This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Researchers at University of Tsukuba have found that river responses to rainfall can reach rivers and appear as runoff much more rapidly than previously assumed in mountainous regions. Through continuous monitoring of precipitation and river water levels, they documented rapid rainfall-related responses not only in moderate-relief catchments but also in high-relief catchments with mountain slopes exceeding 500 m in length.
The findings provide new insights into runoff generation and flood development in mountainous regions. The paper is published in the Journal of Hydrology. In steep mountainous regions with large elevation differences, such as the Japanese Alps, rainfall can trigger substantial runoff and sediment transport.
Improving disaster prevention and supporting sustainable land management in these regions require a better understanding of how rainfall generates river runoff under such conditions. However, hydrological monitoring in these environments is challenging because intense water flow and sediment movement can damage monitoring equipment and restrict access to observation sites. Consequently, direct field observations in high-relief, meso-scale catchments (1–10 km² [0.4–4 square miles]) remain scarce worldwide.
In this study, the researchers continuously monitored river water levels at seven sites and precipitation in the University of Tsukuba's Ikawa Forest in Shizuoka Prefecture, Japan, for approximately 1.5 years. The study catchments ranged from approximately 1 to 10 km² (0.4 to 4 square miles) and were located within a tributary basin of the Oi River, an area characterized by steep, rugged terrain. By analyzing rainfall and subsequent river responses across 65 rainfall events, the researchers found that rainfall-related effects could propagate across mountain slopes to rivers in as little as 160 min in some events, even across mountain slopes exceeding 500 m (1,640 feet) in length.
These findings indicate that rapid hydrological responses can occur even in high-relief mountain catchments. The results could improve understanding and prediction of floods and sediment-related hazards in steep mountainous regions. Takafumi Hajika et al., Hillslope lag times for flood runoff are extremely short in high-relief meso-scale catchments, Journal of Hydrology (2026).
DOI: 10.1016/j.jhydrol.2026.136369 MA in English, copy editor since 2021 with experience in higher education and health content. Dedicated to trustworthy science news. Full profile → Master's in physics with research experience.
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