The current water cycle research program described in this report will provide the nucleus of a new comprehensive U.S. initiative to foster and accelerate global water cycle research. A few U.S. programs related to the water cycle, which are already sponsored by NASA, NOAA, NSF, DOE, USDA, USGS, EPA, and the U.S. Army, are briefly discussed below. Besides laying the groundwork for future understanding of the water cycle, these projects are contributing to better international scientific understanding of the global hydrologic cycle. Before turning to a review of these U.S. programs, we first describe briefly international global water cycle research.

The World Climate Research Program (WCRP) coordinates international research programs to promote better understanding of global climate variability and change. Specifically, WCRP objectives are "to develop the fundamental scientific understanding of the physical climate system and climate processes [that is] needed to determine to what extent climate can be predicted and the extent of man's influence on climate." WCRP is one of 10 major programs of the World Meteorological Organization (WMO), which is a "specialization agency" of the United Nations. Except in rare circumstances, WCRP does not fund research but rather plays a coordinating role.

However, its priorities carry considerable weight in the design and functioning of international climate research, both within and outside the United States. WCRP sponsors five major projects, all of which are relevant to global water cycle research: the Global Energy and Water Cycle Experiment (GEWEX), Stratospheric Processes and their Role in Climate (SPARC), Arctic Climate System Study (ACSYS), Climate Variability and Predictability (CLIVAR), and World Ocean Circulation Experiment (WOCE). Each of these international activities is briefly described below.

GEWEX is the scientific locus within WCRP "for studies of atmospheric and thermodynamic processes that determine the global hydrological cycle and water budget and their adjustment to global changes such as the increase in greenhouse gases." GEWEX coordinates research designed to understand, model, and predict radiative processes involving clouds, aerosols, water vapor, and their impact on radiation transfer and radiation flux divergence in the atmospheric column.

GEWEX also has a major focus on hydrometeorological processes involving the transport and release of heat in the atmosphere, precipitation, evapotranspiration and land surface exchanges, including water storage on and near the surface, and runoff. Within WCRP, GEWEX is the sole program with a major focus on land surface processes; GEWEX activities concentrate on understanding and modeling land surface hydrology at continental and regional scales. The coordination of land surface modeling activities in GEWEX is now being handled by GLASS, the Global Land-Atmosphere System Study.

GEWEX is not an experiment in the traditional sense; rather, it is an integrated program of research, observations, and science activities, ultimately for the prediction of variations in global and regional hydrological regimes. GEWEX initially encouraged a suite of exploratory studies over relatively small experimental sites involving intensive field observations and theoretical process modeling. One such study, organized by the GEWEX International Satellite Land Surface Climatology Project (ISLSCP), was the First ISLSCP Field Experiment (FIFE), conducted at a Kansas grassland site in the mid-1980s.

Small-scale field projects like FIFE were originally expected to continue until about 2000 and then merge into a new phase of global atmospheric/hydrologic studies relying on expected new global satellite data sets. As the understanding of the small-scale aspects of the hydrologic cycle progressed, however, it became clear that the next scientific priority was to address interactions across the spectrum of spatial scales. GEWEX therefore began to focus on the aggregation of processes from micro- or mesoscale to synoptic or planetary scales and the inverse disaggregation of phenomena from the larger meteorological scales to the smaller scales meaningful to hydrological scientists.

The initial concept of the GEWEX Continental-Scale International Experiment (GCIP), which is now taking place in the Mississippi River basin, was formulated in 1990. The goal is to measure, study, and model coupled atmospheric and hydrologic processes on all scales between those captured by intensive field studies at various experimental sites (e.g., FIFE and BOREAS) and planetary scales that can be observed by global observing systems. Four additional GEWEX continental-scale experiments (CSEs) have been formed elsewhere globally, more or less following the GCIP "template." All the CSEs seek to close the energy and water budgets over large land areas representative of various climatic regimes.

Among the other CSEs is the Mackenzie GEWEX Study (MAGS) undertaken by Canada over the north-flowing Mackenzie River basin. MAGS emphasizes cold season processes, such as snow, ice, permafrost, and arctic clouds. BALTEX (Baltic Sea Experiment) focuses on the interactions between the Baltic Sea and the land areas that drain into this sea from northern Europe. The Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) focuses on the Amazon River basin, while the GEWEX Asian Monsoon Experiment (GAME) is ongoing at several sites in eastern Asia. A sixth CSE may eventually be started in western Africa.

CLIVAR is a relatively new WCRP effort to improve global climate prediction. If successful, it will provide a basis for international cooperation in modeling, and for the field studies and process understanding needed to support improved climate prediction. CLIVAR's goals are to identify major modes of climate variability, mechanisms that lead to key modes of variability, and predictability of variability, and to begin a demonstration of climate variation predictions. The program will further understanding of how these variations contribute to and are affected by any mean climatic changes induced by the addition of greenhouse constituents to the atmosphere.

The overall scientific objectives of CLIVAR are to describe and understand the physical processes responsible for climate variability and predictability on seasonal, interannual, decadal, and centennial time scales. These objectives are being met through the collection and analysis of observations and the development and use of models of the coupled climate system, in cooperation with other climate research and observing programs. CLIVAR also aims to extend the record of climate variability over the time scales of interest by assembling quality-controlled paleoclimatic and instrumental data sets; to extend the range and accuracy of seasonal to interannual climate predictions through the development of global coupled predictive models; to understand and predict the response of the climate system to increases of radiatively active gases and aerosols; and to compare these predictions to the observed climate record to detect anthropogenic modification of the natural climate signal.

