Evaluation of the Amount of Water Resources in the Forest Watershed of the Mekong River, Cambodia

Forestry Administration, Cambodia

FY 2015–2018 Research Grant from Kurita Water and Environment Foundation

KABEYA, Naoki

The Cambodian Civil War continued for many years, resulting in the dissipation of scientific data that had been obtained from environmental observations and the loss of a research basis, so that the country became known as a "data blank area." However, the proportion of forested land has remained relatively highly, with Cambodia being the only country in the Indochinese Peninsula to have retained “lowland” (flat ground) evergreen and deciduous forests, which have been lost from many other countries due to economic development in this region. However, the functions of these precious forests in the water cycle remain unclear.

This research aimed to examine the functions of the evergreen and deciduous forests of Cambodia in the water cycle using two methods: the river hydrological method and the micrometeorological method (Fig). A modified discharge rating curve was created by observing the river flow rate in situ. This was then applied to hydrological data over a long period of time, allowing a long-term catchment water budget to be calculated. Comparison of the result with micrometeorological observations allowed the role of evergreen and deciduous forests in the water cycle to be clarified.

We examined the evapotranspiration process in an evergreen forest and a deciduous forest using the micrometeorological method to understand how differences in the evapotranspiration quality between the two forest types has affected the water budget at the watershed scale.

In the deciduous forest site, instrumentation for recording atmospheric measurements using the bandpass eddy covariance method was installed in a tower at heights of 5 m and 30 m, and the obtained data were observed along with other fine meteorological elements. It was found that the amount of latent heat exchange (which is equivalent to the amount of evapotranspiration) decreased rapidly during the first half of the dry season (November 2009 to April 2010) but then gradually increased again after the middle of the dry season as the net radiation increased. This suggested that many trees produced leaves and initiated transpiration in the middle of the dry season when there was a severe soil water content deficit, resulting in seasonal changes in the amount of evapotranspiration.

Comparison of the long-term water budgets of an evergreen and deciduous forest watershed through the collection of hydrological data showed that annual water resources in the evergreen forest were secured by the deep soil, resulting in almost constant evapotranspiration activity throughout the year. By contrast, the deciduous forest had a thin layer of soil and unevenly distributed water resources through the year, so that although trees started to develop leaves in the second half of the dry season, both the wood and grass carried out evapotranspiration activity during the rainy season when water resources were abundant, resulting in seasonal changes.