Evaporation and Transpiration

Evaporation and Transpiration Definitions

Evaporation (E) is the change of water from its liquid (or solid) phase to its vapor phase. The two main factors influencing evaporation for the earth's surface area:
  • Supply of energy (solar radiation)
  • Transport of vapor away from the surface (i.e., winds near the surface)

Transpiration (T) is the evaporation occurring through plant leaves (stomatal openings). Transpiration is affected by plant physiology and environmental factors, such as:

  • Type of vegetation
  • Stage and growth of plants
  • Soil conditions (type and moisture)
  • Climate and weather

Evapotranspiration (ET) is evaporation from the land surface --- a combination of evaporation from open water surfaces (e.g., lakes, rivers, puddles) and transpiration by plants.

Note: In practice, the terms E and ET are often used to mean the same thing --- the evaporation from the land surface. Therefore, you must use the context to determine what the term evaporation means in a specific case (i.e., is it just from an open water surface or the entire land surface?).

Potential evaporation (PE) is the climate controlled E from an open water surface with unlimited supply (and no thermal capacity).

 Potential evapotranspiration (PET) is the ET that would occur from a well vegetated surface when moisture supply is not limiting (often calculated as the PE). [climate controlled]

 Actual evapotranspiration (AET) drops below its potential level as the soil dries. [soil controlled]

Pan Evaporation

Perhaps the simplest way of measuring evaporation is with an evaporation pan. Historical records of daily pan evaporation are available from the National Climatic Data Center (NCDC) for U.S. Weather Buruau Class A Land pans. The Class A plan has the following characteristics:
  • 4 foot diameter
  • 10 inches deep
  • well-ventilated (bottom raised 6 inches off ground)
  • filled to an 8 inch depth
Of course, hydrologists and engineers are not really  interested in what evaporates from a pan; instead they wish to know the regional evaporation from land surface or the evaporation from a nearby lake. Unfortunately, pan evaporation is often a poor indicator of these variables (due in part to pan boundary effects and limited heat storage).

Evaporation from an open water surface (E) is usually estimated from the pan evaporation (Ep) as:

E = K Ep

where K is the pan coefficient. Similar expressions are also used in practice to estimate potential evapotranspiration from pan data.
Last changed on 01/31/01 by aab.