Runoff And Sediment Load Changes And Potential Influencing Factors In The Jialing River Basin Based On SWAT Modelling

River runoff and sediment are the carriers of hydrological cycle,material migration and energy changes in the basin,which are related to the spatial and temporal allocation of water resources,construction of water conservancy projects and health of rivers and lakes.To explore the changes of river runoff and sediment,and to analyze the impact of climate change and human activities on runoff and sediment changes in the basin,we can provide scientific basis for scientific management of water and soil resources and ecological environment in the basin,downstream water environment and water security.Based on the hydrological data of the Jialing River Basin from 1960 to 2015,the spatial and temporal variability of runoff and sediment transport in the main and tributary basins of the Jialing River Basin are analyzed with the M-K trend test,Pettitt test,and Budyko elasticity coefficient method to assess the impact of climate change and human activities on runoff and sediment in the basin quantitatively.The SWAT model was constructed using DEM,land use/cover,soil properties,meteorology and other data,and the applicability of the model was evaluated.The impact of land use/cover changes on the spatial and temporal variability of river runoff and sediment was explored by simulating runoff and sediment changes in the upper Jialing River after land use pattern changes by setting up different converting slope arable land to forest scenarios.The main research findings are as follows:(1)The runoff depth and sediment transport in the Jialing River basin from 1960 to 2015are decreasing,and the change of sand transport is significantly decreasing(p<0.01),with the rate of 1.52-2.11 mm/a and 0.0034-0.034×10~8 t/a,respectively,and the abrupt change of runoff sediment transport is mostly concentrated in 1984-1990,in which the upper part of the main stream and the Xihanshui basin,the changes of runoff depth and sand transport modulus are dramatic.The runoff and sediment transport series are divided into base period and change period according to the year of abrupt change,in which the runoff series in the base period are mostly decreasing,and the runoff series in the change period are all increasing;the sediment transport series in the base period are mostly decreasing,and the decrease rate of sediment transport series in the change period is slowed down.The impact of human activities on runoff-sediment change is significantly higher than that of climate change,in which the change of subsurface caused by human activities is the dominant factor of runoff change,and its contribution to runoff change is more than 70%,followed by rainfall,while the contribution of potential evapotranspiration is the smallest,none of which is more than 20%;compared with runoff,the contribution of human activities to the decrease of sand transport is generally higher than 90%.(2)Based on the delineation of the runoff and sediment variation points in the Jialing River basin,the SWAT model of the upper Jialing River was constructed by combining the meteorological and hydrological data of the basin with the DEM,land use/cover and soil data of the basin,and the monthly runoff and sediment transport data of the benchmark station Guangyuan were used for model calibration and validation.The model can reflect the monthly variation of runoff and sediment in the upper Jialing River.The conversion area of cropland and forest land in the upper Jialing River for the four periods of land use/cover data(1980/1990/2000/2010)accounted for less than 5%,and the conversion area of cropland and forest land in the scenario of returning farmland to forest prepared by the land use/cover data in 1980 accounted for nearly 20%.(3)Grassland,as the main land use/cover type in the upper Jialing River,always accounts for more than 45%.The monthly average reduction of runoff and sand transport during 1975-1984 under the same fallow scenario was significantly different,and the reduction was much greater in the abundant water period than in the dry water period,and the reduction of sand transport was higher than that of runoff.The annual reduction rates of runoff and sand transport were higher in the sloping land>15°fallow scenario than in the sloping land>25°fallow scenario.The reduction of runoff and sediment transport in the sloping land>15°fallow scenario at different time scales was greater than that in the sloping land>25°fallow scenario,and the reduction of sand transport was much higher than that of runoff.In the sloping land>15°fallow scenario,the area of cultivated land in the downstream of the main stream and the Xihanshui watershed is significantly reduced to 1/3 of the original,the area of forested land is increased to 47.10%,and the area of flow production reduction of 10 mm or more accounts for 91.07%.Under scenario RFR25,the area of cultivated land is reduced by less than 10%,and the area of grassland increases to 50.69%,and the percentage of area with flow production of 120-160 mm is reduced by 1.67%,of which the area with a reduction of less than 10 mm accounts for 85.23%,and the increase in the area of woodland has a positive impact on runoff and sediment retention.