species extinction are highest for freshwater organisms. At the core of the solutions associated with these complex challenges is hydrologic science.

Catalyzed in part by the 1991 National Research Council (NRC) report Opportunities in the Hydrologic Sciences, the field of hydrologic science (or “water science” or hydrology) has developed into a distinct Earth science discipline with the goal of understanding the movement of water at all scales and environments and its interaction with climate and life on Earth. This understanding is motivated as much by scientific curiosity as by the desire to address critical societal problems related to water and its impact on human welfare and the environment. Over the past 20 years, new scientific understanding has been enabled by unprecedented measurements and observations of hydrologic processes, made possible through technological and scientific advances in chemical analytical instrumentation, new sensor development, remote sensing and geophysical techniques, increased computation capabilities, and improved hydrologic modeling. Today, hydrologic science is a distinct and critical component of geosciences, linking the atmosphere, land, and oceans and contributing to understanding life on Earth. By its very nature, hydrologic science stands at the interdisciplinary interface with other geosciences, such as atmospheric, ecological, and biological sciences. As a result, new subdisciplines have emerged or old subdisciplines are maturing that advance the frontiers of interdisciplinary research, e.g., hydroclimatology, hydrometeorology, geobiology, hydroecology, hydrogeomorphology, ecogeomorphology, and Earth-surface dynamics. Hydrologic science is central to all of these fields and, therefore, is becoming itself redefined and enriched.

The National Science Foundation (NSF) requested that the NRC (1) review the current status of hydrologic science and its subfields and the coupling with related geosciences and biosciences, and (2) identify promising new opportunities to advance hydrologic sciences for better understanding of the water cycle that can be used to improve human welfare and the health of the environment (Box S-1).

In response, the NRC formed the Committee on Challenges and Opportunities in the Hydrologic Sciences, which authored this report. The report is written for the members of the hydrologic community, mainly the research community, which includes not only academics but also scientists and engineers from the private sector, federal agencies (most notably the Hydrologic Science program and other Earth Science programs within NSF, when appropriate), decision makers interested in water research and policy, and those with Earth sciences and water resource-related missions interested in where hydrologic science fits into the surface-earth sciences. The report is also written for graduate and undergraduate students seeking inspiration, general knowledge of the field, or guidance when selecting a focus within the field. Although the primary audience is the hydrologic

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