Tatsuhiko Nobuhiro

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.

Development of the IRGA enclosed-chamber system for soil CO2 efflux measurement and its application to a spatial variation measurement

A new system was developed for measuring soil CO2 efflux. The chamber in this system contains a small infrared CO2 gas analyzer. This system does not need air tubes or pumps for circulating air, so it is expected to offer the advantages of mobility and durability. This system was verified by a comparison with measurements made by using a closed-dynamic-chamber (CDC) system. The spatial variation in the soil CO2 efflux in a broadleaved deciduous forest was measured using the new system. The soil CO2 efflux at sampling locations 50–70 cm apart varied within a range of 60%–150%. This variation was smaller than the variation due to differences in soil characteristic reflected in different moisture conditions, etc.

Automated foliage chamber method for long-term measurement of CO2 flux in the uppermost canopy

Direct monitoring of CO2 exchange in forest ecosystems is necessary for evaluation of the annual net CO2 flux. We developed an automated foliage chamber system for long-term in situ measurement of the CO2 flux in foliage. The chamber closes automatically for 5 min in every 30. We found a high correlation between the photosynthesis rates measured by this system and by a portable H2O/CO2 analyzer (LCA-3, ADC). We used the new system to continuously monitor the CO2 flux in foliage of Quercus serrata. There was a positive correlation between CO2 flux and short-wave radiation. The daily photosynthesis rate varied widely depending on the environmental conditions. It increased from May to July and decreased temporarily in August. The seasonal pattern of photosynthesis was almost the same as that measured by the eddy covariance method. These results suggest that the new system is effective for tracking long-term seasonal change of the CO2 flux in foliage.

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.

Changes of Vertical Soil Moisture Conditions of a Dry Evergreen Forest in Kampong Thom, 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.

Runoff Characteristics and Observations on Evapotranspiration in Forest Watersheds, Central Cambodia

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.

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.

Effect of Soil Water Content on Water Storage Capacity: Comparison Between the Forested Areas in Cambodia and Japan

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.

Stomatal Response Characteristics of Dry Evergreen and Dry Deciduous Forests in Kampong Thom, Cambodia

We explored diurnal and seasonal variations in stomatal conductance in dry evergreen and dry deciduous forests in Cambodia and examined the stomatal response characteristics at two sites using a Jarvis-type model. Although stomatal conductance had maximum values at 9:00 (0900) or 10:00 (1000) in the morning and decreased continuously during the evening, transpiration showed peak values in the daytime and minimum values in the morning or evening at both sites in correspondence with the vapor pressure deficit. Stomatal conductance decreased in the rainy season to the late dry season; the pattern was clearer in the dry evergreen forest than in the dry deciduous forest. Stomatal conductance and volumetric soil water content had similar seasonal patterns, although these patterns differed between the dry evergreen and dry deciduous forests. The seasonal patterns of stomatal conductance and transpiration were different in the dry evergreen forest in the rainy season as a result of the moist air conditions. Clear differences were observed in maximum stomatal conductance and the function of the vapor pressure deficit between the two sites. In particular, compared to the results of other studies, the two sites showed large differences in their responses to the vapor pressure deficit. The functions of photosynthetically active radiation and the vapor pressure deficit showed wide daily change, suggesting that these factors may greatly impact the diurnal change of stomatal conductance. The vapor pressure deficit and volumetric soil water content also showed large seasonal variations and remarkable differences in function. The vapor pressure deficit had a large influence on stomatal conductance in the early dry season, whereas volumetric soil water content had a large effect in the late dry season.