evapotranspiration - Evapotranspiration and the water cycle, Estimating evapotranspiration, Potential evapotranspiration

The transfer of water vapour to the atmosphere from vegetation and soil surfaces through evaporation and transpiration. Rates of evapotranspiration are determined by factors such as wind velocity, water availability, vapour pressure gradient, and energy availability. Actual evapotranspiration is the observed rate, and differs from potential evapotranspiration, which is what would occur if there were no limiting factors such as supply of moisture.

Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapour through stomata in its leaves. Evapotranspiration is an important part of the water cycle.

Potential evapotranspiration (PET) is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop, completely shading the ground, of uniform height and with adequate water status in the soil profile. It is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapour from the ground up into the lower atmosphere. Evapotranspiration is said to equal potential evapotranspiration when there is ample water.

Evapotranspiration and the water cycle

Evapotranspiration is a significant water loss from a watershed. Types of vegetation and land use significantly affect evapotranspiration, and therefore the amount of water leaving a watershed. Because water transpired through leaves comes from the roots, plants with deep reaching roots can more constantly transpire water. Also, woody plants keep their structure over long winters while herbaceous plants must grow up from seed in the spring in seasonal climates, and will contribute almost nothing to evapotranspiration in the spring. Factors that affect evapotranspiration include the plant's growth stage or level of maturity, percentage of soil cover, solar radiation, humidity, temperature, and wind.

Through evapotranspiration, forests reduce water yield, except for in unique ecosystems called cloud forests. These trees still contribute to evapotranspiration, but often condense more water than they evaporate or transpire.

In areas that are not irrigated, actual evapotranspiration is usually no greater than precipitation, with some buffer in time depending on the soil's ability to hold water. An exception is areas with high water tables, where capillary action can cause water from the groundwater to rise through the soil matrix to the surface.

Evapotranspiration can never be greater than PET, but can be lower if there is not enough water to be evaporated or plants are unable to readily transpire.

Estimating evapotranspiration

Evapotranspiration cannot be measured directly.

Catchment water balance

Evapotranspiration may be estimated by creating an equation of the water balance of a catchment (or watershed). The equation balances the change in water stored within the basin (S) with inputs and exports:

The input is precipitation (P), and the exports are evapotranspiration (which is to be estimated), streamflow (Q), and groundwater recharge (D). If the change in storage, precipitation, streamflow, and groundwater recharge are all estimated, the missing flux, ET, can be estimated by rearranging the above equation as follows:

Hydrometeorological equations

The most general and widely used equation for calculating ET is the Penman equation.

Potential evapotranspiration

Potential evapotranspiration (PET) is the amount of water that could be evaporated and transpired if there was sufficient water available.

PET is expressed in terms of a depth of water, and can be graphed during the year (see figure).

Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such free water (for lakes and oceans), the soil type for bare soil, and the vegetation. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying with a surface coefficient.