Measurement of evapotranspiration in a winter wheat field

Daily evapotranspiration from a winter wheat field on the North China Plain measured by large-scale weighing lysimeter was linearly related to that measured by the Bowen ratio energy balance (BREB) technique. Soil evaporation averaged about 23.6% of evapotranspiration from the post-winter dormancy revival stage to the grain ripening stage in 1999. On clear days during winter dormancy, about half of the net radiation flux R-n was used to warm soil. During the revival stage, conductive heat flux G also used most of the incoming R-n, but the ratio of latent heat flux; E to R-n increased. During the stem-extension stage, lambdaE was about 50% of R-n; thereafter, lambdaE/R-n increased continually, but G remained less than 10% of R-n. During the ripening stage, lambdaE was almost 90% of R-n. Evaporative fraction (EF) can be expressed as a function of plant status and atmospheric boundary layer conditions. The relationship between EF and available energy under moderate air temperature and vapour pressure deficit conditions was examined for five combinations of aerodynamic and canopy conductance. Although the theoretical relationship indicates that EF should be highly correlated to soil water content, the correlation has been difficult to identify under field conditions. However, we observed that there exists a threshold value of R-n - G, above which EF is less than 1.0, and that the threshold value is lower under soil-water deficit conditions than under abundant soil-water conditions. Copyright (C) 2002 John Wiley Sons, Ltd.