| Eddy-Covariance (EC) and sap flow method are employed in this study to investigate the energy balance, water budget and CO2 flux from the aspect of soil-vegetation-atmospheric system. The study site is located in Junshan district of Yueyang city, Hunan province. The primary goal is to find out the changes of the ecosystem and environmental factors, calculate the value of water vapor flux, the heat flux and CO2 flux. These will provide the basic to scientific and reasonable management and give full play to the ecological effect of the snail control and schistosomiasis prevention forests ecosystem. Main results and conclusions are the following:The daily variation regularity of energy balance components in different seasons shows signal peak curve but the ratio of energy partitioning in different components are not same in different time scale. The annual net radiation was 2630.50 MJ/m2 in 2009. The latent heat flux was 77% of the net radiation. The sensible heat flux was 16.71% of the net radiation. The soil heat flux and others only took about 6.29% of net radiation. The soil heat flux is negative, which means soil releases heat. The energy closure level was high in 30min scale, but there still had 21% energy missed.Soil moisture has the feature of spatial and temporal scale, and the difference of soil water content in different soil layers is significant. According to the basic situation of the plot, it can be divided into three levels in vertical direction. It also can be divided into three stages in time scale:water collection stage, water-falling stage and water steady stage. Soil water content is influenced by three typical factors (Vi, Ui) (i=1,2,3). Stepwise regression analysis shows: there are differences in influencing factors of soil moisture in different depth. Generally speaking, soil moisture is mainly influenced by air temperature, vapor pressure deficit, soil temperature, rainfall precipitation and radiation. The square of coefficient in regression equation is becoming smaller and smaller with soil depth increasing. It is suggested that the influence of environmental factors on soil moisture in deep soil layer is getting smaller. Vegetation transpiration is estimated in the plant scale and the stand scale. In the plant scale, there are differences of sap flow velocity in different growth stages, different weather conditions, different directions and different height. In sunny days, diurnal variation of the sap flow velocity of Populus has regular single-peak curve, which is high in the daytime, but was low at night. In wet days, daily variations of Poplar sap flow velocity are irregular and the value is low. It was close correlation between rate quarterly variation and precipitation timely distribution. The changing process of transpiration closely related to the time distribution of rainfall. The daily transpiration in seasonal scale shows multi-peak distribution.Water vapor flux in this ecosystem was positive in almost all seasons in 2009, which showed the ecosystem was water sink. Water vapor flux in summer was higher than in spring and autumn, and the water vapor flux in winter is the lowest. The evapotranspiration was 928.175mm, which was 86.75% of evapotranspiration in 2009. The influence of environmental factors on Water vapor flux was different on sunny days and cloudy days. Stepwise regression analysis shows:the water vapor flux in sunny days was affected by net radiation, vapor pressure deficit, wind speed and soil temperature in 5cm, while in cloudy day is net radiation, wind speed and soil temperature in 5 cm.There were obvious differences of the feature of CO2 Flux in seasonal scale:CO2 Flux in summer was higher than in spring and autumn, while it was the lowest in winter. The changes of CO2 in the day time showed signal peak curve. After the sunrise, the value of CO2 flux changed from positive to negative, which means the forest is a carbon sink, while the forest turned into a carbon source before the sunset. Without the photosynthesis at night, the forest releases CO2 through respiration. There were differences in diurnal variation of CO2 flux on sunny day and on cloudy day. The value in sunny day was higher than in the cloudy day, and it was more regular. The value in day time was negative during the growing season, and the forest absorbs CO2, while it was positive during non-growing season, and the forest releases CO2. The value of NEE, Re and GEE in 2009 were-890.12 gC/(m2·a),807.82 gC/(m2·a)和-1697.95 gC/(m2·a) respectively. Water use efficiency of the snail control and schistosomiasis prevention forests ecosystem was affected by many ecological factors. Its average value in the spring was higher than in the summer and autumn, while it the was lowest in the winter.
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