Development of a simple forest evapotranspiration model using a process-oriented model as a reference to parameterize data from a wide range of environmental conditions

It is essential to know the spatial distribution of water resources to maintain sustainable water use. We present a method for developing a simple model of forest evapotranspiration, an essential component of the water budget because it controls water loss from the land surface, that uses a process-oriented model as a reference to parameterize data from a wide range of environmental conditions instead of observed data. Our model considers two major forest evapotranspiration processes, dry-canopy evapotranspiration and wet-canopy evaporation. Dry-canopy evapotranspiration was calculated based on the Priestley–Taylor equation. Wet-canopy evaporation assumed a constant interception ratio. The accuracy of the model performance was determined in two steps. First, we compared the new model with a reference model using meteorological data other than those used in model parameterization. Then, we compared evapotranspiration data based on the annual water budget method for 25 forest sites with data based on the short-term water budget method for eight forest sites in Japan. The model successfully reproduced both the geographical and seasonal patterns of forest evapotranspiration over Japan. The model has a simple structure and requires few meteorological inputs compared with process-oriented models, and it is therefore considered suitable for regional-scale application. The model has the potential to be incorporated into other monthly forest ecosystem models.