Local hydrological processes in a fractured bedrock and the short-term effect on gravity at Matsushiro, Japan

► At the underground station of Matsushiro, a decrease in gravity is observed when it rains. ► We analyze the data of “precipitation” inside the tunnel, as well as soil moisture at the surface of the hill. ► The soil layer plays a role of a water reservoir, whereas the body of the hill acts as a conduit of water. ► The linear rate of gravity recovery is given an explanation from a hydrological viewpoint. ► The underground observatory of Matsushiro provides an ideal labolatory for combined studies of gravimetry and hydrology.