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); WSC, 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 50cm 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. This variance is fundamentally influenced by the difference in climate properties, particularly the seasonal variation of rainfall between Cambodia and Japan. This fact implies that the effect of the SWC on the WSC in forested areas is greater in Cambodia than in Japan.