Distribution of colluvia and saprolites and their physical properties in a zero-order basin in Okinawa, southwestern Japan

Abstract The thickness and physical properties of colluvium and saprolite were investigated to clarify their role in the hydrological processes in a forested basin. We measured their thickness using a handy dynamic cone penetrometer, and surveyed soil profiles to obtain undisturbed cylindrical samples for measuring physical properties, i.e., effective porosity. Saprolite is divided into strongly weathered saprolite and weakly weathered saprolite with their hardness. In the study area, colluvia are mostly over 0.5 m thick, especially in a head hollow, where it is over 3 m thick. On the other hand, they are less than 0.5 m thick in crest flats. Strongly weathered saprolite is thickest on crest flats, and it is also thick along the axis of a hollow. The physical properties that contribute to water storage show different values for each microtopographic unit. The effective porosity of the colluvia in head hollows and terraced bottomland is large, whereas in crest flats and crest slopes it is small. Strongly weathered saprolites show effective porosity mostly as much as in the lower colluvium layer. As the Nc value is strongly related to bulk density and total porosity, we could evaluate water storage capacity in deep sites where soil sampling is difficult. We also found that similar-shaped geomorphic surfaces often have different colluvium-saprolite systems and ages.