Isotopes in the Water Cycle: Regional- to Global-Scale Patterns and Applications

Stable isotope ratios of hydrogen and oxygen have been applied to water cycle research for over 60 years. Over the past two decades, however, new data, data compilations, and quantitative methods have supported the application of isotopic data to address large-scale water cycle problems. Recent results have demonstrated the impact of climate variation on atmospheric water cycling, provided constraints on continental-to global-scale land-atmosphere water vapor fluxes, revealed biases in the sources of runoff in hydrological models, and illustrated regional patterns of water use and management by people. In the past decade, global isotopic observations have spurred new debate over the role of soils in the water cycle, with potential to impact both ecological and hydrological theory. Many components of the water cycle remain underrepresented in isotopic databases. Increasing accessibility of analyses and improved platforms for data sharing will refine and grow the breadth of these contributions in the future. Isotope ratios in water integrate information on hydrological processes over scales from cities to the globe. Tracing water with isotopes helps reveal the processes that govern variability in the water cycle and may govern future global changes. Improvements in instrumentation, data sharing, and quantitative analysis have advanced isotopic water cycle science over the past 20 years.