Nine principal research areas (PRAs) focused on "natural" phenomena and two on anthropogenic climate change have been identified to facilitate CLIVAR's implementation. All are embodied within three main program areas -- the Global Ocean Atmosphere Land System (GOALS), Decadal to Centennial Climate Variability (DecCen), and Anthropogenic Climate Change (ACC) -- and all are closely linked across geographical regions and between time scales. VAMOS [Variability of the American Monsoon Systems], a project noted in Chapter 2, is a CLIVAR-related activity.

ACSYS, in particular its hydrological program, aims to determine the space-time variability of the Arctic hydrological cycle and the freshwater fluxes to the Arctic Ocean. The ACSYS hydrological region is defined as all of the global land area that drains to the Arctic Ocean. ACSYS has made some progress in assembling consistent precipitation data sets through its Arctic Precipitation Data Archive (APDA), which is maintained at the Global Precipitation Climatology Center in Offenbach, Germany. ACSYS has also developed, in cooperation with the Global Runoff Data Center (GRDC) in Koblenz, Germany, the Arctic Runoff Data Base (ARDB).

The ARDB contains historical river discharge measurements at 235 stations at the mouths of major Arctic rivers and at the confluences of major tributaries of these rivers. Although the ARDB provides reasonably complete spatial coverage for continental-scale studies, relatively few data are available beyond 1985, in large part because of political changes in the former Soviet Union and station closures in Canada. Station closures exacerbate the problem of understanding spatial and temporal distributions of Arctic precipitation, which has always been poorly represented by station data.

ACSYS hydrological modeling activities draw heavily on GEWEX, especially for the MAGS, GAME-Siberia, and BALTEX continental-scale experiments (CSEs), which have significant activities in the Arctic and/or cold regions. The land surface models fostered by GEWEX are being used in offline mode (driven with surface atmospheric forcing) to produce daily runoff estimates for the ACSYS hydrological region. ACSYS will participate in the planned GEWEX Hydrometeorological Panel (GHP) CSE transferability studies that will treat the ACSYS hydrological region as equivalent to a CSE, and will coordinate with GHP a macroscale hydrological model intercomparison activity targeted at high-latitude areas.

ACSYS is presently transitioning from a regional (Arctic drainage basin) activity to take a global focus. The new WCRP program, which will eventually replace ACSYS, is Climate and Cryosphere (CLIC). CLIC will incorporate ACSYS sea ice and oceanographic activities in the Arctic, and will be expanded to include Antarctic research in these areas, as well as research on glaciers and ice sheets. Beyond shifting from a boreal to a bipolar focus, CLIC will also include relevant cold season and region processes elsewhere, such as glaciers in temperate regions, permafrost, and ephemeral snow cover. The WCRP Joint Scientific Committee approved the CLIC draft Science and Coordination Plan at its annual meeting in March 2000. The first version of the plan is currently available from the WCRP website.

The World Ocean Circulation Experiment was designed to help coordinate international ocean data collection and experimental efforts, with the goal of improving the ocean models needed for predicting decadal climate variability and change. WOCE aims "to develop models useful for predicting climate change and to collect the data necessary to test them" and "to determine the representativeness of the specific WOCE data sets for the long-term behavior of the ocean, and to find methods for determining long-term changes in the ocean circulation." WOCE consisted of a field phase from 1990 through 1997, which was followed by an analysis, interpretation, modeling, and synthesis (AIMS) activity scheduled to be complete by 2002. At that time, WOCE activities will be subsumed within CLIVAR (see above).

The WCRP Stratospheric Processes and their Role in Climate study emphasizes stratospheric processes relevant to climate, especially the absorption of solar radiation in the stratosphere by ozone, and the role of other some stratospheric gases, including water vapor and carbon dioxide. It also includes studies to better understand the two-way interactions between stratospheric and tropospheric dynamics. Activities organized by SPARC include construction of a stratospheric reference climatology and improvement of understanding of temperature, ozone, and water vapor trends in the stratosphere. One of SPARC's areas of interest directly relevant to global water cycle research is the distribution of water vapor in the upper troposphere and lower stratosphere (UT/LS) which plays a critical role in Earth's radiative budget and therefore in climate.

Although water vapor has not received the attention paid to anthropogenic greenhouse gases, it is the most important greenhouse gas in Earth's energy balance. In addition, water vapor is the source of hydroxyl radicals, which are responsible for much of the oxidizing capacity of the atmosphere and therefore critical for cleansing the atmosphere of many anthropogenic compounds dumped into the atmosphere. Water vapor is also involved in hydrolysis reactions important for the removal of reactive chlorine and nitrogen species. Given the importance of UT/LS water vapor, a number of issues immediately arise. First, and most important, we do not know the present distribution of water vapor and its temporal variations in the tropopause region well enough to understand the fundamental processes affecting the Earth's radiative balance. Such an understanding is required to model the present distribution mechanistically, and more important, to predict its future evolution under changing conditions, such as the continued increase of greenhouse gases in the atmosphere.

The distribution of water in the stratosphere is complicated by the nature of water as a chemical compound and its phase changes. In the high troposphere, water vapor can be in equilibrium with ice crystals and move from one phase to another. Measurements must take this into account and allow for water in both forms in determining the mean distribution and the controlling mechanisms. Available data suggest that water vapor, especially in the tropics, varies on a range of horizontal scales from global to regional, and down to the size of large convective systems. Vertical scales as fine as a few meters may be important in some cases.

