Micrometeorological estimation of wet canopy evaporation from a cloud forest in central Taiwan

A novel theory and method for estimating wet canopy evaporation were proposed based on the Penman–Monteith equation. Forest evapotranspiration consists of dry canopy evapotranspiration (forest canopy transpiration and forest floor evapotranspiration) and wet canopy evaporation (interception loss from forest canopy). Potential dry canopy evapotranspiration was introduced to represent the evapotranspiration from a hypothetical dry canopy under the same meteorological conditions when the canopy is partially or wholly wet. The potential dry canopy evapotranspiration was modeled using the Penman–Monteith equation with a canopy conductance under dry canopy conditions (dry canopy conductance) and extrapolated to wet canopy conditions. The dry canopy conductance was represented by the Jarvis-type model, with the parameters adapted to the canopy conductance and meteorological data obtained under the dry canopy conditions. Contributions of the wet canopy evaporation and the dry canopy evapotranspiration were separated by representing the total forest evapotranspiration as a linear combination of the potential evaporation and the potential dry canopy evapotranspiration using the wet canopy coefficient. The product of the wet canopy coefficient and the potential evaporation represents the wet canopy evaporation. This theory was applied to a Japanese cedar plantation, a montane cloud forest in central Taiwan, and wet canopy evaporation was estimated on a daily basis using the data obtained in 2020. When the evapotranspiration was corrected for energy imbalance, the estimated annual wet canopy evaporation in 2020 was 249.3 mm, which was 34.3% of the annual evapotranspiration (726.1 mm) and 14.4% of the gross annual precipitation (1734 mm). Given that rainfall partitioning measurements at this site reported a canopy interception of 9.1% of the gross rainfall, the wet canopy evaporation in this study was considered a reasonable estimate, and the portion exceeding canopy interception was attributed to fog interception.