Surface Evapotranspiration Characteristics And Their Responses To Climate Change And Land Use In The Huai River Basin

Huai River Basin(HRB)located in a climatic transition zone between north and south China,is a commodity grain production base in China.In recent years,with the frequency of human beings transform natural activities rises and the intensification of climate change,remarkable changes have occurred in water cycle process,which has also brought prominent problems of water resources and ecological environment,and seriously threatened the sustainable economic and social development as well as ecological security of this basin.Evapotranspiration,as an important climatic parameter,plays a pivotal role in surface water balance and hydrological cycle.Therefore,the research on the surface evapotranspiration change characteristics as well as its responses to climate change and land use in the HRB can provide crucial theoretical basis for the rational use of regional water resources.In this study,from the perspective of climate change and land use,their effects on surface evapotranspiration were evaluated respectively.On climate change,this study first discussed the spatiotemporal change trends of four kinds of evapotranspiration and determined the dominant climatic factors affecting their changes in the HRB from the regional scale.Then,analyzed the evapotranspiration characteristics of winter wheat and rice in Shouxian country from the farmland scale,evaluated and discussed the applicability of the three typical models in simulating crop evapotranspiration in this area,and determined the optimal estimation model.On the land use,this study analyzed and explored the evapotranspiration change characteristics,the land use type change trends,the evapotranspiration change characteristics of different l and use types and the correlation mechanism between evapotranspiration and meteorological factors as well as land use change of the HRB in last 14 years using remote sensing product data of MODIS(MODerate-resolution Imaging Spectroradiometer).The main results are shown as follows:(1)Reference evapotranspiration(ET_ref)showed a significant decreasing trend in the upper-middle HRB and Yi-Shu-Si River Basin,especially at the annual time scale,in growing season and summer,while a generally increasing trend in ET_ref was detected in the lower HRB,and the significance only showed in spring.The most sensitive factor to ET_refwas relative humidity(RH)in most sub-regions and most time scales,except in the growing season and summer.Based on the developed differential equation method,the dominant factor of the decreasing ET_ref was the significant decreasing wind speed at 2-meter height(u₂)in the annual time scale,spring,autumn,and winter in most sub-regions,except the lower HRB,which then shifted to significant decreasing solar radiation(R_s)in the growing season and summer.However,in the lower HRB,the significantly decreasing RH was the most dominant factor,especially in the annual time scale,growing season,and spring.(2)Pan evaporation(E_pan)significantly decreased(p<0.001)at an average rate of-8.119mm·a^-2 at annual time scale in the whole HRB,with approximately 90%of stations occupied.The values of the aerodynamic components in the Pen Pan model were much greater than those of the radiative components,which were responsible for the variations in the E_pan trend.The significantly decreasing u₂ was the first dominant factor that controlled the decreasing E_pan trend at each time scale,followed by the notable decreasing net radiation(R_n)in growing season and summer.However,the second dominant factor shifted to the mean temperature(T_a)during the spring,autumn and winter and the RH in the lower HRB,especially in growing season,spring,autumn and annual time scale.These phenomena demonstrated a positive link between the significance of climate variables and their control over the E_pan trend.(3)Actual evapotranspiration(ET_a)exhibited a significant increasing trend before 1990and then decreased significantly.However,potential evapotranspiration(ET_p)decreased significantly before 1990 and then declined slightly.During 1961-1990,except for significant increasing RH,other meteorological variables exhibited decreasing trends.The aerodynamic component dominated ET_a and ET_p trends in general.The u₂ dominated ET_a trends except for summer and growing season and also ET_p trends except for summer,when the dominant factor is R_n.During 1991-2014,T_a and RH showed distinct increasing and decreasing trends,respectively,whereas significant decline in u₂ palpably slowed.The absolute value of the radiative component was larger than that of the aerodynamic one.The dominant factor of ET_atrends shifted from u₂ to RH in spring and to R_n in autumn,winter and annual time scales.Moreover,the dominant factor of ET_p trends changed from u₂ to RH in spring,winter and annual time scales and to R_n in growing season and autumn.R_n always played a pivotal role in both ET_a and ET_p trends in summer.(4)The daily evapotranspiration of winter wheat general showed a slow decreasing change trend then increasing rapidly,while that of rice exhibited a trend of increasing first and then decreasing with fluctuation.The daily crop coefficient of winter wheat decreased at first,then increased and decreased afterwards generally,while that of rice showed a dereasing trend after increasing.In the R-K model,the optimal a and b values obtained by fitting the observation data of winter wheat in two growing seasons were 1.266 and 0.514,0.360 and0.628,respectively;while that of rice in two growing seasons were 1.166 and 1.255,1.240and 1.104,respectively.Comparing with the R-K model and calibrated AA model,the simulation result of daily evapotranspiration of winter wheat and rice estimated by the PM-K_cmodel is better.Compared with the obvious underestimation phenomenon of the R-K model,the AA model showed a general good performance due to the fitted trend line slope is all around 1.In terms of its statistical parameters,however,the difference of the simulation performance between the AA model and R-K model is little,compared with the R-K model,the simulation performance of the AA model is poor when simulating the daily evapotranspiration of winter wheat in two growing seasons and rice in 2008,but better when simulating the daily evapotranspiration of rice in 2009.(5)In the past 14 years,evapotranspiration(ET)in the HRB increased slightly in spring,but decreased in other seasons,especially in winter.The Hurst index indicates that ET projects a continuous change trend in the future.The land use types of HRB are mainly croplands,and those of the high elevation areas in the southwest and northwest are forests and grasslands.Croplands decreased at a rate of-176.2 km²·a^-1(mainly converted to grasslands and urban/build-up lands);forests increased at a rate of 40.9 km²·a^-1(transformed from grasslands);grasslands presented a W-shaped reduction with fluctuation at a rate of 35.8km²·a^-1(converted to croplands and forests);urban/build-up lands increased significantly at a rate of 138.3 km²·a^-1(transformed from croplands and grasslands);water bodies decreased at a rate of-1.38 km²·a^-1(converted to wetlands);wetlands increased significantly at a rate of43.6 km²·a^-1(transformed from grasslands and croplands);barren reduced gradually at a rate of 9.5 km²·a^-1(converted to wetlands,grasslands and urban/build-up lands).The average annual evapotranspiration of the HRB is closely related to land use types,and the evapotranspiration of each land use type has profound difference,but showing a significant downward trend generally,the corresponding magnitude is as follows:forests>grasslands>croplands>wetlands>water bodies>urban/build-up lands>barren.Water conditions(Pre reduction and RH decline)may be the main controlling factors for ET decline in the HRB.