Research On Evapotranspiration And Water Use Efficiency In Secondary Oak Forest

Eddy covariance technique was used to measure the carbon dioxide and water vapor flux in secondary oak forest at Xiashu forest farm. Evapotranspiration(ET) and water use efficiency(WUE) characteristics were analyzed and the impacts of environmental factors on their diurnal and seanonal changes were also investigated. The main research conclusions are described as follows:1. The water vapor flux value during daytime was positive, but that in nighttime was very small or nearly zero in the whole year of 2008. Water vapor flux value in sunny days was obviously larger than that in cloudy days. Seasonal variation of water vapor flux was observed. Its value was largest in summer, second to that in spring and autumn, and smallest in winter.2. The total evapotranspiration quantity in Xiashu secondary oak forest was about 709mm in 2008, accounting for 66% of all-year precipitation (1067mm). The forest evapotranspiration mainly occurred during the growing season (from May to October). The amount of ET in that period was about 533mm and make up 75% of total evapotranspiration in the whole year.3. The factors influencing half-hourly water vapor flux were different in various weather conditions. Net radiation, soil heat flux, soil temperature, air temperature, wind velocity and saturated vapor pressure deficit(VPD) were important environmental factors in sunny days, and net radiation was the most important factor. On the other hand, soil temperature, net radiation, soil volume water content, VPD, soil heat flux and wind velocity were important environmental factors in cloudy days, and soil temperature was the most important factor. The factors affecting daily ET in the growing season were mainly net radiation, air temperature, VPD and soil volume water content.4. Diurnal WUE variation processes were similar from May to October and presented the change trend that WUE was larger in morning than that in afternoon. Monthly average WUE did not have big difference in the growing season. The average WUE value in the growing season was 7.85±0.55 mg.g-1.5. WUE increased with the increase of net radiation, howeveer when net radiation was larger than 450W.m-2, WUE would decrease as net radiation increased. WUE also increased with the increase of air temperature, but it would decrease as air temperature increased when air temperature was larger than 25℃. There existed an obvious negative relationship between saturated vapor pressure deficit and WUE.