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We assessed evapotranspiration in the Mekong River basin with a focus on the distribution of forested areas using geographic information system (GIS) datasets. We developed a new model to estimate evapotranspiration, a major component of the forest water budget. The model calculates transpiration (including forest floor evaporation) and interception loss separately. Transpiration was calculated based on the Priestley-Taylor equation. Interception loss assumed a constant interception ratio. After clarifying distributions of climatic conditions and forested area in the basin, we calculated the evapotranspiration rate distribution. We then identified significant factors to consider in accurate estimation of evapotranspiration by comparing evapotranspiration rates based on the model and those based on the original form of the Priestley-Taylor equation. Consequently, we concluded that the contribution of evergreen and deciduous broadleaf forests in the southern part of the basin is one of the dominant components of evapotranspiration from the whole basin, because those forests are distributed in an area with high evaporative potential and the forests cover a large area. Furthermore, it is essential to evaluate the transpiration control of evergreen broadleaf forests in the lower part of the basin because of decreases in soil moisture during the dry season. [ABSTRACT FROM AUTHOR]