Identification of critical sediment source areas across the Gharesou watershed, Northeastern Iran, using hydrological modeling

In this study, the process-based watershed model, Soil and Water Assessment Tool (SWAT), was used for simulating hydrology and sediment transport in the Gharesou watershed and for identifying critical areas of soil erosion and water pollution. After model calibration and uncertainty analysis using SUFI-2 (Sequential Uncertainty Fitting, ver. 2) method, the outputs of the calibrated model were used for assessing critical sediment source areas. Three pollution quantifying indices including a Load Impact Index (LII), a Concentration Impact Index (CII), and a load per nit area impact index (LUII), were computed based on the model outputs. The results indicated that despite lack and uncertainty of available data, SWAT model performance in simulating sediment transport in Gharesou watershed is quite acceptable. During calibration, the simulated monthly sediment loads matched the observed values with a Nash-Sutcliffe coefficient of 0.24 and PBIAS of - 17%. The values for validation period were 0.2 and -12.1% respectively, indicating the model’s weakness in simulating sediment dynamics and its capability in predicting average sediment load. Assessing spatial pattern of sediment indices showed that, in general, critical sub-watersheds based on LII are located in downstream areas of the watershed while sensitive subwatersheds in terms of CII are situated in the middle part and critical areas with respect to LUII are in upstream. On the basis of LUAII, eight percent of the watershed area yields about 60% of sediment load. Implementation of appropriate conservation practices in the critical areas has the potential to significantly reduce erosion and sediment transport.