NONPOINT-SOURCE (NPS) MODEL SIMULATION OF TILLAGE EFFECTS ON WATER-QUALITY

Three agricultural non-point source (NPS) models, GLEAMS, EPIC, and WE PP, were used to simulate the effects of two tillage systems on runoff and losses of sediment, N, and P from a field-sized watershed in the Tennessee valley region of Alabama. The field was cultivated with thre e years of conventional tillage (CVT) followed by three years of conse rvation tillage (CsT) of cotton. GLEAMS and EPIC underpredicted NO3-N losses in runoff for both tillage systems. EPIC simulated tillage effe cts on soluble-P losses better than GLEAMS. However, EPIC poorly predi cted annual organic-N and P losses in sediment, mainly due to overpred icted sediment losses. The GLEAMS prediction of annual organic-N and P losses in sediment was more acceptable than that of EPIC. WEPP predic ted sediment losses close to observed data for both tillage systems. H owever, EPIC simulation of sediment loss was not as accurate, because its limited definition of watershed profile prevented the model accoun ting for sediment deposition on the watershed depressional area.