Study On The Response Of Hydrological Processes To Climate Change In The Nu River Basin

Mountain ranges are the water towers of the world,supplying a substantial part of water resources for its downstream socioeconomic development.In the context of climate change,the responses of hydrological processes in the mountainous areas are sensitive and complex,which directly influence the sustainable utilization of water resources in the basins.The researches on hydrological simulations and hydrological processes are difficult in mountainous areas because of high meteorological variability,the lack of observational data,and the complex interplay among atmosphere,hydrosphere,cryosphere,and biosphere.Hence,quantitative assessment of hydrological processes changes in mountainous basins in the context of climate change is of great significance for the sustainable development of social economy and ecological environment protection in the basins.In this study,we firstly assessed the capability of precipitation and evapotranspiration products to analyze the spatial distribution patterns of major hydrological factors in the Nu River Basin,and then explored the interannual and seasonal variation characteristics of temperature,precipitation,and flow from 1970 to 2011.The DEM,land use,soil and meteorological data were used to construct the SWAT model in the Nu River Basin.By setting various temperature and precipitation change scenarios,the hydrological responses over the upper,middle,and lower reaches of the Nu River Basin to climate change were identified.The main conclusions were summarized as follows:(1)The mean annual temperature in the Nu River Basin increased from north to south,and the low-high-low-high precipitation and evapotranspiration were found along the river.The warming tendency of the Nu River Basin was significant from the 1970 to2011.All the mean annual and seasonal temperature had significant increasing trends,and the increasing rate in winter was higher than those in other seasons.Different from the increasing trend of temperature in the whole basin,the trends of precipitation were inconsistent,and increasing trends were found in the Tibet Autonomous Region and decreasing trends were found in the Yunnan Province.The annual flow at the Daojieba station showed an insignificant increasing trend with an increasing rate of 39.5(m~3/s)/10a,and a main oscillation period of 28 a.Flow in the rainy season also showed an insignificant increasing trend with an increasing rate of 52.9(m~3/s)/10 a.In the dry season,the flow increased significantly at the rate of 19.7(m~3/s)/10 a,and flow changed abruptly in 1988.(2)During the calibration and validation periods,the Nash-Sutcliffe efficiency(NSE)values at five hydrological stations on the monthly scale were all above 0.75,and the percent bias(PBIAS)values were mostly within the range between-10%and 10%.The SWAT model could be used effectively in the Nu River Basin because of considering the spatial heterogeneity of climate and underlying surface and the influences of snowmelt on the hydrological cycle.(3)The response of water yield to temperature change in the Nu River Basin was more complex than that of precipitation,and the variation of water yield in the upstream,midstream,and downstream was quite different,among which water yield in the upstream was the most sensitive to temperature change,followed by the midstream and smaller in the downstream.Under the same climate warming scenario,the increase of potential evapotranspiration in the upstream was larger than that in the midstream and downstream,and the increase of evaporation capacity led to the changes of evapotranspiration,soil moisture,and baseflow.Affected by the evapotranspiration,the decrease of baseflow in the upstream was larger than that in the midstream and downstream.On the other hand,when the temperature rose,more snow fell in the upper reaches in the form of rain,leading to decreasing trend in the amount of snowmelt in spring.At the same time,the earlier thawing time of the frozen soils also weakened the impermeability and increased the infiltration.The processes of melting and thawing resulted in a significant decrease of surface flow in the upstream in spring.As there were less snows and frozen soils in the midstream than in the upstream,the variation of surface flow in the midstream was relatively small.(4)Baseflow and snowmelt runoff are important sources of water supply in the dry season of the great rivers on the Tibetan Plateau,especially for the utilization of water resources in the downstream areas.The evaporation capacity and hydrological processes of snow and ice in the upper reaches of the Tibetan Plateau are more susceptible to climate change,and the hydrological processes changes in theses areas are more sensitive and complex.