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Evaporation is the process by which a liquid or a solid (sublimation) enters the gas phase. In the hydrologic context, it refers to the conversion of water and ice at the earth's surface to water vapor, and its dissipation into the atmosphere. |
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Transpiration is the process by which water vapor escapes from living plants and enters the atmosphere. It includes water which has transpired through leaf stomata, as well as intercepted water which has re-evaporated. When the soil is covered by a growing crop, transpiration geatly exceeds evaporation. |
Evapotranspiration refers to the combined effect of evaporation and transpiration.
Pan evaporation, the water loss per unit time from a standard container, is often measured at weather stations. It is not necessarily equal to actual soil evaporation or evapotranspiration. When soils are wet, actual soil evaporation and pan evaporation are nearly equal. When soils are dry, soil evaporation rates are typically lower than pan evaporation rates because water is less available at the soil surface.
Evapotranspiration rates may be higher than pan evaporation rates if soil moisture is not limiting. This is because plants (depending on canopy architecture, Leaf Area Index, and maturity, among other factors) have a greater evaporative surface area per unit area of ground, compared to the pan.
Potential Evapotranspiration (PE) refers to the evapotranspiration from "an extended surface of short green crop, actively growing, completely shading the ground, of uniform height and not short of water".
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