ABSTRACT
The water-circulation function of the atmosphere is a prerequisite for terrestrial life. The atmosphere may be considered to be a low-viscosity fluid within the hydrologic system. The atmospheric portion of the hydrologic cycle includes four variables: evaporation, transpiration, evapotranspiration, and Precipitation. The evaporation process accounts for a significant loss of water from storage reservoirs and irrigation projects. Transpiration involves the transfer of moisture from the ground to the atmosphere via the root systems of vegetation. The amount of transpiration that occurs is primarily a function of climate, the availability of soil moisture, and the type and density of growing vegetation. Precipitation involves the condensation of water vapor to rain and the return of the moisture to the earth. The atmospheric circulation of water is an important factor in balancing the hydrologic equation. Hydroclimatology deals with the longer-term fluctuations in average weather conditions.
