| Environmental changes driven by both natural evolution and human activities are the main driving forces affecting the hydrological cycle,and distinguishing the effects of climate change and human activities on watershed hydrological processes is a key element of sustainable water resources management in a changing environment.The Yangtze River is the first longest river in China,and the upstream area is the main source area of Yangtze River runoff and an important hydropower development base in China.The upstream of the Yangtze River is a sensitive area for climate change,and coupled with the greatly enhanced construction of the Three Gorges upstream gradient reservoir since the 21st century,the hydrological processes in the upstream of the Yangtze River have received extensive attention.The runoff,precipitation and potential evapotranspiration data from six control stations in the upper reaches of the Yangtze River,Xiangjiaba,Gaochang,Fushun,Beibei,Wulong and Yichang stations from 1958 to 2016 were used to analyze the variation patterns of hydro-meteorological elements by using the trend-free white Mann-Kendall test and Sen’s slope method.The cumulative distance level method,Pettitt,SNHT and Buishand test were used to identify the abrupt change years of hydrometeorological elements.The complex Morlet wavelet transform and Hurst index were used to analyze the periodicity and persistence of hydrological series.The precipitation-runoff double accumulation curve and the slope rate of change of precipitation-evaporation accumulation were used to quantify the contribution rate of natural and anthropogenic factors to the runoff variability at each control station.The effects of climatic elements and subsurface characteristics on runoff variability were estimated using the runoff elasticity coefficient method based on Budyko’s hypothesis.The hydrological effects of land use/cover change on the upper Yangtze River basin were analyzed based on land use type and normalized vegetation index(NDVI)data in1980,2000 and 2015.The effects of Three Gorges and the upstream reservoir complex on intra-annual runoff process are discussed by analyzing the changes in intra-annual distribution of runoff at different time periods.The main conclusions are as follows:(1)Evolutionary characteristics of hydro-meteorological elements in the upper Yangtze River basinThe trend test shows that from 1958 to 2016,except for Xiangjiaba,where the annual runoff showed a non-significant increase,the annual runoff of all control stations showed a decreasing trend,and the Gaochang showed a significant decreasing trend.Annual precipitation showed a non-significant decline in all basins except Jinshajiang River,which showed a non-significant increase,and the upper reaches of Yangtze River showed no significant change.The annual potential evapotranspiration,except for the Tuojiang River,is not significantly increasing,and the rest are significantly increasing,with the most obvious increase in the Wujiang River.The annual runoff at Gaochang station changed significantly and abruptly in 1969 and 1994,and that at Fushun station changed significantly and abruptly in 1969.The annual precipitation of the Tuojiang and Wujiang rivers changed significantly and abruptly in 1991 and 1985,respectively.Except for the Jinshajiang River,which had significant abrupt changes in 2002 and2005,all other basins had significant abrupt changes in 2001.The annual runoff of Xiangjiaba has three scales of 28 a,22a and 13 a,and the rest of the stations have 28-32a and 12-13a scales of variation.The first main cycle of annual precipitation in each basin is 28 a,and the second main cycle in the Jinshajiang,Tuojiang,Jialingjiang and the upper Yangtze is 13-14a.The first main cycle of annual potential evapotranspiration in each basin is 28-32a,and the second main cycle is insignificant.The Husrt index in annual runoff of Gaochang,Fushun,Beibei and Wulong stations are all greater than 0.7,with Gaochang and Fushun exceeding 0.8.The annual precipitation Husrt index of Jinshajiang,Minjiang,Tuojiang,Jialingjiang and Wujiang rivers are all greater than 0.6,with Wujiang exceeding 0.7.The annual potential evapotranspiration Hurst indices all exceed 0.8,with positive persistence.(2)Quantitative contributions of climate change and human activities to runoff changes in the upper Yangtze RiverTaking 1958-1969 as base period and 1970-2016 as variation period,the variation period was divided into four stages Ⅰ-Ⅳ.Considering two climatic factors,precipitation and evaporation,the annual runoff of Xiangjiaba and Gaochang is mainly influenced by the evaporation variation.Climate change and human activities increase the runoff from Xiangjiaba,and climate change increases the Gaochang runoff while human activities decrease that.Climate change is the main factor for change in annual runoff in Fushun variation period Ⅰ-Ⅲ,Beibei and Yichang variation period Ⅰ and Ⅲ-Ⅳ,and WulongⅢ-Ⅳ.Runoff from each control station has the highest sensitivity to precipitation and similar sensitivity to subsurface characteristics and potential evapotranspiration.Runoff at Xiangjiaba variation period Ⅱ-Ⅳ is mainly influenced by climate change,with a contribution of 53.07%-98.04%.The contribution rate of climate change to runoff change in variation period Ⅰ-Ⅱ of Gaochang is 52.93%-60.85%;the contribution rate of human activities in the variation period Ⅲ-Ⅳ is 59.95%-57.73%.Climate contributes about 64.52%-82.42% to runoff change in variation period Ⅰ-Ⅳ at Fushun station,and precipitation contributes about 61.05%-74.09%.Runoff change in variation period Ⅰ-Ⅳat Beibei station is mainly influenced by climate change,the contribution rate is about51.21%-84.30%,and exceeds 80% in variation period Ⅱ.Human activities contribute76.49% to runoff changes in variation period Ⅲ at Wulong,and the rest of the period is mainly influenced by climate change,with a contribution rate of 51.76%-89.93%.The contribution rate of human activities to runoff changes in Yichang variation period Ⅰ-Ⅱ is 50.89%-63.66%,and variation period Ⅲ-Ⅳ is mainly influenced by climate change,with the contribution rate of 62.04%-96.97%.(3)Hydrological effects of land use/cover and water conservancy projects on the upper Yangtze River basinArable land in the upper reaches of the Yangtze River decreased continuously from1980 to 2015 and decreased more after 2000.The overall change of forest land and grassland is not obvious.Water area decreases and then increases and decreases overall.Construction land continues to increase and the increase is greater after 2000.Unused land increased.Land use shifts are mainly the conversion of cropland,forest land and grassland to each other,the conversion of cropland,forest land and grassland to watershed and construction land,the conversion of forest land,grassland and watershed to unused land,and the conversion of watershed to grassland and unused land.The integrated land use dynamic attitude in the last 35 years is 1.90%,and the degree of land type change is low.The vegetation cover of Wujiang and Jialingjiang rivers shows an increasing trend in 1982-2015,and the increase is greater after 2000.Jinshajiang and Minjiang rivers rose slightly and showed a decreasing trend after 2000.The changes in the Tuojiang river were not obvious.The upper Yangtze river shows an upward trend,but the increase is weakened after 2000.The intra-annual distribution of runoff change obviously after 2000,the average runoff from January to March is higher than before2000,with 30.69%,27.80%,56.38%,27.36%,25.70%.After 2000,the average runoff from July to September was low,with 6 stations decreasing by 1.68%,16.57%,10.79%,9.50%,14.68% and 12.26%,respectively,and Xiangjiaba decreasing the least.In the 21st century,the Three Gorges and upstream reservoirs have redistributed intra-annual runoff of upper Yangtze river,resulting in an increase in runoff during the dry period and a decrease in runoff during the wet period in the upper Yangtze river basin,and the intra-annual distribution of runoff has become more uniform. |
