Journal of the Japan Society of Erosion Control Engineering,
Vol.57,No.3,,2004
Combined penetrometer]moisture probe for measuring water content distribution in hillslope
Ken'ichirou KOSUGI, Daizo TSUTSUMI, Takahisa MIZUYAMA, Shuzo HASEGAWA
Abstract
A combined penetrometer-moisture probe (CPMP) was developed for simultaneous measurements of penetration resistance and soil water content distributions in a hillslope. The probe uses the time domain reflectometry (TDR) technique to determine soil water content. The TDR moisture sensor consists of two parallel stainless wires, each 0.3 mm in diameter and 20cm long, coiled around a polyvinyl chloride (PVC) column 18mm in outer diameter. A calibration curve relating the dielectric constant measured by CPMP to water content was obtained in the laboratory for Toyoura standard sand, which was packed in a container together with the moisture sensor. The obtained data were described well by the mixing model approach. However, a different calibration curve was obtained for the same Toyoura standard sand, when the dielectric constant was measured during penetration tests. It was presumed that the difference could be attributed to inadequate contact between sand and moisture sensor. Subsequently, field experiments were conducted at a forested hillslope consisting of weathered granite soil to measure simultaneously the penetration resistance and water content along soil profiles. Results showed that penetration resistances measured with CPMP and commonly-used Hasegawa-type penetrometer were about the same. By using a specific calibration curve, the soil water content estimated by CPMP was similar to the water content obtained by the gravimetric method (R2=0.791). CPMP succeeded in measuring detailed water content distributions in the slope under various wetness conditions. The accuracy in the water content measurement was slightly increased when the soil-probe contact was evaluated by penetration resista
Key wordsFCone penetrometer, TDR moisture sensor, Water content distribution, Spatial variation, Forested hillslope
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