Application of SWAT model to investigate nitrate leaching in Hamadan-Bahar Watershed, Iran

Application of large amounts of mineral and organic fertilizers in intensive agricultural regions of Hamadan-Bahar watershed in western Iran contributes to excessive nutrient loads in soils and groundwater bodies. Groundwater supplies approximately 88% of the water consumed in Hamadan. The objective of this study was to investigate the temporal and spatial variability of nitrate leaching in Hamadan-Bahar watershed. We employed the Soil and Water Assessment Tool (SWAT) to model the amount and dynamics of nitrate leaching from a typical crop rotation in this watershed. The SWAT model was calibrated and validated with uncertainty analysis using SUFI-2 (Sequential Uncertainty Fitting, ver. 2) based on measured daily discharge data from 7 hydrometric stations, wheat and potato yield, and measured daily nitrate at the outlet of the watershed. The calibration using crop yield increases the confidence on soil moisture and evapotranspiration. The calibration (R-2 = 0.83, NS = 0.77) and validation (R-2 = 0.70, NS = 0.70) results were quite satisfactory for the outlet of watershed. Spatial variations in nitrate leaching were also found to agree reasonably well with measured nitrate concentrations in groundwater (73% overlap based on a defined criterion). Also, nitrate leaching was found to be more significant under potato (Solanum tuberosum L) rotation (254-361 kg N ha(-1) year(-1)), representing 30-42% of nitrogen applied to the soil. About 36% of Hamadan-Bahar aquifer has a nitrate leaching rate higher than 100 kg N ha(-1) year(-1). The presented model and its results have the potential to provide a strong base for considering different scenarios to reduce nitrate leaching and suggest a BMP (best management practice) in Hamadan-Bahar watershed. (C) 2010 Elsevier B.V. All rights reserved.