Evapotranspiration

Because evaporation and transpiration occur when water moves into the air, levels of evapotranspiration in a given area are primarily controlled by:[9]

• the amount of water present;
• the amount of energy present in the air and soil (e.g. heat); and
• the ability of the atmosphere to take up water (humidity).

Vegetation type impacts levels of evapotranspiration.[10] For example:

• Herbaceous plants generally transpire less than woody plants, because they usually have less extensive foliage.
• Plants with deep reaching roots can transpire water more constantly, because those roots can pull more water into the plant and leaves.
• Conifer forests tend to have higher rates of evapotranspiration than deciduous broadleaf forests, particularly in the dormant winter and early spring seasons, because they are evergreen.[11]

Transpiration is a larger component of evapotranspiration (relative to evaporation) in vegetation-abundant areas.[12] As a result, denser vegetation, like forests, may increase evapotranspiration and reduce water yield.

Two exceptions to this are cloud forests and rainforests. In cloud forests, trees collect the liquid water in fog or low clouds onto their surface, which eventually drips down to the ground. These trees still contribute to evapotranspiration, but often collect more water than they evaporate or transpire.[13][14] In rainforests, water yield is increased (compared to cleared, unforested land in the same climatic zone) as evapotranspiration increases humidity within the forest (a portion of which condenses and returns quickly as precipitation experienced at ground level as rain). The density of the vegetation blocks sunlight and reduces temperatures at ground level (thereby reducing losses due to surface evaporation), and reduces wind speeds (thereby reducing the loss of airborne moisture). The combined effect results in increased surface stream flows and a higher ground water table whilst the rainforest is preserved. Clearing of rainforests frequently leads to desertification as ground level temperatures and wind speeds increase, vegetation cover is lost or intentionally destroyed by clearing and burning, soil moisture is reduced by wind, and soils are easily eroded by high wind and rainfall events.[15][16]

In areas that are not irrigated, actual evapotranspiration is usually no greater than precipitation, with some buffer and variations in time depending on the soil's ability to hold water. It will usually be less because some water will be lost due to percolation or surface runoff. An exception is areas with high water tables, where capillary action can cause water from the groundwater to rise through the soil matrix back to the surface. If potential evapotranspiration is greater than the actual precipitation, then soil will dry out until conditions stabilize, unless irrigation is used.

Design for a lysimeter

Evapotranspiration can be measured directly with a weighing or pan lysimeter. A lysimeter continuously measures the weight of a plant and associated soil, and any water added by precipitation or irrigation. The change in storage of water in the soil is then modeled by measuring the change in weight. When used properly, this allows for precise measurement of evapotranspiration over small areas.

Because atmospheric vapor flux is difficult or time-consuming to measure directly,[8]^{: Ch. 1} evapotranspiration is typically estimated by one of several different methods that do not rely on direct measurement.

Evapotranspiration may be estimated by evaluating the water balance equation for a given area:. The water balance equation relates the change in water stored within the basin (S) to its input and outputs:

${\displaystyle \Delta S=P-ET-Q-D\,\!}$

In the equation, the change in water stored within the basin (ΔS) is related to precipitation (P) (water going into the basin), and evapotranspiration (ET), streamflow (Q), and groundwater recharge (D) (water leaving the basin). By rearranging the equation, ET can be estimated if values for the other variables are known:

${\displaystyle ET=P-\Delta S-Q-D\,\!}$

A second methodology for estimation is by calculating the energy balance.

${\displaystyle \lambda E=R_{n}-G-H\,\!}$

where λE is the energy needed to change the phase of water from liquid to gas, R_n is the net radiation, G is the soil heat flux and H is the sensible heat flux. Using instruments like a scintillometer, soil heat flux plates or radiation meters, the components of the energy balance can be calculated and the energy available for actual evapotranspiration can be solved.

The SEBAL and METRIC algorithms solve for the energy balance at the Earth's surface using satellite imagery. This allows for both actual and potential evapotranspiration to be calculated on a pixel-by-pixel basis. Evapotranspiration is a key indicator for water management and irrigation performance. SEBAL and METRIC can map these key indicators in time and space, for days, weeks or years.[17]

Given meteorological data like wind, temperature, and humidity, reference ET can be calculated. The most general and widely used equation for calculating reference ET is the Penman equation. The Penman–Monteith variation is recommended by the Food and Agriculture Organization[18] and the American Society of Civil Engineers.[19] The simpler Blaney–Criddle equation was popular in the Western United States for many years but it is not as accurate in wet regions with higher humidity. Other equations for estimating evapotranspiration from meteorological data include the Makkink equation, which is simple but must be calibrated to a specific location, and the Hargreaves equations.

To convert the reference evapotranspiration to the actual crop evapotranspiration, a crop coefficient and a stress coefficient must be used. Crop coefficients, as used in many hydrological models, usually change over the year because crops are seasonal and, in general, plant behaviour varies over the year: perennial plants mature over multiple seasons, while annuals do not survive more than a few, so stress responses can significantly depend upon many aspects of plant type and condition.