Study On The Evapotranspiration And Water Balance Of Lake Basin In The Hinterland Of Badain Jaran Desert

Water balance of the lake basin in the hinterland of Badain Jaran Desert is an important part of exploring the mechanism of lake maintenance.Water balance studies include discussions of important components in the water cycle such as lakes,groundwater,precipitation,and evapotranspiration,in which obtaining accurate evapotranspiration in desert areas is one of the key and difficult issues.In this study,based on the comprehensive long-term meteorological and hydrological observation in10 lake basins of the desert hinterland,the groundwater dynamic method and groundwater evaporation empirical model were used to estimate the evapotranspiration rates in various vegetation covers and the groundwater evaporation rate of the bare land.According to the previous experimental results of salinity effect on water evaporation,a regression model of water evaporation and TDS(Total Dissolved Solids)was established,and the annual evaporation rate of the 105 permanent lakes in the Badain Jaran Desert was calculated using this method,respectively.Meanwhile,in combination with vegetation cover and meteorological data,the main influencing factors and control mechanisms of water surface evaporation and groundwater evapotranspiration in the Badain Jaran Desert were discussed.Finally,based on high-resolution images and DEM data,the water surface area,vegetation coverage area,and bare land with evaporation area of 105 permanent lakes were extracted respectively,and the total amount of water surface evaporation and groundwater evapotranspiration in various vegetation cover and groundwater evaporation in bare land were estimated.The total amount of the true evaporation and the magnitude of groundwater recharge in these 105 lakes were obtained.The main conclusions in this study can be listed as follows.(1)The observed series indicated that diurnal groundwater level(GWL)fluctuations were consistent with the vegetation growing season,which was driven by groundwater consumption by phreatophytes.The amplitudes of diurnal GWL fluctuations showed marked differences among the sites,which was attributed to vegetation type and vitality.On a seasonal scale,the air temperature was an important external factor affecting the diurnal GWL fluctuations by controlling the vegetation phenology and groundwater consumption.(2)The estimated groundwater evapotranspiration was associated with vegetation type and exhibited a degree of association with the vegetation growth stage.The groundwater evapotranspiration of the three categories of sites were 600~900 mm for Phragmites australis-covered sites,600~650 mm for Achnatherum splendens-covered sits,and 500~650 mm for Nitraria tangutorum and Achnatherum splendens-covered sites.Depth to groundwater table and potential evapotranspiration are both important factors controlling the groundwater evapotranspiration rate,while their influence varied among observation sites.(3)Within a certain TDS range of water body,the evaporation of the water surface in the desert hinterland decreases with the increase of TDS.The temperature,wind speed and sunshine hours were all important factors affecting the evaporation rate of freshwater and brine,but the coupling effects of the different TDS water evaporation and meteorological factors are different between freshwater and brine.The evaporation of freshwater is more sensitive to wind speed,while the dominant factor affecting the evaporation of brine water is temperature.(4)Based on the regression model of water TDS and evaporation,the annual evaporation rates of 105 permanent water lakes are in the range of 1200~2000 mm,and most of them are in the range of 1200~1500 mm.This method is convenient and reliable,and has certain practical significance for the calculation of water balance of lakes with high water salinity in such as desert hinterlands that lack field observation.(5)The estimated total amount of water discharge in the 105 perennial lakes,that is,the true evaporation is 5.65×10~7 m~3/a,and the total amount of groundwater recharge is 5.45×10~7 m~3/a.The amount of water evaporation and groundwater evapotranspiration is of the same order,and the water consumption of evapotranspiration under the vegetation cover area is less than that.(6)The total amount of groundwater evapotranspiration in the Sumujilin Lake Area,including vegetation coverage and bare zone with shallow groundwater depth,accounts for 12.51%of the total groundwater drainage in the lake area,which proves that groundwater evapotranspiration is one of the key items of water cycle of Badain Jaran Desert lakes,and could not be negligible in the water balance studies.Under the same atmospheric conditions,the total amount of groundwater evapotranspiration with vegetation cover in the Sumujilin Lake Area during the growing season is 16%to 18%more than that without vegetation coverage.Therefore,using only bare land groundwater evapotranspiration formulas or models to calculate the groundwater balance of the lake basin would produce large error.