For the purpose of developing a quantitative method to evaluate effects of forest on hydrological cycle, we analyzed spatial variability in soil pore size distribution on a natural forested hillslope. Undisturbed samples were collected at five plots distributed from downslope (Plot 1) to upslope (Plot 5) segments. The slope is underlain by granitic bedrock and soil is classified as Cambisols. The surface layer of Plot 1 consists of deposited soils that were moved from middle slope region by an old landslide. Plot 2 is characterized by a thin soil layer because of the landslide. Plots 3, 4, and 5 have residual soil layers not affected by the landslide. At each plot, soil samples were collected from soil surface through top of weathered bedrock layer. Retention curve for each sample was measured. The frequency of pore (F value) was computed as the slope of the soil water retention curve expressed as the relationship between the volumetric water content and the pF value (the common logarithmic of the matric pressure head). Results showed that patterns in vertical change in the pore size distribution in the downslope region (i.e., Plots 1 and 2) were different from those in the upslope region (i.e., Plots 3 thorough 5). In the upslope region, while surface soils had greater F values for every pore size class than subsurface soils, the mode pore size was the same for every soil depth. On the other hand, surface soils in the downslope region had greater F values for large pores and smaller F values for mid and small pores than subsurface soils. Thus, the surface soils had greater mode pore size than the subsurface soils. These differences in vertical profiles in the pore size distributions were attributable to the differences in rock weathering and geomorphological processes.
