J. Water Resource and Protection, 2009, 1, 325-335 doi:10.4236/jwarp.2009.15039 Published Online November 2009 (http://www.scirp.org/journal/jwarp) Copyright © 2009 SciRes. JWARP 325 Evapotranspiration Characteristics of a Lowland Dry Evergreen Forest in Central Cambodia Examined Using a Multilayer Model Tatsuhiko NOBUHIRO 1* , Akira SHIMIZU 2 , Katsunori TANAKA 3 , Koji TAMAI 1 , Naoki KABEYA 2 , Eriko ITO 4 , Takanori SHIMIZU 1 , Makoto ARAKI 1 , Sophal CHANN 5 1 Forest Hydrology Laboratory, Forestry and Forest Products Research Institute, Tsukuba, Japan 2 Kyushu Research Center, Forestry and Forest Products Research Institute, Kumamoto, Japan 3 Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan 4 Hokkaido Research Center, Forestry and Forest Products Research Institute, Hokkaido, Japan 5 Forestry Administration, Forest and Wildlife Science Research Institute, PhnomPenh, Cambodia E-mail: tatuhiko@ffpri.affrc.go.jp Received July 6, 2009; revised September 9, 2009; accepted September 15, 2009 Abstract Development pressure has led to serious deforestation on the Indochina Peninsula. Particularly rapid defor- estation has occurred in easily accessible lowland areas, and it is thus important to accumulate knowledge about these forests immediately. We measured evapotranspiration rates for a lowland dry evergreen forest in Kampong Thom Province, central Cambodia, using the energy balance Bowen ratio (EBBR) method based on meteorological data collected from a 60-m-high observation tower. Daily evapotranspiration was higher during the dry season than during the rainy season of the Asian monsoon climate. The seasonal variation in evapotranspiration generally corresponded to the seasonal difference in the vapor pressure deficit. A multi- layer model was used to simulate the seasonal variation in evapotranspiration. The multilayer model also reproduced the larger evapotranspiration rate in the dry season than in the rainy season. However, observed values substantially exceeded model-calculated results during certain periods at the beginning of the dry season and in the late dry season. Moreover, during the rainy season, the model tended to overestimate evapotranspiration. The differences between these observed and simulated values may have been caused by seasonal characteristics of photosynthesis and transpiration in the lowland dry evergreen forest that were not considered in the model simulation. Keywords: Cambodia, Energy Balance Bowen Ratio Method, Evapotranspiration, Lowland Dry Evergreen Forest, Multilayered Model 1. Introduction The Indochina Peninsula, including Cambodia, belongs to the Asian monsoon climate zone characterized by dis- tinct rainy and dry seasons. Pressures including increases in population, farmland development, and illegal logging have led to serious deforestation problems across the Indochina Peninsula. For example, the area covered by forest in Thailand decreased from 54% in 1960 to 25.3% in 1998 [1], and the forested area in Cambodia dropped from 73% in 1969 to 58% in 1997 [2]. Since artificial afforestation of commercial species such as teak has also been occurring in Indochina [3,4], very few forests exist in their natural state. To develop suitable forestry management practices for this region, a first urgent task is to assemble information on the various natural and artificial forest types, considering the cli- matic characteristics of the seasonal tropics. Several studies have examined water cycles and re- sources in the Asian monsoon region, focusing on defor- estation and certain regional climatic features. Using a gradient model, Pinker et al. [5] reported that evapotran- spiration rates in the dry evergreen forest at the Sakaerat