2025 年 16 巻 2 号 p. 19-24
The formation of a freshwater lens, a distinctive lens-shaped body of underground freshwater, is a marvel in areas surrounded by saltwater, such as islands and peninsulas. Despite saltwater infiltration beneath underground freshwater, the density difference ensures that the freshwater floats on the saltwater and remains separate. This freshwater lens, replenished by rainwater infiltration and temporarily storing freshwater, is a precious water resource of immense significance. A freshwater lens has been used for agriculture in the Yumi-Hama Peninsula in Tottori, Japan. However, the limited number of studies on the availability of freshwater resources and their fluctuations in this area is a cause for concern for future sustainability. Moreover, droughts have occurred frequently. Therefore, unstable surface water, such as rivers, reinforces the importance of sustainable groundwater usage. This study investigated the effects of precipitation on groundwater levels using a cross-correlation analysis of the Antecedent Precipitation Index (API) and groundwater levels. The API is an indicator of precipitation involved in groundwater level fluctuations, which is calculated using an exponential function to describe underground infiltration and storage processes. We found positive correlations between the API and groundwater levels at some sites. However, the correlation coefficients were relatively low at sites close to canals, large-scale farms, and shorelines. Possible factors contributing to the low correlation coefficients were the effects of infiltration from canals, pumping for agricultural purposes, and tides. In addition, the time lags for rainwater to reach the groundwater table from the land surface were within 1 day at most sites. These can be explained by the high hydraulic conductivity of sandy soil in the study area.
