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The Global Water Cycle
Report to the United States
Climate Change Global Research Program
Water Cycle Study Group, 2001*
USGCRP-supported research on the global water
cycle focuses on:
(1) the effects of large-scale changes in land use and climate on the capacity
of societies to provide adequate supplies of clean water; and
(2) how natural processes and human activities influence the distribution and
quality of water within the Earth system and to what extent the resultant
changes are predictable.
Specific areas include: identifying trends in
the intensity of the water cycle and determining the causes of these changes
(including feedback effects of clouds on the global water and energy budgets as
well as the global climate system); predicting precipitation and evaporation on
timescales of months to years and longer; and modeling physical/ biological
processes and human use of water, to facilitate efficient water resources
management.
Figure 5.1. Conceptualization of the water
cycle.

Water vapor is a greenhouse gas that maintains
temperatures in a range required by life on Earth. Many of the uncertainties in
the current projections of the effects of the atmospheric buildup of carbon
dioxide are related to the feedbacks between the climate and the water cycle.
On a national basis, near-crisis situations have occurred
in several dry southwestern river basins, including the Colorado and Rio Grande,
where over-allocation has taken place. Recent drought conditions and rapid
development in these basins have exposed the
intensity of competition that exists over the available water resources. The
development of a capability to predict where water management crises will emerge
due to a drought or extended flood conditions is a priority for the Global Water
Cycle program. The ability to provide probabilistic forecasts of rainfall and
snowfall at various time and space scales is at the center of all potential
applications of climate change science and climate information systems.
Human activity is an integral part of the water cycle. A
recent USGCRP-commissioned report concluded that, among other priorities, there
is a pressing need to determine the causes of water cycle variations on both
global and regional scales, and to what extent these variations are induced by
human activities. In view of this emerging link between water science and water
resource issues, the USGCRP global water cycle strategic plan addresses two
major questions: (1) What are the effects of large-scale changes in land use and
climate on the capacity of societies to provide adequate supplies of clean
water, and (2) how do natural processes and human activities influence the
distribution and quality of water within the Earth system and to what extent are
resultant changes predictable?
The USGCRP Global Water Cycle program focuses on
characterizing, explaining, and predicting variability and long-term changes in
the global water cycle and their impacts. To address the issues arising from the
intimate role of the water cycle in controlling climate variability on seasonal
to multidecadal timescales, the program investigates the pathways of water
movement between the biosphere and surface hydrologic systems, the atmosphere,
and the oceans, as well as feedback processes between climate, weather, and
biogeochemical cycles.
The Office of Science and Technology
Policy, the Office of Management and Budget, and the Council on Environmental
Quality provide oversight on behalf of the Executive Office of the President.
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* Excerpts from the USGCRP Program Element
Courtesy of United States Climate Change Global Research
Program.
Suite 250, 1717 Pennsylvania Ave, NW, Washington, DC 20006.
Telephone: 1-202-223-6262.
The U.S. Global Change Research Program was Codified by Congress in 1990
under P.L. 101-606.
See original at < http://www.usgcrp.gov/usgcrp/ProgramElements/water.htm >.
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