| The regional climate of the Loess Plateau(LP)has been experiencing a significant change under the background of global warming.Meanwhile,environment construction in this area has greatly changed the conditions of the underlying surface.Climate change and underlying surface change have profoundly affected the hydrological processes in this area.Scientifically assessing the impact of climate change and underlying surface change on the hydrological processes,and rationally predicting the future climate and runoff can provide theoretical support for environment construction and water resources management for the LP.In view of the current problem in the field on influence of climate change and underlying surface change on hydrological processes in the LP,this paper selects the Wuding River Watershed(WRW)in the LP as the study area.We adopted the Budyko model as the theoretical basis and tools,and utilized long-term hydro-meteorological and remote sensing data.First,this study analyzed the spatiotemporal changes of vegetation cover and its driving force.Secondly,we explored the characteristics of climate change and hydrological processes change,and further assessed the impact of climate change and vegetation cover change on runoff and evapotranspiration in the WRW.Then we investigated the interrelationship among climate,vegetation cover,runoff,and evapotranspiration.We used the region climate model,called Weather Research and Forecasting Model(WRF),to simulate the historical climate of the LP.We also used the downscaling model to downscale output of general circulation.At last,we predicted the future trend in the main hydro-meteorological variables of the LP.The following main conclusions are drawn:(1)The spatiotemporal variation of vegetation cover and its driving force in the WRW was analyzed.The vegetation cover in the WRW has experienced a significantly upward trend over the period of 1982-2011.The vegetation cover change has two stages with completely different features,and the abrupt change point was around the year of 2000.Before the abrupt change point,vegetation changed slowly but has larger amplitude.However,vegetation increased significantly after the abrupt change point.The vegetation change has distinct change features at seasonal scale,of which the summer and fall seasons have remarkable growth.These two seasons made dominant contribution to the vegetation cover growth.The area with significant uptrend occupies 83% of the total watershed.The influence of climate on vegetation cover are becoming weaker,while that of human activities are becoming stronger.Due to human activities,the relationship between climate and vegetation cover has turned into complex.The reforestation activities have become the main driver of vegetation cover change at a short-term temporal scale.(2)A new sensitivity method based on the Budyko model was introduced to determine the time needed to reach a water balance status in the WRW.With this method,we constructed a semi-empirical model to estimate the Budyko parameter including two explanatory variables.One is the vegetation cover factor,and another is the rate of relative infiltration.Based on the semi-empirical model,we separated the contribution of climate change and vegetation change to the streamflow change.The streamflow of the WRW experienced a remarkable downward trend,the change process of which has three distinct stages divided by two abrupt change point.Moreover,the sub-basins of the WRW have the similar change features.The first abrupt change points distributed around the year of 1970 and the second change points distributed around the year of 1999.The relationship between precipitation and streamflow in the WRW become weaker.The dominant influence of precipitation become weaker and that of other factors become stronger.The relationship between precipitation and streamflow become more complex.Both the SPI and SPEI of the WRW tell that the drought in the watershed become more frequent and severer.The performance of SPEI is better than SPI;SPEI can realistically reflect the water condition.The two explanatory variables can largely estimate the Budyko parameter accurately at multi-year scale.The climate change is the dominant factor contributing to the decrease of streamflow and its contribution is 76% averagely.The vegetation also has a relative high contribution(24%),and its contribution is becoming higher.