Koji Tamai

Year-Round Observation of Evapotranspiration in an Evergreen Broadleaf Forest in Cambodia

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 (<25 hPa). The evapotranspiration rate was estimated from these observed meteorological parameters using the heat-balance method incorporating the Bowen ratio. The evapotranspiration rate was higher in the dry season than in the rainy season. Seasonal variation in evapotranspiration corresponded to the variation in the saturation deficit above the forest canopy. The amount of year-round evapotranspiration was 1139.7 mm. The water budget calculations from observation data suggested a water loss of 1202.8 mm for the experimental watershed. Thus, the observed evapotranspiration and water loss amounts were similar.

Radioactive cesium discharge in stream water from a small watershed in forested headwaters during a typhoon flood event

Abstract We investigated radioactive cesium (Cs) export in stream water during a typhoon flood event (No. 1204, Guchol) in a small forested headwater catchment (1.2 ha), which was contaminated by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) disaster. The experimental watershed is covered with hardwood and conifer trees. Hourly stream water samples were collected by an automatic water sampler, and turbidity was measured every 10 min. Downstream transport of radioactive Cs was recorded through the typhoon, during which the total rainfall was 168 mm. Total cesium-134 (134Cs), cesium-137 (137Cs) and suspended solid (SS) concentrations and turbidity were elevated on the rising limb of the hydrograph. When maximum rainfall intensity, maximum runoff peak were observed, and maximum radioactive Cs and SS concentrations occurred. Total 134Cs and 137Cs concentrations were 1.5 and 2.4 Bq L−1, respectively, at peak flow. However, concentrations of radioactive Cs, SS and turbidity dropped sharply on the receding limb. After filtration (0.7 μm), the concentrations of 134Cs and 137Cs were either not detectable (ND) or below 0.09 Bq L−1. Concentrations of dissolved radioactive Cs in stream water were very low, and radioactive Cs was mainly discharged with suspended materials. Total 134Cs and 137Cs exports from the study watershed were 43 and 72 Bq m−2, respectively. These exports represented approximately 0.07% of the 134Cs and 137Cs fallout around the study site (180 kBq m−2). We therefore concluded that discharge of radioactive Cs from forested headwaters was negligible even though a typhoon flood event.

Stable Isotope Studies of Rainfall and Stream Water in Forest Watersheds in Kampong Thom, Cambodia

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.

Measurements of Wind Speed, Direction, and Vertical Profiles in an Evergreen Forest in Central Cambodia

The wind characteristics of speed, direction, and vertical profile were studied to determine the effects of undulations in the local topography and canopy surface on flux observations made from a tower in an evergreen forest in Kompong Thom Province, Cambodia. Three seasonal patterns of wind speeds and directions were identified. The first occurred in December and January and was characterized by northerly monsoons that persisted all day, as well as a diurnal variation in wind speed, with a maximum and minimum around noon and near sunset, respectively. A second pattern, in February, was characterized by southerly to westerly prevailing monsoon winds, along with an easterly mountain wind observed in the early morning. Wind speed was low throughout the day. The third pattern was similar to the second but included brief, strong winds associated with squalls. Thus, regional effects on circulation were limited, and monsoon winds were found to dominate the meteorological system above the evergreen forest of central Cambodia. In the forest, the estimated roughness length and zero plane displacement height averaged 18.3 m and 7.5 m, respectively, and the average canopy height was 27.2 m. The dependence of roughness length and zero plane displacement height on the wind direction was within the standard deviation. Thus, the undulating canopy surface had little effect on the tower flux observations.

Seasonal and height-related changes in leaf morphological and photosynthetic traits of two dipterocarp species in a dry deciduous forest in Cambodia

Background: Dry deciduous forest (DDF) is distributed throughout the monsoon area of South-east Asia. Leaf morphological and photosynthetic traits are essential for adaptation to the environmental variability and usually change with tree height and season. Aims: To clarify the differences in leaf traits as influenced by tree height, light conditions and season for two dominant dipterocarps of DDF: Dipterocarpus tuberculatus and Shorea obtusa. Methods: We measured changes in leaf traits with tree height, namely leaf mass per area (LMA), maximum photosynthetic rate (Amax), stomatal conductance (gs) and nitrogen concentration (N), in the rainy and early and mid-dry seasons. To identify the effects of height and light intensity on leaf traits, we conducted multiple regression analyses using leaf traits as the dependent variables and the height, light and season as independent variables. Results: Tree height was more important for gradients in most leaf traits, especially in LMA in both species, than light intensity. Tall canopy tree leaves had greater LMA, Amax and N than shorter tree leaves in the rainy and early dry seasons, although gs was almost constant. The lowest Amax and gs were recorded in immature leaves, sampled in the mid-dry season, at all tree heights. Conclusions: The two dipterocarp species achieved high Amax by having high gs, LMA and N in a high-light canopy environment, whereas larger, thinner leaves with low respiratory costs may contribute to efficient light capture under low-light conditions in the understorey.

Experimental Estimation of the Effect of Rainfall Interception on Soil Respiration in a Broad-leaved Deciduous Forest in Western Japan

To estimate the effect of rainfall on soil respiration, soil efflux was measured with a manipulation of rainfall interception at a broad-leaved deciduous forest in western Japan from 20 February to 19 November 2001. The diurnal patterns of soil respiration observed at the intercepted subplot () were quite similar to those of soil temperature at 0.2 m depth with a maximum around midnight and a minimum from noon to early afternoon. Such diurnal patterns have not been observed in the previous studies at the same study site under natural conditions (which manifested no clear diurnal variations). Furthermore, the magnitudes of showed substantial differences (e.g., ~50% reduction under water-limited conditions) compared to those of the potential soil respiration under natural conditions (). These findings demonstrate that rainfall events not only affected the magnitude of soil efflux but also modified the vertical structure of soil temperature, thereby altering diurnal patterns of soil respiration.

Two phenological variants of Terminalia alata coexist in a dry dipterocarp forest

Two morphological variants of Terminalia alata (Combretaceae) differed in leaf flushing phenology and spatial distribution in a Cambodian deciduous forest. The hairy-type trees displayed leaf exchange behavior in the middle of the dry season. The glabrous type flushed new leaves 3 months after the wet season started. The leafless period of the hairy type was estimated to be <1 month, whereas that of the glabrous type lasted more than 5 months. The landscape-scale leaf exchange behavior was similar to that of the hairy type. The two types showed clear spatial separation. The hairy type was limited to flat areas with deep soils. The dominance of the glabrous type in hilly areas with shallow soils suggests that it is adapted to water-limited environments. The abundance of the glabrous type in hilly areas and its unique leaf phenology probably influence the carbon, energy and water balance at the landscape level.