Developing new modeling techniques to evaluate the environmental and economic impacts of individual management practices at the field and watershed scales

Excess sediment yield as nonpoint source pollution from agricultural lands is a major threat to water quality in lakes and streams in the United States. Watershed modeling can provide data about the source and amount of excess sediment yield. Identifying sediment yields at the field and watershed scales allows watershed planners to make better decisions concerning water quality. The specific objectives of this study were to: 1) compare three watershed-scale models (Soil and Water Assessment Tool (SWAT), Field SWAT, and High Impact Targeting (HIT)) against a calibrated field-scale model (RUSLE2) in estimating sediment yields from fields in the River Raisin Watershed; 2) evaluate the statistical significance among models; 3) assess the watershed models' capabilities in identifying the areas of concern at the field level; 4) evaluate the reliability of the watershed-scale models for field-scale analysis; 5) design and test multiple methods for quantifying the impacts of field-scale management changes the watershed outlet; 6) compare the true costs of BMPs and those from government programs. SWAT was the only model found to be not significantly different from the calibrated RUSLE2. All the models were incapable of identifying priorities areas similar to RUSLE2. SWAT provided the most estimates within the uncertainty bounds of RUSLE2 (51%). A hybrid RUSLE2-SEDMOM-SWAT model proved to be the best method to predict the effects of field-scale management decisions at the watershed outlet. The true costs of sediment reduction at the field and watershed scales were greater than government defined program costs in five out of six BMP categories.