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We measured precipitation, runoff, and several meteorological factors associated with evapotranspiration in evergreen broadleaf forest watersheds in Kampong Thom Province, central Cambodia. All the studied watersheds have flat topography, with Vatica odorata and Mynistica iners as the primary plant species. The mean tree height in the upper crown layer was 27 m and the maximum tree height was 45 m. Meteorological factors were observed from a 60-m-high meteorological observation tower. The heat budget method, which incorporates the Bowen ratio, was used to calculate the energy balance above the forest canopy. To estimate evapotranspiration, meteorological data were collected during two sampling periods: October 2003, near the end of the rainy season, and March 2004, in the middle of the dry season. Average daily evapotranspiration levels calculated for the late rainy season and middle of the dry season were 4.4 mm/day and 4.9 mm/day, respectively. A continuous simulation model (modified HYCY model) was then applied with the obtained streamflow data for the watersheds. Evapotranspiration calculated using the tower observations was included as a model parameter. The estimated runoff matched observed runoff comparatively well for small watersheds. The model parameters varied in correspondence with the watershed size.

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 (

Stable isotopes, such as deuterium (D) and oxygen-18 (18O), are widely used in hydrology as environmental tracers because they move with the water itself. Information obtained from stable isotope data can improve our understanding of the processes associated with the source of water and system dynamics and also provide quantitative estimates related to flow dynamics and transport parameters. In this chapter, stable isotope ratios (δD, δ18O) of rainfall and stream water were studied from 2003 in four forest watersheds in the Kampong Thom Province of Cambodia. The stable isotope ratios of rainfall during the dry season from November to April lined up below the local meteoric water line (LMWL), implying that rainfall during the dry season may be affected by secondary evaporation during its descent. When these data were discarded, the slope and the intercept of the LMWL were 7.95 and 9.11, respectively, and close to those of the global meteoric water line (GMWL). The volumetric weighted means of δD and δ18O values in rainfall were −6.7‰ and −43.9‰, respectively. During the period from January to March, when little rainfall occurred, the δD values of stream water were near the volumetric weighted mean of δD in the rainfall. During the period from April to December, when a considerable amount of rain fell, the temporal variation in δD in rainfall was less evident in stream water. The range of variation in the δD value of stream water differed among the watersheds, which may indicate that the residence time of stream water differs from watershed to watershed.

Water storage capacity (WSC), which is based on effective porosity in a soil profile or watershed scale, is one of the indicators for evaluating the water conservation function in a forested area. The effect of soil water content (SWC) on WSC was compared in this study between Cambodia and Japan. We studied four experimental plots. The DEF-plot is located in dry evergreen forest, the DEFlog-plot, in selectively logged dry evergreen forest, and the MF-plot, in mixed (evergreen and deciduous trees) forest in Kampong Thom Province, Cambodia. The JPN-plot is located in a natural forest consisting of fir and Japanese hemlock in Kochi Prefecture, Japan. The effect was evaluated using the index for the effect of SWC on WSC (ESW = WSCb/WSCa); WSCa is the typical WSC calculated from the effective porosity (estimated by the difference in SWC at saturation and -49 kPa), examined using the pressure plate method and soil thickness based on a soil survey. WSCb is a modified WSC that considers soil water in a WSC evaluation, which is computed by removing the effective porosity filled with water from WSCa. SWC was measured using a soil moisture gauge and was observed at depths of 30 and 100 cm from the surface in the three plots in Cambodia and at depths of 10, 30, and 50 cm in the JPN-plot. The ESW in the three plots in Cambodia was in the range of 0.6 to 0.8 from January through April and below 0.5 from June through October. In contrast, the ESW for the JPN-plot remained almost constant at 0.5 to 0.7 throughout the year. Seasonal variations in the ESW were considerable in the three plots in Cambodia and small at the JPN-plot. These results suggest that although the capacity for temporal rainwater storage was almost the same throughout the year in Japan, it decreased greatly in the rainy season in Cambodia.

Changes of soil water conditions in a soil profile were observed and estimated using a one-dimensional vertical soil water movement model for a dry evergreen forest area of Kampong Thom Province, Cambodia. The research site was in a dry evergreen forest where a meteorological observation tower had been established. Soil water matric potentials were measured at 20-, 50-, 100-, 150-, 200-, and 250-cm depths in an observation plot. Groundwater levels were observed at the site. Soil water matric potentials at each observation depth in a soil profile were simulated using a one-dimensional water movement model that was based on Richards’ equation. Results of observations and simulation revealed the following. (1) The site’s water-saturated zone was close to the ground surface during the rainy season. Water conditions in the unsaturated zone, which was above the groundwater level, were influenced strongly by groundwater. The groundwater level was 400 cm deep even in the dry season. The entire soil profile, from the surface to the bottom, never dried completely. (2) At the beginning of the rainy season, at the surface and subsurface depths of 20 cm and 50 cm, respectively, soil matric potentials were increased by rainfall events, which often supply water to the ground surface. Meanwhile, matric potentials at 200 and 250 cm depths were influenced directly by groundwater; they retained high potential values even during the dry season. In the middle zone, at depths of 100 and 150 cm, the soil was quite dried; the minimum matric potentials in the rainy season resembled those of the surface zone. (3) At the beginning of the dry season, matric potentials at 20- and 50-cm depths decreased because of soil water loss by transpiration of trees and evaporation from the ground surface.

Leaf area index (LAI) is a key biophysical variable in most process-based models of forest ecosystems and water cycles. We compared the LAI of two types of tropical seasonal forest in Kampong Thom Province, Cambodia. The two forest types are extremes of crown-cover density, i.e., closed dry evergreen forest (DEF) and open dry deciduous forest (DDF), suggesting marked spatial variation in forest site conditions such as soil moisture. Monthly changes in LAI were estimated indirectly using a plant canopy analyzer and hemispherical photographs. Both methods of LAI estimation showed instrument errors, i.e., low reproducibility in the plant canopy analyzer data and LAI-saturation in hemispherical photograph data; nevertheless, LAI values differed between DEF and DDF. The average LAI from three years of measurements was about 4.6 times higher in DEF than in DDF. DDF exhibited much greater seasonality than DEF. The annual minimum LAI averaged 76% and 84% of the annual maximum LAI for DDF and DEF, respectively. LAI showed high peaks in the rainy season and decreased in the dry season. However, in DEF, LAI decreased twice annually, at the beginning of the dry season and the beginning of the rainy season. Seasonal changes in LAI could be approximated using a third-degree Fourier-series equation.