Forest categorization according to dry-canopy evaporation rates in the growing season: comparison of the Priestley-Taylor coefficient values from various observation sites

Summarizing observed dry-canopy evaporation (hereafter, evaporation) data from earlier papers, we developed a scheme for forest categorization according to evaporation rates in the growing season. Evaporation rates were represented by the Priestley-Taylor coefficient alpha calculated for daytime. We examined relationships between forest properties (e.g. climatic regions, leaf types) and alpha values. We obtained alpha data for 67 forest sites from earlier papers. Based on these data, we found (i) a clear difference in a values between broad-leaved and coniferous forests, (ii) a greater variation in alpha values between individual coniferous forests than between individual broad-leaved forests, and (iii) a clear relationship between canopy height and alpha values for coniferous forests. These three results were supported by surface conductance data summarized from earlier papers. We concluded that forests should be primarily classified into broad-leaved and coniferous forests, and that coniferous forests should be further classified according to canopy height. This classification scheme is applicable only to forests with projected leaf area index (LAI) >= 3.0. Regardless of this LAI limitation, this classification will be useful because many forests satisfy this LAI limitation. This paper shows valuable results in the following two respects. First, this paper explicitly shows the difference in evaporation rates between broad-leaved and coniferous forests. Although this difference would have been implicitly recognized, this difference has not been shown based on adequate amounts of observed data. Second, it is shown that classifying coniferous forests according to canopy height is as important as classifying forests according to leaf type (broad-leaved or coniferous). Although studies have recognized the effect of canopy height on evaporation rates, the significant effect of canopy height on evaporation rates, compared with the effects of other factors on evaporation rates, has not previously been shown. Copyright (c) 2005 John Wiley & Sons, Ltd.