cools the basin in summer (Roads et al., 1997). Latent heating and radiative effects of the water substance provide strong coupling between the water and energy cycles.

The picture of net atmospheric water vapor convergence, net transfer to the surface, and runoff to the ocean obscures several important features of water and related energy cycles in the Mississippi basin. The net convergence of water over the basin is actually the difference between very large influxes over the Rocky Mountains and the Gulf of Mexico and effluxes over the Appalachian Mountains (Rasmusson, 1967). Similarly, the net transfer from the atmosphere to the surface is much smaller than either precipitation (about 3000 km3 per year) or evapotranspiration (about 2500 km3 per year) taken separately. To understand the balances, therefore, it is necessary to understand how atmospheric moisture, provided by vapor inflow and evapotranspiration, is partitioned into precipitation and vapor outflow, and how precipitation is subsequently partitioned into evapotranspiration and runoff. An analogous issue for the energy balance is the question of how the heat generated by precipitation is partitioned into radiative cooling and atmospheric heat flux divergence.

Additionally, fluxes and stored amounts of water and energy vary in space and time. Parts of these variations are regular, following the annual cycle of solar forcing in time and the physical controls of geography in space. Superimposed on these regular variations are irregular fluctuations or changes caused by the chaotic dynamics of the atmosphere-land-ocean system (e.g., the interannual variability of Mississippi River flow shown in Figure 1.1). Such chaotic behavior is generated both internally in the basin and externally (e.g., by the general circulation of the atmosphere). Storage processes within the basin modulate both the regular and the irregular variations in water and energy fluxes. Much of the internal modulation of the system response is associated with the storage of water on and beneath the land surface (Delworth and Manabe, 1989; Milly and Dunne, 1994; Koster and Suarez, 1995). In a region corresponding roughly to the Mississippi basin, the seasonal change in total water storage has been estimated to be on the order of a 10-cm depth of water (Rasmusson, 1968; Mintz and Serafini, 1981; Roads et al., 1994). GCIP will seek to describe and predict both the regular and the chaotic components of water and energy flux variations.

Geographic and seasonal variability of water and energy cycling are considerable in the Mississippi basin. Annual precipitation is greatest (about 1600 mm) in the southeast of the basin and decreases markedly toward the west, generally following the decreasing trend in vertically integrated atmospheric water content and transport. Under orographic influences, precipitation increases in the Rocky Mountains. The seasonality of precipitation varies from a weak winter maximum in the southeastern part of the basin to a strong summer maximum in the west, and these seasonal patterns follow the respective southern and western vapor inflows.

The geographic distribution of runoff is qualitatively similar to that of precipitation, which partially explains the disproportionate contribution of eastern

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