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Page 3 1 Setting Priorities Study of the water cycle is a priority in global environmental research because this cycle is central to the working of the Earth system. Furthermore, the availability of unpolluted water to sustain life and fuel economies is perhaps the most important recurrent constraint in human history, and it will remain so for the foreseeable future. During this past decade, concomitant with consideration and implementation of several of the findings in Opportunities in the Hydrologic Sciences (NRC, 1991), an in-depth understanding of the water cycle, especially at regional scales, has emerged as a major scientific challenge. This is reflected in the research priorities highlighted in several National Research Council (NRC) reports dealing with global change, climate variability, environmental quality, and hazards mitigation (NRC, 1998b–e). Moreover, along with carbon, water is one of the two major themes in the NRC report Global Environmental Change: Research Pathways for the Next Decade (NRC, 1998a; the so-called Pathways report), and water now has been established as one of the key program elements of the U.S. Global Change Research Program (USGCRP, 1999). The NRC and USGCRP reports generally present water as a critical component of other systems. From this perspective, many but not all priorities for hydrologic sciences emerge as components in cross-cutting programs such as the USGCRP. The Pathways report and the USGCRP implementation plan, Our Changing Planet (USGCRP, 1999), identify some key hydrologic science challenges, but there are also critical hydrologic issues that are not contained therein. The USGCRP has progressed in its recognition of hydrology as a key issue. In the FY1999 edition of Our Changing Planet (USGCRP, 1998), hydrologic research and applications are discussed only in the context of climate variability and change research on seasonal-to-centennial time scales. In the FY2000 edition (USGCRP, 1999), however, the hydrologic cycle is identified as one of USGCRP's six fundamental program elements: (1) understanding Earth's climate system, (2) biology and biogeochemistry of ecosystems, (3) composition and chemistry of the atmosphere, (4) paleoenvironment/paleoclimate, (5) human dimensions of global change, and (6) the global water cycle. Terrestrial hydrologic processes, specifically the storage and movement of water on and within land and within the terrestrial biosphere, are important across all these elements and should serve as a unifying physical process of USGCRP activities. The global water cycle element contains two primary research challenges. The first challenge relates to land surface interactions and the need to develop a better understanding of (1) the
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Page 4 coupling of land surface hydrologic processes to atmospheric processes over a range of spatial and temporal scales; (2) the role of the land surface in climate variability and climate extremes; and (3) the role of the land surface in climate change and terrestrial productivity. The second relates to atmospheric processes and the need to develop a better understanding of (1) the role of clouds and their influence in the coupling of the atmospheric water and energy cycles and (2) the vertical transport and mixing of water vapor on scales ranging from the local boundary layer to regional weather systems. The global water cycle program element bridges the gap in the spatial-scale spectrum between large-scale atmospheric research and smaller-scale hydrologic research. Its proposed endeavors that are related to the ''fast component" of the climate system should simultaneously draw upon and feed into international programs such as the Global Energy and Water Cycle Experiment (GEWEX) and the Biological Aspects of the Hydrologic Cycle (BAHC). This program element also addresses important "slow component" aspects of the climate system, therefore relating it strongly to the Program on Climate Variability and Predictability (CLIVAR) as well. These challenges are important, but limited. Broader challenges for hydrologic sciences that address cross-disciplinary research and recognize the integrative nature of terrestrial hydrology would strengthen the USGCRP. Two strategic research areas are identified in this report: (1) predictability and variability of regional and global water cycles and (2) coupling of hydrologic systems and ecosystems through biogeochemical cycles, including the characterization of water and chemical flow pathways at the surface-atmosphere and the surface-subsurface interfaces. Within the first topic, predictability directly addresses the USGCRP priority of identifying possible future environmental changes and of defining the limits of prediction. The emphasis on variability, particularly in regional hydrologic systems, is designed to link understanding of the global water cycle with the emerging regional and local issues that are receiving increasing emphasis in USGCRP. Variability (and memory) in the global water cycle and regional hydrologic systems is due to both the cycling of water between reservoirs with various storage capacities (e.g., atmosphere, surface waters, near-surface soil moisture, and groundwater systems) and the development of feedback dynamics resulting from linkages among the reservoirs. Presently, for example, linkages to the groundwater reservoir are not considered in the USGCRP. The ultimate aim is to use the enhanced understanding of linkages in the variability of global and regional systems as the basis for producing improved and useful hydrologic predictions. The second topic addresses priorities identified in both Our Changing Planet (USGCRP, 1999) and the Pathways report and highlights the need to understand linkages between the cycles of water, energy, nutrients, and carbon through ecosystems. Terrestrial ecosystems exert a strong influence on the water cycle through evaporation processes. Evaporative flux linking the surface and atmospheric systems and the recharge flux linking the surface and subsurface systems are two key components of the hydrologic cycle that are poorly understood and are very poorly monitored. It is imperative that the characterization of these two fluxes be recognized as grand challenges for hydrologic science. Finally, ecosystem disturbances are likely to be a major pathway for any changes and shifts in water and chemical cycles resulting from human activity. This cross-discipline research area is key to addressing challenges identified under both the USGCRP global carbon cycle (contained within the biology and biogeochemistry of ecosystems USGCRP program element) and the global water cy-
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Page 5 cle element. The foundation for this research must be a better understanding of water and chemical pathway, and of hydrologic system-ecosystem linkage, and a new means of achieving that understanding. It will then be possible to address the combined influences of climate change and land use change, which occur both in the context of natural and human-induced variability, on hydrologic systems and ecosystems.
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