Study On Effects Of Climate Change And Human Activities On The Vegetation Water Use Efficiency In The Tao River Basin

The interwoven impacts of climate change and human activities have increased the complexity and variability of the river basin eco-hydrology dramatically.Understanding roles that the two factors play in the basin eco-hydrological process is of great significance for sustainable utilization and rational protection of basin resources and environment.In this study,we selected the Tao River Basin(TRB)as the study area.Diverse regions located in the upper,middle and lower reaches were set as for the analysis and comparisons.To do this,we collected and organized the reanalyzed hydrometeorological data and the remote sensing based ecological data across the basin from 1980 to 2015.Spatial and temporal variation of the key hydrometeorological factors and the underlying conditions(i.e.Land Use/Cover Change,LUCC;the Gross Primary Productivity,GPP)of the basin were analyzed,the effects of climate change and human activities on the main and functional variables of the basin's ecohydrological system,such as Actual Evapotranspiration(AET)and GPP,were quantitatively separated.Water Use Efficiency(WUE)was selected as the substitute index to couple carbon-water cycle for analyzing comprehensive responses of ecohydrological processes to climate change and human activities.Main conclusions are listed as the follows.1.Remarkable variability of hydrometeorological series in time and space were found during the time period from 1980 to 2015.The temporal variability of precipitation was largest,followed by radiation,and that of the air temperature was the smallest.The temporal uniformity of the temperature in the upper reaches was evidenced the highest,followed by that in the middle reaches,the uniformity of the radiation in the lower reaches was the lowest.Increases were found for many hydrometeorological variables such as precipitation,temperature,radiation,and AET,with the largest variation amplitude of precipitation and AET,followed by radiation and the air temperature.River discharges in the three diverse regions showed decreasing trends except in the downstream,mainly occurred in decades of the 1980 s and the 2010 s.2.The LUCC in the TRB was greatly affected by implementations of policies and projects like agricultural expansion,grain for green,natural forest protection,and so on.The land use intensity in the upper and lower reaches presented increasing,while in the middle reaches there was a fluctuating variation.Land use transfers in the basin were mainly occurred among cultivated land,grassland,woodland and built-ups.Area of built-up showed a continuous expansion across the whole TRB.The variation trend of GPP in the three diverse regions were found primarily same.GPP values were averaged as the highest in the middle reaches while the smallest ones were found in the lower reaches.GPP values of different vegetation were various and numerically ranged in an order of cultivated land > forestland > grassland in the upstream.In the middle and lower reaches,which was in an order of forestland > cultivated land > grassland.The monthly variation of GPP among different land cover followed a single-peak curve and the highest value in a year was found in July.The vegetative GPP in the TRB was found rising during the time period from 1982 to 2015.3.Effects of climate change and human activities on AET and GPP were found remarkably different in the three diverse regions.Climate change positively dominated AET's increase across the whole basin.Human activities functioned less in the upstream but more in the middle and lower reaches,when compared before and after the changeable breakpoint.For GPP variations,it was climate change that caused more GPP in the upper and lower reaches,while human activities negatively led a decrease of GPP in the middle reaches.Human impacts became stronger when compared before and after the changeable breakpoints of GPP series in all the three diverse regions.4.WUE was mainly controlled by AET in the upper and middle reaches,while in the lower reaches,it was GPP.The variation of WUE was dominated by climate change,and the contribution rates in the upper,middle and lower reaches quantitatively resulted in numbers of-0.76,-0.76 and 0.65,respectively.Effectiveness of human activities on WUE's variation spatially increased from the upper reaches to the lower ones,during the time period from 1982 to 2015.