Modeling nitrogen transport with the ANSWERS model

Abstract

Nonpoint source pollution from cropland has been identified as the primary source of nitrogen and sediment, and a significant source of phosphorus in the Chesapeake Bay. These pollutants, whether from point or nonpoint sources, have been found to be the primary cause of declining water quality in the Bay. Numerous studies have indicated that, for many watersheds, a few critical areas are responsible for a disproportionate amount of the nutrient and sediment yield. Consequently, if pollution control activities are concentrated in these critical areas, then a far greater improvement in downstream water quality can be expected with limited funds.

In this research a nitrogen transport model is incorporated into ANSWERS, a distributed parameter watershed model. The nitrogen model simulates nitrogen transformations of applied fertilizer and soil nitrogen in the soil. Dissolved nitrogen transport in surface runoff is modeled by assuming complete mixing of the soil surface layer and surface runoff. Sediment-bound nitrogen transport is modeled as a function of the clay content of transported sediment.

The extended ANSWERS model was verified using water quality data from rainfall simulator plot studies conducted on the Prices Fork Research Farm in Blacksburg, Virginia. The four plots were 5.5 m wide by 18.3 m long with average slopes ranging from 6.2 to 11 percent. Two of the plots were tilled conventionally, and two were no-till. Simulated rainfall at an intensity of 5 cmlh was applied to the plots and runoff samples were analyzed for sediment and nitrogen. The model was then verified by comparing the simulated response with the observed data. The model predicted sediment-bound nitrogen losses within a factor of two. The model tended to overpredict dissolved nitrogen losses by a factor of five. The model shows potential as a best management practice planning tool, however, further verification of model predictions versus observed data is required.