The Soil nutrient Loss Simulation in Gheshlagh Dam Basin Using SWAT Model

One of the harmful effects of erosion processes is soil nutrient loss by runoff and sediment. Nitrogen and phosphorus are the main nutrients of soil in watersheds, but the accumulation of these elements in rivers and channels is the one of the most important issues in the nutrient soil loss, that can lead to the growth of aquatic plants, algae and led to Eutrophication phenomena. The aim of this study is to simulate the amount of total phosphorus and nitrogen that carried by runoff and sediment in the sub-basins of the Gheshlagh dam basin, mapping of nutrient loss in the basin and determining the critical sub-basins using the Soil and Water Assessment Tool (SWAT) model. To simulate the sediment load and the amount of soil nutrients loss, continuous and semi-distributed SWAT model was used. For this purpose, at first the DEM, network of streams, land use and soil maps were collected. The climate, soil and management databases were prepared for Gheshlagh dam basin. The observation data of runoff, sediment and water quality of 1993 to 2003 years were used for model calibration and those for 2004 to 2007 years were used for model validation using the SUFI-2 algorithm. The results showed that the SWAT model has simulated discharge, sediment, phosphorus and nitrogen in Gheshlagh dam basin very well. For example, 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. The coefficients for phosphorus in Chehelgazi station are 0.68, 0.63, 0.39 and 0.55 respectively and in Khalyfetarkhan station 0.69, 0.66, 0.55 and 0.49. The organic nitrogen, nitrate, organic phosphorus, soluble phosphorus, and mineral phosphorus were estimated to be 323, 12, 48, 0.18 and 71 kg per hectare, receptively. The sub-basins with highest sources of soil loss are 50, 47, 43, 51, 48, 34 and 31 sub-basins, respectively, which are the source of about 30 percent of total nitrogen and phosphorus load in the basin. The results showed that the model can be effectively applied to determine the critical sub-basins with regard to nitrogen and phosphorus loss. The cultivated lands on steep slopes in west of basin have critical situation in terms of soil nutrient loss. In order to control soil nutrient loss of Gheshlagh dam watershed, the best management practices are reduction and control of nitrate and phosphate fertilizers, conversion of agricultural land to rangeland or forest in slopes and also creating a buffer zone along the river to reduce nitrogen and phosphorus losses entering to the reservoir basin