Introduction

Demonstrate skill in predicting changes in water resources on time scales up to seasonal and annual as an integral part of the climate system.

During the past century, many anomalous climatic events have disrupted American lives. Persistent Great Plains droughts, such as those of the 1930s and the recent one in 1988, ruined Midwest crops and farmland. The Mississippi floods of 1927 and 1993 and the Northwest floods of 1996–1997 were equally devastating. Even larger regional climatic variations may occur in the future, especially if the global climate is seriously influenced by the rise in greenhouse gases, as models and some observations indicate (IPCC, 1996b). To what extent are such events predictable? Do we have the capability to predict future water resources under present or modified climate conditions? The benefits of accurately predicting regional climatic variations over seasons and longer time periods are so great (NRC, 1995) that the scientific community is now strongly motivated to meet the challenge of developing a real-time prediction capability on climatic time and space scales. However, both the application and the eventual utility of seasonal to interannual climate predictions also depend upon the ability to translate general circulation forecasts into significant hydrological information relevant to water resources management, agriculture, and forestry for specific regions.

A number of scientific questions must be resolved to develop such a prediction capability. Do we adequately understand the mechanisms that underpin natural hydrologic variations in our climate system? Is it possible to predict these variations accurately at seasonal and longer time scales? How do hydrologic and energy cycles vary over the United States, and how will they change in the near and long-term future? Water and energy budgets are understood qualitatively but are not known quantitatively on regional or even continental scales, partly because



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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities Introduction Demonstrate skill in predicting changes in water resources on time scales up to seasonal and annual as an integral part of the climate system. During the past century, many anomalous climatic events have disrupted American lives. Persistent Great Plains droughts, such as those of the 1930s and the recent one in 1988, ruined Midwest crops and farmland. The Mississippi floods of 1927 and 1993 and the Northwest floods of 1996–1997 were equally devastating. Even larger regional climatic variations may occur in the future, especially if the global climate is seriously influenced by the rise in greenhouse gases, as models and some observations indicate (IPCC, 1996b). To what extent are such events predictable? Do we have the capability to predict future water resources under present or modified climate conditions? The benefits of accurately predicting regional climatic variations over seasons and longer time periods are so great (NRC, 1995) that the scientific community is now strongly motivated to meet the challenge of developing a real-time prediction capability on climatic time and space scales. However, both the application and the eventual utility of seasonal to interannual climate predictions also depend upon the ability to translate general circulation forecasts into significant hydrological information relevant to water resources management, agriculture, and forestry for specific regions. A number of scientific questions must be resolved to develop such a prediction capability. Do we adequately understand the mechanisms that underpin natural hydrologic variations in our climate system? Is it possible to predict these variations accurately at seasonal and longer time scales? How do hydrologic and energy cycles vary over the United States, and how will they change in the near and long-term future? Water and energy budgets are understood qualitatively but are not known quantitatively on regional or even continental scales, partly because

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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities of deficiencies in available observations and partly because of a lack of accurate models. However, more than qualitative information is needed for application to the management of water and other natural resources. Precipitation events occur over a range of spatial scales, from small afternoon showers to synoptic-scale storms that develop within the planetary circulation. The heterogeneous land surface integrates these transient precipitation events over longer periods, so that streamflows tend to vary on daily to weekly time scales, depending on the size of the drainage basin. Except for short-term recharge events, the characteristic time scale of soil moisture fluctuations is weeks to months. The feedback between the components of the hydrological system and the recycling of water in the atmosphere generates variability on similar time scales. Observing, understanding, and modeling these processes through the full range of spatial and temporal scales are essential for developing long-range predictive capability for the water and energy cycles. Scientists recognize that such regional variations are part of the natural variability and/or change of the global climate system. GEWEX, an international research program to study fast climate processes in the atmosphere and at the Earth's surface, approaches the problem of climate variability from a global perspective. The GEWEX research strategy aims to study and parameterize generic processes representative of global climate. The GEWEX Continental-Scale International Project and comparable studies in other regions of the world (see Appendix A) constitute an apparent departure from this global outlook, motivated by the need to find a common ground between atmospheric and hydrological sciences. The spatial scales covered by GCIP are those at which both atmospheric circulation dynamics and hydrological process models can be formulated meaningfully. Nevertheless, the objective of relevance to global climate is not lost, ensuring that the transportability of GCIP results to comparable climatic regions is an explicit concern of the project. NOAA has taken the lead in organizing a GEWEX continental-scale research project (WCRP, 1992) that aims to determine quantitatively the energy and water budgets of the Mississippi River basin (Figure I.1). "Mississippi" is a native American Indian word meaning "Great River," and the Mississippi is sometimes referred to as "Old Man River." Spanning about 2350 miles from its headwaters in Lake Itasca, Minnesota, to its mouth in the Gulf of Mexico, south of New Orleans, Louisiana, the Mississippi is one of the largest river systems in the world. Major rivers, including the Illinois, Missouri, Ohio, Arkansas, and Tennessee, among others, contribute to the Mississippi flow. At the mouth, the flow of the Mississippi constitutes the major freshwater discharge from North America. The Mississippi River basin, bounded by the Rocky Mountains in the West and the Appalachian Mountains in the East, covers most of the continental United States (area: 3.2 × 106 km2). This huge continental basin contains several distinct

