Theurer 2001) AnnAGNPS is a continuous-simulation, daily time-step, pollutant loading model designed to simulate long-term chemical and sedi-ment movesedi-ment from agricultural watersheds (Bingner et al 2003) The spatial vari-ability of soils, land use, and topography within a watershed is accounted for by dividing the watershed into many user-specified, homogeneous, drainage-area-determined cells For individual cells, runoff, sediment, and pollutant loadings can be predicted from precipitation events that include rainfall, snowmelt, and irrigation

Each day, AnnAGNPS simulates runoff, sediment, nutrients, and pesticides leaving the land surface and being transported through the watershed channel system

to the watershed outlet before the next day is considered The model routes the physical and chemical constituents from each cell into the stream network and finally

to the watershed outlet and has the capability to identify the sources of pollutants

at their origin and to track them as they move through the watershed system The AnnAGNPS model has evolved from the original single-event Agricultural Nonpoint Source (AGNPS) model developed in the early 1980s by the USDA-ARS (Young et al 1989, 1995) The AGNPS model was developed to simulate runoff and water-quality response of agricultural watersheds ranging from a few hectares to 20,000 hectares from a single rainfall event The AGNPS model has been applied throughout the world to investigate various water quality problems The AnnAGNPS model includes significantly more advanced features but retains many of the impor-tant features of AGNPS (The complete suite of AnnAGNPS model, composed of programs, pre- and post-processors, technical documentations, and user’s manuals,

is currently available at http://www.ars.usda.gov/Research/docs.htm?docid=5199.)

9.2 WATERSHED PROCESSES CONSIDERED

IN AnnAGNPS

The hydrology components considered within AnnAGNPS are rainfall, interception, runoff, evapotranspiration (ET), infiltration/percolation, subsurface lateral flow, and sub-surface drainage The runoff from each cell is calculated using the Soil Conservation Service (SCS) curve number (CN) method (Soil Conservation Service 1985) The mod-ified Penman equation (Jenson et al 1990; Penman 1948) is used to calculate the potential

ET, and the actual ET is represented as a fraction of potential ET The fraction is a linear

function of soil moisture between wilting point and field capacity For percolation, onlythe downward drainage of soil water by gravity is calculated (Bingner et al 2003) Lateralflow is calculated using Darcy’s equation, and subsurface drainage is calculated usingHooghoudt’s equation (Freeze and Cherry 1979; Smedema and Rycroft 1983) Amount of sheet and rill soil erosion loss — not field deposition — for eachrunoff event is calculated using the Revised Universal Soil Loss Equation (RUSLE)model (Renard et al 1997) A delivery ratio, which quantifies the amount of sedimentdeposited in the field and the amount of sediment delivered to the stream, is calcu-lated using the Hydrogeomorphic Universal Soil Loss Equation (HUSLE) model(Theurer and Clarke 1991) Ephemeral gully erosion is based on the EphemeralGully Erosion model (Merkel et al 1988) The model uses the Bagnold equation(Bagnold 1966) to determine the sediment transport capacity of the stream and amodified Einstein equation to determine the sediment transport in the stream system(Bingner et al 2003) Sediment is partitioned into five classes: clay, silt, sand, smallaggregates, and large aggregates The model estimates particle-size distribution ofdeposited sediment by taking into account the density and fall velocity of each class The AnnAGNPS model calculates a daily mass balance within each cell for soilmoisture, nitrogen (N), phosphorus (P), organic carbon (OC), and pesticides Plantuptake of nutrients, fertilization, residue decomposition, mineralization, and trans-port are major factors considered to determine the fate of nutrients in the watershed.Both soluble and sediment adsorbed nutrients are considered by the model.The pesticide component is adopted from the Groundwater Loading Effects ofAgricultural Management Systems (GLEAMS) model (Leonard et al 1987) TheAnnAGNPS model allows simulation of any number of pesticides and treats eachpesticide separately with independent equilibrium assumed for each pesticide Bothsoluble and sediment-adsorbed fractions of each pesticide are calculated on a dailytime scale Factors affecting fate and transport of pesticides include foliage wash-off, vertical transport in the soil profile, and degradation

