Al- Shakil, Andrew Charles Whitaker
Received 18 July, 2024
Accepted 5 September, 2024
Published online 31 January, 2025

Al- Shakil¹⁾, Andrew Charles Whitaker²⁾

1) Graduate School of Science and Technology, Niigata University, Japan
2) Institute of Science and Technology, Niigata University, Japan

Historically, the Japan Sea region of Honshu has experienced some of the heaviest snowfalls in the world, though snowfalls are now decreasing due to global warming. This study examined changes in streamflow seasonality and trends in monthly runoff during the past 60 years at nine river basins located across Niigata, Yamagata, Akita and Aomori Prefectures. A streamflow seasonality index (Center Time, CT) was adopted, and meteorological stations paired with gauging stations to analyze the dependence of streamflow seasonality on air temperature. Overall, there is a strong tendency for winter and early snowmelt season flows to increase (December to March), while the peak snowmelt season flows in April are decreasing. In most cases, CT shows a trend for earlier seasonal runoff, and we confirm a strong linear relationship between CT and temperature during snow cover season (December to April). The relationship between CT and temperature appears to be stronger in the south of the study region (mean R² = 0.64) than in the north (mean R² = 0.37). In addition, the regression slope (temperature sensitivity of CT) is greater in the southern region, with an average value of –6.3 days per degree Celsius compared to –4.2 days per degree Celsius in the northern region.

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