(3)The impacts of climate change and vegetation cover change on the coupling hydrothermal relationship in the WRW was explored.We derived a new elasticity method based on the Budyko model to assess changes in evapotranspiration.The wind speed,relative humidity,and net radiation have experienced significant downward trends over the period of 1980-2011.The decrease in the net radiation is the main reason for the remarkable downward trend in potential evapotranspiration.The actual evapotranspiration in the WRW generally display a significantly downward trend,but it has increased after the year of 2000.The evapotranspiration ratio of the WRW continues to increase,which lead to the decrease of streamflow.Our elasticity method can estimate changes in the actual evapotranspiration to a large extent.The coefficients of actual evapotranspiration to precipitation,net radiation,temperature,wind speed,relative humidity are 0.79,0.13,0.06,0.05,-0.10,respectively.The reforestation in the WRW after 2000 made the actual evapotranspiration increased,meanwhile the water storage of the watershed is decreasing.The combined effect of precipitation and temperature contributes to the large variation of winter albedo.The reforestation can increase the energy received by the watershed through making the albedo become little,which also compensate the diming of the WRW.(4)The impacts of resolutions and physical parameterizations on the climate simulations over the LP were investigated.An optimal WRF model was used to simulate the long time series of the climate in the Loess Plateau.The horizontal resolution can affect the simulation of precipitation.A coarser resolution cannot capture the complicated topographic characteristics,which results in a large precipitation output.The 10 km and 5 km resolution have a better performance of simulation of the precipitation.WRF simulations for precipitation over the LP were most sensitive to cumulus schemes,moderately sensitive to planetary boundary layer schemes,and less sensitive to microphysics schemes.Through comparisons among three categories of parameterization schemes directly associated with precipitation processes,our results indicate that the combination of the Dudhia shortwave radiation scheme,the RRTM longwave radiation scheme,the MM5 Monin-Obukhov land surface scheme,the CAM5.1 microphysics,MYNN2.5 planetary boundary layer,and Kain-Fritsch cumulus schemes can capture precipitation in the LP quite well.The WRF model not only adequately captured the spatial distribution of precipitation in the LP but also reproduced its magnitude and variability at different time scales.Moreover,the WRF model generated a realistic simulation of extreme precipitation,which causes sub substantial soil erosion in the LP.Errors in the WRF precipitation simulations resulted mainly from those in the simulated extreme precipitation.The complicated topography can influence the simulation.The realistically representation of the water vapor transport is the main reason for the well performance over the climate simulation over the LP.(5)The future trend of the main hydro-meteorological variables over the LP were predicted.We also predicted the streamflow of the WRF in the future based on the elasticity method.The applied downscaling method can reproduce the spatiotemporal distribution of the historical climate of the LP.The precipitation over the LP will increase in the future with a rate of 6.6 mm/decade,and the variability in the precipitation will become larger.The precipitation will increase large at north of the Loess Plateau and increase little at south of the Loess Plateau,almost of the Loess Plateau will have an increase of 50 mm.The increase in some regions will reach at 150 mm.The temperature will significantly increase at the end of this century.The increase rate is 0.32 ?/decade.The area having an obvious upward trend distributes at north and south of the Loess Plateau,but the temperature has little growth at the mid part of the Loess Plateau.The growth of the temperature will large than 2? at almost area of the Loess Plateau.In addition,we highlighted the prediction of climate over the WRW.The precipitation and temperature of the WRF will significantly increase in the future.Compared to the base period of 1979-2005,The precipitation will decrease 10 mm over the period of 2011-2040,and increase 29 mm and 67 mm over the periods of 2041-2070 and 2071-2100,respectively.Moreover,the streamflow of the WRF will decrease 16% over the period of 2011-2040,and increase 7% and 30% over the periods of 2041-2070 and 2071-2100,respectively.This paper analyzed and discussed the influence of climate change and vegetation cover change on runoff and evapotranspiration in WRW of the LP.We proposed a new method for determine the time needed by a watershed to reach the water balance status.We also expanded the influence factors of Budyko parameter at the long-term scale.In addition,an elasticity method was derived to estimate the changes in actual evapotranspiration.We found an optimal parameterizations combination in the WRF model,and it can realistically reproduce the historical climate of the LP.Combining the downscaling model and Budyko theory,the future trend of the main hydro-meteorological factors in the LP were predicted.The research results can provide scientific basis for the sustainable ecological environment construction and theoretical support for the rational management of regional water resources in the LP. |
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