Developing the soil moisture sensor using optical fiber technique (Joint research)

Komatsu, Mitsuru*; Nishigaki, Makoto*; Seno, Shoji*; Hirata, Yoichi*; Takenobu, Kazuyoshi*; Tagishi, Hirotaka*; Kunimaru, Takanori; Maekawa, Keisuke  ; Yamamoto, Yoichi; Toida, Masaru*; Nakano, Katsushi*

This research focused on methods of estimating the amounts of groundwater recharge, which are normally required as upper boundary conditions in groundwater flow analyses, based on measurements of infiltrating water in the ground, to systematize the methods and establish systems which are stable and measurable on site over a long time. Regarding developing measurement systems, fiber-optic strain measurement methods that enable multiple-point and long-distance measurement were used for measuring three quantities: suction pressure, soil moisture and volumetric water content rate obtained by applying water absorption swelling material, and each measurement was discussed. The results showed that the fiber-optic soil aquameter has two types of practical application: one for measuring suction pressure (Type I), and the other for measuring volumetric water content rate obtained by applying water absorption swelling material (Type III). Furthermore, by using measurement instruments in actual fields, the validity of the two methods for estimating the rainfall infiltration capacities of shallow-layer soils, that is, estimating the capacities either directly by measurements of soil water or from unsaturated hydraulic conductivities, was confirmed.