A combined penetrometer-moisture probe (CPMP) for investigating hydrological properties of forested hillslopes

To predict shallow landslides on steep hillslopes, adequate and precise information on the soil structure and soil water movement are indispensable. A combined penetrometer-moisture probe (CPMP) was developed for measuring vertical profiles of soil water content and penetration resistance simultaneously, and applied to figure out internal structure of the soil mantle. In this study, we measured the profiles of water content and penetration resistance with the CPMP at each of 57 points on a forested valley-side slope in Hirudani research catchment, Hodaka, central Japan. Additionally, we installed tensiometers at each point for continuous measurements of pressure heads at soil-bedrock interface. Thickness of the soil layer was highly variable throughout the slope and the bedrock depression apparently shaped a hollow. Soil water content measured with the CPMP showed high values in the lower section of the slope. The high water content region extended to the upper slope section along the hollow, roughly corresponding with the region with high values of the topographic index. Pressure heads measured with tensiometers were positive throughout the observing period at most of the points where high water contents were detected with the CPMP. At some points in the upper slope section, water contents were especially high in spite of relatively small values of the topographic index. At these points, pressure heads showed delayed peaks in comparison with hyetographs and slow recession limbs, suggesting an existence of groundwater seepage from the bedrock. The other saturated points exhibited almost no responses to hyetographs, and all of the unsaturated points had rapid peaks coinciding with rainfall peaks. In conclusion, the CPMP has a large potential to reveal hydrological processes within hillslopes by providing simultaneous observations of soil water content and penetration resistance.