One of the major causes of groundwater pollution in Hamadan-Bahar aquifer in western Iran is nonpoint sources resulting from agricultural activities. Withdrawal of over 88% of drinking water from aquisers necessisates a better management of water supplies. The objectives of this study were to: construct groundwater vulnerability maps based o DRASTIC model, investigate temporal and spatial variability of nitrate leaching with SWAT model, and consider different scenarios for reducing nitrate leaching in critical areas of Hamadan-Bahar watershed. The results showed that performance of the conditioned DRASTIC model improved significantly (R 2 = 0.52) over the intrinsic (R 2 = 0.12) and specific (R 2 = 0.19) models in predicting the groundwater nitrate concentration. SWAT model was calibrated and validated using SUFI-2 procedure based on measured discharge and nitrate at the outlet of the watershed and crop yield. The calibration and validation results were quite satisfactory. About 36% of Hamadan- Bahar plain has a nitrate leaching rate of 100-595 kg N ha -1 year -1 . Among the considered scenarios, the S 6 scenario (application of common amount of urea without hen manure) was selected as the best management practice for this watershed. Key Words : Nitrate pollution, groundwater, SWAT, DRASTIC, SUFI2, BMP
