EVAPOTRANSPIRATION

The term evapotranspiration combines two words: evaporation of water from the soil, and transpiration of water from plants into the air. Evapotranspiration means the total loss of water from a crop into the air. Water evaporates from any moist surface into the air unless the air is saturated. Water surfaces in contact with air, such as lakes, plant leaves, and moist soils, all evaporate water.

Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and water bodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves.

CONSUMPTIVE USEpart of water withdrawn that is evaporated, transpired by plants, incorporated into products or crops, consumed by humans or livestock, or otherwise removed from the immediate water environment. Also referred to as water consumed.

POTENTIAL EVAPOTRANSPIRATIONAccording to Thornthwaite’s definition (1943), Potential Evapotranspiration is “water loss which will occur if at no time there is a deficiency of water in the soil for use by vegetation”.

Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired if there were sufficient water available. This demand incorporates the energy available for evaporation and the ability of the lower atmosphere to transport evaporated moisture away from the land surface.

Mean Annual Potential Evapotranspiration

Factors Affecting EvapotranspirationEnergy availability. The more energy available, the greater the rate of Evapotranspiration

The humidity gradient away from the surface. The rate and quantity of water vapor entering into the atmosphere both become higher in drier air

The wind speed immediately above the surface. The process of evapotranspiration moves water vapor from ground or water surfaces to an adjacent shallow layer that is only a few centimeters thick. When this layer becomes saturated evapotranspiration stops. However, wind can remove this layer replacing it with drier air which increases the potential for Evapotranspiration. Winds also affect evapotranspiration by bringing heat energy into an area.

Water availability. Evapotranspiration cannot occur if water is not available.

Physical attributes of the vegetation. Such factors as vegetative cover, plant height, leaf area index and leaf shape and the reflectivity of plant surfaces can affect rates of evapotranspiration.

Estimation of Actual and

Potential

Water-Budget Determination of Mean basin EvapotranspirationThe hydrologic cycle, how water falls

to earth, flows over and into the earth, then returns to the atmosphere, is evaluated by means of a water budget. Water Budgets have four components. These are (1.) precipitation, (2.) evapotranspiration (i.e., the return of water to the atmosphere by surface evaporation and plant transpiration) (3.) surface runoff and (4.) groundwater recharge.

These components may add water to an environment. These are inputs. They may remove water from an environment. These are called outputs. Or water may be held in these environments. These are called storage.

Field-Plot Determination of EvapotranspirationApplication of a water budget to field plots

produces satisfactory results only und

Lysimeter