In this paper, we have measured many meteorological elements associated with evapotranspiration using a 60-m-high meteorological observation tower, runoff and precipitation of broad-leaf forest watersheds in the Kampong Thom Province of central Cambodia. Moreover, an interception plot was established for calculating the rainfall interception ratio by forest coverage close to the tower. Analyzing many kinds of observed data, we evaluated Stung Chinit watershed, which had the largest drainage area and an annual loss of ~1200 mm. This value was in agreement with annual evapotranspiration estimated in 2004 by the Bowen ratio energy balance method. The annual canopy interception rate, which is one of the most important hydrological factors in the forest, was approximately 15%. Also, the rainfall interception process was reproduced well according to the multilayer model. The available renewable freshwater resources in forested areas of the Mekong River basin were estimated by modeling. Annual available renewable freshwater resources in forested areas of the Mekong ranged from 300 to 2200 mm/year. The mountainous area of west Cambodia had vast renewable freshwater resources, and almost all regions had 500 mm/year. This result mostly corresponded to the amount of water resources in the largest evergreen forest experimental watershed including several small watersheds in central Cambodia.