Simulation Study On Runoff And Sediment From Gaozhou Station In Jianjiang River Basin

The issue of ecological protection and its governance has always been the focus of society’s attention.In recent years,the rate of land use change in the watershed has accelerated with the increasing intensity of human modification of land use types.At the same time,the impact of land use changes on regional flow and sand production has also emerged,and soil erosion has been one of the problems facing the development of regional ecological protection.Therefore,in order to clarify the effects of different types of land use on flow and sand production and water-sand processes in the watershed,this study takes the upper reaches of the Jianjiang River basin above the Gaozhou control station as the research object,and simulates the flow and sand production at the Gaozhou control station using the SWAT hydrological model based on the sub-basin conditions and hydro-meteorological data.Using two periods of land use data in 2005 and 2018,we analyzed the effects of flow production and sand production in the region and different land use types in each sub-basin in the region.The soil erosion modulus in the upper part of the Jianjiang River basin was estimated by GIS and RS technology,and the soil erosion characteristics of the study area under different elevation and slope conditions were analyzed by combining the United States Universal Soil Loss Equation(USLE).The main findings of the study are as follows:(1)Parameter sensitivity analysis by the global approach in SWAT-CUP shows that the most sensitive parameter for runoff is runoff curve number CN2,and the most sensitive parameter for sediment is USLE soil and water conservation measure factor USLE＿P.Sand production is more sensitive to changes in land use type compared to flow production.The R~2of Gaozhou hydrological station in the upstream region of the Jianjiang River basin during the runoff simulation calibration and validation periods were 0.73 and 0.69,NS were 0.71 and 0.67,and PBIAS were-2.8%and+4.1%,respectively;In the sand production simulation,the R~2 was0.58 and 0.69 for the calibration and validation periods,respectively,the NS was 0.58 and 0.67,respectively,and the PBIAS was-3.2%and-12.4%,respectively.Although there was an abnormally high rainfall year in 2008,the simulation results of the Gaozhou control station were better in terms of evaluation indicators.(2)Based on the effect of land use types on the production of flow and sand at the sub-basin scale,it can be concluded that forest land has the most obvious effect on water and sand reduction compared with other land use types,followed by grassland.Cultivated land has a sediment-producing effect.According to the secondary classification,the increase in the area of rural settlements in both urban and rural residential land has led to the destruction of soil structure and caused serious soil erosion.Therefore,the land use type of the watershed can be adjusted to increase the concentration of forested land to avoid the increase of sediment production due to human activities as much as possible.(3)The study area belongs to the mild erosion class,with an average soil erosion modulus of 1908.35t/(km~2·a)and a total annual erosion volume of about 5.66 million tons.Soil erosion is predominantly slight and mild.Analysis of soil erosion at different elevations and slopes shows that soil erosion is more serious in the[0,400)m elevation zone,with an erosion area of2384.49 km~2,accounting for 80.46%of the total erosion area.In the<5°and[8°,15°)slope zone,the erosion area is 1268.36km~2 and 709.26km~2 respectively,accounting for 42.80%and23.93%of the total erosion area respectively,and the soil erosion is more serious.The priority areas for soil and water conservation in the study area can be identified as[0,400)m elevation range,<5°and[8°,15°)slope zones.Strengthening soil erosion prevention measures in these areas can effectively reduce the overall soil erosion in the region.(4)Under the extreme scenario,the sand production of each class of land is from high to low:cultivated land>grassland>forest land.The future scenario assumes that the implementation of reforestation and the expansion of land for urban and rural residents will increase the flow production and decrease the sand production in the study area.