Hydrological modeling of the water quality of an agricultural watershed

Degradation of surface water quality is an important problem in Quebec. In agricultural areas, pollution sources are related to agricultural practices . Hydrological modeling is a tool which can help establishing less pollutin g farming practices. The HSPF model was tested for a period of two years on a 5.3 km(2) agricultural watershed and results presented on a weekly basis for outflow, sediment, total nitrogen, nitrate and nitrite, and ammonium. The first year served for calibration and yielded correlation between obser ved and simulated values of 0.87, 0.88, 0.95, 0.96 and 0.62 for outflow, se diment, total nitrogen, nitrate and nitrite, and ammonium. The second year was used for verification and correlation coefficients of 0.66, 0.54, 0.63, 0.73, and 0.62 were obtained for the same variables. For the two years of the study, HSPF underestimated outflow by 18%, sediment by 38%, total nitro gen by 25%, and overestimated nitrate and nitrite by 23%, and ammonium by 4 5%.