Groundwater-Lake Interactions And Associated Responses To Changing Environment In The Typical Floodplain Area Of Poyang Lake

Poyang Lake is the largest freshwater lake and an important floodplain wetland in China,which is a gulping,seasonal shallow lake.The hydraulic connection between lake water and groundwater of Poyang Lake floodplain wetland is close,and the large change of lake water level has some impacts on the groundwater level of floodplain wetland.2022 extreme drought event led to the return of dryness of Poyang Lake in flood season and extreme dryness in dry season,which has some impacts on the groundwater environment of Poyang Lake floodplain wetland.On this basis,Jiangxi Provincial Government proposed to build a hydraulic project to improve the increasingly dry hydrological phenomenon of Poyang Lake.The study of groundwater in the floodplain of Poyang Lake under the extreme drought and the proposed hydraulic project is a further extension and supplement to the current work on the assessment of surface hydrology and hydrodynamics in the lake area,which helps to comprehensively understand the comprehensive hydrological impacts of the extreme drought and the construction of the hydraulic project.Therefore,this paper takes Poyang Lake floodplain wetland as the study area,focuses on the response of Poyang Lake floodplain wetland to different changing environments,carries out the analysis of the influence of changing environments on lake water-groundwater in Poyang Lake floodplain wetland,constructs a numerical model of groundwater dynamics,reveals the characteristics of groundwater dynamics in Poyang Lake floodplain wetland,and explores the influence of scenarios such as extreme drought and proposed Poyang Lake hydraulic project on groundwater in Poyang Lake floodplain wetland.This study emphasizes the importance of the changes in hydrological response between the lake and groundwater,and the results can provide scientific support for the optimal allocation of water resources and sustainable development of ecological environment in the wetland of Poyang Lake under different environmental changes,as well as provide an important basis for the relevant local management departments to carry out relevant work.The main conclusions are as follows:(1)Analysis of Lake water-groundwater interaction of Poyang LakeThe fluctuation of lake level is the key factor affecting the dynamic recharge pattern between groundwater and lake water in floodplain wetland.The groundwater recharge to the lake in floodplain mainly occurs in the low dry water level season of Poyang Lake(e.g.dry and recession water periods),while the lake recharge to groundwater mainly occurs in the medium to high water level season(rising and flooding water periods).The variability of groundwater level in floodplain wetlands is large within the year and has strong spatial variability,and the variability of groundwater level in time and space is mainly influenced by the topography and geomorphology as well as the dynamic changes of lake level together.The groundwater level flow rate in the whole study area is generally less than 0.1m/d,but during the high water period,the groundwater flow rate in the eastern part of the floodplain is larger,up to 0.3m/d.(2)The water balance equation of Poyang Lake was constructed,and the groundwater recharge and discharge amounts and percentages were determined.From the perspective of groundwater equilibrium,the groundwater in the floodplain wetland of Poyang Lake is basically kept in equilibrium during the flat water year.The main recharge sources of groundwater in floodplain wetland are precipitation recharge and lake water recharge to groundwater in floodplain wetland,which account for 50%and 40%of the total recharge respectively.The main discharge term is evapotranspiration,which accounts for 70%of the total discharge in the study area,and groundwater recharge to the lake accounts for 24%of the total discharge.In addition,the groundwater recharge-discharge relationship in the study area shows strong temporal variability,with the groundwater system receiving external recharge mainly during the spring and summer months,and discharging mainly to the outside during the autumn and winter months.(3)The impact of the 2022 extreme drought event on the groundwater in the floodplain wetland of Poyang Lake was investigatedThe extreme drought event in 2022 causes the groundwater level in the floodplain of Poyang Lake to drop significantly in the second half of the year,with the maximum water level drop up to 5 m compared with the base year.the rapid drop of groundwater level causes the groundwater flow rate to accelerate generally,with the maximum groundwater flow rate reaching 0.3 m/d,which is about three times of that in normal years.In addition,extreme droughts also lead to an increase in the overall intensity of groundwater discharge in the study area,exacerbating the degree of groundwater attenuation in the floodplain.From the annual scale analysis,the extreme drought causes a large discharge of groundwater storage in Poyang Lake(1.62×10~7 m~3),which is about 14.5 times of the groundwater discharge in the base year.(4)Predicted the impact of the proposed Poyang Lake hydraulic project on the groundwater in the floodplain wetland of Poyang LakeThe proposed Poyang Lake hydraulic project will play a significant role in raising the groundwater level in the floodplain wetland of Poyang Lake,and the intensity of the effect near the eastern main lake area(1-3 m)is significantly stronger than that in other areas of the floodplain(<0.5 m).On a temporal scale,the construction of the hydraulic project has a stronger effect on groundwater levels during the dry water period(2-3 m)and recession water period(1.5-2.5 m)periods than during the rising and flooding water periods(<1.5 m).Changes in groundwater levels also lead to a reduction in groundwater flow velocity in the floodplain and changes in groundwater flow direction during certain periods(e.g.dry and recession water periods),resulting in an east-to-west trend in groundwater flow in the floodplain throughout the year.In addition,the hydraulic project lead to a shift in the water balance of the groundwater system in the floodplain,which affects the recharge and discharge state of the groundwater system,resulting in a long-term stable pattern of groundwater recharge from the lakes.