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We measured the sap flux densities of 12 deciduous trees in a tropical dry deciduous forest with high seasonality of available water located in Cambodia and evaluated the seasonal trends in transpiration and leaf phenology. For all trees, the minimum transpiration was recorded in the middle of the dry season, and almost all trees restarted transpiration before the first monsoon rain. The occurrence of the ‘paradox’ of leaf phenology was confirmed. During the dry season, transpiration was controlled by leaf phenology and decreased with an increase in the duration of the leafless period. In contrast, during the wet season, daily changes in transpiration were determined by changes in evaporative demand. Transpiration during the dry season accounted for more than 30% of the annual total among trees, and at the stand scale, the dry season contribution was 38%. The dry season transpiration was not negligible for the water balance in this ecosystem. The soil water condition in the shallow layer, where the main root system is located, was not the source of transpiration during the dry season. This implied that the root probably extended to a deep layer and absorbed water. The relationships between the mean canopy stomatal conductance and vapour pressure deficit revealed that most trees were isohydric. Isohydric behaviour controlling stomatal openness to avoid xylem hydraulic failure was also confirmed at the stand scale and was advantageous for these trees, allowing them to continue transpiring under the high evaporative demand during the dry season. Copyright © 2015 John Wiley & Sons, Ltd.

Although information about the hydrologic and ecological features of lowland evergreen forests in central Cambodia has been collected since the beginning of the 21st century, measurements of the transpiration process remain very limited. This paper describes the differences detected in transpiration (q) and crown-level stomatal conductance (GS) between Calophyllum inophyllum (which undergoes successive leaf exchange) and Drypetes sp. (which performs irregular leaf exchange) (hereafter referred to as Calophyllum and Drypetes, respectively), and the analysis of these differences in consideration of their contrasting leaf phenologies. We evaluated q using sap f low measurements and obtained daily GS values. Calophyllum and Drypetes had high and low periods between which q and GS differed significantly. Within high/low periods, smaller scatter in the plot of GS versus vapour pressure deficit (D) was found in Calophyllum compared to Drypetes. For a given value of D, q in high periods was 1.3 and 1.9 times larger than in low periods for Calophyllum and Drypetes, respectively. The smaller scatter for Calophyllum was the result of relatively constant physiological activity that was maintained by successive leaf fall. For both species, high periods were recorded after remarkable leaf-fall events; thus, our current data implied that leaf phenology is one of the most important factors affecting transpiration. Discipline: Forestry and forest products Additional key words: leaf phenology, riparian zone, sap flux density, transpiration, vapour pressure deficit

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.

In this paper, we have measured many meteorological elements associated with evapotranspiration using a 60-m-high meteorological observation tower, runoff and precipitation of broad-leaf forest watersheds in the Kampong Thom Province of central Cambodia. Moreover, an interception plot was established for calculating the rainfall interception ratio by forest coverage close to the tower. Analyzing many kinds of observed data, we evaluated Stung Chinit watershed, which had the largest drainage area and an annual loss of ~1200 mm. This value was in agreement with annual evapotranspiration estimated in 2004 by the Bowen ratio energy balance method. The annual canopy interception rate, which is one of the most important hydrological factors in the forest, was approximately 15%. Also, the rainfall interception process was reproduced well according to the multilayer model. The available renewable freshwater resources in forested areas of the Mekong River basin were estimated by modeling. Annual available renewable freshwater resources in forested areas of the Mekong ranged from 300 to 2200 mm/year. The mountainous area of west Cambodia had vast renewable freshwater resources, and almost all regions had 500 mm/year. This result mostly corresponded to the amount of water resources in the largest evergreen forest experimental watershed including several small watersheds in 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.

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.