There is also a range of temporal scales, from a few hours to seasonal and annual, corresponding to the spatial scales. These facts imply that more than one type of observing campaign and a wide range of observing techniques will be required. They also indicate that considerable scientific imagination will be needed to construct empirical models and develop a mechanistic understanding of the ways water distribution is maintained and its natural variations. In addition, a vigorous modeling effort must accompany any measurement program. The goals are to aid in data interpretation, suggest new measurements needed, and ultimately allow mechanistic and parameterized calculations to be carried out within large climate models.

Beyond WCRP, several other World Meteorological Organization programs are directly relevant to global water cycle research. These include the Global Climate Observing System (GCOS) and the Hydrology and Water Resources Program.

GCOS is a WMO program at the same level as WCRP, established in 1992 to ensure that the observations and information needed to address climate-related issues are obtained and made available to all potential users. GCOS is cosponsored by the Intergovernmental Oceanographic Commission (IOC) of UNESCO, the United Nations Environment Program (UNEP) and the International Council for Science (ICSU), as well as WMO. GCOS is intended to be a long-term, user-driven operational system to provide the comprehensive observations required for monitoring the climate system, detecting and attributing climate change, assessing the impacts of climate variability and change, and supporting research to improve understanding, modeling, and prediction of the climate system.

The program addresses the total climate system, including physical, chemical, and biological properties, and atmospheric, oceanic, hydrologic, cryospheric and terrestrial processes. GCOS does not itself make observations or generate data products. It stimulates, encourages, coordinates, and otherwise facilitates the needed observations by national and international organizations in support of their own requirements as well as common goals. It provides an operational framework for integrating and enhancing observational systems of participating countries and organizations to produce a comprehensive system focused on the requirements for resolving climate issues.

The Global Terrestrial Observing System (GTOS) is a UN Food and Agriculture Organization (FAO) program established in 1996 to provide data for detecting, quantifying, locating, and giving early warning of changes in the capacity of terrestrial ecosystems to sustain development. Of particular interest to the present water cycle initiative is a permanent observing system for managed and natural ecosystems, one envisaged to include agricultural and ecological research centers, field stations, and derived data products to better represent global soils, vegetation, and related land cover conditions.

In addition to linking with GTOS, GCOS builds on and works in partnership with other existing and developing observing systems, such as the Global Ocean Observing System (GOOS) and WMO's Global Observing System and Global Atmospheric Watch. GCOS will build on existing operational and research observation, data management, and information distribution systems, which will be enhanced as necessary to meet GCOS goals. Within the United States, the newly established U.S. GCOS office should serve as the principal interface between international GCOS activities and the U.S. water cycle program.

The WMO Hydrology and Water Resources Program (HWRP) promotes activities in operational hydrology and cooperation between national meteorological and hydrological services. In particular, HWRP concentrates on the measurement of basic hydrological elements from networks of hydrological and meteorological stations; the collection, processing, storage, retrieval, and publication of hydrological data, including data on the quantity and quality of both surface water and groundwater; the provision of such data and related information for planning and operating water resource projects; and the installation and operation of hydrological forecasting systems.

HWRP also promotes improvements in developing country capabilities, through technology transfer and technical cooperation, to enable continuous assessment of water resources, to respond to threats of floods and droughts, and thus to meet the requirements for water management for a range of purposes. HWRP takes into consideration the existence of global change and its hydrological impacts and the need to provide more information to the general public and to governments so they can better understand the importance of hydrology and the role of national hydrological services (NHSs) in their activities. The HWRP also promotes increased collaboration between NHSs and national meteorological services (NMSs), particularly in providing timely and accurate hydrological forecasts.

Hydrological elements are encompassed by several other WMO programs: the Tropical Cyclone Program, Education Training Program, and World Climate Program (WCP, in the efforts known as WCP-Water). There are strong links between hydrology and meteorology in study of the hydrological cycle where WMO has a particular interest and responsibility in promoting close coordination of the methods and activities of the two disciplines. GEWEX is one noteworthy example. The International Association of Hydrological Sciences (IAHS) and WMO jointly convened a working group on GEWEX. It was this working group that proposed the study of a large river basin, which subsequently focused on the Mississippi River basin in the GEWEX Continental-Scale International Project (GCIP).

The International Hydrological Program (IHP) is a UNESCO natural sciences program that grew out of the International Hydrological Decade (IHD; 1965-1975). Its purpose is to improve the scientific and technological basis for the development of methods and the human resource base for rational management of water resources, including environmental protection, and to integrate developing countries into the worldwide ventures of research and training.

In February 1999, the 5th Joint UNESCO/WMO Conference on International Hydrology unanimously endorsed a new global initiative, HELP (Hydrology for Environment, Life and Policy), which will establish a global network of catchments to improve the links between hydrology and societal needs. The overarching purpose of HELP is to deliver social, economic, and environmental benefits to stakeholders through sustainable and appropriate use of water, a development made possible by applying hydrological science in support of integrated catchment management. Because the catchment is the natural unit of hydrology, HELP is specifically catchment-based. However, HELP is people- and environment-centered, problem-driven, and demand-responsive: it takes questions of environment, life, and policy as the starting points, and uses hydrology as the vehicle for their solution.

