Estimation of sediment transport in agricultural drainage channels using the swat model: A case study of the Akarsu water user association area

This study was carried out in the Lower Seyhan Plain located in the Çukurova Region in the south of Türkiye. In the research, an agricultural area with a hydrological extent of approximately 11,308 ha, which includes the Akarsu Irrigation Union area, was considered as a model domain, and the SWAT (Soil and Water Assessment Tool) model was applied in order to simulate drainage discharges (irrigation return flows, IRFs) and sediment transport in agricultural drainage channels. The main objective of the study is to integrate daily drainage discharge data (IRFs) and sediment data obtained from flow monitoring stations within the Agricultural Drainage Basin into the SWAT model, to estimate drainage discharge and sediment transport at the outlet of the basin, and to evaluate the applicability of the model to agricultural catchments. For model setup, observed drainage discharges, sediment and meteorological data for the period 2008–2012 were used. Meteorological records were obtained from the meteorological station located within the basin. Data on soil series and their properties, land use, climate, land management practices, digital elevation model and the existing agricultural drainage system (network of drainage canals) in the study area were employed as model inputs. Model calibration was performed using the SUFI-2 algorithm within the SWAT-CUP program. In the evaluation of the calibration and validation results, the coefficient of determination (R²), Nash–Sutcliffe efficiency coefficient (NSE) and percentage bias (PBIAS) between observed and simulated values were used. According to the model performance statistics, a good agreement was obtained between observed and simulated drainage discharges. However, the parameter sets used for sediment transport were not found to be adequate, since they did not reach an acceptable level of performance. Considering the data set used in the study, the model assumptions and the complex sediment dynamics of agricultural drainage channels in the Agricultural Drainage Basin, this result indicates that the ability of the SWAT model to represent sediment transport in such systems requires further and more detailed calibration efforts.