Phosphorus Modeling in Soil and Water Assessment Tool (SWAT) Model
The Soil and Water Assessment Tool (SWAT) model was developed by the U.S. Department of Agriculture Agricultural Research Service (USDA-ARS). It is a theoretical model that functions on a continuous time step. Model components include weather, hydrology, erosion and sedimentation, plant growth, nutrients, pesticides, agricultural management, channel routing, and pond and reservoir routing. Agricultural components in the model include crop cycles from planting to harvesting, fertilization, tillage options, and animal production and have the capability to include point source loads (Neitsch et al. 2001a, 2001b). All model calculations are performed on a daily time step. The SWAT model predicts the inﬂuence of land-management practices on constituent yields from a watershed. SWAT is the continuation of over 30 years of model development within the USDA-ARS. The Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS), Groundwater Loading Effects of Agricultural Management Systems (GLEAMS), and Erosion Productivity Impact Calculator (EPIC) models (Knisel 1980; Leonard et al. 1987; Williams et al. 1984) have each contributed to the scaling up of past ﬁeld-scale models to one that includes large river basins. Large-area simulations are possible due to the advances in computer software and hardware, including speed and storage, geographic information science (GIS), and spatial analysis and debugging tool software. SWAT model development primarily emphasizes (1) impacts of watershed management and climatic conditions; (2) ﬂow and water quality loadings and fate; (3) ﬂexibility in how a basin is descretized into smaller geographic areas; and (4) continuous time simulation. SWAT is a public domain model that is actively supported by the USDA-ARS at the Grassland, Soil, and Water Research Laboratory in Temple, Texas.