Comparison of three models to estimate evapotranspiration for a temperate mixed forest

Forest evapotranspiration is one of the main components in the regional water budget. A comparison between measured and estimated eddy covariance (EC) data, considering, the Katerji-Perrier (KP), Todorovic (TD) and Priestley-Taylor (PT) actual evapotranspiration methods, was carried out. These models, relying on more easily obtainable data, are valuable when long-term direct measurements are not available. The objective of this paper is to compare the effectivity of these three models. In this paper, experimental data were obtained within the temperate mixed forest of broad-leaved and coniferous trees of the Changbai Mountains in northeastern China during the growing seasons of 2003 to 2005. The KP method gave the most effective values for half-hourly and daily evapotranspiration computed by Summing Up half-hourly estimates, and the TD method overestimated evapotranspiration by about 30%. The diurnal courses of estimated and measured evapotranspiration showed bell curves, similar to that of net radiation. except for a slight increase at about 14:30 solar time due to a peak value of vapour pressure deficit (VPD). For the case of daily evapotranspiration using daily mean micrometeorological variables, the PT method presented the closest values to the measurements. Accuracy of estimation related to VPD negatively (especially for VPD > 1.5 kPa). The KP parameters, considered to be vegetation dependent, were a = 0.545 and b = 1.31 at the experimental site. A constant PT parameter (alpha = 1.18) was applied to estimated evapotranspiration. Daily values of alpha responded to VPD (negatively) more strongly than to soil moisture (positively) in this forest. The experiment showed the inherent limits and advantages of the three methods. The KP method, a semi-empirical approach, was preferred to estimate half-hourly evapotranspiration. The TD method Was a mechanistic approach to estimate reference evapotranspiration and always overestimated actual evapotranspiration. The PT method, being site dependent and the simplest approach, was effective enough to estimate large time-scale (at least daily) evapotranspiration. Copyright (C) 2008 John Wiley & Sons, Ltd.