Study Of The Climate Change And Underlying Surface Change As Well As Their Runoff Effects In The Yarlung Zangbo River Basin

Crossing China,India and Bangladesh,the Yarlung Zangbo River Basin is ex-tremely sensitive to environmental changes and plays an important role in national and regional ecological balance,environmental protection and socio-economic devel-opment.Water resources change is an important index reflecting the climate change and underlying surface change in the basin.However,due to the unique geographical location and environmental conditions of the Yarlung Zangbo River Basin,the scarcity of basic data hinders conducting a systematic research on climate-change-driven un-derlying surface change and associated runoff effects in the basin.Utilizing the Global Land Data Assimilation Systems(GLDAS)and hydrometeorological observation data,this study analyzes the characteristics of climate change and the resulting runoff evolu-tion in the Yarlung Zangbo River Basin from 1961 to 2010,which served as the stage division basis for analyzing the characteristics of underlying surface change.By tak-ing full advantage of GLDAS and remote sensing data,a new generation of ESSI-RSM equipped with snow and ice response mechanism was developed.The developed model was employed to simulate the hydrological processes of the basin from 1980 to 2010 in which 1980-1985 and 1986-1990 were used for warming up and calibrating the model states,respectively.Likewise,the period 1991-2010 was divided into four sections with16 simulation scenarios to quantify the contributions of climate change and underlying surface change to total runoff and spatial distribution of runoff.In the actual simulation scenario,the snow-ice runoff series is obtained by dynamically embedding the snow-ice melting module into the ESSI-RSM model.We examined the long-term impact of climate change on areas covered with snow-ice and the resulting runoff in the Yarlung Zangbo River Basin.Finally,based on the future meteorological data under the sce-narios of RCP4.5 and RCP8.5,the high-accuracy daily meteorological dataset with the spatial resolution of 1 km×1 km was obtained by using Quantile-Mapping(QM)and downscaling method improved by present study;then climate change was analyzed be-tween 2036-2065 in the Yarlung Zangbo River basin under IPCC RCP 4.5 and RCP8.5 scenarios,and the corresponding runoff response is obtained by ESSI-RSM model,which provides the basis for water resources protection and water resources manage-ment in the basin.The main conclusions of this study are as follows:(1)Based on the GLDAS and in-situ data,combined with the spatial downscal-ing method improved by using GIDS and neighborhood algorithm,the long-term high-precision and high-resolution meteorological data in the Yarlung Zangbo River Basin was obtained from 1961 to 2010.During the period,there was an overall increasing trend in temperature,precipitation and actual evapotranspiration.According to the analysis result of using EEMD algorithm,temperature and evapotranspiration had re-spectively strong quasi-45-year and 13-year low-frequency oscillation periods,while precipitation had no significant oscillation period.(2)The annual runoff discharge of the Yarlung Zangbo River Basin is unevenly distributed,with 53.0%of annual runoff occurring in summer and 6.9%in winter.The interannual variation shows a decreasing-increasing non-linear change with the turning point in the mid-1990s.According to the analysis result of using EEMD algorithm,runoff had a significant low-frequency oscillation period with quasi-12 and quasi-45years during the study period.The annual runoff variation coefficient C_vof the Yarlung Zangbo River Basin is small,which indicates that the annual runoff fluctuation of the Yarlung Zangbo River Basin is relatively stable compared with other rivers.(3)Grassland is the main land use in the Yarlung Zangbo River basin,accounting for 62.6%of the area.According the analysis results of using transition matrix,during1990-2010,a large proportion of the grassland area was mainly transformed into urban and rural constructions and woodland.Even though the change of other land use types was not significant,but there was a significant mutual transformation process among various land use types.There are more snowpacks in the upstream source area and downstream area of the basin,and the days with snow-cover tend to decrease due to the combined effect of the snow-cover starting days(SCS)and of snow-cover melting days(SCM).(4)On the basis of the ESSI model,a new generation of ESSI-RSM model with snow-ice response mechanism is developed on the Visual Studio 2010 platform,using C#as the development language and Arc Object 10.2 as the GIS component,by improv-ing the three main aspects of precipitation aggregation state determination,snow-ice melting module and infiltration process and other functions(i.e.adding spatial-driven data interface,dynamic underlying surface,etc.).The Nash coefficient(NSE)of the runoff in Yarlung Zangbo River Basin can reach more than 0.8 in all simulation peri-ods,which indicates that ESSI-RSM can be well adapted to the hydrological simulation in the plateau cold region.(5)Since the 1990s,the snowmelt runoff in the basin has shown an increasing trend,with the contribution rate to annual runoff ranging from 21.1%to 48.6%,and the average annual contribution rate of 33.6%.Climate change is the dominant controlling factor of the runoff in the Yarlung Zangbo River Basin,with its impact being much higher than that of underlying surface change.From period 1(1991-1995)to period 4(2006-2010),the contribution rate of climate change to runoff change is 99.4%,while the contribution rate of underlying surface change to runoff change is only 0.6%.(6)Under RCP4.5 and RCP8.5 scenarios,the temperature of the Yarlung Zangbo River basin increased significantly in the 2050s(2036-2065).However,precipitation increased significantly under RCP4.5 and non-significantly under RCP8.5,while evap-otranspiration decreased significantly.Compared with the RCP 4.5 scenario,RCP8.5scenario resulted in a bigger amplitude of climate factors variation.Runoff showed a significant increasing trend under both scenarios,being indicated more pronouncedly under RCP 8.5 scenario.Moreover,it was found that the contribution of subsurface flow to total runoff is higher than that of surface runoff.