Strategic Plan for the
Climate Change
Science Program
Review draft, November 2002

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See also the white paper:
The Global Water Cycle and Its Role in Climate and Global Change  [PDF]
(posted 27 Nov 2002).

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Chapter 7:
Water Cycle

The global water cycle is an integral part of the Earth/climate system, manifesting itself through many processes and phenomena, such as clouds, precipitation, mountain snow packs, groundwater, droughts, and floods. The cycling of water exerts an important control on climate variability as a result of its complex feedbacks and interactions with other components of the climate system. Many of the uncertainties with respect to long-term changes in the climate system and their potential impacts, as described in Intergovernmental Panel on Climate Change (IPCC) reports, arise from our inadequate understanding of, and inability to model, water cycle processes as they feed back on the climate system. In particular, clouds, precipitation, and water vapor produce feedbacks that alter surface and atmospheric heating and cooling rates, and redistribution of the associated heat sources and sinks lead to adjustments in atmospheric circulation and precipitation patterns. The current inability to adequately represent these complex multiscale processes in climate models is a major source of uncertainty in long-term climate change projections and impacts, and seasonal to interannual climate forecasts.

The needs for adequate supplies of clean water and advance preparations for extreme hydrologic events, such as floods and droughts, pose major challenges to social and economic development and to the management of natural resources and ecosystems. Water supplies are subject to a range of stresses, such as population growth, pollution and industrial and urban development. These stresses are exacerbated by variations and changes in climate that alter the hydrologic cycle in ways that are currently unpredictable. These concerns are documented in a recent report on research needs and opportunities, A Plan for a New Science Initiative on the Global Water Cycle (Hornberger et al., 2001). This report identified questions and strategies for research on climate change and water cycle trends, prediction, and the linkages between water and nutrient cycles in terrestrial and freshwater ecosystems.

Advances in observing techniques, combined with increased computing power and improved numerical models, now provide new opportunities for significant scientific advances through a concerted, integrated Global Water Cycle research effort. Recently, reasonably accurate predictions of variations in the water cycle have been produced for some years in some regions. This new capability to produce credible predictions provides a basis for dialogue between the scientific community and water system and land managers. This dialogue is enabling the research community to understand decisionmakers' management processes and information needs. It will also identify opportunities for improving the adaptability of infrastructure and management practices to runoff variations, long-term changes and extremes.

To address the urgent need for better information on the water cycle, the Climate Change Science Program (CCSP) is planning its Global Water Cycle research program around two overarching questions, namely:

Recent observations suggest that there have been notable changes in critical water variables: precipitation amounts, location, and type; surface and subsurface runoff; cloud cover, both amount and type; atmospheric water vapor; soil moisture; groundwater; etc. Although techniques for measuring many of these variables have improved, the number of observations is limited and, in some cases, new sensors are needed. Current models cannot properly simulate the global water cycle. Moreover, we cannot definitively attribute observed trends to human-induced climate changes as opposed to natural variability.

New observing capabilities, both satellite and in situ, will be critical to detecting patterns and quantifying fluxes, especially instruments for global measurement of terrestrial water cycle variables such as soil moisture. Existing in situ networks need to be maintained and enhanced, and data sets developed to ensure consistency between historical and new observations. Network enhancements and open data exchange are needed to address water quantity issues in critical areas such as high mountain areas and river deltas. Also needed are new data assimilation techniques that combine different kinds of data, and data with varying spatial and temporal characteristics, to produce consistent data products for research and process studies of key water cycle variables, such as clouds, precipitation, and soil moisture. Complementary research is planned under the Land Use/Land Cover Change program (Chapter 8).

As global temperatures warm, the atmosphere will hold more moisture. Given the same carbon dioxide (CO₂) increase, climate models produce different rates of warming and drastically different patterns of circulation, precipitation, and soil moisture depending on their parameterizations (simplified representations) of basic water cycle processes. This large discrepancy in model predictions indicates that the representation of key water cycle processes is rudimentary at best. A better understanding of these changes and the consequences of sub-grid processes (processes occurring at smaller scales than the model grid size) are needed to improve the reliability of climate projections. In particular, while some progress has been made in cloud parameterizations, the representation of clouds and cloud processes remains the greatest uncertainty in climate models. Further, cloud processes are inextricably linked to other critical water cycle processes.

Model development will be accelerated by interdisciplinary field studies over regional test beds that provide much needed understanding of scaling effects. New parameterizations of water cycle/climate feedbacks (e.g., cloud-aerosol and land-atmosphere) and sub-grid scale processes (e.g., clouds, precipitation, evaporation, etc.) will have to be developed and validated, and the sensitivity of global models to these new parameterizations will have to be evaluated. Complementary research is planned under the Atmospheric Composition (Chapter 5) and Climate Variability and Change (Chapter 6) programs, and components of these programs, accelerated through the Climate Change Research Initiative (CCRI), are described in Chapter 2.

Current global and regional models demonstrate limited skill in predicting precipitation, soil moisture, and runoff on time scales beyond a few days. One of the most critical deficiencies in climate change projections involves precipitation and soil moisture -- essential parameters for assessments of the impacts of climate change and variability. While the large scale conditioning of the atmosphere by El Niño-Southern Oscillation (ENSO) events has been documented, memory effects of land conditions on the atmosphere are not fully quantified, and cloud and precipitation feedbacks and the interactions of the lower boundary layer (lower 500 meters of the atmosphere) with land and ocean surface conditions are not well understood. In addition, data sets are needed for the calibration of global coupled climate models and the development of regional downscaling and statistical forecasting techniques.