9.3 MODEL INPUTS AND OUTPUTS

A complete list of AnnAGNPS input data sections is shown in Figure 9.1 Thesedata can be grouped into the following categories: climate, watershed physicalinformation, land-management operations, chemical characteristics, and feedlotoperations Daily precipitation, maximum and minimum temperatures, dew pointtemperature, sky cover, and wind speed are climate data required by the model toperform continuous simulation Climate data used with AnnAGNPS can be histor-ically measured, synthetically generated using the climate generator program(Johnson et al 2000), or a combination of the two

Geographic information systems (GIS) data layers of a watershed are needed tocharacterize the watershed The GIS data layers must be in sufficient spatial detail

to permit the model to accurately reflect the real landscape it represents Using theGIS layers of digital elevation model (DEM), soils, and land use, a majority of thelarge data input requirements can be developed using a customized ArcView GISinterface Those input requirements include watershed and cell delineation, cell landslope, slope direction, cell land use and soil type, and stream reach data, can be

Modeling Phosphorus in the Environment

FIGURE 9.1 A complete list of AnnAGNPS input data sections.

AnnAGNPS

Identifier

Watershed Data

Simulation Period

Daily Climate

Verification Data

Global Output

Gully Point

Source Feedlot

Feedlot Management Field Pond

Field Pond Management

Soils

Management Field Tile Drain Impoundment Reach Channel

Geometry

Reach Nutrient Half Life Management

Strip Crop Contours Crop

Runoff Curve Number Irrigation

© 2007 by Taylor & Francis Group, LLC

developed by using a customized ArcView GIS interface Additional input ments, which include developing the soil layer attributes to supplement the soilspatial layer, describing crop operations and management practices, defining channelhydraulic characteristics, and entering many other optional data sections as needed

require-by the watershed (Figure 9.1), can be organized using the AnnAGNPS Input Editor.The Input Editor is a graphical user interface developed to aid users in selectingappropriate input parameters Much of the information needed to characterize cropcharacteristics, field operations (e.g., crop rotation, tillage, planting, harvesting),chemical characteristics, feedlots, and soils can be obtained from databases importedfrom RUSLE or from other USDA-NRCS data sources

Feedlot information includes daily manure production rates, manure istics, amount of manure removed from the field lot, and residual amount of manureavailable from previous operations The model outputs include runoff, sediment,nutrient, and pesticide at a temporal scale ranging from daily to yearly All modeloutputs can be obtained at any desired location such as specific cells, stream reaches,feedlots, gullies, or point sources The model also has capabilities to provide sourceaccounting information in terms of the fraction of a pollutant loading passing throughany reach location that originated from a user-specified pollutant source area Cronsheyand Theurer (1998), Geter and Theurer (1998), and Theurer and Cronshey (1998)provide detailed information on available model outputs

character-9.4 AnnAGNPS MODEL OF PHOSPHORUS PROCESSES

Simulation of P transport and transformation processes at a watershed scale is verychallenging because of the complexities and uncertainties related to the processes

A complete understanding of the relationship of various P pools and their chemical,physical, and biological interactions in the soil profile is essential for a full descrip-tion of the P cycle in soils and plants (Jones et al 1984) A model based onmathematical descriptions of fundamental chemical, physical, and biological mech-anisms of the soil P behavior would be ideal for P modeling

In general, the chemical component in AnnAGNPS exists in two phases: solved (solution) in the surface runoff and attached (adsorbed) to clay-size particlesresulting from sheet and rill erosion To simulate P loading, daily soil mass balances

dis-of P in a cell are maintained for each computational layer The daily mass balances

of P are adapted from the Erosion Productivity Impact Calculator (EPIC) model(Sharpley et al 1984; Sharpley and Williams 1990)

The P processes simulated in AnnAGNPS are shown in Figure 9.2 Morespecifically, P is partitioned into inorganic P and organic P, and a separate massbalance is maintained for each Inorganic P is further broken down into (1) labile

P, or P readily available for plant uptake; (2) active P, or P that is more or lessreversibly adsorbed to the soil; and (3) stable P, or adsorbed P that is fixed orrelatively irreversibly chemisorbed to the soil adsorption complex or as discreteinsoluble P minerals The model simulates the effect of P adsorption that controls Pavailability to plant uptake and runoff loss, and the model also simulates P movementsbetween labile P and active P and between active P and stable P Sediment-attached

