Evaporation and Transpiration in Plants

Evaporation is a process whereby water (liquid) become vapor, decreasing as it evaporates from surfaces such as lakes, rivers, pavements and soil. This process needs energy to change the molecular state from liquid to vapor, energy partially from solar radiation and partially from the air temperature. The driving force is the difference between the water vapor pressure at the water surface and the air pressure.

As evaporation continues, the air becomes saturated, and the process slows down

Transpiration is the vaporization of water (liquid) from plant tissue to the atmosphere. Generally, crops lose water through stomata (small openings on the leaf), water is taken up from the roots, and mostly it is lost because of transpiration. The energy is from the vapor pressure and wind; therefore, air temperature, air humidity and winds are important in measuring transpiration.

Evapotranspiration is considered the main component of the hydrologic cycle concerning the water used for agriculture. Agricultural water use planning and management are becoming fundamental due to increasing populations, particularly in arid and semiarid regions. Evapotranspiration and irrigation management are also becoming important for avoiding the contamination of groundwater.

It is possible to distinguish weather parameters, crop factors and other environmental factors that can affect evapotranspiration. Among the weather parameters, we shall consider air radiation, air temperature, humidity and wind speed. Among the crop factors are crop type, crop height, crop roughness, reflection and ground cover. In particular, crop evaporation at standard conditions is the evaporating demand from crops in a large field with optimum soil water. Other factors that can affect evapotranspiration are soil characteristics such as salinity and fertility, the use of fertilizer, the presence of pests and soil management, and plant characteristics such as leaf anatomy, stomatal characteristics, aerodynamic properties and albedo.

In arid and semiarid regions, the use of greenhouses is suggested for avoiding ET; the use of greenhouses changes the radiation balance in the environment and prevents moisture loss.