New paper published on climate change impacts on seasonal runoff in the Japan Sea region

Climate change impacts on seasonal runoff in the snowy Japan Sea region of Honshu, Japan

Al- Shakil, Andrew Charles Whitaker
Hydrological Research Letters 19(1), pp 8-14
DOI: 10.3178/hrl.24-00017

Abstract
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.

Figure 1. Locations of the streamflow gauging stations in the Japan Sea region of eastern Japan

Figure 2. (a) Long-term temperature trend during snow accumulation season (DJFMA) and seasonal snowfall trend at Tokamachi FFPRI, Niigata Prefecture (Shinano River Basin), (b) Monthly streamflow fractions (monthly runoff/ annual runoff) for the Uono River (1408 km2), a major tributary of the Shinano River. Comparing the early period with the later period, dominant spring snowmelt runoff (Apr–May) is decreasing, while winter season runoff (Dec-Mar) is increasing

Figure 3. Comparison of seasonal runoff trends for the Uono River, showing significant increasing runoff in winter to early spring (Dec–Mar) is balanced by significant decreasing runoff in April, as indicated by the Sen’s slope of the Mann-Kendall trend test

Figure 4. Relationship between (y) seasonal runoff timing (CT) and (x) mean temperature during snow cover season (DJFMA) for river basins Uono (a), Shinano (b), Agano (c) and Akagawa (d) in the southern region (37.24–38.63 degrees N) and Mogami (e), Omono (f), Yoneshiro (g) and Iwaki (h) in the northern region (38.76–40.81 degrees N)