Using SWAT to determine and model water regime, water quality and sediment yield in the watershed of Borcka dam

The upper sections of forest watersheds, playing crucial role in producing water resources, have been undergoing some negative ecologic and environmental changes lately due to many investments and projects including mining, quarrying, run-of-river type hydroelectric plants and road constructions. The study area of this research, Borcka Dam Watershed (BDW), has also been facing similar disturbances listed above. However, there is not sufficient scientific research made on how these mentioned interferences having impacts upon BDW. Therefore, determining regime and quality of water and sediment amount reaching to the reservoir of BDW both with field measurements and by modelling them with SWAT (Soil and Water Assessment Tool) program constitutes the aim of this thesis. Values of pH, dissolved oxygen (DO), total dissolved solids (TDS), total suspended sediment (TSS), ammonium nitrogen (NH4-N), nitrate nitrogen (NO3-N), salinity, conductivity, temperature, sulphate (SO4), orthophosphate as phosphorus (PO4-P) and total nitrogen (TN) were measured monthly from July 2016 to June 2017 in the total of 43 sampling points determined within the four sub-watersheds (Murgul, Hatila, Fabrika and Godrahav) of BDW. Although results showing that waters throughout BDW were generally clear, it was revealed that Murgul and Fabrika sub-watersheds had significantly more polluted surface waters especially in some sampling points compared to Hatila and Godrahav sub-watersheds. The annual averaged values of conductivity (227 μs/cm), salinity (0.15 ppt), TDS (209 mg/l), TSS (64 mg/l) and SO4 (202 mg/l) were statistically higher in Murgul sub-watershed, as expected, than the others as it exposed to serious interferences such as mining, run-of-river type hydroelectric plants and road constructions. In addition, it was revealed that the SWAT model with high R2 rates ranging from 0.85 to 0.91 could predict the values of water regime and quality as well as sediment yield confidingly. Moreover, the estimation of sediment yield demonstrated that with a mean of 61855 t/yr, Murgul carries significantly higher sediment than the other sub-watersheds into the reservoir.