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.

Akaike is a small temporary pond located at the northern foot of Mt. Fuji, which appears following heavy rainfall events. However, the paucity of water quality and isotope data prevents us from elucidating the details of the formation mechanism of Akaike Pond. In this study, we examined hydrogen (δD) and oxygen (δ¹⁸O) stable isotope ratios and dissolved major ion contents in water samples from Akaike Pond and the surrounding area in August 2021, and compared the results with those in 2020 to reveal the origin of Akaike Pond. The appearance of Akaike Pond in 2021 is closely related to the fluctuation of the water level difference between Lake Sai and Lake Shoji, suggesting that the appearance of Akaike is not solely attributed to the rise in the water level of Lake Shoji. The δD and δ¹⁸O of Akaike Pond were higher than those of the surface and groundwater samples in the surrounding area, showing values closer to those of the rainfall sample in the same period. In addition, the concentrations of Ca²⁺and HCO₃^- in Akaike Pond were lower than those in the water samples around the pond, suggesting that the water in Akaike Pond in 2021, as in 2020, was mainly from the preceding rainfall infiltrated and flowed in a relatively short period. Furthermore, even though the amount of rainfall in 2021 was about half of that in 2020, the concentrations of major dissolved ions in Akaike Pond in 2021 were much lower than those in 2020. Such a relationship cannot be explained by the dilution effect of rainfall, suggesting that different amounts of rainfall likely affected the flow rate and the volume of groundwater that flowed into Akaike Pond.

In this study, a water quality survey was conducted to clarify the existence of hot springs as a geothermal manifestation in the Kara River catchment. This catchment flows through the northeastern part of Akakuradake, which is part of the North Hakkoda volcanoes, where a small volume of hot spring discharge has been observed. The study findings showed that springs emerging from gaps in the gravel on the collapsed slope on the side of the Kara River and the springs flowing out of the fracture in the bedrock in the Kara River riverbed had a water temperature below 25°C. However, H₂SiO₃ concentration exceeded the limit set by the Hot Spring Act. Thus, the presence of cold mineral springs in the Kara River was confirmed.

In this study, we aimed to obtain basic data for understanding the characteristics of oxygen and hydrogen stable isotope ratios (δ¹⁸O·δD) of precipitation in the Seto Inland Sea climate region. In an island with a maximum elevation of about 400 m and an area of about 30 km², precipitation was observed at three sites at different elevations for more than two years to estimate the Local Meteoric Water Line (LMWL), seasonal variations in d-excess values, and the altitude effect. It was estimated that the LMWL was δD=8.9δ¹⁸O+21 (R²=0.92), and lapse rate was 0.21‰. Groundwater recharge line was estimated to be δ¹⁸O=-0.0021H−7.82 based on the 2-year average of a nearby spring. These results could provide important information for the studies about hydrologic cycle and water resources in the target area.

The seasonal variation of stable isotope ratios in precipitation and the influence of typhoons on Ishigaki Island, Okinawa Prefecture were considered based on observational data from January 2013 to March 2017. As a result, both stable isotope ratios and d-excess in precipitation showed seasonal variation, low in summer and high in winter. The slope of the meteoric water line was 8.4 throughout the year, 7.6 in summer, and 6.2 in winter, and the intercepts were 15.56 throughout the year, 6.15 in summer, and 13.91 in winter. The precipitation amount effect was observed throughout the year (-0.76‰/100 mm for δ¹⁸O; -6.1‰/100 mm for δ²H), but a negative correlation was observed between the isotope ratios and temperature, which is the opposite of the temperature effect. Similar results were obtained for partial correlation coefficients excluding relationships between temperature and precipitation. In addition, the stable isotope ratios of precipitation when typhoons approached tended to be lower than usual. A negative correlation was observed between typhoon precipitations and stable isotopes, and the precipitation amount effect was significantly larger than usual (-1.67‰/100 mm for δ¹⁸O; -13.0‰/100 mm for δ²H). In particular, when the typhoon precipitation contribution is greater than 70% to the monthly precipitation amount, the stable isotope ratios tend to be lower than the average for each month.