RIVER CUTOFF AND SPRING DEPLETION IN MAKI, OZU, KUMAMOTO

Abstract

 The irrigation areas in the Maki District have decreased since the Yagogawa river was cut off following the 2016 Kumamoto earthquake. Then, owing to the Shimoyama spring depletion in February 2018, the winter flooding areas decreased. We aimed to understand the causes of the river cutoff and spring depletion, with the goal of regenerating irrigation and winter flooding patterns to promote groundwater recharge. Thus, in this study, we evaluated hydrological data, including rainfall and groundwater level, combined with the status of a closed-type sabo dam constructed in 2012 on the Yagogawa river. We discovered that drainage holes in the dam became clogged, decreasing river discharge below the dam. A rapid rise in groundwater levels following the 2016 Kumamoto earthquake may have caused forceful drainage of mountain area groundwater, significantly reducing upstream runoff. Therefore, the sabo dam blockage and earthquake effects on groundwater redistribution might have been the primary causes of the river cut-off. The spring water was not depleted immediately after the earthquake owing to the contribution of groundwater draining from the mountain and underflow water from the Yagogawa river, which recovers during the rainy season. However, the spring was depleted more than one year after the earthquake since the mountain groundwater, which formerly supplemented the underflow water, gradually flowed downstream.