Spatial Variations In Evapotranspiration Over East Asian Forest Sites. I. Evapotranspiration And Decoupling Coefficient

Evapotranspiration (ET) is not only a vital component of water budget, but also plays an important role in the energy budget of the earth-atmospheric system, ultimately driving many regional and global scale climatological processes. This paper describes the ET characteristics and factors controlling ET across the 17 forest sites in East Asia (2 degrees S to 64 degrees N latitude). ET was measured using the eddy covariance technique at each site. Daytime dry-canopy data for the growing season were used in this study. Growing season mean ET gradually decreased as latitude increased, with a range of 4.4 to 1.2 mm d(-1). The growing season mean decoupling coefficient (Omega) ranged from 0.42 to 0.11 across the studied sites. At low-latitude forest sites, Omega was close to 0.50, indicating that the bulk surface was partially decoupled from the atmosphere and ET was strongly controlled by net radiation and vapour pressure deficit. At high-latitude forest sites, Omega was low (similar to 0.12), indicating that the bulk surface was well coupled to the atmosphere and ET was mainly controlled by surface conductance. The value of O was determined mainly by the ratio of aerodynamic conductance to surface conductance across the studied forests of East Asia.