Abstract:Soil layers play an important role in hydrological processes, particularly in forested basins. For example, thick soil layers can store rainwater temporarily and drain it gradually. In the Mekong River Basin, especially in Cambodia, evergreen and deciduous forests are widely distributed. However, we have no data on soil conditions such as thickness and hardness that affect water storage capacity in forested areas. To clarify the water storage role of forest soils in the lower reaches of the Mekong River Basin, we investigated the distribution of soil thickness using a handy dynamic cone penetrometer in three Cambodian provinces: Kampong Thom, Kratie, and Mondul Kiri. Soil thickness was greater than 8 m in a deciduous forest in Kampong Thom. In some evergreen forests, apparent soil thickness varied from the rainy to dry seasons because of specific grain size distributions; for example, the soils were rock hard in the dry season and soft in the rainy season. In Kratie, deciduous forests prevailed and soil thickness was shallow at less than 3 m. In Mondul Kiri, forests were mainly formed on a basaltic plateau, and soil thickness was greater than 7 m. Soil thickness and hardness differed among the soil types in these three provinces, as did water storage capacity. The maximum water storage capacity was 1350 mm in an evergreen forest distributed on Acrisols in Kampong Thom. In contrast, the minimum water storage capacity was 225 mm in a deciduous forest distributed on Leptosols in Mondul Kiri. We constructed a soil thickness map for the study area in Kampong Thom that was mainly covered with evergreen forest; the distribution of soil thickness was related to elevation, with soil thickness increasing 0Ð13 m per 1 m increase in elevation, except in a deciduous mixed swamp forest area.

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.

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 AE3%. 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 cm 3 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.

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