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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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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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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.
Representative terms from entire chapter:
global water