P estimated from soil erosion is assumed to be associated with the clay-sizefraction of the soil and consists of both organic and inorganic P Major processesconsidered are residue decomposition and mineralization, fertilizer application,plant uptake, runoff, and erosion losses Plant uptake of P is modeled through asimple crop-growth stage index either specified by the user or by the model(Bingner et al 2003)

Phosphorus losses from each AnnAGNPS cell within a stream reach are added

to an AnnAGNPS reach Phosphorus is reequilibrated between dissolved P andsediment-attached P in the reach during transport to the watershed outlet

The initial soil P content is needed to initialize AnnAGNPS simulation Usually,calibration is recommended to define the initial soil P content

The input P levels in the soil profile are input as concentrations, but AnnAGNPSperforms calculations on a mass basis To convert a concentration to a mass, AnnAGNPS

uses a conversion factor, conv (Equation 9.1) The conversion factor converts nutrient

concentration in soil to mass (in kilograms) using Equation 9.1:

(9.1)

where conv is the intensive unit-to-extensive-unit conversion factor (kg), ρb is the

bulk density of composite soil layer (g/cm3 or mg/ m3), D is thickness of soil layer (mm), and Acell is the AnnAGNPS cell area (ha)

FIGURE 9.2 Phosphorus processes simulated in AnnAGNPS.

Erosion

loss

Erosion loss

Organic fertilizer

Active

Stable

Stable (Humic)

9.4.2 O RGANIC P S IMULATION P ROCESSES

All AnnAGNPS mass balances are based on AnnAGNPS cells and are maintainedfor two composite soil layers The first soil layer is 203 mm in depth from thesurface, typically defined as the tillage layer by RUSLE The second soil layer isfrom the bottom of the tillage layer to either an impervious layer or the user-supplieddepth of the soil profile

The mass balance equation for organic P simulation is as follows:

(9.2)

where orgPt is organic P concentration in the composite soil layer for the current

day (mg/kg), orgP t–1 is organic P concentration in the composite soil layer for the

previous day (mg/kg), resP is organic P addition to a cell from decomposed fresh crop residue (kg), orgPfer is organic P addition to a cell from fertilizer application

(kg), hmnP is the mineralization from the humus active organic P pool (kg), and

orgPsed is organic P loss from a cell by attaching to sediment (kg)

Decomposition is calculated once a day Equations for residue decompositionwere adapted from RUSLE Only surface decomposition is calculated for crop land.Cell organic P from fertilizer application is the product of the fertilizer applied forthe current day and the organic P fraction in the fertilizer The organic P fractioncan be obtained from the fertilizer reference database in AnnAGNPS

The P mineralization equation is adapted from the EPIC model (Sharpley andWilliams 1990) Temperature and aeration, represented by soil moisture, are con-sidered for P mineralization (Sharpley and Williams 1990) AnnAGNPS assumesthat organic phosphorous is associated with the clay fraction of the soil Sediment-attached organic P is calculated by Equation 9.3:

(9.3)

where f orgP is a decimal fraction of organic P in clay in soil layer (g/g), and sedclay isthe amount of clay in the mass of sediment (mg) The decimal fraction of organic P is:

(9.4)

where orgP is the organic P concentration in the composite soil layer (mg/kg), and

fclay is the fraction of clay to total composite soil, provided by the soil database.Organic P mass balance is maintained for the second soil layer the same way

as the first layer except that fertilizer application and rainfall-induced runoff andsediment loss are not considered AnnAGNPS assumes that fertilizer application,rainfall-induced runoff, and sediment loss are associated only with the top soil layer.Equation 9.5 represents the mass balance for the second layer:

9.4.3 I NORGANIC P S IMULATION P ROCESSES

AnnAGNPS simulates three different pools of inorganic P in the soil It adapts theprinciples of the soil mineral P model developed by Jones et al (1984) Mineral P

is transferred among three forms: labile P in solution (available for plant use andrunoff loss), active P, and stable P AnnAGNPS assumes that inorganic P added fromfertilizers initially goes to the labile P pool and the active P pool, based on a value

of the P sorption coefficient Fertilizer P that is labile at application may be quicklytransferred to the active mineral pool Many studies have shown that after an appli-cation of inorganic P fertilizer, solution P concentration in the soil decreases rapidlywith time due to reaction with the soil This initial fast reaction is followed by amuch slower decrease in solution P that may continue for several years (Barrow andShaw 1975; Munns and Fox 1976; Rajan and Fox 1972; Sharpley 1982) Flowbetween the active and stable mineral pools is governed by a P exchange rate Within each inorganic P pool, addition from fertilizer application is calculatedfirst, followed by the mineralization of organic P Then, losses through runoff,erosion, and plant uptake are calculated At the end of each day, the mass balance

is updated for each P pool The simulation is a sequence of adjusting the massbalance of each inorganic P pool

