The Global Water Cycle

Updated 9 April, 2004

The Global Water Cycle
USGCRP Program Element

The Global Water Cycle

Overview

Recent Accomplishments

For long term plans, see Water Cycle chapter of the Strategic Plan for the Climate Change Science Program (2003) posted on CCSP web site.

The Water Cycle
Basic background information from NASA's Earth Observatory Reference section.

Proceedings of the Tenth U.S. -Japan Workshop on Global Change: Climate and Water. Workshop held 15-17 January 2003, Irvine, California. Draft dated 30 May 2003. Also available as MS Word file. (link posted 30 May 2003)

MontBlanc CCSP-supported research on the global water cycle focuses on how natural processes and human activities influence the distribution and quality of water within the Earth system, whether changes are predictable, and on the effects of variability and change in the water cycle on human systems. 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 and socioeconomic processes to facilitate efficient water resources management.

Strategic Research Questions

5.1. What are the mechanisms and processes responsible for the maintenance and variability of the water cycle; are the characteristics of the cycle changing and, if so, to what extent are human activities responsible for those changes?

5.2. How do feedback processes control the interactions between the global water cycle and other parts of the climate system (e.g., carbon cycle, energy), and how are these feedbacks changing over time?

5.3. What are the key uncertainties in seasonal to interannual predictions and long-term projections of water cycle variables, and what improvements are needed in global and regional models to reduce these uncertainties?

5.4. What are the consequences over a range of space and time scales of water cycle variability and change for human societies and ecosystems, and how do they interact with the Earth system to affect sediment transport and nutrient and biogeochemical cycles?

5.5. How can global water cycle information be used to inform decision processes in the context of changing water resource conditions and policies?

See Strategic Plan for the U.S. Climate Change Science Program, Chapter 5, for detailed discussion of these research questions.

The water cycle is essential to life on Earth. As a result of complex interactions, the water cycle acts as an integrator within the Earth/climate system, controlling climate variability and maintaining a suitable climate for life. The water cycle manifests itself through many processes and phenomena, such as clouds and precipitation; ocean-atmosphere, cryosphere-atmosphere, and land-atmosphere interactions; mountain snow packs; groundwater; and extreme events such as droughts and floods.

Conceptualization
of the water cycle.

Inadequate understanding of and limited ability to model and predict water cycle processes and their associated feedbacks account for many of the uncertainties associated with our understanding of long-term changes in the climate system and their potential impacts, as described by the Intergovernmental Panel on Climate Change (IPCC). For example, clouds, precipitation, and water vapor produce feedbacks that alter surface and atmospheric heating and cooling rates, and the redistribution of the associated heat sources and sinks leads to adjustments in atmospheric circulation, evaporation, and precipitation patterns.

Clean water is an essential resource for human life, health, economic growth and the vitality of ecosystems. From social and economic perspectives, the needs for water supplies adequate for human uses, such as drinking water, industry, irrigated agriculture, hydropower, waste disposal, and the protection of human and ecosystem health, are critical. Water supplies are subject to a range of stresses, such as from population growth, pollution, and industrial and urban development. These stresses can be affected by climate variations and changes that alter the hydrologic cycle in ways that are currently not predicted with sufficient accuracy for decisionmakers.

Advances in observing techniques, combined with increased computing power and improved numerical models, now offer new opportunities for significant scientific progress. Recently, for example, credible predictions of seasonal variations in the water cycle have been produced for the western United States and Florida. Along with the growing ability to provide advance notice of extreme hydrologic events, this forecast capability provides new options for social and economic development and resource and ecosystem management. In addition, recently launched NASA satellites such as Terra, Aqua, GRACE, and IceSAT, among others, will substantially increase the detailed data needed to better understand and model global and regional water cycle processes.

See also:

Water Cycle [also available: PDF Version]. Chapter 5 from the Strategic Plan for the Climate Change Science Program (July 2003). See also the draft white paper, The Global Water Cycle and Its Role in Climate and Global Change [PDF] (posted 27 Nov 2002).

Water Cycle. Presentation from Breakout Session 8 of the US Climate Change Science Program: Planning Workshop for Scientists and Stakeholders, 3-5 December 2002, Washington, DC.

Climate Variability -- Atmospheric Composition -- Water Cycle. Presentation from Breakout Session 19 of the US Climate Change Science Program: Planning Workshop for Scientists and Stakeholders, 3-5 December 2002, Washington, DC.

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