Applying Hydrus-1D Model And Stable Isotopes To Study Wetland Water Transformation Process Of Lake Poyang

Poyang Lake is the largest fresh water lake in China.Under the influence of the five rivers and the Yangtze River,the water level of Poyang Lake presents a highly dynamic seasonal change.Poyang Lake wetland is an important wetland in China,which plays an important role in maintaining the ecological balance of the surrounding areas.However,in recent years,under the comprehensive influence of global climate change and human activities,frequent drought events have occurred in Poyang Lake.The continuous low water level threatens the health and stability of the local vegetation ecological environment.Therefore,it is of great theoretical significance to study the water transfer process of Poyang Lake Wetland and clarify the water use patterns of dominant species for regional wetland ecosystem protection.In this study,a typical wetland reed community in Poyang Lake is taken as the research object.Firstly,the numerical model HYDRUS-1D of vertical movement of soil water is used to simulate the water transfer process at the interface of soil vegetation atmosphere system（SPAC）.And the seasonal variation characteristics of water flux at different interfaces of SPAC system are analyzed.Then,with the help of hydrogen and oxygen stable isotope tracing technology,the transformation relationship of various water sources（precipitation,groundwater,river water,lake water,soil water）in Poyang Lake wetland is clarified on the scale of slope,and the water use law of wetland plant reed is further revealed.The main results are as follows:（1）In 2018,the main water supply of root zone soil at reed community in Poyang Lake wetland were precipitation infiltration and the upward water fluxes from deep soil layer.The annual rainfall infiltration is 1023 mm,which mainly concentrated from March to June,accounting for 52.7%of the total annual rainfall amount.The upward water fluxes from deep soil into root zone were801 mm,which mainly occurred from June to August,accounting for 67%of the total annual amount.The main drainage way of soil water in root layer is evapotranspiration and water leakage.The total amount of transpiration is 1273mm,the evaporation of soil surface is 140 mm,and the total amount of annual deep leakage of water in root zone is 457 mm.（2）The process of wetland soil water replenishment and drainage is significantly affected by the seasonal change of lake water regime.During the wet season,the upward water fluxes from deep soil is the main water supply,accounting for 63%of the total water supply.While during the dry,the flood and the retreating season,precipitation is the main water supply,accounting for58%～77%of the total water supply.Plant transpiration is the main drainage way of soil water（42%～77%）in the dry,flood,wet and retreating season.（3）In 2018,the LMWL of the study area isδd=7.68δ¹⁸O+8.33（n=33,R²=0.94）,and the soil water evaporation line equation isδd=6.75δ¹⁸O+5.27（n=69,R²=0.86）.During the flood season,the lake water is supplied by the precipitation and river water,the wetland groundwater is affected by the precipitation accumulation in the early stage and the river water supply in the flood season of the previous year.During the wet season,there is a close hydraulic connection among the groundwater,lake water and soil water of the wetland.The soil water in each layer exchanges frequently.The groundwater is supplied by the lake water and the river water.The river water is supplied by the precipitation,the flow in the soil and the regional underground runoff.During the retreating season,the groundwater is discharged to the river,and the river water is supplied by groundwater.The deep soil water in three hydrological seasons is supplied by the capillary rising water of groundwater.（4）Reed mainly used 0-80 cm soil water at different depths and groundwater.And the water used of reed can be flexibly converted among different water sources.From April to May,0-40 cm shallow soil water was the main water sources of reed,with the utilization rate of 59.7%～75.6%.In June,soil water at60-80 cm depth and groundwater were the main water sources of reed,with utilization rate of 32.1%and 33.1%respectively.In August,reed mainly used soil water at 0-15 cm depth and groundwater,with a total utilization rate of 84%.In September and October,the proportions of soil water used by reed from different soil layers and groundwater were similar.The utilization rate of water from each layer was 16.4%～25.9%in September and 10.5%～23.7%in October.