HELP therefore undertakes new interdisciplinary studies at a range of appropriate scales for integrated solutions to a range of water-related environment, life, and policy problems. From the outset of the program, HELP has involved physical and social scientists from the operational and research communities, water policy experts, managers, and users. Where expertise is lacking, HELP will endeavor to create it through education and capacity-building. HELP addresses the following six global freshwater policy issues: (1) water and food; (2) water quality and human health; (3) water and the environment; (4) water and climate; (5) water and conflicts; (6) communication between hydrologists and society.

At the global level, HELP is guided by a steering committee of international experts on water-related policy, management, and science, and representatives from partner organizations (e.g., WMO, IAEA, IGBP, GEWEX, CLIVAR, IAHS, and NGOs). The structure of HELP regional coordinating units (RCUs) is necessarily flexible to accord with local institutional arrangements, most likely established in existing national or regional institutions. Fundamentally, HELP represents a global network of catchment organizations, which benefit from communication between hydrologists and stakeholders within and between participating basins. Benefits include knowledge of new technologies of data acquisition and analysis, the opportunity to address existing and emerging conflicts through access to external expertise and experience in conflict management, and learning from the experience and knowledge of other HELP basins.

To contribute to the HELP program, some catchment attributes are deemed essential. HELP catchments must provide an opportunity to study a water policy or water management issue for which hydrological process studies are needed; relevant national and local agencies must agree to cooperate in the execution of HELP; adequate local capacity must exist to participate in the program as a full partner; a minimum range of key variables and parameters must be monitored; data, information, and technological expertise must be shared openly; and HELP data standards, quality assurance, and quality control must be adhered to.

The International Geosphere Biosphere Program (IGBP) was established in 1986 by the International Council of Scientific Unions (ICSU). Its objective is "to describe and understand the interactive physical, chemical, and biological processes that regulate the total Earth system, the unique environment that it provides for life, the changes that are occurring in this system, and the manner in which they are influenced by human activities." IGBP is an interdisciplinary research endeavor. Like WCRP and other WMO activities, IGBP only rarely provides direct funding for research; its primary function is to coordinate research and data collection activities within cooperating countries.

Emphasis is placed on the interactions of biological, chemical, and physical processes that govern change in the Earth system and that are most susceptible to human perturbation. IGBP has developed detailed plans for the conduct of science in its 11 component program elements. Eight of these consist of broad, discipline-oriented projects, on such topics as atmospheric science, terrestrial ecology, oceanography, hydrology, and links between the natural and the social sciences. The three IGBP core projects are of particular interest to global water cycle research: Biospheric Aspects of the Hydrologic Cycle (BAHC); Global Change and Terrestrial Ecosystems (GCTE); and Land Use and Cover Change (LUCC). These three are described briefly below.

The BAHC core project addresses the nature of the interaction between vegetation and the hydrologic cycle. BAHC is an interdisciplinary project combining and integrating expertise from many disciplines, in particular, ecophysiology, pedology, hydrology, and meteorology. In this respect, BAHC cuts across disciplines as well as across spatial scales. At smaller scales, BAHC is involved in developing techniques and algorithms to provide climatic data needed at the scales of hydroecological research to study changes of land surface conditions. At larger scales, BAHC provides soil-vegetation-atmosphere transfer models, in particular, on the areal pattern of heat and moisture fluxes according to land-surface heterogeneity. BAHC is involved in these activities in a number of selected areas in the world that represent major ecosystems.

The GCTE core project aims at predictive understanding of the effects of changes in climate, atmospheric composition, and land use on terrestrial ecosystems (both natural and managed), and at determining feedback effects to the atmosphere and physical climate system. Ecosystem responses are being investigated both through manipulative studies and long-term studies at selected sites. The latter activity is being developed in collaboration with the developing Global Terrestrial Observing System (GTOS). Modeling studies are focused on the construction of dynamic vegetation and agricultural systems models at a variety of scales, to be linked to global biogeochemical models, physically based GCMs, and direct impact studies.

The LUCC core project is sponsored by the International Human Dimensions Program (IHDP) on Global Environmental Change. It addresses the ways that land use, and thus land cover and surface properties, are affected by socioeconomic factors, and aims to integrate the driving forces of land cover change into a global land use and land cover change model. The primary objectives of LUCC are to obtain a better understanding of global land use and land cover driving forces; to investigate and document temporal and geographical dynamics of land use and land cover; and to define the links between sustainability and various land uses.

Over the coming decades, the global effects of land use and land cover change may be as significant, or more so, than those associated with any climate change. Unlike climate change per se, land use and cover change are known and undisputed aspects of global environmental change. These changes and their impacts are with us now, ranging from potential climate warming to land degradation and biodiversity loss, and from levels of food production to spread of infectious diseases. The scale and pace of such changes, their human and biophysical origins, and their linkages to other global changes are all inadequately understood. In fact, no accurate global map of agriculture exists; we have no good measures of change in such land covers as forests and grasslands; and we cannot model or project well ongoing land use and cover changes in an integrative way.

The Intergovernmental Panel on Climate Change (IPCC) is a joint activity of WMO and the United Nations Environment Program (UNEP). IPCC was established in 1988 and is open to all members of UNEP and WMO. IPCC's charge is "to assess the scientific, technical and socioeconomic information relevant for understanding the risk of human-induced climate change." The IPCC provides scientific, technical, and socioeconomic advice to the world community, in particular to the 170-plus parties to the UN Framework Convention on Climate Change (UNFCCC), through its periodic assessment reports on the state of knowledge of causes of climate change, potential impacts of climate change, and options for response strategies.

