Comparison of methods for estimating evapotranspiration rate of dry forest canopy: Eddy covariance, Bowen ratio energy balance, and Penman-Monteith equation

A multiyear, multitechnique study was conducted to estimate latent heat flux within a temperate mixed forest of broad-leaved and coniferous trees of Changbai Mountains in northeastern China. Three different methods were used, including eddy covariance (EC), Bowen ratio energy balance ( BREB), and Penman-Monteith ( PM), during the growing seasons ( May to September) of 2003-2005. BREB- and PM-based latent heat fluxes calculated with micrometeorological variables of different reference levels were analyzed and those of the topmost reference level were the most similar to the EC measurements. The latent heat fluxes ( LE) estimated with the three methods showed similar diurnal and seasonal courses. The maximum of monthly averaged daily LE appeared in July and August. Although the three methods gave the roughly consistent result, dispersion among them still existed in the experiment. PM method usually gave the highest latent heat flux among the three methods. Sum of the half-hourly values from BREB and PM methods of the three growing seasons took 81.2% and 131% of that from EC measurement. The discrepancy between BREB estimates, as well as PM estimates, and EC measurements was analyzed with regard to vapor pressure deficit. Several reasons leading to the uncertainty of BREB and PM methods were discussed, including the assumptions in the two methods, source area and the influence of environmental factors.