Simulated Phosphorus and Sediment Loadings in Two Representative Subbasins of the Illinois River

Quantification of the factors affecting phosphorus (P) loading to surface waters is important in assessing the contribution from agricultural activities to water quality. This study investigated the long-term impact of applications of P in poultry litter on the loadings of P and sediment to streams in two subbasins of the Illinois River Watershed. The influences of application rate and environmental characteristics were examined utilizing a transport model, a geographical information system (GIS), and 30 years of daily weather data. Simulated runoff and sediment concentrations of P increased linearly with poultry litter application rate. Most P loadings to streams were in the dissolved form, as overall sediment transport was low, particularly in areas with excellent stands of forage. Because only pasturelands received poultry litter and the initial concentrations of P in the soils in the forested areas were low, the forested areas contributed little runoff and sediment P to streams. Areas of high P loadings coincided with soils having high Curve Numbers (CN) and poor internal drainage. Therefore, subbasins of the watershed containing a greater proportion of pasture and soils with poor drainage characteristics have higher loading rates to streams. Predicted runoff, sediment, and P loadings in both subbasins were highly variable.