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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities FIGURE I.1. The Mississippi River basin is bounded by the Rocky Mountains in the West and the Appalachians in the East. Understanding the water and energy cycles of this basin, including the heavily regulated Mississippi and other rivers, is a major objective of GCIP. (Courtesy of National Geographic.)

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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities regional climate regimes that have become separate foci for GCIP. These include (1) the Arkansas-Red River basin, a focus for arid summertime hydrologic processes; (2) the headwaters of the Mississippi River in Minnesota and Illinois, a focus for wintertime hydrologic processes; (3) the upper Ohio and Tennessee-Cumberland Rivers, a focus for semihumid processes and water resources management associated with the Tennessee Valley Authority; and (4) the upper Missouri and Yellowstone Rivers, a focus for orographic processes. A multiplicity of GCIP data acquisition and assimilation activities are funded both in government laboratories and in universities. The operational phase, aiming to provide the best information available in near real time, is part of a NOAA core project, whereas the research phase, which aims to develop even better products from more comprehensive models and reanalyzed data, will constitute the basic research component of GCIP. Because of GCIP, our nation's regional assimilation capabilities have been upgraded to synthesize a wider diversity of inputs and to produce consistent gridded fields of aerological and hydrological variables over the region on a systematic daily schedule. For the first time, these regional operational products will be 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. Extensive regional observations are also being gathered, including many upper-air radiosondes, surface weather stations, rain gauges, and stream gauges, in addition to a dense meteorological radar network and satellite observing system [e.g., Geostationary Operational Environmental Satellite (GOES)] in the Mississippi that may be unique in the world. In concert with National Weather Service modernization, GCIP is developing new information from the NEXRAD WSR-88D (Weather Surveillance Radar 1988-Doppler) radars, wind profilers, and automatic weather stations. Because of GCIP, new measurements of snow and soil moisture may become part of the nation's climatic information system. GCIP also sponsors basic research to (1) characterize the time and space variability of the energy and water budgets from catchment to continental scales and predict how future climate variability will affect these budgets across the spectrum of scales relevant to atmospheric and hydrological processes; (2) develop global and limited-area atmospheric models and hydrologic models ranging from the highest feasible resolution to regional or "macroscale" models and apply these models to the estimation of energy and water budgets; (3) develop information retrieval schemes to integrate existing and future satellite observations and ground-based measurements, including procedures for generating an operational "national precipitation climatology" over the continental United States; (4) develop and disseminate a comprehensive GCIP data base including in situ, model, and remote sensing information; and (5) apply predictive hydrological models utilizing GCIP data sets and models.

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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities FIGURE 1.2 The large-scale areas (LSAs) of GCIP provide a focus for two-year intensive observation activities that will allow GCIP to ultimately scale up to full continental scope. GCIP received initial funding in 1994 to prepare a program of core and research activities. The various components of the program are described in a succession of planning documents (IGPO, 1993, 1994a,b). Current progress is described in a series of Major Activities Plans for 1995, 1996, and 1997, also produced by the International GEWEX Project Office (IGPO). The GCIP Implementation System Test (GIST) was carried out in 1995, and the data were disseminated widely. The earliest activities were discussed in a series of papers published as a special issue of the Journal of Geophysics Research (see Coughlan and Avissar, 1996). GCIP is now entering its second and most resource-intensive phase, a five-year (1996–2000) Enhanced Observing Period (EOP) and associated Enhanced Seasonal Observing Periods (ESOPs) during which a wide range of special data sets will be assembled for further analyses and model validation. A five-year period was chosen for EOP because it is highly probable that this will include at least one wet year and one dry year as well as several relatively "normal" years. In addition, the complexity of scale interactions that permeate all aspects of the hydrologic cycle has led GCIP initially to adopt a multiscale approach spanning:

