9:45 AM - 10:00 AM
[HTT16-04] Effects of 20 years of climate change on the spatial distribution of stable isotope ratios of stream water in the Japanese archipelago
Keywords:Stream water, Stable isotope, Spatial distribution, Climate change, the Japanese archipelago
Stream water reflects the water cycle processes within a watershed from precipitation to runoff. The stable isotope ratios of stream water are known to be generally equal to the long-term weighted average of precipitation, and also reflect regional differences observed in precipitation. In the Japanese archipelago, ISOSCAPE was created in 2003 based on samples collected throughout Japan, and the determinants of the distribution have been clarified (Katsuyama et al., 2015). On the other hand, with the progression of climate change, changes in precipitation processes are occurring, such as an increase in the frequency of short, intense rainfall events. Changes in precipitation processes are expected to affect the subsequent runoff processes, with regional differences in the degree of impact. The purpose of this study is to show the distribution of oxygen and hydrogen stable isotope ratios in stream water based on new samples collected nationwide in 2022, and to clarify the changes in relation to climate change over a 20-year period since 2003.
Samples collected from March to December 2022 at 1430 mountain streams in 45 prefectures, excluding Okinawa and Chiba prefectures, were used. δ18O and δ2H were measured and compared with data from 2003. Multiple regression analysis with latitude, elevation, and annual mean temperature was used to develop isotopic ratio prediction equations for 2003 and 2022, respectively. The d-excess values were compared to the cumulative distribution function of values and number of sites.
A clear linear relationship was found between δ18O and δ2H. The spatial distribution pattern of δ18O in 2022 was similar to that of 2003, with values decreasing from south to north, and even smaller in high-elevation areas such as the Japanese Alps and the Daisetsu Mountains in Hokkaido Island. In other words, the distribution clearly reflected latitude and altitude effects. δ18O values were larger nationwide in the comparison between 2003 and 2022. In relation to air temperature, 91% of all sites showed an increase both in air temperature and δ18O value. This fact means that the temperature effect is clearly evident.
The cumulative distribution function of d-excess showed that the curves generally overlapped, but the curve shifted to the right in the d-excess range of 10 to 14, and the number of sites with large d-excess values increased. These sites were mostly located on Kyushu, Shikoku, and the Pacific side of Chubu region. The d-excess is an index that reflects the evaporation process of the source water vapor, and an increase in d-excess in precipitation indicates an increase in the evaporation rate. In recent years, it has become clear that the number of short-duration, concentrated heavy rainfall events, called extreme rainfall events, has been increasing. The distribution of d-excess may reflect regional differences in the effects of such extreme rainfall events.
As described above, the distribution of stable isotope ratios in stream water across Japan not only clearly shows the increase in temperature associated with climate change over the past 20 years, but also reflects changes in precipitation processes and water cycle processes associated with climate change. ISOSCAPE should be considered as a snapshot of a single point in time of dynamic change, and should be updated periodically to examine its causes and effects.
Samples collected from March to December 2022 at 1430 mountain streams in 45 prefectures, excluding Okinawa and Chiba prefectures, were used. δ18O and δ2H were measured and compared with data from 2003. Multiple regression analysis with latitude, elevation, and annual mean temperature was used to develop isotopic ratio prediction equations for 2003 and 2022, respectively. The d-excess values were compared to the cumulative distribution function of values and number of sites.
A clear linear relationship was found between δ18O and δ2H. The spatial distribution pattern of δ18O in 2022 was similar to that of 2003, with values decreasing from south to north, and even smaller in high-elevation areas such as the Japanese Alps and the Daisetsu Mountains in Hokkaido Island. In other words, the distribution clearly reflected latitude and altitude effects. δ18O values were larger nationwide in the comparison between 2003 and 2022. In relation to air temperature, 91% of all sites showed an increase both in air temperature and δ18O value. This fact means that the temperature effect is clearly evident.
The cumulative distribution function of d-excess showed that the curves generally overlapped, but the curve shifted to the right in the d-excess range of 10 to 14, and the number of sites with large d-excess values increased. These sites were mostly located on Kyushu, Shikoku, and the Pacific side of Chubu region. The d-excess is an index that reflects the evaporation process of the source water vapor, and an increase in d-excess in precipitation indicates an increase in the evaporation rate. In recent years, it has become clear that the number of short-duration, concentrated heavy rainfall events, called extreme rainfall events, has been increasing. The distribution of d-excess may reflect regional differences in the effects of such extreme rainfall events.
As described above, the distribution of stable isotope ratios in stream water across Japan not only clearly shows the increase in temperature associated with climate change over the past 20 years, but also reflects changes in precipitation processes and water cycle processes associated with climate change. ISOSCAPE should be considered as a snapshot of a single point in time of dynamic change, and should be updated periodically to examine its causes and effects.