Quantifying The Impacts Of Climate Change And Human Activities On Runoff In The Upper Reaches Of The Yangtze River

In recent years,global climate change and intensified human activities play an important role in the process of global water cycle,and also pose a huge threat to the already scarce water resources.The research on the impact of climate change and human activities on regional water resources is also increasing.The upper reaches of the Yangtze River is the ecological barrier of the Yangtze River Basin.In addition,40%of the water resources in the Yangtze River Basin are also concentrated here,which is of great significance to the socio-economic and ecological construction of the entire river basin and even the whole China.In the past few decades,human activities,including ecological engineering,reservoir construction,urban transformation,and water withdrawal,have intensified,which has a significant impact on water resources of the upper reaches of the Yangtze River.In this context,“How much has water resources changed?How much has human activities and climate change affected water resources?”has become a major concern of scholars and policy makers at home and abroad,to solve these problems effectively will be helpful to the scientific management and prediction of water resources in the future.In this study,the upper reaches of the Yangtze River and its four sub-watersheds were selected as the study area,and using the longitude,latitude,and elevation as covariates to interpolate the 1956～2017 meteorological data;the Penman-Monteith formula recommended by FAO was used to calculate the regional potential evapotranspiration;Mann-Kendall trend test with Sen’s slope are used to analyze the trend of interannual precipitation,potential evapotranspiration and runoff time series in all study areas from 1956 to 2017,using Pettitt’s test to identify the locations of change points in the runoff in each region,and use the Mann-Whitney U test to test the difference in the runoff sequence before and after the change point;the parameterωin the Fu’s Budyko type equation was calculated based on the annual water balance equation and Budyko framework,and the precipitation elasticity,potential evapotranspiration elasticity and parameterωelasticity of runoff were also calculated;the contribution of climate change and human activities to runoff change in each study area was quantitatively analyzed by elasticity method and decomposition method based on Budyko framework;the relationship between precipitation,potential evapotranspiration and runoff in different periods was established to analyze the impact of climate change on runoff;the human activities in the upper reaches of the Yangtze River were discussed by using land use change,leaf area index,water use,and reservoir construction.The main conclusions of this study are as follows:（1）The runoff of Jialing River Watershed and Minjiang River Watershed showed a significant downward trend,with the change rates of-1.46 mm yr^-1 and-1.48 mm yr^-1,respectively,while the runoff changes in other regions were not statistically significant.The potential evapotranspiration only changed significantly in the Wujiang River Watershed with a change rate of-0.58 mm yr^-1,and the change of precipitation in all regions was not statistically significant.The runoff of Wujiang River,Jialing River,Minjiang River,Jinsha River,and the whole upper reaches of the Yangtze River changed in 2004,1993,1993,1997,and 2000,respectively.Only the runoff of Jinsha River had no significant difference before and after the change point,while the runoff of other regions had significant differences in the two periods.（2）From 1956 to 2017,ωincreased in all regions except the Jinsha River Watershed.Theωof Jialing River Watershed,Minjiang River Watershed,and the whole upper reaches of the Yangtze River increased significantly at the rate of 0.003 yr^-1,0.002 yr^-1,and 0.001 yr^-1.The change rate ofωin period II was higher than that in period I in all regions,which proved that human activities were more intense in period II than in period I,and theωin Min River Watershed and Jinsha River Watershed had an obvious increasing trend in period II,the growth rate both are 0.006 yr^-1.Theωchanges in the period I over all regional were not statistically significant.（3）The climate elasticity and parameterωelasticity of runoff in other areas outside Jinsha River Basin in periodⅡwere less than those in periodⅠ.In all regions,the precipitation elasticity coefficient of runoff was greater than 0,while the potential evapotranspiration elasticity coefficient and the parameterωelasticity coefficient of runoff were less than 0.In Wujiang River Watershed,Jialing River Watershed,Minjiang River Watershed and the upper reaches of the Yangtze River,the precipitation elasticity>the parameterωelasticity>the potential evapotranspiration elasticity;in Jinsha River Watershed,the parameterωelasticity>the precipitation>the potential evapotranspiration elasticity.（4）There is little difference between the contribution of climate change and human activities to runoff calculated by the elasticity method and the change of actual runoff,and the contribution calculated by decomposition method is consistent with the change of actual runoff.The results of the elasticity method and decomposition method are similar:In Wujiang River Watershed,Jialing River Watershed and Jinsha River Watershed,climate change is the dominant factor of runoff change;In the Minjiang River Watershed and the upper reaches of the Yangtze River,human activities are the dominant factors of runoff variation.（5）In all study areas,precipitation is the main climatic factor leading to runoff change,while potential evapotranspiration has little effect on runoff.Land use change,vegetation change,regional water use change and reservoir construction have a certain regulating effect on runoff.