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Assessments of potential environmental impacts of non-point source (NPS) pollutants at local and regional scales are necessary as a basis for effective management strategies to protect precious resources such as land and water. Intensive watershed scale study is therefore necessary to develop management strategies for abating the agricultural NPS pollution. The major goal of the present study was to identify the critical areas of an agricultural watershed and recommend the best management practices using a physical process based watershed scale model, soil water assessment tool (SWAT). A 973-ha agricultural watershed located in Midnapore district of West Bengal state in eastern India was monitored to quantify the hydrologic parameters such as runoff, sediment yield and the quality of surface water. The model was calibrated and validated using observed hydrologic and water quality data of the watershed monitored during the rainy seasons of 2002 and 2003, respectively. Besides these data, micro-meteorological data, topographical map, soil map, land resources data and remote sensing data (satellite imagery) of the watershed were used as input to the model. The study revealed that SWAT model simulates daily runoff, sediment yield and nutrient concentration in runoff satisfactorily throughout the entire rainy season, as evident from standard statistical tests. The calibrated model was then successfully used for identifying the critical sub-watersheds and for development of best management practices. Several simulations were performed to determine the best out of the 48 different combinations of treatments for the management of the critical sub-watersheds. The study revealed that no other crop could replace the rice (Oryza sativa) crop during the rainy season and the existing conventional tillage practice needs to be replaced by conservation tillage in order to minimize the sediment yield and nutrient losses. Fertilizer application rate of 80:60 kg ha-1 of N:P is also recommended to minimize the surface water pollution in the watershed due to NO(3)-N and P.
