Response Of Streamflow And Sediment Load To Human Activities And Climate Change In Xinshui River Watershed Of The Loess Plateau,China

Due to the influence of climate change and human activities,the streamflow and sediment load in the Loess Plateau decreased significantly.To reveal the response mechanism of water yield and sediment yield on watershed scale to climate change and human activities is of great significance to the implementation of the Grain for Green Project(GFGP),the restoration of ecological environment and the sustainable development of social economy.In this paper,the Xinshui river watershed of the Loess Plateau was taken as the research object.Based on the measured data of rainfall stations,meteorological stations and hydrological stations and four periods of land use data from 1958 to 2015,the linear trend method,Mann Kendall nonparametric statistical test method,Pettitt test method,double cumulative curve method,water balance method,sediment identify factor method and SWAT model were applied to detect the hydro-climatic variables,and evaluated the contribution of climate factors and land use change to streamflow and sediment load.Combined with the calibrated SWAT model,the spatiotemporal variation characteristics of hydrological process components and soil erosion characteristics caused by the project of returning farmland to forest and grassland were discussed.Combined with the future climate scenarioes of Xinshui river watershed,the future streamflow and sediment load in the watershed were researched.The main conclusions were as follows:(1)The annual precipitation and potential evapotranspiration of Xinshui river watershed from 1958 to 2015 had no significant trend,while streamflow and sediment load decreased significantly.1986,1995,2006 and 2015 land use data of Xinshui river watershed showed that forest land,shrub land,construction land and residential area increased significantly,cultivated land decreased significantly.(2)The breakpoints of annual streamflow were 1974 and 2000.And we divided the streamflow series into three stages.Based on the ecological hydrology method(TS method),the water balance method and SWAT model method,the results were as follows: Compared to the period 1,climate change was the main driving factor(69±6%)of streamflow reduction in the period 2,while land use change was the main driving factor(81±32%)of streamflow reduction in the period 3.The impact of land use change on streamflow was becoming more and more important,which is attributed to the increase of vegetation coverage in the watershed caused by the implementation of the GFGP.The SWAT model was better than the simple ecological hydrological method and the water balance method,due to its spatial and attribute data input and can reflect the actual hydrological cycle process of the Xinshui river watershed.TS method can qualitatively analyze the impact of climate factors and human activities on runoff;Water balance method and SWAT model are used to quantify the response of runoff to climate factors and human activities;SWAT model is superior to the other two methods in reflecting the actual hydrological cycle process and predicting future climate and land use scenarios.(3)The breakpoints of annual sediment load were 1981 and 2003.And we also divided sediment load into three stages Different streamflow-sediment mutation points are attributed to the influence of human activities.Based on the double curve method and the SWAT model method,the results were as follows: Compared to the period 1,climate change played a leading role in sediment load transport reduction(63±3%)in the period 2,while land use changes played a leading role in sediment load transport reduction(92±27%)in the period 3.Although the sediment identify analysis method cannot study the impact of climate and land changes on sediment transport,it can explain the impact of precipitation,water yield and sediment yield on sediment load.The results showed that the change of water yield factor and sediment yield factor in the whole period contributed more than 99% of the change rate of sediment transport proportion;the change of water production factor from the period 1 to the period 2 contributed65.24% of the change rate of sediment transport proportion;the change rate of sediment transport proportion from period 2 to period 3 was contributed by water yield factor(40.02%)and sediment yield factor(68.44%),while precipitation contributes to sediment transport proportion that had a negative effect(-8.46%).Attributable to the implementation of the GFGP project,the increase of the vegetation area covered by perennial vegetation in the watershed had enhanced its functions in stabilizing soil and reducing erosion.The quantitative results of these three methods are not completely the same,each method has its advantages.The sediment identify analysis method can explore the influence of water yield factor,sediment yield factor and precipitation on sediment load;The double cumulative curve method and SWAT model can explore the effects of climate factors and human activities on sediment load;In addition,SWAT model needs a lot of spatial and attribute data input,but it can better reflect the physical process of Xinshui River Basin at different spatial and temporal scales.(4)The results of the temporal and spatial response characteristics of runoff and sediment before and after the implementation of the GFGP showed that after the implementation of the GFGP,the contribution rate of the GFGP to the runoff and sediment transport reduction were 68.7% and 74.9%,respectively.It showed that GFGP played a leading role in runoff and sediment transport reduction,which was attributed to the decrease of cultivated land area and the increase of grassland,shrub land and forest land area.Under the joint action of climate and GFGP,the hydrological process components have changed significantly at the sub basin scale.Controlling climate factors unchanged,when cultivated land was converted to grassland and woodland,the erosion rate of the sub-watershed decreases the fastest.This is attributed to the fact that grassland and woodland have better water and soil conservation effects than cultivated land.(5)Based on 15 future climate scenarios predicted by 5 atmospheric circulation models under 3RCPs scenario models,compared with the current year,the range of future streamflow changes were-42.39%?113.07%(-6.55mm?17.47mm),indicating that future streamflow changes are not only affected by precipitation but also by potential evapotranspiration;the future sediment transport rate varied from-71.6% to-18.15%(-4.16t/ha to-1.08t/ha),indicating that the future sediment transport will show a decreasing trend.This may be attributed to the increase in temperature,which has led to serious soil warming and drying in the Loess Plateau,slowing the rate of soil erosion.