IPCC does not carry out new research or monitor climate-related data. It bases its assessment mainly on published and peer-reviewed scientific technical literature. The IPCC has three working groups and a task force. Working Group I, on scientific aspects of the climate system and climate change, and Working Group II, on vulnerability of socioeconomic and natural systems to climate change and adaptation options, are most relevant to the U.S. water cycle research program.

Several questions being addressed by Working Group I may benefit from the U.S. water cycle program: How have precipitation and atmospheric moisture changed in the recent past? Have climate variability or climate extremes changed (e.g., variability of droughts, wet spells, and hail)? What are the types and magnitudes of water vapor and cloud feedbacks? The needs of people for freshwater dictate that Working Group II address possible changes in hydrologic conditions. Thus, Working Group II stands to benefit from progress made by the U.S. water cycle program in furthering understanding of: the current state and potential changes in the hydrological cycle, including in precipitation, evaporation, runoff, soil moisture, groundwater, and extreme hydrological events; and of management implications and adaptation options, including responses to extreme hydrological events.

A number of U.S. programs that are run collaboratively by two or more agencies already contribute to water cycle studies and research. As the U.S. water cycle program evolves, these existing water cycle efforts will likely be expanded, while new ones will be developed to fill identified gaps. A few aspects of these ongoing efforts are summarized below. The list is neither comprehensive nor complete; it simply highlights some important current efforts.

NOAA and NASA have supported the ongoing GCIP project (currently slated to terminate in 2001) and its successor, GAPP. The primary focus of GCIP is the Mississippi River basin, where the project aims to quantify atmospheric energy and water budgets. GCIP includes an operational pathway, designed to transfer GEWEX research results into near-real-time weather and climate forecasting activities. This is the so-called NOAA core project. A parallel (and larger) research phase addresses GCIP science goals, which include development of better products from more comprehensive models and reanalyzed data from the basic research of the GCIP project. Among GCIP's major contributions to date are improvements in NCEP's regional data assimilation capabilities and the ability to produce consistent gridded fields of aerological and hydrological variables over the continental United States on a systematic daily schedule.

For the first time, these regional operational products are being archived and distributed as a basic resource for investigations of coupled atmospheric and hydrologic climate processes on spatial scales from local to continental and on time scales from hourly to interannual. GCIP has also facilitated integration of data from a range of sources, including upper-air radiosondes, surface weather stations, rain gauges, and stream gauges. It is also assembling a five-year (1996-2000) research-quality data set from precipitation radar (based on NEXRAD WSR-88D), as well as supporting data from wind profilers, and automatic weather stations. New observations of soil moisture have also been initiated under GCIP sponsorship and will become part of the nation's climatic information system.

Through GCIP's directions, basic research has been sponsored leading to characterization of time and space variability of energy and water budgets from catchment to continental scales. Across the spectrum of scales relevant to atmospheric and hydrological processes, GCIP has sponsored development of global and limited-area atmospheric and hydrologic models ranging from the highest feasible resolution to regional or "macroscale." These models have been applied to estimate energy and water budgets, and to develop information retrieval schemes that integrate existing and future satellite observations and ground-based measurements. In addition, GCIP is also developing and disseminating a comprehensive database that includes in situ, model, and remote-sensing information.

Finally, GCIP has funded development of macroscale hydrological models, applicable to large continental river basins like the major tributaries of the Mississippi. These models have been used to predict the land surface water and energy budgets of these major tributaries, and have proved useful as a diagnostic tool for other water balance assessments (e.g., based on reanalysis data). GCIP received initial funding in 1994 to explicitly prepare a program of core and research activities. GCIP is now transitioning to a project covering the entire United States and linking to the U.S. CLIVAR PACS program. This GEWEX America Prediction Project (GAPP) is expected to be a central element of the U.S. water cycle initiative.

The U.S. CLIVAR effort, which is the U.S. contribution to the international CLIVAR, is designed to understand seasonal to interannual climate variability and prediction. Decadal variability and anthropogenic change are also subjects of high priority. U.S. CLIVAR will have a strong focus on decadal modulation of El Niño -- Southern Oscillation (ENSO), and seasonal to decadal variability of the North Atlantic Oscillation. In the Pacific sector, the Pan American Climate Study (PACS) and the Variability of the American Monsoon System (VAMOS) programs are under active development and will be included within U.S. CLIVAR.

The overall goal of PACS is to advance the understanding of seasonal and longer time scale phenomena needed to extend the scope and skill of climate prediction over the Americas, with emphasis on warm season precipitation. PACS is concentrating on the North American monsoon, including the structure and variability of the continental-scale mode and the mechanisms that generate warm season precipitation anomalies. PACS is specifically concerned with explaining climatological characteristics of the atmospheric hydrologic cycle, including the relationship of the eastern Pacific coastal stratus and continental precipitation, as well as the influence of land and ocean surface on seasonal predictability. U.S. CLIVAR activities are further linked with World Weather Watch (WWW), the Global Climate Observing System (GCOS), the Global Ocean Observing System (GOOS), and the Global Ocean Data Assimilation Experiment (GODAE), the Global Energy and Water Cycle Experiment (GEWEX) and Past Global Changes (PAGES). Of particular interest to this water cycle initiative are the CLIVAR and GEWEX efforts to determine surface fluxes, including evaporation, from in situ observations and satellite measurements. In addition, understanding the role of the oceans in global water cycle variability will be critical for climate predictability relating to the water cycle.

