Hydrologic cycle in mountainous regions with an Asian monsoon climate based on spring runoff characteristics

Kamikochi area in the Japanese Northern Alps For mountains with high precipitation and hydraulic gradients, understanding the hydrologic cycle is important for disaster prevention (i.e., floods and landslides) and water resource management. Mountainous regions with an Asian monsoon climate have higher precipitation than other parts of the world, and the runoff processes that accompany significant precipitation remain largely unresolved. We conducted hydrological observations to clarify the runoff process in mountainous regions with an Asian monsoon climate using the difference in runoff characteristics between mountain-block and floodplain springs in the Kamikochi area. Hydrological observations and analysis under dry/wet conditions have revealed that the floodplain spring has a higher seasonally variable water temperature than the mountain-block spring. Under steady-state conditions, mountain-block groundwater has a larger discharge and helps maintain the baseflow. During precipitation events, the mountain-block spring exhibits a more pronounced increase in runoff than the floodplain spring under wet conditions. This is because the latter interacts with the mainstream through permeable sand/gravel beds, whereas the former is pushed out of the bedrock by precipitation. Thus, the hydrologic cycle in mountainous regions with an Asian monsoon climate is driven by the supply of high-potential mountain-block groundwater to the valley floor. Mountain-block groundwater not only maintains the baseflow but also drives the hydrologic cycle system in mountainous regions.