Application Of Pilot Point Method In The Inversion Of Hydrogeological Parameters In Zhongwei Plain

Zhongwei Plain is located in northwest China,with arid climate,drought and little rain,intense evaporation and water shortage.Zhongwei Plain is an important economic development zone and major grain producing area in Ningxia.With the development of urbanization,the water shortage is serious and the contradiction of water supply is increasing day by day.Therefore,finding out the aquifer parameters in Zhongwei Plain is of great practical significance to realize the effective utilization of water resources and ensure ecological security in Zhongwei area.Hydrogeological parameters of aquifer play an important role in groundwater resources calculation and prevention and control of groundwater pollution.Pumping test is often used to determine hydrogeological parameters of aquifer in field hydrogeological work,but it takes a lot of time and costs a lot.In order to solve this problem,this paper introduces PEST algorithm into the field of groundwater parameter identification based on the simulation-optimization theory framework,and combines it with the groundwater flow simulation software Visual MODFLOWFlex to establish the groundwater flow numerical model under certain conditions,and use it to solve aquifer parameters.An ideal model of random two-dimensional confined aquifer is established by using Visual MODFLOW Flex software.By understanding the permeability coefficient field in advance,the initial permeability coefficient field is obtained by assigning the initial value to it.Using PEST software,the calculated water level of the model is as close as possible to the actual observed water level,and the estimated field of permeability coefficient is obtained after inversion.RMSE(root mean square error)between the initial permeability coefficient field and the actual permeability coefficient field is denoted as R1,which indicates the prior information accuracy.RMSE(root mean square error)between the estimated permeability coefficient field and the actual permeability coefficient field is recorded as R2,and the smaller the R2 value,the higher the inversion accuracy.When the proportion of observation wells in the total area of the study area increases,the corresponding R2 value first decreases in turn and finally remains stable;With the increase of the distribution range of the pilot points,the R2 value decreases first and then reaches a stable value,and the number of calls to the MODFLOW.exe program and the number of optimization iterations also decrease.With the increase of R1 value,the corresponding R2 value also increases,and the inversion accuracy will decrease.With the increase of the distribution range of observation wells and pilot points,the inversion accuracy will improve and finally reach basic stability;At the same time,with the increase of the distribution range of pilot points,the gradient descent rate of the objective function increases,and the number of calls to MODFLOW.exe software and the number of optimization iterations decrease The closer the permeability coefficient field to be estimated is to the real value,the higher the inversion accuracy will be.Meanwhile,the Visual MODFLOW Flex software was used to establish a three-dimensional groundwater unsteady flow numerical model of Zhongwei Plain,and the pilot point method was used to identify and verify the model.The simulation results show that the main sources of groundwater replenishment in this area are field irrigation infiltration and rainfall infiltration replenishment;the main sources of discharge are artificial mining,evaporative discharge and discharge to the Yellow River.