Potential Effects of Grassland Restoration on the Water Resources in Nango-Dani, Aso, Japan

The semi-natural grasslands of the Aso Caldera, Japan, have historically played a key role in maintaining biodiversity, tourism, and water resources. However, they are now in decline due to a decrease in the number of agricultural workers and an aging workforce, as well as structural changes and stagnation in the agricultural and livestock industries. This study focused on the water resource maintenance function of grasslands by applying a water balance model to quantify the potential impact of grassland restoration on water resources in Nango-Dani, located in the southern part of the Aso Caldera. We simulated groundwater recharge, storage, spring discharge, and baseflow under multiple scenarios involving the conversion of coniferous trees to grasslands. According to the calculation results, replacing 10% of coniferous trees with grassland increased groundwater recharge by approximately 0.86 million m3. This increase is due to grasslands having a higher groundwater recharge capacity, owing to their higher canopy permeability and lower evapotranspiration. The storage volume increased by approximately 0.54 million m3, which is equivalent to the annual water usage of 6700 people. Furthermore, grassland restoration increased spring discharge and baseflow. These results quantitatively demonstrate a significant enhancement of regional water resource sustainability and provide scientific evidence to inform land-use policies.