Study On Groundwater Circulation Mechanism And Dynamic Prediction Of Lake Level In Hongjiannao Basin

Hongjiannao Lake is the largest desert freshwater lake in China,and its wetland is a natural habitat and important breeding ground for the endangered species--the Relic Gull.In recent years,the area of Hongjiannao Lake have gradually reduced may be due to frequent human engineering activities and unreasonable development and utilization of water resources.Meanwhile,the water quality of the lake has deteriorated and the ecological environment has been seriously damaged,which threatens the survival and reproduction of endangered species.At present,many studies main focus on the area change Hongjiannao Lake and its climate driving factors,and there is very little research of groundwater circulation in Hongjiannao Basin and water level of forecasting Hongjiannao Lake.In this paper,we focus on the groundwater in Hongjiannao Basin and water level of Hongjiannao Lake,through field surveys,monitoring data,and soil and water sample test,combining chemical analysis,isotope analysis,cluster analysis,wavelet coherence,machine learning,and numerical simulation to predicting water level of lake and investigating hydrogeochemical characteristics and mechanism of groundwater circulation.The following understandings are obtained:1.The area change of Hongjiannao Lake can be divided into three phases:stationary phase?1988-1998?,shrinking phase?1998-2015?and rising phase?2015-2018?.Combining the area of Chahannao Lake and meteorological data during the same period,it can be found that the area change of Hongjiannao Lake during the stationary phase were affected by climatic factors,and that during the shrinking phase and rising phase were affected by both of climatic factors and human activities.Closely hydraulic connection and higher significance existed between water level of Hongjiannao Lake and groundwater level in the lakeshore area.Moreover,the regression model of gradient boosting decision tree revealed that evaporation has the greatest effect on the dynamic change of lake level,and the relative importance of precipitation was the smallest.Dynamic changes of predicted values and measured values of lake level were consistent,and their mean-square error was 0.004.2.The water chemistry types consisted of HCO₃-Ca type,HCO₃-Na type and Cl-Na type,and the formation of water chemistry was main controlled by water-rock and evaporation.The composition of groundwater was dominant by the halite,gypsum and fluorite,and most of the dolomite and calcite were precipitated or reactive minerals in the groundwater environment.The cluster analysis of SOM revealed that the seasonal changes of groundwater chemistry weren't large during rain and dry season,and nine groundwater samples had obvious seasonal changes.3.The Quaternary groundwater was mainly recharged by atmospheric precipitation and had a fast refresh rate.Groundwater was controlled by evaporation concentration and laterally recharged the rivers and lakes in groundwater circulation,whose depth was shallow with the maximum of 76 m.The Cretaceous groundwater was older and had a slower refresh rate.Groundwater was recharged by precipitation and the lateral flow from the basin boundary,and the water-rock action was strong in groundwater circulation.The Cretaceous groundwater was eventually drained into the Hongjiannao Lake due to groundwater runoff from west to east was obstructed by Jurassic strata.4.Based on the characteristics of groundwater circulation,we constructed a numerical model of groundwater system by FEFLOW software in the Hongjiannao Basin.The results of water balance revealed that the total recharge of the groundwater system was 33464.44×10⁴ m³,the total discharge was 13807.8×10⁴ m³,and the groundwater discharge into the Hongjiannao Lake in the simulation period was546.2×10⁴ m³.