The Earth Observing System (EOS), in planning since the 1980s, is a NASA program, with national and international collaborators, which entered a new stage with the launch of Terra (previously known as EOS-AM) in December 1999. A significant part of the EOS program is focused on observations of atmospheric and land surface phenomena, to better understand the dynamics of the Earth's physical climate. NASA has been a major supporter of field projects, modeling, and data assimilation activities aimed at better representing the coupled land-ocean-atmosphere system. These studies have included, for instance, intensive field campaigns like FIFE, BOREAS, and LBA, which integrated in situ observations with aircraft and satellite remote sensing. The International Satellite Land Surface Climatology Project (ISLSCP) has had major support from NASA.

NASA also provides data products and analyzed fields essential to the success of GCIP, notably diagnostics of cloud amount and properties through the International Satellite Cloud Climatology Project (ISCCP), surface radiation flux estimates (Langley Research Center), and soil/hydrology/vegetation data (Huntsville Global Hydrology and Climate Center). Conversely, it is expected that GCIP multidisciplinary studies and data products will provide a high-quality benchmark to validate EOS observations for Terra, EOS-PM, and other missions like the Tropical Rainfall Monitoring Mission (TRMM).

The U.S. Weather Research Program (USWRP) provides a research focus for the ongoing modernization of the National Weather Service. USWRP is attempting to improve the specificity, accuracy, and reliability of weather forecasts using the best possible mix of modern observations, data assimilation, and forecast models. In particular, USWRP's goal is to improve forecasts of high-impact weather for agriculture, construction, defense energy, transportation, public safety (emergency management), and water resource management, including floods. USWRP is especially concerned with studies related to quantitative precipitation forecasting.

These include the measurement, estimation, and depiction of water vapor, representation of convection in forecast models, and estimation of precipitation amount and type by radar and satellite. USWRP has also begun to consider the control on extreme events by surface effects, including soil/vegetation and canopy. These weather prediction research efforts complement GCIP's regional climate activities. In addition, USWRP's studies related to quantitative precipitation forecasting will help GCIP understand how to better use NEXRAD products. USWRP is beginning to coordinate its activities with the World Weather Research Program, which is currently exploring a formal linkage with GEWEX through WMO/WCRP.

The National Assessment of Potential Consequences of Climate Variability and Climate Change ("U.S. National Assessment") was called for by the 1990 Federal Climate Change Act. The first assessment was initiated in 1998 and is currently nearing completion. It consists of regional assessment reports for eight regions of the United States, sector reports for agriculture, water, human health, and coastal and marine resources, and an overview report. Some of the regional reports have been released in draft or final form, and the sector and overview reports have recently (or are about to be) released for public comment.

The U.S. National Assessment was conducted by a large group of researchers and practitioners. Primary responsibility for each of the regions and sectors was assigned to a federal agency (e.g., NOAA was the lead agency for the Pacific Northwest report, while USGS took the lead for the water sector report). The summary document was written by the National Assessment Synthesis Team and a set of lead authors, who were drawn from private and public sectors, and a range of disciplines.

The current assessment is the first component of what is expected to be a continuing activity, somewhat parallel in function and structure to the IPCC, which operates on the international level. The National Assessment is not expected to be a research activity; rather, it is intended to report regularly on understanding of "what we presently know about the potential consequences of climate variability and change for America in the 21st Century."

The cooperating agencies within USGCRP all have some interest in water resources. Thus, there are a number of research efforts within agencies that can contribute to a coordinated water cycle initiative. Again, the list below is neither complete nor comprehensive, but represents a sample of ongoing programs.

The mission of NASA's Earth Science Enterprise (ESE) is to develop a scientific understanding of the Earth system and its response to natural or human-induced changes, and to improve prediction capabilities for climate, weather, water resources, the Earth's ecosystems, global air quality, and natural hazards. To this end, the research program seeks a deep understanding of Earth system components and their interactions ranging from short-term weather to long-term climate time scales, and from local and regional to global space scales. ESE is driven by the recognition of the impacts of natural variability of the planetary environment on society, and the realization that humans are no longer passive participants in the evolution of the Earth system, a view shared by the world's scientific authorities. The strategic objective of ESE is to provide the scientific basis and answers to the overarching question: "How is the Earth changing and what are the consequences for life on Earth?" Particular attention is given to forcings, responses, and processes that link the two and/or constitute feedback mechanisms.

ESE's research coverage maps across the broad themes of the USGCRP, addressing needs to better understand the Earth's climate system on all time scales; the composition and chemistry of the atmosphere; the global water cycle; ecosystems and the global carbon cycle; the human dimensions of global change; and the geological history of environmental change (slow physics processes). ESE's research efforts emphasize, but are not limited to, space-based studies of the Earth as an integrated system. All of ESE's programs contribute directly or indirectly to the GCRP water cycle initiative, in particular, ESE's Global Water and Energy Cycle (GEWEC) program. The main science questions addressed by GEWEC are the following:

The satellites developed and launched under the auspices of NASA's Earth Observing System (EOS) contribute directly to GEWEC and the USGCRP Water Cycle Program, in particular, NASA's TRMM, EOS-Terra, EOS-Aqua, and others. NASA is now planning new space-borne missions in the post-EOS era (for roughly 2002-2010). Among the mission concepts considered are sensor packages that would support scientific investigations in surface water hydrology, global precipitation, soil moisture, and cold season processes. All of these missions, as well as currently planned cloud and radiation sensors and various current and planned land cover missions, are directly applicable to the water cycle science plan.

