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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