The effect of agricultural and conservation management on surface runoff and sediment load in Dashte Bozorg catchment using the ArcSWAT model

Prioritizing critical source areas and using the best management practices, including agricultural and conservation management, are effective methods to reduce erosion in catchments.The main objective of this research was to evaluate the impact of agricultural and conservation management on surface runoff and sediment yields in Dashte Bezorg catchment, Khuzestan, Iran using the ArcSWAT model.The data was collected in 2021. The Sequential Uncertainty Fitting was applied for Calibration and validation. The model was calibrated from 2004 to 2015 and validated from 2016 to 2021 for surface runoff. Furthermore, calibration and validation of sediment yields were performed for the statistical periods of 2004-2013 and 2014-2019, respectively. The performance of the model was evaluated by four objective functions (NS, R2, BIAS, and RSR). The model was then applied to predict critical source areas for sediment yields and surface runoff. Agricultural management practices in four crop rotation scenarios (“wheat-wheat-wheat”, “wheat-rice-wheat-mung bean-wheat”, “rice-mung bean-wheat” and “wheat-potato-tomato”), residue management scenarios (No residue and 50 percent of the residue) and three tillage scenarios (conservation tillage, no-tillage, and conventional tillage) were evaluated. Conservation management scenarios were focused on contouring, strip cropping, terracing, vegetated filter strip, and the grassed waterway scenarios (width of 5 and 10.4 meters).The sensitivity analysis showed that ALPHA_BF (Baseflow alpha factor) and RCHRG_DP (Deep aquifer percolation fraction) parameters were identified as the most effective base flow parameters. The objective function values (NS, R2, BIAS, and RSR) were 0.7, 0.72, 3.7, and 0.55 for surface runoff during calibration, and 0.74, 0.75, 2.1, and 0.51 during the validation period, respectively. These results indicated that the ArcSWAT model performed well in estimating surface runoff but was not satisfactory for sediment yields.Collecting sediment data only during floods resulted in large uncertainty in the input data, and the uncertainty in the inputs produced a large uncertainty in the 95 Percent Prediction Uncertainty (95PPU) bands. Subcatchments 5 and 17 were critical source areas for surface runoff and subcatchments 4, 9, 14, and 16 were also identified as critical source areas for sediment in the catchment. The application of agricultural management practices showed that the cultivation of wheat for three consecutive years increased surface runoff and sediment loss under no-tillage and Conservation tillage. The result of conservation management scenarios indicated that the difference in the width of the grassed waterway had no significant effect on reducing the sediment load. The terracing and vegetated filter strip scenarios were more effective than the other conservation scenarios on sediment reduction.The findings of this study showed that the application of conservation management scenarios can significantly reduce sediment yields compared to agricultural management. It is also recommended to avoid continuous cultivation of the same crop as much as possible.