Simulation Study On Evapotranspiration In Seasonally Flooded Reed Wetland

Water is an important factor in maintaining the stability of wetland ecosystem in Poyang Lake.However,in recent years,wetland hydrological characteristics have been changed because of human activities and the climate change.As for wetland,evapotranspiration is the main loss of water and energy in wetland,which affects water balance.Hence,simulating the response of evapotranspiration to different weather,soil,vegetation and water conditions is meaningful for understanding water balance and water movement processes,as well as maintaining the stability of wetland ecosystem in Poyang Lake.In this paper,the study site is a reed wetland in Poyang Lake National Nature Reserve.With the meteorological,hydrological,soil and vegetation data in 2013,Hydrus-1D software is used to construct one-dimensional water transport model to simulate the response of evapotranspiration to different radiation,relative humidity,wind speed,groundwater depth,groundwater periodical variation,LAI(Leaf Area Index)conditions,in order to explore the influence of different factors on evapotranspiration.Furthermore,the principle of water balance and the simulation results of SPAC(Soil-Plant-Atmosphere Continuum)system can be used to explain the variation of evapotranspiration.The results are as follows:(1)Evapotranspiration and the water movement processes are all affected by radiation,wind speed and relative humidity.The evaporation of soil surface increases with radiation.When soil water is enough,transpiration increases with radiation,on the contrary,the stomata of vegetation will reduce or close,which weakens the transpiration.Similarly,transpiration and the evaporation of soil surface increase with wind speed when the soil water is enough.However,evapotranspiration is weakened when the wind speed is too big,because the strong wind makes the leaves protect themselves and increases the resistance of soil surface.Transpiration,evaporation of soil surface and evapotranspiration are all negative correlated with relative humidity.This paper takes July and August as the simulation period,and the results show that soil water is enough in July,evapotranspiration is not limited by water,and it increases with radiation and wind speed.But soil water is insufficient in August,the transpiration will be weakened when radiation exceeds 18MJ/(m~2·d),and the evapotranspiration will be weakened when wind speed exceeds 270km/d.(2)With the increase of groundwater depth,the supply to upper soil and vegetation is decreasing,correspondingly,evapotranspiration is decreasing.But when groundwater depth is lower than 3m,evapotranspiration will not be affected by groundwater,because the supply to the upper layer soil is zero.When the soil is not flooded,the change of groundwater depth has little effect on the evaporation of soil surface.(3)Without changing the potential evapotranspiration conditions,a peak of evapotranspiration always appear in July 13th,and this time is affected more by cliamte and vegetation conditions,it is not decided by groundwater when groundwater depth is less than3m.With the change of the cycle of groundwater,the results show that the evaporation of soil surface is mainly affected by the peak of groundwater if it appears before July,on the contrary,if the peak appears after July,it mainly affects transpiration.In vegetation growing season,the reduction of groundwater level decreases evapotranspiration significantly,however,this effect is not obvious in other seasons.The evaporation can responses quickly to the changes of groundwater level,but transpiration delays.The reduction of groundwater cycle can increases the total evapotranspiration,what's more,the change of the water content in the root layer is more obvious.However,if the groundwater cycle is too small,transpiration can't response quickly,and the trend of curve is not obvious.(4)Increasing the potential evapotranspiration can increase the actual total evapotranspiration.The function of the groundwater peak to evaporation is weakened,but the function to transpiration is enhanced.Whether or not in the vegetation growing season,the reduction of groundwater level will significantly weaken evapotranspiration.Compared to the previous,evapotranspiration and transpiration can response to groundwater level more obviously and quickly.Affected by the climate and vegetation conditions,a peak of evapotranspiration will also appears in summer,however,after increasing the potential evapotranspiration,the time of the peak is less affected by climate and vegetation conditions,and the groundwater level can advance or delay it for few days,but this peak still appears in June or July.What's more,aftering increasing the potential evapotranspiration,the water content in the root layer is affected by the change of groundwater more obviously.(5)Transpiration and evapotranspiration increase with LAI,however,the growth rate is decreasing gradually,and the curve becomes flat.Evaporation decreases with LAI.If the study site is mudflat,the total evapotranspiration is 106.38mm.If the study site is water,the total evapotranspiration is 186.8mm.As for the wetland which is covered by vegetation,when LAI is 0.5,it's total evapotranspiration is less than mudflat,only when LAI is no less than 1,it's total evapotranspiration is more than mudflat,and when LAI exceeds 2.5,it's total evapotranspiration is more than water.