Abstract

To survey heterogeneous hydrological properties in a footslope area of a mountainous watershed, we applied a new type of combined penetrometer–moisture probe (CPMP) for simultaneous measurements of soil water content, θ, and penetration resistance, Nc, of soil mantles. We examined the usefulness of CPMP data for inferring hydrological properties in a footslope area by comparing the spatial distributions of θ and Nc with pressure head, ψ, obtained by continuous monitoring using densely nested tensiometers. By comparing θ and the topographic index of the bedrock surface obtained by the CPMP, we could effectively detect the heterogeneous water distribution that exists independently of topographic flow convergence. Such anomalous water distribution patterns suggest the existence of a preferential pathway, a soil layer with low permeability, and bedrock groundwater exfiltration. The CPMP data corresponded well with the tensiometer-observed ψ under no-rainfall conditions. The results show that we can estimate the complex shape of the water flow domain, hydrological baseline, and point of bedrock groundwater exfiltration using CPMP measurements, without measuring ψ by tensiometers and soil hydraulic conductivity by collecting soil samples. Moreover, complicated water movement phenomena that occur during storm events, such as preferential water flow and variations in the hydrological baseline, could be inferred from the CPMP data under no-rainfall conditions. The CPMP technique was proven to be effective for understanding the heterogeneous hydrological properties of a footslope area.

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