In order to contribute to climate change adaptation, we conducted hydrological observations and examined runoff characteristics with meteorological conditions during the cold season (November to May) from 2020 to 2022 at the Nagasaka Experimental Watershed (NEW) covered by Japanese cedar plantation in Akita Prefecture. According to observations by AMeDAS Takanosu in the past 43 years near the NEW, the midwinter (January to February) of 2020 was the second warmest and the snow depth in early March of 2020 was the lowest on the record, making it a rare warmer and less snowy year. The daily runoff in the midwinter dispersed among plentiful, ordinary, low, and scanty runoffs in the discharge duration curve, and the slope of the curve was gentler than other years. In the midwinter of 2022, the average temperature was lower, and the daily runoff during the midwinter was relatively constant and concentrated on low runoff, resulting in an increase in snowmelt water in spring (March to April). In the warmer with less snowy year, it was suggested that the risk of disasters due to flooding was low and that the risk of drought was high in early spring for years with the less amount of precipitation. Observation data in this study would contribute to evaluate the impact of climate change on runoff characteristics and to establish suitable adaptation plans.

From the Global Network of Isotopes in Precipitation (GNIP) dataset, Reykjavik and Espoo in the Northern Europe, where there is a negative and a positive correlation between the North Atlantic Oscillation (NAO) index and stable isotopes in precipitation, respectively, were extracted and revealed the differences in the fluctuation factors between them. The influence of temperature effect on the total variation of δ¹⁸O in precipitation was less than 30% in Reykjavik, while it was more than 85% in Espoo. And composite analysis on the distribution of δ¹⁸O in precipitation and atmospheric circulation fields were conducted using the long-term data of an isotope-incorporated atmospheric general circulation model. Furthermore, the difference of moisture flux was analyzed separately high NAO years and low NAO years. As a result, the δ¹⁸O in precipitation is low (high) due to moisture flux which has higher (lower) stable isotopes from west (south) in high (low) NAO years in Reykjavik. While, the δ¹⁸O in precipitation is high (low) due to the strong (weak) westerly wind nearby 50°–60°N, and the moisture flux from the Atlantic Ocean which has higher stable isotopes is a large (small) amount in high (low) NAO years in Espoo.