Development of a continuous, physically-based distributed parameter, nonpoint source model

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Virginia Tech


ANSWERS, an event-oriented, distributed parameter nonpoint source pollution model for simulating runoff and sediment transport was modified to develop a continuous nonpoint source model to simulate runoff, erosion, transport of dissolved and sediment-bound nutrients, and nutrient transformations. The model was developed for use by nonpoint source pollution managers to study the long-tenn effectiveness of best management practices (BMPs) in reducing runoff, sediment, and nutrient losses from agricultural watersheds.

The Holtan's infiltration equation used in the original version of ANSWERS was replaced by the physically-based Green-Ampt infiltration equation. Soil evaporation and plant transpiration were modeled separately using the Ritchie equation. If soil moisture exceeds field capacity, the model computes percolation based on the degree of soil saturation. Nutrient losses include nitrate, sediment-bound and dissolved ammonium; sediment-bound TKN, and sediment-bound and dissolved phosphorus. A linear equilibrium is assumed between dissolved and sediment-bound phases of ammonium and phosphorus. Nutrient loss is assumed to occur only from the upper cm of the soil profile.

The model simulates transformations and interactions between four nitrogen pools including stable organic N, active organic N, nitrate and ammonium. Transformations of nitrogen include mineralization simulated as a combination of ammonification and nitrification, denitrification, and plant uptake of ammonium and nitrate. The model maintains a dynamic equilibrium between stable and active organic N pools.