It is the primary process that moves moisture from Earth's surface to the atmosphere. Evapotranspiration (ET) is thus a compound term that describes the collective effect of evaporation of water and transpiration of plants.
EvapotranspirationĮvaporation is defined as "the rate of liquid water transformation to vapor from open water, bare soil or vegetation with soil beneath" (Shuttleworth, 1993), and transpiration is the rate of water added to the atmosphere as it moves from soil through the stomata of vegetation. Examples of such work are discussed in Smith (Chapter 24), Bales and Cline (Chapter 25), Sorooshian et al. Recent advances are rapidly improving the situation by merging satellite and radar with gage information. Rain gages have been the primary mechanisms for observation, but their sparse distributions and other limitations do not provide the spatial and temporal resolution needed for various modeling and research efforts. Measurements and estimates of precipitation (volume and intensity) are critical to any study or modeling effort involving the hydrologie cycle. In some regions, where dry air dominates the weather conditions, precipitation may fall from the clouds but evaporate before ever reaching the ground this is a phenomenon known as virga. The intensity and frequency of precipitation vary considerably both spatially and temporally, and the effects of precipitation can be both welcome (e.g., during droughts) or undesirable if it occurs in excess and causes subsequent flooding. The occurrence of precipitation over land is typically cited as the driving force of the hydrologie cycle, since it triggers the commencement of other fluxes (évapotranspiration, runoff, infiltration) by providing a new source of moisture to the system.
Precipitation is the process by which liquid and solid-phase aqueous particles, such as rain, snow, sleet, and hail, fall from the atmosphere to Earth's surface.
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