An Evergreen Forest Largely Evapotranspires in Northern Thailand
under Driest Conditions of Late Dry Season
-New Forest Model Simulation Agrees with Observation Results-

December 26, 2001

  Dr. Katsunori Tanaka, the Hydrological Cycle Process on Land Group of the Hydrological Cycle Research Program in the Frontier Research System for Global Change (a joint project of NASDA and JAMSTEC), first showed that evapotranspiration from the evergreen forests in northern Thailand into the atmosphere peaks during the late dry season when conditions are the driest, with a newly-developed forest model.(fig. 2) Further, the prediction results agreed with the results obtained from observing sap flow rates, which provide clues to the seasonal variation in evapotranspiration (fig. 1).
Details on the new forest model will be published in the January 1, 2002 edition of Ecological Modeling.

  Thailand has both deciduous and evergreen forests. Trees in a deciduous forest defoliate in the dry season, and evapotranspiration from the leaves ceases. Owing to the dry ground surface, water vapor is not released into the atmosphere from the ground. In contrast, the trees in an evergreen forest, which retain foliage all year round, close their stomata when both the air and soil are dried out in the dry season (ref. 1) to prevent loss of water from within the plant. The significant decline in evapotranspiration has been attributed to this aspect. In fact, Pinker et al. reported that evapotranspiration was little detected when they conducted their observation study in 1970 (Journal of Applied Meteorology 19, pp 1341 to 1350, 1980) (ref. 2). Thereafter, no further tests were conducted towards this research.

  When the newly developed forest model was used to simulate evapotranspiration, evergreen forests were shown to release a large quantity of water vapor into the atmosphere due to evapotranspiration in the late dry season (fig. 1). These findings led to the conclusion that the trees in an evergreen forest pull water from deep underground and then dispere it into the atmosphere as if they were pums. The most recent observation results obtained under the GAME-Tropical Project, (i.e. seasonal variation in sap flow, which is a guide to seasonal variation in evapotranspiration) were also shown to agree with the model results. These findings demonstrate that an ergreen forest releases large amounts of water vapor into the atmosphere during the late dry season. The results obtained from this research should assist in improving the precision of hydrological cycle predictions in the Asia Monsoon regions, in addition to serving as important reference material toward considering the role performed by forests in affecting water resources. (Explanation of Terms)

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