Estimating Groundwater Evapotranspiration And Rainfall Infiltration Using Groundwater Water-level Fluctuation Method

Evapotranspiration is the main discharge in shallow groundwater system in arid and semi-arid areas,and rainfall infiltration is the main recharge for groundwater.They are both the important terms in groundwater balance,and play a key role in groundwater resources evaluation and exploit reasonably,vegetation protection and other fields.In this paper,the study area is loess area at the southern bank of the Wei River,in Baoji,Shannxi.7 representative observation boreholes were used observing groundwater level,and a database of high frequency groundwater level for one year was established.Then on this basis,the groundwater level variations regularity was analyzed,the variation of groundwater level caused by the net lateral recharge and barometric effect were separated one by one using water-level fluctuation method,and Lisse effect was removed,thereby the methods of estimating evapotranspiration and rainfall infiltration.The accuracy of the two methods was verified by water balance method,in the end.The main conclusions of this paper are as follows:(1)the tendency of the groundwater level changes caused by the net lateral recharge per day is “bigger in summer,and little in winter”,the biggest value is 4.96 cm/d in September,the minimum value is 1.41 cm/d in January,the net lateral recharge per day is steady in spring.(2)The barometric effect in study area is obvious,is the key factor influencing the water level fluctuation.There exist delay feature of the groundwater level responses to barometric pressure,the lag times of water level in 7 observation wells are in the range 0 to 80 minutes.The confidence interval of barometric effect coefficient is(-0.558,-0.442).In this study area,the groundwater level changes caused by barometric effect is bigger in the area that groundwater level is shallow,little in in the area that groundwater level is deep,the influence depth of groundwater level about 3.5 m in study area.(3)Annual variation of evapotranspiration is reflected by results by 7 observation hole,can be simply described as “ bigger in summer,little in spring and bigger in autumn,little in winter”,the values of evapotranspiration are in May,June,July and August.(4)There exists response-delay effect of groundwater level response to rainfall events.The delay time is related to the size of the rainfall intensity and groundwater level depth.The fist rainfall after the long drought without rain,the delay time is logger.The bigger of the groundwater level depths,the longer of the delay time.The stronger of the rainfall the shorter of the delay time.(5)Average annual coefficients of rainfall infiltration of 6 efficient observation wells respectively are 0.37,0.29,0.64,0.39,0.43 and 0.23.The rainfall infiltration coefficients of the first rainfall after long drought without rain is very small or is 0,they will be bigger with the continued occurrence of rainfall.Rainfall infiltration recharge to groundwater is continuous,and the continuous time is in range of 5-40 days in the period of study.