A Water Cycle Initiative

Abstract

In August 1999, the U.S. Global Change Research Program (USGCRP) appointed a Water Cycle Study Group (WCSG) to advise the USGCRP agencies on the development of a Global Water Cycle Program. The charge to the WCSG was to define a USGCRP initiative for fiscal years 2001 and beyond. After more than a year of work, which included soliciting views from the broad community of scientists and engineers interested in water cycle research, the WCSG issued its report in 2001 (http://www.usgcrp.gov/usgcrp/Library/watercycle/wcsgreport2001/default.htm). The report describes the rationale for research intended to provide critical fundamental knowledge about the water cycle—the amount of water in various reservoirs, the fluxes among these reservoirs, and the role of the changing amounts and fluxes in climate dynamics. Such research will provide information and tools to improve the use of water resources and to reduce losses associated with hydrologic extremes. The details of a research agenda to improve knowledge about the water cycle are necessarily involved. The major questions guiding such research can be stated quite succinctly, however. First, how much water of what quality is available, and how have these measurements of quantity and quality changed over time? The ability to assess the resource depends critically on knowing the amount of water in each water cycle component, how much water is moving among the components, the quality of water, and how amounts and quality have changed over time. Second, how are amounts and quality of water in various water cycle components likely to change in the future—over the next few days in response to weather, over the next few months in response to seasonal change, and over longer times in response to changes in water use, climate, and land use and land cover? Answering these deceptively simple questions will require dedicated work by scientists from a range of earth science disciplines, including hydrogeology, geochemistry, and geophysics. The water cycle research agenda is not restricted to the role of surface and near-surface water on climate. To be sure, more work on land-atmosphere linkages and feedbacks influenced by surface water and soil moisture is needed to understand the climate system. But ground water is the largest reservoir of liquid fresh water on the earth and is certain to become ever more important as a resource. Demand for fresh water is steadily increasing, and at the same time, the overall quality of water for humans and ecosystems is decreasing because of pollution and other human-imposed stresses. The pressure to use ground water will continue to increase, and if the resource is to be used wisely and sustainably, ground water scientists will have to be active participants in water cycle research. The status of our knowledge of ground water components of the global water cycle is not good. We hardly have enough data to determine amounts of stored water for critical and overstressed aquifers in the United States, let alone internationally. Sustainable use of ground water resources demands knowledge of recharge and discharge, fluxes for which adequate measurement techniques are not available in many instances. It is precisely these recharge-discharge processes that must be the focus of a major water cycle research initiative related to ground water (e.g., National Research Council. 2004. Groundwater Fluxes across Interfaces. Washington, D.C.: National Academy Press). Data and information with enhanced resolution and increased precision relative to that collected in the past must be developed. Techniques borrowed from neighboring disciplines must be adapted to provide new methods for measuring key variables. Data from new sensors and from existing networks will have to be integrated, and new observation networks will have to be established. Existing long-term records must be archived and preserved, and observations must be continued indefinitely at sites with long high-quality records, so that patterns of temporal variability, including long-term, low-frequency fluctuations, can be identified and studied. All of the measurements—at various scales—will have to be interpreted within the context of conceptual models and their mathematical implementations. Ultimately these models must evolve so that reliable predictions of responses to stresses in the future can be made. To improve predictions, a comprehensive program that includes observations, field experiments, and numerical modeling will be needed. The challenges of these research efforts for ground water scientists are formidable. But it is essential that the challenges be met. The rewards associated with gaining improved understanding are obvious. So are the penalties for failing to do so.