Effects Of Climate And Land Use And Land Cover Change On Streamflow And Sediment In Fu River Watershed

Climate change and landuse change are two driving factors affecting streamflow and sediment yield within a watershed.The Fu river watershed(FRW)was selected as the study area due to its critical location to the upper Yangtze river.This study aims to quantify streamflow and sediment reduction under the isolated and combined effects of climate change and landuse change at multiple spatial scales.Two combined used techniques,Mann-Kendall test and Sen's slope estimator,were selected for trend analyses.Double Mass Curve(DMC)and Pettitt test were used for the abrupt point detection.Concentration Index was selected to analyse the distribution of annual runoff and sediment concentration.Transition matrix was adopted to analyze the variation of land use type before and after the detected abrupt year.Two time periods were divided by the abrupt year and four different scenarios were set up to simulate the isolated and combined effects of climate change and land use change.Streamflow and sediment discharges for different scenarios were derive by SWAT simulation.The main results were as follows:(1)During the entire period,annual streamflow showed a significant reduction trend with a confidence level of P<0.01,annual sediment showed an extremely significant reduction trend with a confidence level of P<0.001 and annual precipitation exhibited a downward trend,respectively.The year 2000 was detected as the abrupt point after considering a variety of factors.The concentration of streamflow and sediment transport shows an upward trend,which indicates that the phenomenon of large water and sediment is more obvious in recent years,and the concentration of sediment transport is significantly higher than that of streamflow.(2)The primary land use type of FRW was farmland(58.80%),which was decreased by 427.98km²during the study period.The areas of unutilized and urban land were relatively small,yet with largest growth rates(218.80%and 195.58%,respectively).The value of NDVI in FRW had an uptrend since the Grain for Green Project implemented.It means that the plant here has been recoveried.The value of NDVI in summer(June to August)is bigger than that in Autumn(September to November)and spring(March to May),as well as winter(December to February).The spacial distribution of NDVI also displayed a heterogeneity because of the terrain and soil type.(3)SWAT model could be satisfactorily used in FRW.The R²,E_ns,and PBIAS were selected as model evaluation indices,the values for streamflow were 0.89,0.88,and-0.2%for calibration period,respectively,and 0.82,0.82,and 6.8%for validation period,respectively.The values for sediment were0.78?0.75 and-29.7%,for calibration period,respectively,and 0.39,0.31,and 4.6%for validation period,respectively.(4)Four scenarios were constructed during the basic period(1980-2000)and the varied period(2001-2015),and results showed that:at the whole basin scale,water yields under scenario 1(with isolated variation of precipitation),scenario 2(with isolated variation of land use)reduced by 10.47%and 1%,respectively.However,water yield under scenario 3(with combined variations of precipitation and land use)reduced by 16.01%.At the sub-basin scale,the increase of unutilized in the upstream increased the risk of soil erosion.On the other hand,the expansion of urban land at downstream increased the water yield,while the effects of Grain-to-Green project on both bank regions of the river channel significantly reduced streamflow discharge.As the farmland in the upper reaches decreased,the woodland and grassland increased,the ability of water and sediment retention becomes stronger,and the increase of rainfall promotes the growth of vegetation more luxuriant and the soil structure is more stable with the biomass increases.Therefore,the sediment yield is greatly reduced.Both climate change and land use change will affect water yield and sediment yield of FRW at basin scale and sub-basin scale.It is remarkable that the combined effects of climate change and landuse change were not equal to sum of corresponding isolated effect,which is potentially due to the interactions between the climate and land use types.This research can provide reference for water resource management,soil and water conservation and reservoir streamflow forecast of the Three Gorges Dam.