9.4.3.1 Calculation of Inorganic P Additions to a Cell

Fertilizer additions are simulated in one of two ways: well mixed with the top soillayer or unincorporated on the soil surface On a daily basis, AnnAGNPS checks ifthere is a tillage operation and the percentage of soil disturbance from the tillageoperation If the soil disturbance exceeds 50% of the top soil layer, any fertilizerapplications are considered as mixed Otherwise, it assumes the applied fertilizerstays on the soil surface In addition, when the soil disturbance exceeds 50% of thesoil, it incorporates not only the applied fertilizer on the current day but also anyfertilizer left on the soil surface from previous applications Therefore, when soildisturbance exceeds 50% of the top soil layer,

(9.6)

where mnaP is the mass of inorganic P added to the soil profile from the current operation (kg) (and it is assumed to be well mixed with the first soil layer), and surf_inorgP is the

surface inorganic P in a cell, added through fertilization at the soil surface (kg)

If a fertilizer is applied in the current operation, then

(9.7)

where inorgPfer is inorganic P applied during the current operation (kg) It is lated using the rate of fertilizer applied for the current day times the inorganic Pfraction (from the fertilizer reference database mass/mass)

calcu-When soil disturbance is less than 50% of the soil, the fertilizer on the soilsurface remains on the soil surface and nothing is incorporated into the soil profile

If a fertilizer is applied for the current operation, then

(9.8)

mnaP=surf inorgP_

mnaP=mnaP+inorgPfer

surf inorgP_ =surf inorgP_ +inorgPfer

Then, AnnAGNPS checks if a rainfall event occurred, and if so, soil inorganic

P is adjusted to reflect the rainfall impact When a rainfall event occurs, it dissolvesthe soluble P on the soil surface When the rainfall generates runoff, AnnAGNPSassumes that inorganic P on the soil surface is totally dissolved in the water and iseither carried away with runoff or is carried into the soil profile with infiltration.The amount of inorganic P carried away with runoff or carried into the soil profilewith infiltration is determined based on the amount of runoff and infiltration fromthe rainfall event

(9.9)

(9.10)

where surf_sol_P is mass of inorganic P in runoff (kg), inf_sol_P is the amount of inorganic P carried into the soil profile by infiltration (kg), Q is the amount of surface runoff (mm), and inf is the amount of infiltration (mm) Then, the amount of inorganic

P carried into the soil profile by infiltration is added to the mnaP value to reflect

the impact of the current rainfall event

9.4.3.2 Calculation of Intermediate Inorganic P Mass Balance

The intermediate inorganic P mass balance refers to P pools with P additions butprior to any P losses to runoff, erosion, and plant uptake Bottom soil-layer inorganic

P does not change with this operation

A portion of the incorporated inorganic P is added into the labile P pool:

(9.11)

where labPi is the concentration of intermediate labile inorganic P in the composite

soil layer (mg/kg), labPstart is the concentration of labile inorganic P at the beginning

of a day, and it is equal to the labile P at the end of the previous day (mg/kg), mpr

is the flow rate of P between labile and active P pools on the current day (+ impliesflow from labile to active pool; – implies flow in the opposite direction) (mg/kg/d)

(Sharpley and Williams 1990), Psp is the soil type-dependent P sorption coefficient (dimensionless) (Sharpley and Williams 1990), and mnaP is mass of inorganic P

added to a cell soil profile (kg)

The rest of the incorporated inorganic P is added into the active P pool:

inf sol P inf

Q inf surf inorgP

=+

labP labP mpr Psp mnaP

where actPi is the concentration of intermediate active inorganic P in the composite

soil layer (mg/kg), actPstart is the concentration of active inorganic P at the beginning

of a day (equal to the active P at the end of the previous day) (mg/kg), and aspr is

the flow rate of P between active and stable P pools on the current day (+ impliesflow from active to stable pool; – implies flow in the opposite direction) (mg/kg/d)(Sharpley and Williams 1990)

Stable P pool size is calculated as follows:

(9.13)

where stbPi is concentration of intermediate stable inorganic P in the composite soil

layer (mg/kg) and stbPstart is the concentration of stable inorganic P at the beginning

of a day (equals to the stable P at the end of the previous day) (mg/kg)

Then, the inorganic P pools are further adjusted to add the organic P frommineralization This mineralized P is partitioned among three inorganic P poolsbased on the fraction of each inorganic P pool to total inorganic P

sum of labile P, active P, and stable P), fact is the fraction of active P to total P, and

fstb is the fraction of stable P to total P

9.4.3.3 Calculation of Inorganic P Losses from the Soil Profile

This calculation includes sequential adjustments to the P pool size to reflect lossesfrom a cell

9.4.3.3.1 Loss through Surface Runoff

When a rainfall event occurs, runoff interacts with soil and carries soluble inorganic

P in the soil profile away from fields AnnAGNPS assumes the effective depth ofrunoff interaction with soil to be 10 mm All soluble inorganic P in the top 10 mm

of soil is carried away by the runoff

Soil soluble inorganic P in the top soil layer available for runoff loss is calculated as

1

where soil_sol_inorgP is the concentration of soluble P available for runoff loss in

a cell soil profile on the current day (mg/kg) and Kd_inorgP is the linear partitioning

coefficient for inorganic P (the ratio of the mass of adsorbed P to the mass of P insolution)

Soluble inorganic P removed by runoff from the top 10 mm of soil is calculated as

(9.18)

where cell_soil_sol_inorgP is the inorganic P removed from the top soil layer through runoff (kg), edi is the effective depth of interaction factor, AnnAGNPS uses

10 mm, and depth is the depth of the top soil layer (mm)

The labile P pool is adjusted to reflect the loss to surface runoff

(9.19)

9.4.3.3.2 Loss to Soil Erosion

Soil erosion also carries inorganic P away from fields The inorganic P loss througherosion is calculated the same way as organic P AnnAGNPS assumes that theinorganic P is also associated with clay fraction The amount of sediment-attachedinorganic P is calculated first; then it is partitioned between the active and stable Ppools based on the amount of each pool

(9.20)

(9.21)

where sed_inorg_actP is the sediment loss from active P (kg) and sed_inorg_stbP

is the sediment loss from stable P (kg)

9.4.3.3.3 Loss through Plant Uptake of Inorganic P

In AnnAGNPS, the amount of crop nutrient uptake is calculated in a crop-growthstage subroutine that determines the crop-growth stage based on crop data specified

by a user Four growth stages —initial, development, mature, and senescence — aresimulated by AnnAGNPS The length of each growth stage can be specified by auser or by the model (Bingner et al 2003) The amount of nutrient uptake iscalculated based on the crop-growth stage and differs by growth stage The cropnutrient uptake is also limited by available nutrients in the composite soil layer Thecalculated crop uptake P in the crop-growth stage subroutine affects the inorganic

P mass balance Phosphorus uptake on a given day is calculated as follows:

(9.22)

cell soil sol inorgP_ _ _ =edi soil sol inorgP Conv_ _

D Depth 1,000,000

labPi+2=labPi+1−cell soil sol inorgP_ _ _ 1,000,0000

where uptP is the amount of inorganic P taken up by the plant on the current day (kg),

growth_P_uptake is the fraction of P uptake for the current growth stage, yield is

the yield at harvest (kg/ha), P_uptake_harvest = P uptake per yield unit at harvest

(mass of P/mass of harvest unit, dimensionless), and stage_length is the the number

of growing days for the current growth stage (days)

Plant P uptake is adjusted based on the availability of P in the soil If uptP

calculated in Equation 9.22 is greater than the available labile P in the soil layer,then a limited crop P uptake is calculated as

(9.23)

where uptPlimited is the mass of labile P taken up by the plant on the current day (kg)

and labPi+2 is the labile P concentration in the soil (mg/kg) The mass of crop uptake

P is subtracted from the labile P pool at the end of each day

The total mass of inorganic P lost through surface runoff is composed of loss fromthe soil profile (Equation 9.18) and loss from the soil surface (Equation 9.9) Due tothe low mobility of P, leaching loss of soluble P is not simulated Phosphorus lossesfrom each AnnAGNPS cell within a stream reach are added to an AnnAGNPS reach.Phosphorus is reequilibrated between dissolved P and sediment-attached P in the reachbased on the P partitioning coefficient during the process of being transported to thewatershed outlet Detailed P transformation in the reach is not simulated