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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities FIGURE 1.3 GCIP timeline of activities. GCIP's implementation strategy will start off with the study of summertime hydrometeorological processes in the LSA-Southwest and will next study wintertime activities in the LSA-North Central. GCIP will then move to the humid Appalachians LSA-East and finally go west to the more mountainous LSA-Northwest. continental-scale area (CSA) activities, which will continue at a more or less steady level over the whole Mississippi basin for the duration of the EOP. The continental-scale area is well resolved by GCIP's regional analysis as well as global analysis systems. The large area provides another way to link small-scale hydrologic processes to global climate models. large-scale area (LSA) activities, which are being implemented successively, following a phased time table of (about) two-year observing periods for each region. LSAs are adequately represented by GCIP's regional models. Four LSAs have been identified, the aggregate of which covers the GCIP domain. These are the southwestern, north central, eastern, and northwestern LSAs (Figure 1.2). The areal coverage of the LSAs is on the order of 106 km2. The southwestern area is being emphasized first, followed by the north central, eastern, and northwestern areas (Figure 1.3). intermediate-scale area (ISA) activities, which include typical river catchments (104 km2 or less) such as represented by operational hydrological models. The hydrologic processes explicitly represented in these models could potentially be parameterized in regional to global atmospheric models. ISA studies will be phased in accordance with the availability of basic data from LSA and continental-scale activities.

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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities small-scale area (SSA) activities, which include intensive field observation projects over densely instrumented sites, such as the little Washita experimental river catchment. GCIP provides a unique framework for these small-scale experiments. By focusing on the study of hydrometeorological processes occurring in various seasons and different parts of the basin, GCIP aims to develop continental-scale hydrologic models and coupled atmospheric-hydrologic models applicable to seasonal and interannual prediction over a wide range of latitudes all over the globe. In particular, it is planned first to develop various parameterized representations of land surface exchanges of energy and water in mesoscale-resolving models and then to test these schemes in a hierarchy of weather prediction models used operationally by the NOAA National Centers for Environmental Prediction (NCEP) and the National Weather Service River Forecast System run by NOAA's Office of Hydrology, as well as a number of other models. Developing the ability to predict the hydrologic cycle and its components on seasonal to interannual time scales requires the integration of measurements, physical understanding, and models covering many space and time scales. GCIP aims to bridge this spectrum between microscale processes and the synoptic-scale circulation regime by delivering continental-scale field and data sets with the spatial and temporal resolution needed to characterize the continental atmospheric-hydrological water and energy budgets. In order to understand the role of remote forcings on seasonal to interannual variability, GCIP will cooperate with Global Ocean-Atmosphere-Land System (GOALS) and other national programs that bring in remote features of the global climate system affecting the Mississippi River basin. GCIP also aims to foster the closest possible cooperation of research and operational teams and institutions, in order to bridge the gap between understanding the phenomena and applying this knowledge to practical climate forecasting and water resources management on the regional or statewide scales that matter most to end users. The purpose of this report is to review how GCIP is accomplishing its objectives and to recommend additional actions to accelerate progress. Chapter 1 is a review of the general geoscience issues being faced by GCIP in attempting to understand the continental water and energy cycles. Chapter 2 describes model development activities addressed by GCIP. Chapter 3 discusses the problem of the data assimilation using inescapably sparse observations and predictive models. Chapter 4 covers GCIP data collection and management activities. Chapter 5 discusses application of the results expected from the experiment to hydrologic prediction. Each chapter includes a review of existing and future GCIP activities, as well as an assessment of achievements and recommendations.

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GCIP Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities To summarize, GCIP is the first international project to bring together the hydrologic and meteorological science communities on a common research goal. This cooperation has already been beneficial to both communities and is the basis for the initial success of GCIP. Thus, the panel's primary recommendation is that GCIP should stay the course and continue with the implementation of its scientific research plans, with modifications as recommended in this report. When fully implemented, GCIP can be expected to strengthen our nation's capability for climate prediction and water resources management. It will provide a sound basis for hydroclimatological research at the beginning of the twenty-first century. Still, the project does not address hydroclimatic phenomena that are characteristic of the semiarid U.S. Southwest, a region where the availability of water is a critical resource issue as well as a challenging scientific problem. Applying the methodologies and technical facilities developed for GCIP to a study of the Colorado River basin and surrounding mountain regions is a challenge for the future.