In addition to is lead role in the GCIP/GAPP program, NOAA, through its Office of Global Programs (OGP), supports integrated scientific assessments of the effects of climate variability and change on the natural and managed environment. These continuing projects characterize the state of knowledge of climate variations and changes at regional scales, to identify knowledge gaps and linkages in selected climate-environment-society interactions, and to provide an informed basis relating to climate-related risks. At present, there are five regional integrated science and assessment activities funded by NOAA-OGP. These are focused on the Pacific Northwest, the Southwest, California, Inter-Mountain West, and the Southeast regions of the United States.

In addition, NOAA-OGP's Human Dimensions Program supports research on the institutional capacity of water management agencies to plan for climate variability and respond to forecast information. Projects analyze the use of climate and hydrologic information related to competing water uses, transboundary resource management issues, and the ability of water markets, adaptive management, and other mechanisms to enhance the efficiency of water management in the face of global change. The NOAA Paleoclimatology Program and World Data Center for Paleoclimatology serve as a national and international catalyst for understanding and modeling interannual to century-scale environmental variability. Regarding the water cycle, paleoclimatology provides information on an array of scales, including studies on global-scale climate variability that influences regional hydroclimate, reconstructions of regional drought over centuries to millennia, and annual reconstructions of past seasonal streamflow within a watershed.

Within the National Weather Service Office of Hydrology (NWS/OH), the Advanced Hydrological Prediction System (AHPS) is seeking to improve the state of the art of hydrologic prediction as applied primarily to flood forecasting. Although NWS/OH does not formally support extramural research, it is cooperating with the academic community in developing AHPS, in particular, through an evolving partnership with GCIP/GAPP. Also in NWS, the Environmental Modeling Center (EMC) and Climate Prediction Center (CPC) of the National Centers for Environmental Prediction (NCEP) develop and execute retrospective (multidecade) and real-time climate monitoring and prediction systems.

These monitoring and prediction systems include (1) the coupled ocean-atmosphere modeling system with its ocean data assimilation system, (2) the retrospective 50-year atmospheric Global Reanalysis and its real-time monitoring counterpart, and (3) the retrospective 25-year coupled land-atmosphere Regional Reanalysis program and its real-time Eta Data Assimilation System, retrospective and real-time Land Data Assimilation System (LDAS) and companion reanalysis of daily U.S. precipitation.

NSF supports a broad range of disciplinary and interdisciplinary research in the geosciences related to various aspects of the water cycle. NSF has supported water cycle research under the Water and Energy -- Atmospheric, Vegetative, and Earth Interactions (WEAVE) initiative, through special competitions under Biocomplexity in the Environment, and through its core programs. Currently, an Environmental Research and Education initiative is being developed and will include research addressing water and biogeochemical cycles. The program will stress the interconnectedness of earth, atmospheric, and biological systems and the dynamics of coupled natural and human systems.

DOE funds the Atmospheric Radiation Measurement (ARM) program to improve understanding of the transfer of radiation through the atmosphere. A central ARM component is the Cloud and Radiation Testbed (CART), which is currently under way at sites in the Southern Great Plains (SGP) of south-central Kansas and central Oklahoma, the North Slope of Alaska, and a tropical western Pacific site. The CART sites provide surface radiation flux data and boundary layer soundings at multiple observing locations. Enhanced observations are collected during Intensive Observation Periods (IOP) of a few weeks duration each year. At the SGP CART site, observations are coordinated with GCIP studies of summer rainfall and reevaporation. In addition, the Atmospheric Boundary Layer Experiments (ABLE) facility located at the SGP CART site is well suited for studies of the hydrological balance and associated processes in a closed catchment. ABLE has hosted two field campaigns, carried out by the Cooperative Atmosphere-Surface Exchange Study (CASES) consortium.

The Walker Branch Watershed is located on the U.S. Department of Energy's Oak Ridge Reservation in Anderson County, Tennessee. This 100-ha watershed is an instrumented forest watershed research site with a 30-year history of research and modeling of forest ecology, stream ecology, hydrology, and biogeochemistry. The site supports several long-term experiments. Research priorities include biogeochemical cycling, pollutant deposition, climate change, forest productivity, and biodiversity. The watershed is currently home to an AmeriFlux installation managed by NOAA for DOE and the Throughfall Displacement Experiment -- a large-scale manipulation to understand the potential impacts of precipitation changes on forested ecosystems.

Agricultural Research Service (USDA-ARS). USDA-ARS conducts fundamental hydrologic research in support of those aspects of its mission related to management of agricultural crop- and rangelands. One aspect of the ARS research program of particular interest to this water cycle science plan is its national network of instrumented experimental watersheds and field research facilities. The program consists of 12 intensively instrumented catchments across the U.S., and over 140 less intensively monitored sites, some of which have operated continuously since the 1930s. Additionally, hydrometeorological measurements exist for experimental rangeland areas extending back to 1915. The ARS provides public access to the watershed data for all hydrologic researchers.

