SWRRBWQ was developed to simulate hydrologic, sedimentation, and nutrient and pesticide
transport in a large, complex rural watershed. The model operates on a continuous time-scale and allows for
subdivision of basins to account for differences in soils, land use, rainfall, etc. It can predict the effect
of management decisions on water, sediment, and pesticide yield with reasonable accuracy for ungaged rural
basins throughout the United States.
SWRRB-WQ Windows Interface was developed by U.S. EPA in June 1993. Pesticide, Soil and Rainfall Coverage is for the entire U.S., all other site-specific parameters are user supplied.
SWRRBWQ includes five major components: weather, hydrology, sedimentation, nutrients, and pesticides. Processes considered include surface runoff, return flow, percolation, evapotranspiration, transmission losses, pond and reservoir storage, sedimentation, and crop growth. A weather generator allows precipitation, temperature, and solar radiation to be simulated when measured data is unavailable. The precipitation model is a first-order Markov chain model, while air temperature and solar radiation are generated from the normal distribution.
Sediment yield is based on the Modified Universal Soil Loss Equation (MUSLE). Nutrient yields were taken from the EPIC model (Williams et al., 1984). The pesticide component is a modification of the CREAMS (Smith and Williams, 1980) pesticide model. SWRRBWQ allows for simultaneous computations on each subbasin and routes the water, sediment, nutrients, and pesticides from the subbasin outlets to the basin outlet.
Surface runoff volume is predicted using the SCS curve number (USDA, 1972) as a function of daily soil moisture content. Return flow is calculated as a function of soil water content and return flow time. Return flow travel times can be calculated from soil hydraulic properties or user-inputs.
The percolation component uses a storage routing model combined with a crack-flow model to predict flow through the root zone. Evapotranspiration is estimated using Ritchie's ET model. Transmission losses in the stream channel are calculated as a function of channel dimensions, flow duration, and effective hydraulic conductivity of the channel bed. Pond storage is based on a water balance equation that accounts for inflow, outflow, evaporation, and seepage. The reservoir water balance component is similar to the pond component except that it allows flow from the principal and emergency spillways.
Peak runoff rate predictions are based on a modification of the Rational Formula. Sediment yield is computed for each subbasin with the modified Universal Soil Loss Equation (MUSLE). The channel and floodplain sediment routing model is composed of two components operating simultaneously (deposition and degradation). Degradation is based on Bagnold's stream power concept, and deposition is based on the fall velocity of the sediment particles. Sediment is also routed through ponds and reservoirs.
The crop growth model computes total biomass each day during the growing season as a function of solar radiation and leaf area index (LAI). LAI is computed for each day from the maximum LAI and total above ground biomass. The ET component uses LAI to compute plant evaporation. Water and temperature stress factors are used as growth constraints.
SWRRBWQ simulates crop growth for both annual and perennial plants. Annual crops grow from planting date to harvest date or until the accumulated heat units equal the potential heat units for the crop. Perennial crops maintain their root systems throughout the year.
Lake water quality simulation can be applied when a single reservoir is simulated at the basin outlet. The lake water quality computes the toxic balance and the phosphorus mass balance in the lake, the equations for which come from Chapra (1983) and from Thomann and Mueller (1987), respectively.
The major processes in the toxic balance are loading, outflow, reactions, volatilization, settling, diffusion, resuspension, and burial, while in the phosphorus balance, the balances are loading, outflow, and settling. The model tracks the fate of pesticides from their initial applications on the land to their final fate in the lake. This allows decision makers to directly predict the influence of upland agricultural management decisions on lake water quality (Arnold et al., 1991).
SWRRBWQ has been equipped with a Windows-Interface including help screens, which makes editing of input files (in spreadsheet-format) user friendly.