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Research on Evapotranspiration and Energy Budget of Phragmites australis Stand in Oasis

SI Jian-hua1,2, FENG Qi2, ZHANG Yan-wu3, CHANG Zong-qiang2, ZHANG Kai1,2(1.Institute of Arid Meteorology,China Meteorological Administration,Lanzhou 730020,China;2.Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou 730000,China;3.National Climate Center,China Meteorological Administration,Beijing 100081,China)  
Energy exchange is one of the most important processes in ecosystems,because it affects variables,such as temperature,water transport,plant growth and productivity.The main components of surface energy balance are the net radiation,heat stored in water and soil,sensible heat flux and latent heat flux(or evapotranspiration).Evapotranspiration(ET) in vegetated wetlands is frequently the largest consumer of the incoming energy and has a great influence not only on the energy distribution but on water conditions.Previous studies on Phragmites australis stand energy fluxes reported the influence of meteorological factors,such as solar radiation,vapor pressure,wind speed,etc.Limited information,however,is available on the influence of related plant variables,such as leaf area index(LAI).Therefore,it is very important to study the regional energy fluxes in oasis. This study is designed to investigate the energy fluxes of Phragmites australis stand in oasis and quantify the components of the surface energy balance in this important community using Bowen ratio equilibrium energy method.In order to gain a better understand of the energy partitioning,the characteristics of evapotranspiration and energy balance are analyzed.The results are as follows:(1) Under the clear and cloudless term,the daily variation of evapotranspiration rate changes regularly.In the morning,air temperature goes up gradually with the increase of solar radiation,relative humidity is decreased,and evapotranspiration rate is increased gradually.At 12:00,the evapotranspiration rate reaches its highest value.The evapotranspiration rate is decreased thereafter till next day with the decrease of light intensity and temperature and the increase of relative humidity.The average evapotranspiration rate was 0.3~0.4 mm/h during the period from June to August,0.2~0.3 mm/h in September,but 0.1 mm/h only in October.The mean daily amount of evapotranspiration of Phragmites australis was 3 mm in June and July,1.5 mm in August,but 0.5 mm only in September.The values of total monthly evapotranspiration of Phragmites australis during the period from June to September were 68.03 mm,81.29 mm,56.03 mm and 32.09 mm respectively.There was a distinct seasonal variation of evapotranspiration with different growth stages of Phragmites australis,the total evapotranspiration was 252.5 mm;(2) Groundwater is an important source of evapotranspiration when precipitation is not available for the evapotranspiration,and it plays an important role in evapotranspiration in arid oasis.Net radiation and air temperature are the dominant factors affecting evapotranspiration all the year round,and wind speed accelerates obviously evapotranspiration in plant growth season.There is a good correlation between evapotranspiration rate and net radiation,the correlation coefficient is as high as 0.838,and the daily evapotranspiration of Phragmites australis is in a downtrend with drawdown of groundwater level;(3) The peak of average sensible heat flux of Phragmites australis varies in a range of 250~300 W/m2,the average maximum latent heat flux varies in a range of 120~230 W/m2,and the average maximum soil heat flux is generally 20 W/m2.Sensible heat flux plays a significant role in energy exchange process in June.Sensible heat flux and latent heat flux were up to 52.7% and 42.6% of net radiation respectively.Latent heat flux is dominant in July,latent heat flux and sensible heat flux were up to 55.0% and 40.4% of net radiation,they were up to 60% and 30% of net radiation in September,respectively,and soil heat flux was about 8% only.
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