Experimental study of evapotranspiration from Phragmites Australis wetland in Liaohe Estuary

To study the evapotranspiration regularity over the Phragmites Australis wetland,the field experiments were carried out in a test field of the Liaohe Estuary.The evaporation container with water supply was installed to measure the evapotranspiration from the Phragmites Australis wetland,and the transpiration was measured using A PM-5 porometer.Evaporations from water surface and bare soil are compared.The results shows that the water depth is 10 cm above the soil surface,the transpiration rate presents a singlepeak curve pattern with the peak value occurring at around 14:00.While with the time progress,the transpiration rate decreases if Phragmites Australis is submerged in the water for a long time.When the water depth is below the soil surface,the transpiration rate will show a double peak pattern.The diurnal variation of transpiration exhibits a noon break phenomena at noon time (12:00) due to stoma closing.The water depth has a complicate influence on the ratio of evaporation and evapotranspiration.When the groundwater depth is about -20 to -40 cm,the ratio would be the smallest.The evapotranspiration is enhanced when the Phragmites Australis is planted in the wetland.The ratio of evapotranspiration from the open water surface with Phragmites Australis and evaporation of that without Phragmites Australis is about 2.When the water depth is below the soil surface,the ratio of evapotranspiration from the land with Phragmites Australis and from the bare soil will be in the range of 3 to 3.5.The daily evapotranspiration from the Phragmites Australis wetland varies significantly due to changing meteorological conditions.The Phragmites Australis evapotranspiration can be well correlated to climatic factors,such as the daily average of photosynthetic active radiation,the daily average air temperature and daily average relative humidity.The corresponding correlation equation can be established.The evapotranspiration from the Phragmites Australis wetland can be calculated using the equation when the observation is not available.