Erosion intensity zoning by using SWAT model in Gheshlagh Dam Basin

Erosion and sediment are one of the most important problems in the watershed management of Iran. In order to carry out the conservation programs, soil erosion control and sediment yield decreasing, estimating the total of the sediment volume, erosion intensity in watershed and determined the sensitive areas are necessary. In this research, the study area is located in the mountainous watershed in Gheshlagh Dam. Before the arrival of Gheshlagh river to the city of Sanandaj, Gheshlagh dam has been constructed over it, which is the source of drinking water of Kurdish citizens and also water supply for agriculture, industry and services of the area . One of the problems in this reservoir is organic load with upstream sediments through surface currents in the tanks. This issue that is caused by road construction, dry farming on the steep slope, use unprincipled upstream land and the excessive use of chemical fertilizers, in addition to reduce the capacity of the reservoir and raising the costs of cleaning tanks, can cause other side effects such as algal bloom in the water tank which is followed by stink and aversive water. So the aim of this study is modeling and estimating the amount of erosion in Gheshlagh basin and then erosion intensity mapping and identify sensitive areas to control erosion and sediment in these sub basins.
Methods and Material: In this research for estimating the amount of erosion in Gheshlagh dam basin, the Soil and Water Assessment Tool (SWAT) and Geographic Information System (GIS), were used. After preparing and present hydrological data and the required basin plans, model was ruined for 1987 to 2007 years. In order to calibrate and validate the results, SWAT-CUP programs and SUFI2 algorithms were used and sediment discharge data of Chehelgazi and Khalifetarkhan hydrometric stations were applied.

In the calibration and validation of the runoff and sediment, R2, NS, p-factor and r-factor coefficients in outlet of two main rivers have good results. The R2, NS, r-factor and p-factor coefficients for monthly runoff calibration in Chehelgazi station were estimated 0.80, 0.72, 0.78 and 0.52 respectively and in Khalyfetarkhan station 0.82, 0.74, 0.80 and 0.54. Also the R2, NS, r-factor and p-factor coefficients for monthly sediment yield calibration in Chehelgazi station were estimated 0.71, 0.69, 0.60 and 0.55 and in Khalyfetarkhan station 0.74, 0.70, 0.63 and 0.60 respectively. Based on the results, SWAT model has the suitable potential to identify critical areas. Based on the intensity erosion map, the basin was divided in to five classes of erosion with very high, high, medium, low and very low intensity and in general the results of this study showed that from 102 sub basins that have been considered, fifteen sub basins create 55% of the total sediment yield in watershed. The most critical areas are in northern and northeastern of basin and have rangeland and dry farming on sloping lands.

The results of the sensitivity analysis test, calibration and validation of the model using SUFI-2 model showed that using this model, the time it takes to optimize parameters and calibration model is reduced from a few months to a few weeks and the ability to achieve maximum calibrated parameters is more certainty. The model has evaluated erosion condition of basin with the 16.2 ton per hectare for especially amount of sediment. One of the significant results of this study was identification of 15 sedimentation sub basins; therefore management of these sub basins can play significant roles in reduction of imported sediments into Gheshlagh dam. By compliance of land use and slope maps it seems that the erosion in that watersheds affected by soil erodibility due to farmland use and slope of these sub-basins. Based on the results, SWAT model has the suitable potential to identify critical areas. The results of this study can be used in management of sediment in sub basins that are susceptible to erosion.