9.5 MODEL APPLICATION

AnnAGNPS is currently utilized in many locations of the U.S by the EnvironmentalProtection Agency (EPA), NRCS, and others to estimate the impact of best man-agement practices on nonpoint pollution (Yuan et al 2002) Several studies havebeen performed to evaluate the performance of AnnAGNPS in predicting runoff,sediment, and nitrogen losses (Baginska et al 2003; Suttles et al 2003; Yuan et al

2001, 2003) Suttles et al (2003) evaluated AnnAGNPS performance on P simulation

in a coastal plain agricultural watershed in Georgia, and Baginska et al (2003)performed a similar evaluation on a small experimental catchment in the Sydneyregion of Australia This section presents the AnnAGNPS application to the DeepHollow (DH) watershed and evaluates the performance of AnnAGNPS on P simulationusing comparisons with measurements from the DH watershed of the Mississippi DeltaManagement Systems Evaluation Area project (MDMSEA)

Data collected at the DH watershed by Yuan et al (2001) were used to evaluate theperformance of the AnnAGNPS P component The DH watershed, located in LefloreCounty, Mississippi, is one of three watersheds studied in the MDMSEA, whichseeks to develop and assess alternative innovative farming systems for improved

uptPlimited 0.99labPi conv

1,000,000

water quality and ecology in the Mississippi Delta The main crops grown in the

DH watershed are cotton and soybeans The watershed contains 15 soil series varying

in texture from loamy sand to silty clay, but three series cover 80% of the total area(Yuan et al 2001) Detailed records of agricultural operations including tillage,planting, harvesting, fertilization, cover crop planting, and pesticide usages havebeen maintained since 1996 (Yuan et al 2001) A rate of 72.9 kg/ha phosphatefertilizer was applied to cotton fields on October 6, 1998, with equipment that knifes

in the material at a depth of 100 mm without further mixing with soil No fertilizerwas applied to soybean fields or during the winter wheat cover crop-growth period

In the period of 1995 to 1996, the U.S Geological Survey (USGS) installed agauging station to monitor runoff, sediment, nutrient, and pesticide loadings at one

of the inlets to the DH Lake (Yuan et al 2001) Data collected at this monitoringsite were used for this study The drainage area for the monitored site was 11 ha.Runoff was monitored using a critical flow flume Both discrete and compositesamples were taken during rainfall events for sediment and nutrient analyses Rainfallwas monitored at the flume using a tipping bucket rain gauge

Total P and orthophosphate concentrations were determined for water samples.Total P and orthophosphate mass loads were calculated by using discrete sampleswhen available (Rebich 2004) Loads were also calculated by using compositesamples for runoff events when discrete samples were not available (Rebich 2004)

Established input files for model runoff and sediment evaluation — watershed graphy, soil type, climate data, and actual field operations and management (Yuan

topo-et al 2001) — were modified for this study Yuan topo-et al (2001) described thedevelopment of input information for AnnAGNPS simulations (complete informa-tion on input file preparation can be found at the AGNPS website at

(AnnAGNPS cells), land use, soil information, and stream network for the monitoringsite are presented in Figure 9.3 Based on this input file, fertilizer application wastimed according to actual field records Fertilizer application reference informationwas set up based on AnnAGNPS guidelines and databases

Detailed soil information was obtained from the Soil Survey Geographic(SSURGO) Database (Natural Resources Conservation Service 2005) SSURGOprovides most of the soil parameters needed for AnnAGNPS simulation, such assoil texture, erosive factor, hydraulic properties, pH value, and organic matter.However, information on soil nutrient contents was not available from this database Determining initial soil nutrient values needed for the model was a very difficulttask Soil testing is one way of gaining soil nutrient values Location, timing, andmethod of sampling impact the nutrient values obtained from soil testing (Self andSoltanpour 2004) However, soil testing may not be a feasible way to gain soilnutrient values at a watershed scale because of limited resources First, a watershedmay include thousands of fields Second, each field has different soil types and fieldmanagements Third, nutrient level may vary from one location to another within afield Consequently, obtaining representative values for the watershed is challenging

http://www.ars.usda.gov/ Research/docs.htm?docid=5199) The subwatersheds