4.0
AGNPS
The Agricultural Non-Point Source Pollution model (AGNPS) is a system of computer models developed to predict watershed response to water, sediment, and chemical loadings. AGNPS was first developed in the early 1980’s by the United States Department of Agriculture (USDA) as a collaborative effort of the Agricultural Research Service (USDA-ARS) and Natural
Resources Conservation Service (USDA-NRCS). Several versions of AGNPS have been produced throughout its history, with AGNPS V5.0 being the last single event version. The single event version of AGNPS was replaced in the late 1990’s by the Annualized Agricultural
Non-Point Source (AnnAGNPS) model. As an expansion of AGNPS, AnnAGNPS is a continuous simulation model with numerous improvements. The following section describes the framework and application of the AnnAGNPS suite of models that is currently referred to as
AGNPS. The model is nonproprietary and can be downloaded at http://www.ars.usda.gov/Research/docs.htm?docid=5199.
This model is one of a group of watershed models capable of estimating chemical loading, sediment loading, and water flow within a watershed, and its waterbodies, and analyzing water quality of the receiving waters as well. AGNPS is a spatially-distributed, continuous simulation model that predicts the quantity of water, sediment, and chemicals from the land surface through the watershed. AnnAGNPS can be operated in AGNPS mode as a single-event simulation with limited input requirements
4.1
Conceptual Basis
In the AnnAGNPS model, the watershed is divided into subwatersheds or cells. Each cell is a homogeneous drainage area with the same characteristics such as soil type, land surface slope, weather, and land management practices. These cells are connected by streams/channels by onedimensional channel routing that transports runoff and pollutants downstream.
AnnAGNPS models processes that focus on the subwatersheds (cells) and the stream channels
(reaches). The cell processes include weather, precipitation, soil moisture, irrigation, management operations (crops, fertilizer, etc.), sediment yield from erosion, pesticides and nutrients. In the cell processes, input information for the pesticides and nutrients is specified that includes the application rates, decay rates, and the partitioning of the chemical into dissolved and sorbed forms. Management operations specified for the cells assist in source accounting by identifying areas such as feedlots.
The reach processes modeled include water inputs (rain, irrigation, point sources), sediment yield, chemicals, and impoundments. In addition to nutrients and pesticides, which are also modeled in the cell processes, organic carbon is modeled in the reaches. Impoundments are modeled in the reaches as areas of sediment deposition.

4.2
Model Processes
The AnnAGNPS model is comprised of multiple model processes and is supported by several models for specific modeling routines. These processes and the supporting models are discussed below.
4.2.1 Meteorology
Daily weather is generated in the model by a synthetic weather generator, Generation of weather
Elements for Multiple applications (GEM), and additional associated programs. These models can generate daily weather for any number of user specified years, which allows the user to perform risk analyses and set meaningful TMDL's. The model generates weather parameters equivalent to, precipitation, minimum and maximum air temperature, relative humidity, cloud cover, and wind speed.
4.2.2
Pollutant loading
Pollutant loading is modeled by kind of pollutant (water, sediment, chemical) and source (source accounting).
Surface water runoff
The model accounts for surface water generation from natural sources, such as rainfall and snowmelt, and anthropogenic sources such as irrigation. Daily surface water runoff within the cells is calculated using the SCS curve number technique which describes the soil moisture balance. Lateral subsurface flow is computed using Darcy’s equation or tile drain flow.
Sediment loading
Sediments are modeled based on size (clay, silt, & sand) and source (land or stream channel).
Within the cells, AnnAGNPS uses the Revised Universal Soil Loss Equation (RUSLE) to compute land surface erosion such as rill and sheet erosion. A delivery ratio is used to compute sediment discharged into a channel or other receiving water body. Within the reaches, sediment delivery is computed using the Hydro-geomorphic Universal Soil Loss Equation (HUSLE).
Chemical loading
Loading and routing for dissolved and adsorbed chemicals are computed using a mass balance approach. The model includes dissolved and adsorbed forms of nitrogen, phosphorous, organic carbon, and pesticides (any number and any kind). Nutrient concentrations from feedlots and other point sources are modeled. Individual feedlot potential ratings can also be derived using the model.

Source accounting
A feature of AnnAGNPS is "source accounting," which allows the user to determine exactly how much pollutant loading originates from each segment of the landscape. The model accounts for sources such as cells (individual subwatersheds within the model), feedlots, field ponds, gullies, impoundments, point sources, and stream reaches.
4.2.3 Hydrology
AnnAGNPS integrates the results for more comprehensive applications to model hydrologic processes. The model uses CCHE1D to model stream network processes. The Conservational
Channel Erosion Pollutant Transport System (CONCEPTS), as stream corridor model, is used for its greater detail with channel hydraulics, morphology, and transport of sediments and contaminants. Instream water temperature is modeled by SNTEMP.
4.2.4 Salmonid models
Recently added to the model processes of AnnAGNPS are several related salmonid models
(SIDO, Fry Emergence, Salmonid Total Life Stage, & Salmonid Economics).
4.3
Data
AnnAGNPS includes accessory programs for input generation and editing along with associated databases. Data input is facilitated by GIS interface to characterize the watershed and set model parameters. Watershed delineation, cell boundaries, land slope, slope direction, and reach information are generated by TOPAGNPS and AGFLOW. Management information, land characteristics, crop characteristics, field operation data, chemical operation data, feedlots, and soil information can be imported from RUSLE or NRCS sources.
4.4
Strengths and Weaknesses
The greatest strength of AnnAGNPS is its ability to perform continuous simulations for a watershed scale water quality model. In fact, this was the main intent of developing this model from the single-event AGNPS model.
A limitation of AnnAGNPS is that the model does not track adsorbed chemicals deposited from day to day during a simulation run. The model also limits point sources to constant loading rates for the length of the simulation run and does not allow for spatial variability in rainfall.
4.5
Applications
As a continuous simulation, surface runoff model, AGNPS is designed to assist with assessing best management practices (BMPs), developing Total Maximum Daily Loads (TMDLs), and performing risk and cost/benefit analyses.
REFERENCES

ALL THE REFERENCES WERE LISTED WITH THE AQUATOX SECTION