Hydrological Characteristics Of The Nu-salween River And Their Response To Climate Change

The Nu-Salween River(NSR)is the longest free-flowing river in Southeast Asia and plays an irreplaceable role in local ecological protection and social development.However,the climate and topography of this basin are complicated and it is in lacks of observed and simulated data,which restricts hydrologic research in the basin,especially in the lower part.Analyzing the hydrological characteristics of the basin and revealing its response to climate change is of great importance for the rational use of water resources and effective response to water-related disasters.This study first developed a distributed hydrological model in the NSR basin named GBHM-SW based on the hillslope flow production mechanism and reconstructed the hydrological dataset(GBHM-ERA5)for the historical period using this model driven by ERA5 reanalysis data.The validation results against hydrological stations demonstrate that this reconstructed data can well reproduce the spatial and temporal distribution characteristics of streamflow and significantly outperforms other publicly available streamflow datasets.Based on this reconstructed data,the streamflow characteristics of the lower reaches of the basin were revealed,and it was found that during 1981?2014:(1)the surface runoff changes were different between north and south of the downstream,with the northern part showing a downward trend,while the south part upward;(2)The rate of decline in discharge at the outlet of the downstream was significantly higher than at the upstream and midstream,the discharge at the outlet of the downstream was decreased at a rate of about 744 million cubic meters per year;and(3)the dominant factors of surface runoff linear trends showed seasonal divergence,surface runoff trends in the dry season were mainly dominated by temperature changes while surface runoff trends in the rainy season were controlled by precipitation changes.A monthly-scale ensemble dataset with a spatial resolution of 0.25° was generated based on the latest CMIP6 data using Bayesian model averaging(BMA)mothed,containing temperature,shortwave radiation,and precipitation data from 1980 to 2100.The data for the future period includes three scenarios: SSP1-RCP2.6,SSP2-RCP4.5,and SSP5-RCP8.5.The BMA has better applicability in the NSR basin compared with the simple model average(SMA)data.GBHM driven by BMA was used to predict the hydrological processes in the NSR basin under three climate change scenarios,it was found that the water resources situation of the basin will be severe in the near future(2019?2048),with small changes in precipitation,increased evapotranspiration,and decreased surface runoff.In the middle future(2049?2078)and the far future(2079?2100),the basin will experience increased precipitation,increased surface runoff,and increased water supply capacity.The water resources condition in the lower reaches will be relatively better,and the discharge of the downstream will increase significantly from July to August.Spatially,the upstream will experience the greatest increase in temperature,increased evapotranspiration,and significantly lower runoff in the near and middle future than in the historical period.The area of the downstream basin with the greatest increase in surface runoff is located in the southwestern Shan State of Myanmar,where cropland is widespread,and flood control pressure will increase.This study found that the effects of different meteorological data gradually expand in water balance simulations: the BMA and SMA methods are used to generate CMIP6 ensemble data,which predict the similar relative changes in meteorological elements;they drive the hydrological model to simulate evapotranspiration using the same set of parameters with a relative change difference of about 5%;while for streamflow,the signs and magnitudes of relative changes predicted by BMA and SMA are different,with the largest difference reaching 27%.The results suggest that the impacts of different ensemble methods cannot be ignored in the study of assessing the hydrological response of the NSR basin under climate change.