Water balance characteristics of Kashima plateau basins assuming variable areas of underground catchments

Abstract

Agricultural water use on upland fields depends mainly on precipitation and irrigation by small wells in the Kashima plateau basins. To establish a stable supply of water for irrigated upland fields, an evaluation of water-cycle mechanisms on an arable plateau near plain lakes is important. Previous research suggests that groundwater catchments do not necessarily coincide with the corresponding surface catchments in the plateau basins. However, a physical mechanism of groundwater movement has not yet been clearly demonstrated. In this paper, therefore, we aimed to estimate the areas of groundwater catchments that did not coincide with their corresponding surface catchments, by observing groundwater flow in boreholes and by evaluating water balances of four small basins on the Kashima plateau. As a result, hydrological and groundwater observations in the Kashima-Kohoku basin showed: (1) the groundwater catchment was larger than the surface catchment, as inferred from continuous groundwater observations and the observed directions of groundwater flow, (2) groundwater potential may change after intensive rains, especially after precipitation events of more than 50 mm, and (3) estimates of the long-term water balance confirmed that groundwater flowed into the surface catchment from the outside. The area of the Kashima-Kohoku groundwater catchment was 1.3–2.4 times the area of the surface catchment. Furthermore, the same result was obtained for two other basins.