Simulation And Benefit Evaluation Of Water And Sediment Regulation In Yanhe River Basin Based On SWAT Model

The Yanhe River Basin is located in the middle reaches of the Yellow River Basin and is one of the typical watersheds with severe soil erosion in the Loess Plateau region.Conducting water-sediment regulation simulation and benefit evaluation for this watershed can provide a typical example for soil and water conservation and ecological restoration in hilly and gully areas of the Loess Plateau.Best Management Practices(BMPs)are a series of measures to protect the watershed water environment from pollution caused by agricultural production activities,and have been widely used in controlling soil erosion in the watershed.Based on the SWAT model,this paper simulates the runoff and sediment yield in the Yanhe River Basin,analyzes the spatial and temporal variations of water and sediment,sets up multiple scenarios of returning farmland to forest and grassland,evaluates the response of water and sediment to land use change in the watershed,and analyzes the water-sediment regulation effects of 11 single measures and 36 combinations of BMPs at both the watershed and sub-watershed scales.Meanwhile,based on the cost-benefit analysis method,the comprehensive environmental cost-benefit of single and multiple measures under different scenarios is studied,and the optimization scheme for water and sediment management in the watershed is proposed.The main conclusions are as follows:(1)The SWAT model performs well in the simulation of monthly runoff and sediment in the Yanhe River Basin,meeting the requirements for model accuracy.During the calibration period(1958-1992),the R~2and NSE coefficients of monthly runoff were 0.76and 0.74,respectively,while those of monthly sediment were 0.66 and 0.64,respectively.During the validation period(1993-1997),the R~2and NSE coefficients of monthly runoff were 0.66 and 0.63,respectively,while those of monthly sediment were 0.60 and 0.51,respectively.These results meet the requirements for model accuracy and demonstrate the applicability of the SWAT model in simulating and regulating water and sediment in the watershed.(2)There are significant spatial and temporal differences in the distribution of water and sediment in the Yanhe River Basin.The annual and intra-annual fluctuations are large,with more than 80%of the water and sediment produced during the flood season.Nearly20%of the sub-watersheds face severe erosion risks,with cultivated land being the main source of soil erosion in the watershed.The simulation results show that the inter-annual fluctuation of sediment transport is greater than that of runoff and precipitation.The distribution of water and sediment during the year is mainly concentrated in the flood season from May to October,with the highest production in August.In terms of spatial distribution,the amount of runoff and sediment decreases gradually from upstream to downstream,with the spatial distribution difference of sediment yield being significantly higher than that of runoff and rainfall.The maximum sediment yield of the sub-watershed is 188.22t/ha.According to the classification standard of soil erosion,sub-watersheds 1,3,4,20,26,and 27 with sediment yield greater than 140t/ha were identified as high-risk sub-watersheds for soil erosion.This area accounts for 44.13%of the total sediment yield and 51.25%of the total runoff yield in the study area.Cultivated land contributes the most to runoff and sediment production,accounting for 45.28%and 56.90%,respectively,followed by grassland and forest land.(3)The decision of returning farmland to forests and grasslands enhances the effect of water and sediment regulation in the watershed as the gradient of returning farmland moves downward.The regulation effect of returning farmland scenario on sediment is better than that on runoff,and the regulation effect of returning farmland to forests is better than that of returning farmland to grasslands.The effect of returning farmland to forest is better than that of returning it to grassland in terms of water and sediment regulation.In the 2015 land use scenario,the area of cultivated land decreased by 26.4%and the area of forest land increased by 37.0%.Compared with 1980,the total runoff and sediment yield decreased by5.30%and 2.63%,respectively,indicating that the water and soil conservation capacity of the watershed has been improved under realistic conditions.Among the six scenarios of returning farmland to forest or grassland according to slope gradient,for cultivated land with a slope above 15 degrees,returning it to forest or grassland resulted in a reduction of24.56%and 9.83%in total runoff,and a reduction of 34.83%and 28.82%in sediment yield,respectively.However,for cultivated land with a slope above 25 degrees,the same operation resulted in a reduction of 9.26%and 2.24%in total runoff,and a reduction of10.87%and 8.32%in sediment yield,respectively.(4)The evaluation of 11 single measures and 36 combined measures in Yanhe River Basin shows that the regulation effect of the three combined measures is better than that of the two combined measures and single measures.The increase of the number of measures will lead to the surge of economic input,and the improvement of the comprehensive benefits of regulation is weak.In terms of environmental benefits,the sediment reduction effect of single measures in the whole basin is 29.14%and 29.11%,which is good.The combined sediment reduction rate is much lower than the sum of the two measures respectively.The sediment reduction rate of the watershed of returning farmland to forest+stubble mulch tillage+grass channel is 41.73%,which is the best effect among all measures and combinations.In terms of economic benefits,the evaluation based on cost-benefit analysis showed that the cost-benefit value of a single measure was far higher than that of other measures,which was 10864.08 kg/yuan,and the highest value of the three BMPs combinations was only 13.49kg/yuan.The combination would lead to a large increase in economic costs,but the increase of sediment reduction was relatively small.The evaluation of multi-attribute decision method based on information entropy showed that the combined BMPs comprehensive attribute value Z was slightly better than that of a single measure,and GW+ST had the highest Z value of 0.92 among all measures.The BMPs combination of grassland return decision had better comprehensive attribute than forest return.Therefore,the environmental and economic benefits should be taken into full consideration in the allocation of watershed management measures and comprehensive evaluation.