EBBR Observation and fluctuation of evapotranspiration in a Cambodian evergreen forest

In the Mekong River basin, the increase in farming associated with a rapidly growing population has lead to a dramatic reduction in forest area. The incidence of illegal logging and wood collection is also increasing throughout the entire Asian Monsoon area, including Cambodia. According to Cambodian government statistics, the proportion of forested area in Cambodia has declined from 74% in the 1970s to 58% in 1993. Despite this reduction, the area covered by forests in Cambodia remains high compared to that in adjacent countries. We measured several meteorological elements associated with evapotranspiration, runoff, and precipitation in the broadleaf forest watersheds in Kampong Thom Province of central Cambodia. The topography of the watershed studied was relatively gentle. Meteorological factors were observed with a 60-m-high meteorological observation tower to determine the amount of evapotranspiration. The Energy Balance Bowen Ratio (EBBR) system was used to calculate the energy budget above the forest canopy for estimating evapotranspiration. Moreover, an automatic rain gauge was placed at the top of the observation tower and an interception plot was established for calculating the rainfall interception ratio by forest coverage near the tower. The main vegetation species at the research site were Vatica odorata and Myristica iners. The mean tree height in the upper crown layer at the research site was 27 m, and the maximum tree height was 45 m. Meteorological data for estimation of evapotranspiration were collected from October 2003 to September 2004. The SPAC model, used for analyzing characteristics of evapotranspiration variation, is a multilayer model considering factors such as Reynolds stress, temperature and H2O exchanges of leaves and ground surface, radiation transfer within the canopy, atmospheric diffusion within and above the canopy, energy balance for leaves and ground surface, interception of rainfall, and water budget for leaves. Several parameters were identified from the observation data set. The simulations reproduced the variation in evapotranspiration.