Distribution of colluvium and saprolite thickness and soil physical properties in Kahoku Experimental Watershed No. 3

Abstract

Focusing on microtopography units, we investigated the thickness of colluvium soil and saprolite, as well as physical properties of soils at a small watershed (Kahoku Experimental Watershed No. 3) in the northern part of Kumamoto Prefecture, located in the warm temperate zone. Both the surface soil thickness and the weathered layer thickness were thicker above the knick line. The colluvium soil thickness and saprolite thickness were particularly notable in the head hollows with a gentle slope in the middle of the slope. Regarding the general physical properties of the soil, the bulk density was large, the total porosity was low, and the liquid phase ratio was high, especially in the B horizon. Meanwhile, the saturated hydraulic conductivity showed a good value. The effective porosity, which contributes to the soil's water storage function, was generally low except for the upper side slopes, which occupies the widest area in the watershed.