Year-Round Observation of Evapotranspiration in an Evergreen Broadleaf Forest in Cambodia

Abstract

We conducted a year-round observation of meteorological elements using a meteorological observation tower 60 m in height to evaluate evapotranspiration in an evergreen broadleaf forest watershed in central Cambodia. The period of observation was from November 2003 to October 2004. Solar radiation was consistent throughout the year. The integrated values of net radiation and downward and upward shortwave radiation were 5.09, 6.79, and 0.76 GJ m−2 year−1, respectively. The temperature observed above the forest canopy was lowest and highest in the first and latter half of the dry season, respectively. The mean air temperature was 26.4°C. The saturation deficit was high in the late dry season (>30 hPa) and low during the rainy season (<25 hPa). The evapotranspiration rate was estimated from these observed meteorological parameters using the heat-balance method incorporating the Bowen ratio. The evapotranspiration rate was higher in the dry season than in the rainy season. Seasonal variation in evapotranspiration corresponded to the variation in the saturation deficit above the forest canopy. The amount of year-round evapotranspiration was 1139.7 mm. The water budget calculations from observation data suggested a water loss of 1202.8 mm for the experimental watershed. Thus, the observed evapotranspiration and water loss amounts were similar.