Search

Tipping-bucket flow meter and rain gauge (TBFM/TBRG) are widely used for the measurement of gross rainfall (GR), throughfall (TF), and stemflow (SF) to evaluate the amount of interception loss (I). However, TBFM/TBRG cannot measure the inflow rate during tipping and underestimates the inflow rate. To correct this systematic bias, 33 total calibrations were conducted for five types of TBFM/TBRG in the laboratory. The tipping time increased with the bucket volume, and the underestimation during one tip was higher for TBFM/TBRG of larger capacity. With the use of the scaled actual inflow rate and the actual volume of a single tip from the measured static volume of a single tip when the inflow rate is zero, the common calibration curves were obtained as quadratic equations for each of the five types within an error range of ±3%. We measured GR and TF by using TBRG and TBFM with a resolution of 0.2 mm and measured SF by TBRG with a single-tip static volume of 15.7 cm3 in a Japanese temperate coniferous forest (TCF) and a Cambodian tropical deciduous forest (TDF). At both sites, the calibration curves needed to be applied to obtain GR, TF, and SF on an event scale with an underestimation degree of less than 3%. Without applying any calibrations, the higher rainfall intensities in TDF caused larger underestimations of GR, TF, and SF and larger overestimations of I compared with results for TCF. On an annual scale, the degree of overestimation of I relative to GR (ΔI/GR) was 1.2% in TCF and 3.5% in TDF, and ΔI/I was at least 10% at both sites. Copyright © 2012 John Wiley & Sons, Ltd.

Both evergreen and deciduous forests (Efs and Dfs) are widely distributed under similar climatic conditions in tropical monsoon regions. To clarify the hydraulic properties of the soil matrix in different forest types and their effects on soil water storage capacity, the soil pore characteristics (SPC) were investigated in Ef and Df stands in three provinces in Cambodia. Soils in the Ef group were characterized in common by large amounts of coarse pores with moderate pore size distribution and the absence of an extremely low Ks at shallow depths, compared to Df group soils. The mean available water capacity of the soil matrix (AWCsm) for all horizons of the Ef and Df group soils was 0·107 and 0·146 m3 m−3, respectively. The mean coarse pore volume of the soil matrix (CPVsm) in the Ef and Df groups was 0·231 and 0·115 m3 m−3, respectively. A water flow simulation using a lognormal distribution model for rain events in the early dry season indicated that variation in SPC resulted in a larger increase in available soil water in Ef soils than in Df soils. Further study on deeper soil layers in Ef and each soil type in Df is necessary for the deeper understanding of the environmental conditions and the hydrological modelling of each forest ecosystem. Copyright © 2010 John Wiley & Sons, Ltd.

Micrometeorological observations from a tower in a forest in central Cambodia were used to estimate forest water yields. For these estimates, the turbulent boundary layer was identified from tower observations of wind profiles. Indices including the stability length, friction velocity, and Richardson numbers were calculated to diagnose boundary layer characteristics. The calculated stability length indicated that the atmospheric condition was between weakly unstable and weakly stable 63% of the time. The calculated friction velocity exceeded 0·5 m s−1 77% of the time, and the calculated Richardson number was smaller than the critical number (0·2) 72% of the time. The indices calculated in this study indicate that the degree of atmospheric stability over this evergreen forest in central Cambodia is generally suitable for tower flux observations. Copyright © 2008 John Wiley & Sons, Ltd.

In this study, we measured evapotranspiration in an evergreen broadleaf forest watershed in the Kampong Thom Province of central Cambodia with the aid of a 60-m high meteorological observation tower. The main vegetation species at the study site were Myristica iners and Vatica odorata. The mean tree height in the upper crown layer at the study site was 27·2 m, and the maximum tree height was 45·1 m. A heat balance method, which incorporated the Bowen ratio, was used to calculate the energy balance above the forest canopy; this value was subsequently used for the calculation of evapotranspiration. All the equipment necessary for the measurement of meteorological variables was installed in the observation tower. Data were collected during two distinct sampling periods: October 2003, in the late rainy season, and March 2004, in the middle of the dry season. Daily variations in evapotranspiration were strongly correlated with the measured amount of net radiation above the canopy layer. On the basis of our measurements, the mean evapotranspiration levels of the forest watershed during the late rainy season and the middle of the dry season were 4·3 and 4·6 mm/day, respectively. No significant differences in soil moisture were observed between the middle of the dry season and the late rainy season, probably due to the shallow depth of the water table. The results of this study suggest that the high levels of evapotranspiration recorded during the middle of the dry season reflect the presence of sufficient soil moisture (derived from the shallow water table) and a high vapour-pressure deficit. Copyright © 2008 John Wiley & Sons, Ltd.

The Great Lake (Tonle Sap), located in mid-western Cambodia, is the largest lake in southeast Asia and joins the Mekong through the Tonle Sap River. During the annual monsoon peak from about the end of May, the flow direction in the Tonle Sap reverses and river water from the Mekong flows to the Great Lake. When the water level of the Mekong decreases sufficiently, usually in October, the Tonle Sap again flows in its normal direction from the Great Lake to the Mekong. This study quantitatively investigated interactions between the Tonle Sap River and the Mekong River using the stable isotope ratios of oxygen (δ18O) in waters from both rivers, and estimated the contribution rate of Tonle Sap River water to Mekong River water during the peak flow period of the Tonle Sap River. During this period, the Tonle Sap River contributed approximately 70–97% along the right bank and 0–5% in the middle and left banks of the Mekong. We also observed δ18O of the Mekong River along a longitudinal section on the right bank side, where the contribution rate of the Tonle Sap River water remained high (80%) even 21 km downstream from the confluence. This study clearly demonstrates that we can determine the mixing rate of these rivers at each point after they converge by using stable oxygen isotopes as tracers, when the Tonle Sap flows in its normal direction. Copyright © 2008 John Wiley & Sons, Ltd.