INTRODUCTION: STUDIES FROM THE WATER SECTOR OF THE NATIONAL ASSESSMENT

In early 2000, a series of reports are to be presented to the President, the Congress, and the U.S. public as the first National Assessment of the Impacts of Climatic Variability and Change on the United States (the National Assessment). These reports represent only one early step in what promises to be a long and arduous effort to try to understand the vulnerabilities of the country to climatic changes and to evaluate the kinds of economic, technological, institutional, and social responses that might be available or developed to help us cope and adapt to those changes. Hundreds, if not thousands, of people have been involved in the preparation of these reports, ranging from members of the public who have participated in workshops, hearings, and open meetings to atmospheric scientists and modelers interested in the complex nature of ocean-atmosphere interactions and anthropogenic influences. Five major sectoral reports are being prepared: Water, Coastal Ecosystems, Human Health, Forests, and Agriculture. Each of these covers a diverse set of problems and concerns. Two special issues of the Journal of the American Water Resources Association (this one and a second issue to be released in April 2000) represent part of the work underway for the Water Sector. While the principal effort of the sectoral assessments has been to compile, analyze, and summarize the findings of existing research on climate impacts and vulnerability, the Water Sector was fortunate in having some funding support for new research, together with a group of dedicated scientists and water experts concerned about the issues. As a result, analytical papers were solicited in preparation for a major conference on climate change and water resources, held in May 1999 in Atlanta, Georgia, under the auspices of the American Water Resources Association and the National Assessment. From this conference, papers were submitted for publication and underwent comprehensive peer review. The papers in these two special JAWRA issues represent those that offer new science, valuable insights, and policy-relevant background for decision makers, water managers, educators, and the public. The hydrologic cycle naturally consists of large seasonal, annual, and regional variations. A vast infrastructure of dams, reservoirs, canals, pumps, and levees collects, controls, and contains surplus flows and distributes water on demand during lowas well as high-flow periods and to arid as well as humid regions. Current water-use patterns and the infrastructure to regulate and allocate supplies are the results of past hydrologic conditions. Even today, the design and evaluation of alternative water investments and management strategies typically assume that future precipitation and runoff will continue to look like the past. The increasing likelihood that a human-induced greenhouse warming could affect the variability and availability of water quality and supplies as well as the demand for water raises doubts about this assumption and the most appropriate water policies for the future. Among the most important implications of global climatic changes will be widespread effects on the hydrologic cycle, natural aquatic ecosystems, watermanagement infrastructure, and the nature of water supply and demand. Water is critical to a society's welfare — it is widely used by industry, it is a fundamental component of natural ecosystems, it is used both directly and indirectly to produce energy, it is an important part of our transportation system, it provides the basis for much of our outdoor recreation, and it serves as a vehicle for disposing of wastes.