Estimation of Suspended Sediment Loads in Diyala River Watershed, Iraq, using SWAT Model

Suspended sediment loads (SSL) transported from the watershed of the Diyala River (WODR) are the most important and dangerous forms of sediment as they drift to the stream flow of the Diyala River and are then transferred to the reservoirs of Hemren Dam (HD) and Derbendikhan Dam (DD), which are located in the study area, affecting the capacity storage of the reservoirs and reducing electrical energy production. Therefore, it is necessary to apply a hydrological model that can simulate the SSL distribution in the WODR to enable decision-makers to develop an appropriate plan to solve the sediment problem. In WODR, the data of SSL are very rare, as sediment measurements have not been conducted for more than 40 years. Due to the lack of historical data for sediment values for the study area and the need to reduce uncertainty, sediment measurements were conducted from November 2022 to April 2023. The motivation of the present study is to study and address the limitations imposed on the Soil and Water Assessment Tool (SWAT) model during the estimation of SSL in the WODR that have scarce data and whose quality is inaccurate. The observed monthly flow data from two gauging stations, HD and DD, from January 2000 to April 2023 and suspended sediment concentration, which was measured in the field from November 2022 to April 2023, were used for calibration and validation of the model, respectively, using the Sequential Uncertainty Fitting Version 2 (SUFI-2) algorithm and SWAT-Calibration Uncertainty Procedures (CUP). Statistically, using the coefficient of determination (R2), Nash-Sutcliffe efficiency (NSE), and percent of bias (Pbias) the performance of the model was evaluated, with good agreement between observed and simulated values for both stream flow and SSL. The results showed that the values of the SSL in the WODR from January 2000 to April 2023 were equal to 115.240 t/ha/yr. Sub-basins 5 and 12 have the highest SSL values of 15.125 t/ha/yr and 9.098 t/ha/yr, respectively, and the most important factor in SSL formation is the slope of the land, with a correlation coefficient (R2=0.94).