Long-term trend validation and seasonal variation evaluation in groundwater discharge around spring-fed Lake Ezu, Kumamoto City

Kumamoto City relies entirely on groundwater for its municipal water supply, and spring-fed Lake Ezu is a major discharge area of the regional groundwater system. We estimated monthly groundwater discharge to the Suizenji, Kami-Ezu, and Shimo-Ezu zones using river-flow measurements from 13 sites collected between December 1991 and February 2023. Long-term trends were evaluated for the whole period and three subperiods—Period I (December 1991–June 2006), Period II (July 2006–April 2016), and Period III (November 2017–February 2023)—using standardized anomalies smoothed with a 13-month moving average and the Mann–Kendall test. Seasonal variability was characterized using monthly means and coefficients of variation. Total groundwater discharge showed a statistically significant increasing trend over the whole period, and mean discharge rose from approximately 410,000 m3/day in Period I to 520,000 m3/day in Period III. The Suizenji and Kami-Ezu zones together accounted for more than 80% of total discharge and showed statistically significant increasing trends over the whole period, whereas the Shimo-Ezu zone showed a statistically significant decreasing trend. During Period III, monthly discharge remained comparatively high throughout the year in the Suizenji and Kami-Ezu zones, possibly reflecting earthquake-related changes in groundwater flow and recharge. The increase in annual discharge from 2006 to 2007 was evaluated using simple and Antecedent Precipitation Index (API)-weighted three-year precipitation anomalies. Antecedent precipitation was estimated to account for 36–43% of the increase, whereas the remaining 57–64% was treated as a non-precipitation residual that may include artificial recharge in the midstream Shira River area. Because the estimated fractions are sensitive to the selected precipitation metric and regression assumptions, they should be interpreted as indicative rather than as a direct quantification of artificial recharge. These results clarify long-term and seasonal changes in groundwater discharge to Lake Ezu and provide a scientific basis for regional groundwater management.