Geological controls on snowmelt runoff in a snowfall-dominated region of northern Japan: Implications for water resources management using decentralized water supply systems

Abstract Mountainous rural regions are facing population decline and aging water infrastructure, increasing the need for decentralized water supply systems. Implementing these systems requires identifying watersheds suitable as water sources and their spatial distribution. In snowfall-dominated regions, where snowmelt runoff constitutes a major seasonal water resource, understanding the controls on snowmelt runoff is essential for identifying such watersheds. Although geological setting is often a primary control on runoff and therefore a key factor in identifying such watersheds, its influence on snowmelt runoff and its interactions with climatic conditions remain poorly understood. We evaluated the controls of climate, watershed geology, and their interactions on quick and delayed discharge during the snowmelt period across 33 forested watersheds (2.1–21.9 km2) in the Sorachi River basin, Hokkaido, northern Japan, using 153 watershed-year observations. Linear mixed models revealed that watershed geology exerted the strongest control on delayed discharge. In particular, delayed discharge increased with volcanic rock proportion, which also interacted significantly with snowfall water equivalent, indicating stronger geological effects under high-snowfall conditions. In contrast, quick discharge was controlled primarily by climatic conditions. Although volcanic rock proportion proved to be an effective indicator for identifying suitable source watersheds, watersheds with high volcanic rock proportions were concentrated in the eastern basin. In contrast, high-relief watersheds, the second most important control on delayed discharge, occurred mainly in the central and western parts of the basin, where non-volcanic geology predominates. Considering these complementary spatial patterns provides an important framework for decentralized water supply systems in mountainous rural regions.