Change Of Underlying Condition With Its Water Retention Function In The Hei And Bai River Basins

Basin hydrology is deeply influenced by the underlying conditions.Under a background of global warming,declines of land surface ecology,with negative impacts on regional water retention(WR),have been evidenced in China's cold-alpine regions where many rivers originate.Quantifying changes of the underlying condition,together with analysis of its water retention function in the water source area would help people better understanding regional water evolution and benefit basin water management.The Hei and Bai river basins(the HRB and the BRB,respectively),located in northeast of the Tibetan Plateau,featuring lots of cold-alpine meadows and wetlands,are of great importance in water retention and conservation in the upper reaches of the Yellow River Basin.Differentiation of the land surface geo-ecology provides one natural platform for studies of underlying changes and basin eco-hydrology.In this study,data of hydrometeorology,land use and land cover(LUC)and ecologic indicators like the normalized difference vegetation index(NDVI)were collected and treated as into series from 1981-2015 for analyzing variations of regional hydrometeorology and underlying conditions in the two selected river basins.Supported by the data,the InVEST model is parameterized to simulate the basin water retention.Based on the above,WR's variation in time and space,along with its response to the changeable key factors including air temperature(T),local precipitation(P),potential evapotranspiration(Ep)and regional NDVI are quantified for a comprehensive study on changes of underlying conditions and its water retention function in the area.Main conclusions are made as in the following.(1)Geo-differentiations in variations of regional hydrometeorology were found in the two selected basins during the time period from 1981 to 2015.Increasing rate of T was higher in the north HRB than that in the south BRB,corresponding to more remarkable rising of Ep in the north.Trends of local P was not found significant although a 7-year variation period was diagnosed,the same as that of the Ep and river discharges,the last one increased during the time period from 1981 to 1985 and turned into a decreasing trend since then,the changeable breakpoint was tested occurring in the year of 1993.(2)It was found that the cold-alpine meadow with high coverage occupied the largest in the two selected river basins,and then the marsh wetlands.Degrees of land development and utilization there were evidenced rising since the year 2000,mainly featuring grasslands transforming form those with high coverage to the ones with middle and low coverage.Also,parts of wetland surroundings were found changing into kinds of grasslands.Especially,more desertification in the HRB and obvious urbanization in the BRB were characterized,respectively.Multi-yearly averaged NDVI was valued 0.66 in the HRB,while that in the BRB was slightly higher,with the value of 0.67.Vegetation dynamics was evidenced strengthened by an increase of the NDVI value during the statistic time period.(3)InVEST simulations resulted in annual total of 1.61×10~8m~3 WR in the HRB,while that in the BRB,1.75×10~8m~3.Capacity of WR decreased remarkably in the 1980s mainly in the middle reaches of the two selected river basins.During the time period from 1981 to 2015,basin WR presented slight decreasing across the whole study area.(4)WR capacity in kinds of underlying types were sorted into an order of forest>grassland>marsh wetland>cultivated land>built-up>bare land>water area,that in main land cover types like forest,grassland and wetland all experienced a changing process of"decrease-increase-decrease"in both the river basins.Positive effectiveness of T,P and NDVI were found to WR.Given 1%increase of the above 3 variables,additional amount of WR would be 0.61%and 0.52%?2.07%and 1.83%?0.99%and0.82%,respectively in the HRB and BRB.While negative effectiveness was given to WR by Ep,1%increase of that might lead to 3.27%and 1.98%of WR reduction in the two basins.Influence of P and Ep were evidenced remarkable to WR variation in the study area.