Advances in prediction capabilities will depend on improvements in model structure and initialization, data assimilation, and parameter representations. Predictability studies will be required to determine the regions, seasons, lead times, and processes most likely to provide additional predictive skill. Better understanding and improved model representations of less-well-understood processes, such as the seasonal and longer-term interactions of mountains, vegetative cover, soils, oceans, and the cryosphere with the atmosphere are needed. In addition, model evaluation studies with enhanced data sets are needed to improve models and to characterize and reduce uncertainties. Complementary research is planned under the Climate Variability and Change (Chapter 6) and Carbon Cycle (Chapter 9) programs.

Our ability to quantify the role of flowing water as the primary agent for sediment transport that reshapes the Earth's surface, and for nutrient transport that feeds riparian habitats and degrades water bodies, is rudimentary. Currently, we do not have the monitoring framework needed to generate a database to support research on these processes. The priority challenges are to quantify water flow and the various transport rates, biochemical transformations, and constituent concentrations and feedbacks whereby the water cycle alters media and ecosystems. Furthermore, the consequences of variations in water availability and quality for agriculture, energy production and distribution, and urban and industrial uses need to be integrated into a common modeling framework.

Overall, there is a basic need to develop an integrated research vision (complete with hypotheses) for addressing multiple-process (hydrological, physical, chemical, and ecological) interactions between water and other Earth systems. Techniques that scale up processes active at watershed and sub-watershed scales to the larger scales widely used in climate studies must be developed and tested. In addition, it is necessary to refine geophysical methods and the use of tracers, including isotopes, to determine subsurface paths, flow rates, and residence time, and to track pollution plumes. Complementary research is planned under the Land Use/Land Cover Change (Chapter 8), Carbon Cycle (Chapter 9), Ecosystems (Chapter 10), and Human Contributions and Responses to Environmental Change (Chapter 11) programs.

Variability and changes in the water cycle have been shown to lead to profound impacts on human societies and ecosystems (including on human health), but many of the linkages between change and outcome are not yet understood in the detail needed for appropriate policy and management responses. Water management takes place within a set of constraints that include, among other things, stringent flood control standards, federal and state environmental regulations, hydropower production schedules, and increasing irrigation, urban, industrial, and recreational demands for water. There is evidence that the results of recent research on the water cycle can contribute to the decisionmaking capacities of policymakers and water managers who must operate within these constraints. However, advances in water cycle research have found little use in water management and decisionmaking. Factors such as regulatory inflexibility, institutional structures, and time pressures make it difficult to change established management and decision systems. In addition, there is a mismatch between research products and operational information needs. Efforts to eliminate the barriers between research and research users have been initiated and indicate that early collaboration and side-by-side demonstrations may be effective tools for speeding innovation.

In order to make rapid progress in projecting the consequences of variability and change it will be necessary to integrate data from a broad range of sources and disciplines. Basic needs to achieve this goal include frameworks for integration, such as improved mechanisms for integrating remote sensing, GIS capabilities, and existing databases in decision support tools for water managers. In order to determine patterns and trends, it will also be necessary to inventory existing data sources and regional and sectoral studies, especially for data for which regional, national, and global repositories are rare or non-existent, such as for water demand, diversion, use, and consumption. In order for scientific information to have an impact, it will have to rely on refined and extended research on the role, entry points, and types of water cycle knowledge required for water management and policy decisionmaking processes. Complementary research is planned under the Climate Variability and Change (Chapter 6), Land Use/Land Cover Change (Chapter 8), and Human Contributions and Responses to Environmental Change (Chapter 11) programs.

A strong Global Water Cycle research program is essential for, and will derive critical inputs from, the following CCSP elements: Climate Variability and Change, Carbon Cycle, Land Use/Land Cover Change, Ecosystems, Atmospheric Composition, and Human Contributions and Responses to Environmental Change. In particular, the modes of water cycle variability arising from ocean sea surface temperatures will be addressed by the Climate Variability and Change element. Furthermore, to carry out this ambitious Global Water Cycle program, support will be required in the areas of Climate Quality Observations (Chapter 3) and in Decision Support Resources (Chapter 4). In addition, there will be a need to work closely with the Decision Support Resources activity to ensure that Water Cycle research is more effectively used in policy development and decisionmaking. Finally, sustained progress toward answering the questions addressed by the Global Water Cycle research program will depend on development of the modeling, observations, and information systems described in Chapter 12.

There are strong international linkages between the Global Water Cycle program and the World Climate Research Programme's (WCRP) Global Energy and Water Cycle Experiment (GEWEX). Other connections to international programs occur in the observational area with Integrated Global Observing Strategy (IGOS) Partners in terms of its emerging Water Cycle theme as well as the Global Climate Observing System (GCOS) and the Global Terrestrial Observing System (GTOS). In addition, the water cycle program will collaborate with a number of international programs concerned with water cycle research, water resources, and climate. These include the WCRP, International Geosphere-Biosphere Programme (IGBP), International Human Dimensions Programme (IHDP), and Diversitas Joint Water Project; the World Meteorological Organization's Hydrology and Water Resources Programme; and United Nations Educational, Scientific and Cultural Organization's International Hydrology Program and Hydrology for Environment, Life and Policy (HELP), as well as the Dialogue on Water and the 3^rd World Water Forum. Also, the Global Water Cycle program will contribute to work through bilateral treaties, particularly with countries like Japan, that have placed a priority on water cycle research.

Hornberger et al., 2001.  Hornberger et al., A Plan for a New Science Initiative on the Global Water Cycle (Washington, D.C., US Global Change Research Program).