Changing agricultural practices have significant impacts on water resources. The information of these changes is abundant at the plot or field study, but limited at the watershed scale. Therefore, the goal of this dissertation is to quantify the impacts of these changes at a watershed/sub-watershed scale. AVSWAT-X, an extension of the Soil and Water Assessment Tool (SWAT), is used to model the impacts of two tillage systems (No-tillage, NT and Moldboard plow, MP), three crop combinations (continuous corn, CC; corn-soybean rotation, CS; and continuous soybean, SS), and three N application rates (90, 170, and 250 kg N/ha) on flow, nitrogen load (N), phosphorus load (P), and fecal coliform concentration (FC), in a 5- and a 15-year period in two sub-watersheds of the Little Miami River (LMR) basin. In addition, the economic impacts of these farming practices are also examined.
In general, the results show more variation in the 5-year simulation than the 15-year period and indicate that SWAT is capable of providing a realistic and reliable prediction. When other conditions are the same, the environmental quality is better (less flow, N, P, and FC) under NT than MP; SS than CS and CC; and 90 than 170 and 250 kg N/ha application rates. The combined impacts of N application with either tillage or crop system are similar, with higher pollution under higher N application rates, while the pollution levels are lowest under NT/CS, followed by MP/CS, NT/CC, and MP/CC. Considering, the combined impacts of the three farming practices, P and FC levels are highest under MP/CC/250 and lowest under NT/SS/0, while the impacts on flow and N are not consistent. Furthermore, crop types have higher impacts on flow, N, and P, while N applications have the highest impacts on FC. In addition, the annual net incomes are higher under NT than MP, and under corn than soybean. The 2-year net incomes are highest under NT/CC, followed by MP/CC, NT/CS, MP/CS, NT/SS, and MP/SS. Consequently, it seems that NT/CS is the most feasible option for balancing the economic and environmental benefits in a long term (relatively high net incomes and less water pollution).