Advances in precipitation isoscape studies and its basis/application were reviewed with a focus on mountainous regions. A recent study showed that precipitation isoscapes over Japanese Alps region can be mapped by a relatively simple model considering altitude and inland effects. However, accurate modelling of a steady state isoscape requires isotope monitoring longer than a few years to reduce errors associated with year-to-year variations. Published values of isotopic lapse rates are likely to be overestimated because of too much emphasis of snow/ice data and an overlapping of altitude and inland effects. Precipitation isotope maps for mountainous regions have been applied for the following purposes so far: areal estimation of recharge elevation for groundwater and spring, scaling up of hydrograph separation, improvements of transit time estimation and runoff/groundwater models for large basins, and basin-scale evapotranspiration partitioning. These advances are promising especially in detecting changes in alpine environment and in providing scientific basis for river basin managements. Innovations in mapping with finer temporal resolutions and in applying to ecological, anthropological, and forensic studies will be important as well as continuous monitoring for longer time scales.
The data assimilation has been realistic with stable water isotope circulation models and spectroscopic observations for vapor isotopes. This tendency is most likely due to the rapid improvements in both the modeling and the measuring. The recent study revealed that it is possible to constrain the atmospheric fields, such as wind, air temperature, pressure, etc. by putting vapor isotope observations into the atmospheric data assimilation system. This technique would help us to more understand the atmospheric cycle system and/or the land-atmosphere interaction. On the other hand, the data assimilation has been frequently used in climate and paleoclimate studies. In particular, newly developed “offline data assimilation” technique would drastically reduce the computation cost. All of reviewed studies in this paper did not directly use measured isotopic information, but used converted information with empirical relationship to reconstruct the climate. However, it is known that the relationship between isotope and climate is not stationary in time and space. Therefore, it will be more and more important to develop a new method directly using the isotope information in the data assimilation system.
Water vapor origins (i.e., the sources of evaporated moisture) were estimated based on the deuterium excess (=δD-8×δ18O, hereafter d-excess) in precipitation, which was primarily determined by relative humidity, sea surface temperature, and wind speed as water vapor evaporates and is conserved during the atmospheric moisture transportation process. This review summarizes observation studies on d-excess in precipitation and numerical studies about estimation of water vapor origin over Japan. Moreover, this review discusses the potential for using d-excess in precipitation as a tracer of atmospheric hydrological cycles. The results revealed that precipitation originating from the Sea of Japan was most likely not predominant in the region, other than on the Sea of Japan side, despite the fact that d-excess values in winter were above 20‰ or higher than in summer. On the other hand, high d-excess values in precipitation appeared to lead to a high contribution of water vapor evaporated from the Sea of Japan to precipitation during winter. These results suggest that the d-excess in precipitation has potential applications in estimating the contribution ratio of water vapor originating from the Sea of Japan. Combining these key findings with isotope mapping can provide useful information for paleoclimate research and water resource management.
To elucidate the spatial distribution and seasonal variability of stable isotopes in precipitation across Japan, the Isotope Mapping Working Group of the Japanese Association of Hydrological Sciences conducted precipitation sampling during an intensive observation period throughout 2013 (IOP2013). More than 2000 precipitation samples were collected at 56 stations, and their stable water isotopes (δ18O, δD) were measured at Kumamoto University. The time-series of δ18O and d-excess of precipitation showed rapid and significant seasonal variability, affected by precipitation type (i.e., the winter monsoon and Nangan-Low) and by the amount of rainfall during the Baiu rainy season. The monthly mean δ18O and d-excess of IOP2013 were averaged in each of six regions across Japan and compared with previous studies, although the sampling periods varied. The seasonal variability of the monthly δ18O anomaly from the annual mean was not coincident, indicating the deviation depended on the sampling period. Conversely, the seasonal variability of the monthly d-excess was coincident, even though the absolute values were averaged. Based on the results of short-term sampling at each station, the temperature effect was found north to 35°N and also the precipitation amount effect was mostly found south to 37°N. Further observational data will be required to increase the spatial resolution for better understanding of the spatiotemporal variability of stable isotopes in precipitation across Japan.
The monthly precipitation samples have been taken at 8 sites (Ogawa Town, Utsunomiya City, Kumagaya City, Kashiwa City, Tsukuba City, Fukushima City, Matsumoto City and Kyoto City), and the stable isotope of oxygen and hydrogen of these rainfall samples were analyzed. As a result of correlation analysis, the stable isotopes (δ18O and δD) of precipitation and the elevation have a good negative correlation. Since the stable isotope ratio of monthly precipitation at each observation point varies almost same, it is assumed that the water vapor which is origin of rainfall is almost same at Kanto district, south Tohoku district and Chubu district (central Japan). The stable isotope ratios of precipitation at Ogawa, Utsunomiya, Kumagaya and Kashiwa are lowest in January 2013. In this period, heaviest snowfall occurred because of the passage of a Pacific Coast Low (nangan Low pressure). The isotope ratios of snowfall event typically are relatively low, hence the isotope ratios in this period might be low. However, in Fukushima and Matsumoto, the stable isotope ratios of monthly precipitation the previously mentioned month are not so low. It is different from the result of Kanto district. As a result of the temporal variation of weighted-mean annual isotope ratios of each site, the isotope ratios at Ogawa have shown a decreasing tendency. Recently, the rainfall event which show the high rainfall intensity is increasing, so the isotope ratios of precipitation might be changed affected by the rainfall intensity. In the case of Matsumoto should be explored in the future. Since it is expected that the isotope ratios of precipitation will be changed in future because of global climate change, the maintaining a continuous observational survey is critical.
We determined oxygen and hydrogen stable isotope ratios (δ18O and δD) of water in fruits (citrus) and vegetables (ginger) using cavity ring-down spectrometry (CRDS) for assessment of their authenticity. The δ18O and δD values of fruits and straight juice had higher than those of concentrated juice. The citrus fruits from Japan had relatively lower δ18O and δD values of than those from Australia, South Africa and the United States. The δD values and d-excess of ginger samples from Japan were relatively higher than those of ginger samples from China. The δ18O and δD values of water in fruits and vegetables would be representative of the ambient water, depending on geographical parameters such as the latitude and altitude. These results suggested that δ18O and δD values of water in fruits and vegetables by using CRDS would be potentially useful for assessment of their authenticity.