The global hydrologic cycle is a logical unifying theme for hydroclima-tology. For practical purposes, the global hydrologic cycle is a closed circulation for water's three phases. Within the structure of the general systems perspective commonly employed in the earth sciences, the hydrologic cycle is a subsystem and centerpiece of the global climate system. Consequently, the occurrence and movement of water assumes a primary role in both climatology and hydrology even though some illustrations of the hydrologic cycle lack comprehensive portrayal of the atmospheric transport role. Recognizing the full natural array of phenomena included in the hydrologic cycle is aided by conceptualizing the hydrologic cycle as consisting of two branches. The terrestrial branch encompasses continental processes and the atmospheric branch provides an energy and moisture redistribution mechanism (Fig. 1.3).
The terrestrial branch of the hydrologic cycle consists of the inflow, outflow, and storage of water in its various forms on and in the continents and in the
oceans. The primary focus of the terrestrial branch is the natural processes at or near the land surface that ultimately produce surface and subsurface runoff and directly influence cycles of other materials that shape the Earth's surface (Stricker et al., 1993). It is evident that the terrestrial branch is concerned with those processes commonly associated with hydrology.
The atmospheric branch of the hydrologic cycle consists of precipitation, evaporation, and the atmospheric transport of water mainly in the vapor phase. The two branches join at the interface between the atmosphere and the Earth's surface. The outflow of water from the Earth's surface through evaporation and transpiration is the inflow of water for the atmospheric branch. Precipitation, the atmospheric output, is a gain for the terrestrial branch of the hydrologic cycle. The atmosphere's mobility and ability to induce phase changes leading to precipitation establish the atmospheric branch as the forcing for the terrestrial branch of the hydrologic cycle. The dynamics of the hydrologic cycle are regulated by sources and sinks of atmospheric water vapor, by the thermodynamics of phase transitions, and by the dynamics of atmospheric general circulation (Peixoto, 1995)
The atmospheric branch of the hydrologic cycle is coupled with atmospheric general circulation and the transport of water vapor and the liquid and solid water in clouds. Recognizing the transport function of the hydrologic cycle is necessary to comprehensively portray how the hydrologic cycle is sustained. In addition to transporting moisture from the oceans to the continents, the atmosphere has an important role transporting energy vertically and horizontally as well as modulating the radiative forcing at the Earth's surface. The mobility of the atmosphere, and its capacity to force phase transitions of water establish the atmosphere as a forcing function for the terrestrial branch of the hydrologic cycle. The atmospheric branch of the hydrologic cycle is based on the dynamics of the general circulation of the atmosphere and is linked to the terrestrial branch by precipitation and evaporation (Peixoto, 1995). The role of vertical and horizontal transport of energy and mass and the delivery of precipitation places the atmospheric branch within the general framework of climatology and meteorology. However, the hydrologic cycle must be viewed as a whole and not in parts to comprehend its complete behavior and its complex non-linear feedback processes (Chahine, 1992).
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