Water Cycle Evolution And Its Response To Climate Change In The Upper Reaches Of Nujiang River Basin

The upper reaches of Nujiang River Basin is located in the hinterland of Qinghai-Tibet Plateau,which is a typical alpine climate region and a sensitive area of climate change.The close relationship between water and energy cycle result in the complex phase change characteristics of the water cycle in the Basin.The aim of this study is to analyse the evolution of water cycle and its response to climate change.Firstly,the nonstationary evolution for temperature and precipitation has been investigated.Then,the simulation methods of snowfall separation and snowmelt process have been improved according to the meteorological and remote sensing data,and the evolution of snowfall,snow cover area and frozensoil depth were analyzed.On this basis,a distributed water-heat coupled(DWHC)model has been constructed,and the simulated water cycle has been verified and analyzed in the basin.Finally,the DWHC model has been drived by GCMs outputs,and the future evolution trend of water cycle has been explored.The main conclusions are as follows:(1)There is a significant increasing trend of average air temperature,which is obviously larger than the increasing trend of air temperature over China.The difference of spatial distribution of extreme temperature will be more significant in the basin.The increasing trend of surface temperature is larger than that of air temperature.The increasing trend of the precipitation is insignificant overall basin,and the change rate of precipitation increases with elevation.Meanwhile,the temporal variability of the air temperature and precipitation of meteorological stations was analysed.There is a different influence of monsoon on the air temperature and precipitation of meteorological stations.(2)Based on the relation between precipitation phases and daily air temperature,a separation scheme with exponential function structure was applied to estimate the snowfall,which performs well in snowfall simulation.The influences of air temperature and precipitation on snowfall show that the increasing probability of precipitation at low air tempetature increases the snowfall under the background of climate change.However,the significant increase of air temperature decreases the snowfall,especially after 1980.Remote sensing inversion snow data shows that more than half of the basin with snow cover last more than 50d.The significant increasing air temperature decreases snow depth after the 1979.There is a good power function relationship between snow cover area and snow depth,which correlates well with the average slope of sub-basin.In addition,the maximum frozen soil depth shows a significant decreasing trend,and is negatively correlated with the accumulated negative air temperature.The maximum frozen soil depth and its change rate increases and decreases with the altitude,respectively.(3)A distributed water-heat coupled(DWHC)model has been constructed and applied into upper reaches of Nujiang River Basin by coupling the soil water-heat coupled model and the distributed water cycle model.The results show that both of meterological stations data and grid data perform well in driving DWHC model.The water cycle simulation derived from the meterological station data shows that the average annual runoff is 23.35 billion cu.m,among which the snowmelt runoff and glacial runoff are 3.97 and 1.19 billion cu.m,accounting for 18.78%and 5.09%of annuall runoff,respectively.The increasing air temperatures increases the glacier runoff by 18 million cu.m pre year.Freezing time of soil is shortening and soil moisture content is increasing,which promotes evapotranspiration and decreases the runoff outside glacier runoff by 2 million cu.m pre year.Total runoff increases by 16 million cu.m pre year dut to the increase of glacial runoff.The decrease of snowfall directly result in the reduction of snowmelt runoff.The annual distribution of glacier runoff and total runoff is consistent,flood peak occurred in flood season(June-Septemper).meanwhile,annual snowmelt has two peak occurred at May and Septemper accordingly.(4)The suitable GCM output has been screened from the 5 GCMs output provided by ISI-MIP according to the meteorological station data and grid data.The future evolution of air temperature and precipitation shows that the future increases of air temperature are 0.14?/10a,0.41?/10a and 0.45?/10a and the future changes of precipitation are 8.34mm/10a,3.28mm/10a and-4.23mm/10a under the RCP2.6,RCP4.5 and RCP8.5 scenarios,respectively.The future evolution of water cycle is analyzed based on DWHC model.The results show that under RCP2.6,RCP4.5 and RCP8.5 scenarios the future annual runoff decreases by 14 million cu.m,16 million cu.m and 60 million cu.m;the snowmelt runoff decreases by 24 million cu.m,33 million cu.m and 28 million cu.m;the glacier runoff increases by 4 million cu.m,6 million cu.m and 9 million cu.m,respectively.The ice-storage capacity of the glacier in the basin will be reduced to 32.3-33.1 billion cu.m,and the area of the glacier will be 712-724 km2.