Abstract

Information on spatial distributions of soil water content and mechanical strength is fundamental to hydrogeomorphological studies in mountainous watersheds. For simultaneous measurements of soil water content, θ, and penetration resistance, Nc, of soil mantles on natural hillslopes, we developed a new type of combined penetrometer–moisture probe (CPMP). The new CPMP has a robust configuration to reduce the frequency of moisture probe breakdowns during penetration into gravelly and rocky natural soils, and it has a penetration depth of up to 552 cm, which is about four to 14 times greater than the maximum measurable depth of any previous CPMP developed for agricultural soils. Laboratory calibrations and field validations showed that the CPMP succeeded in producing vertical distributions for θ and Nc similar to those measured with conventional methods. The CPMP provided less time-consuming and less destructive measurements of θ profiles than conventional methods that require excavation of deep trenches. Because θ profiles consider the stratified characteristics of soil mantles as well as the depths of groundwater tables, the CPMP was more effective than the conventional cone penetrometer for surveying the hydrogeomorphological structure of soil mantles. The CPMP was successful in determining the spatial distributions of θ and Nc in a headwater basin underlain by weathered granitic bedrock. From a direct comparison between two θ and Nc values measured on two different days at nearby points and the same depth, it was shown that Nc tended to decrease when θ increased by more than 0.15.

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