Identifying Critical Sources and Evaluate the Best Management Practices to Control Nutrient Load of the Dez River Basin Using the SWAT Model

   Using the best management strategies (BMPs) is a useful way to control pollutions at the watershed outlets, which, of course, requires spending costs. Prioritizing these strategies (especially in areas with scarce resources) will greatly contribute to the efficiency and adequacy of these methods in terms of reducing pollution and their application in critical areas. In this research, the SWAT conceptual model has been selected as the simulating model, using the long-term data including meteorological, dam, agricultural management and Dez river flow. The results of SWAT model calibration and validation indicate that this model is reliable for simulating the Dez catchment area. These results were quite satisfactory for discharge, total phosphorus and nitrate, but not very suitable for sediment load. Also, the separation of the parameters affecting the variables, on the quality of the results of the calibration and validation has been desirable. Examining the modeling results in normal, wet and dry water years indicate more confidence in the results of the model in wet periods. Identification of critical areas has also been done with a single index methods based on the amount of pollution load produced in the basin, as well as three combined indicators. Finally, the practice of plant filtration is the most effective method in reducing the nitrogen load and the practice of increasing the D50 of soil particles is also the most effective method in reducing the phosphorus load. The sub-basin downstream of the city of Dezful, which has a major agricultural activities and included the sugarcane factory effluent, has been identified as the most critical sub-basin.