Forest Service (USDA-FS). USDA-FS supports hydrologic cycle and related research on forested watersheds as part of its mission to sustain the health, diversity, and productivity of the nation's forests and grasslands. The watershed research program is managed at 34 locations, with studies conducted via a nationwide network of experimental forests and watersheds. Six sites (Baltimore, Bonanza Creek, Coweeta, H. J. Andrews, Hubbard Brook, and Luquillo) are intensive research sites within the NSF Long-Term Environmental Research (LTER) Program. Current program components focus on developing the science base to manage and restore watershed and riparian ecosystems, assess effects of forest roads on streamflow and erosion, protect municipal water supplies, evaluate effects of air pollutants and urbanization on water quality, rehabilitate watersheds following wildfire, and contribute to integrated ecological and hydrologic studies at LTER sites. Ongoing studies focus on global change impacts on forest ecosystems and watersheds, including development of national and regional models to evaluate ecological, hydrologic, and economic impacts of climate change.

USGS carries out a broad program of monitoring, data collection, and investigations in water resources, biological resources, mapping, and geology. Results relevant to the global water cycle include archives on U.S. streamflow, groundwater, and water-quality data; maps and digital geospatial data, including digital elevation models and digitized stream networks and drainage divides; archives of land remote-sensing imagery and derived products; and models and syntheses of information about regional hydrologic systems throughout the United States. Through its Water, Energy, and Biogeochemical Budgets (WEBB) Program, USGS supports five sites, in collaboration with scientists from universities and other federal and state agencies, that are used to investigate watershed processes, including the effects of atmospheric and climatic variables. In addition, through the National Research Program (NRP), other national programs, and district offices, the USGS conducts research in support of its mission of characterizing the quality and quantity of the nation's water resources. Relevant to this initiative are NRP's studies on processes governing the sources, sinks, and transport of carbon and nitrogen; chemistry, microbiology, and movement of water, solutes, and gases in porous and fractured materials and the unsaturated zone; and interaction of groundwater and surface water. USGS also supports a small extramural research program that provides funds to the 54 Water Resources Research institutes and other universities.

USGS's NRP conducts hydrologic research in support of its mission responsibilities to characterize the nation's water resources. NRP is designed to encourage the pursuit of a diverse agenda of research topics to obtain new knowledge and insights on land surface hydrologic processes that are not well understood, and which involve water quantity and water quality. The emphasis of the NRP evolves over time, reflecting the emergence of new areas of inquiry and the potential for new tools and techniques. In addition to its research programs, USGS carries out a broad program of monitoring, data collection, and investigations in water resources, mapping, geology, and biological resources

EPA's Global Change Research Program is conducting assessments of the consequences of global change (changes in climate, land use, and UV radiation). The program's Water Quality focus area examines how changes in the hydrologic cycle might affect the ability of water resource managers to meet water quality standards. In particular, EPA is studying the possible impacts on drinking water treatment and quality, wastewater treatment requirements, and surface water quality. The Human Health focus area is examining changes in the incidence of waterborne diseases, while the Ecosystems focus area is exploring impacts of changes in water quantity and quality on aquatic ecosystem health.

The U.S. Army, through its Army Research Office (ARO), supports basic research to advance scientific and technological knowledge for enhancing Army capabilities. ARO's terrestrial science program sponsors academic research in the broad area of land-atmosphere processes, including the hydrologic regime.

The Army Corps of Engineers, through its Engineer Research and Development Center (ERDC), Institute for Water Resources (IWR), and Hydrologic Engineering Center (HEC), conducts significant applied research on more effective decision making for water resource management. This work emphasizes coupled physical and ecological process simulation at systemwide (watershed) scales, regional sediment management, innovative flood protection, water control infrastructure rehabilitation, coastal storm protection, and environmental risk assessment. Much of this research is accomplished jointly with the academic community and related initiatives of other federal agencies. Climate change and the effects of climate variability have thus far not been a specific research focus area; however, the Corps has participated in studies and technical forums concerning these issues. The Corps remains highly interested in this area, including the value of new knowledge for the execution of its mission in the future.

The Bureau of Reclamation (Reclamation) mission is to manage, develop, and protect the planet's most valuable natural resource -- water -- and related resources in an environmentally and economically sound manner in the interest of the American public. As the primary resource management agency, responsible for managing 350 major dams from Grand Coulee in the Columbia basin to Hoover Dam in the Colorado River basin, Reclamation serves over 100 million Americans. Reclamation supports applied science and technology to develop and improve science-based decisions for reservoir and river system operations. Reclamation's water resource managers are charged with balancing the allocation of limited water supplies among competing needs for agricultural, municipal, and industrial users, while maintaining the quality of riparian habitat for fish, recreation, and cultural activities.

Reclamation's Science and Technology Program conducts research related to water and water resource management, watershed modeling, water quality, ecosystem and riparian habitats, precipitation forecasting, delivery system enhancements, and technology research and development, all toward improving water delivery and riverine ecosystems. Reclamation's science program seeks to integrate emerging practical technologies like those proposed in this water cycle research initiative into its operations through the Watershed and River System Management Program. This program serves water operations managers in the Colorado, Yakima, Rio Grande, and Truckee River basins, and provides state-of-technology links to NWS river forecasts, WSR-88D precipitation estimates, USGS stream gauge and watershed runoff forecasts, and other water supply and demand information.

There are numerous traditional publications associated with each of the programs listed above. However, rather than attempting to list all relevant documents, we instead list the main website for each program. These websites can be used as the starting point to obtain far more information than is normally available from a single traditional publication.