Estimating Groundwater Evapotranspiration And Rainfall Infiltration Rates Using Fourier Transform

Evapotranspiration is the main discharge in shallow groundwater system in Northwest China,and rainfall infiltration is the main recharge for groundwater,It is important to calculate the evapotranspiration and rainfall infiltration rates to evaluate and exploit the groundwater resources in this area.The water level fluctuation is one of the basic methods to estimate evapotranspiration and rainfall infiltration rate,but estimating evapotranspiration of groundwater is often disturbed by barometric pressure effect,and estimating rainfall infiltration rate will be interfered by Lisse effect.Taking shallow groundwater system in loess area of the Shaanxi Baoji as an example,this paper studies the methods of eliminating barometric pressure effect and Lisse effect by means of Fourier transform,correlation analysis and water level prediction,in order to accurately estimate evapotranspiration of groundwater and rainfall infiltration rate.The paper has obtained the following main research results.(1)When the water level changes,it is affected by barometric pressure effect and evapotranspiration,it is possible to select the barometric pressure independently to influence the time period of water level variation(the weakest period of evapotranspiration usually from 22:00 to 6:00),the optimal barometric pressure coefficient is obtained by calculating the relation between barometric pressure change and water level change.The barometric pressure effect is eliminated in the change of water level,and the traditional water level fluctuation method is used to estimate evapotranspiration of groundwater.In this method,the accuracy of the computation can be increased by using the Fourier transform to de noise process.(2)During the rainfall infiltration recharge,the Lisse effect caused the water level to rise,but the rising value of the water level is equal to the decline of water level caused by trap gas escaping.Using this relationship,we can choose a certain period of time,so that the two can be eliminated,to eliminate Lisse effect.There are three key steps to implement this method,Firstly,eliminate barometric pressure effect during rainfall by using the above method;secondly,the end time of rainfall infiltration recharge is determined by the change rate of water level before rainfall;thirdly,according to the variation rule of water level before rainfall,the basic water level(including net lateral recharge and evapotranspiration)during rainfall infiltration is obtained by Fourier transform.Through the above changes,the rainfall infiltration recharge rate of a rain can be obtained.(3)The above method is compared with the Qi Lijun(2017)method for the same observation boreholes.The results of the two methods are similar to estimate evapotranspiration and rainfall infiltration recharge rate.But the principle of the improved method is more clearly,the calculation process is simple,and the practicability is strong.(4)When the change of water level is affected by barometric pressure effect and evapotranspiration,Fourier transform cannot separate the barometric pressure effect individually,either by periodic separation or phase separation.