What is Evapotranspiration? Types and Importance

What is Evapotranspiration?

Evapotranspiration (ET) is the sum-up of water loss through the evaporation and transpiration of water from a surface area to the atmosphere. Water is lost from the soil through evaporation from the surface of the soil and by transpiration from the leaves of plants that grow on the soil. Therefore evapotranspiration is a very crucial part of the water cycle. Evapotranspiration is the process of the hydrologic cycle that sums up the water lost during evaporation and transpiration.

Evapotranspiration involves the transpiration from plants and the evaporation from soil
Photo credit: https://serc.carleton.edu

Evaporation accounts for the movement of water from sources such as canopy interception, soil, and water bodies to the air. Transpiration, on the other hand, accounts for the movement of water within a crop and the subsequent exit of the water from the crop as water vapor. In vascular plants, water exits the plants through the stomata in the leaves whereas, in nonvascular plants, it exits through the phyllids. This means that evapotranspiration (ET) accounts for the majority of water that is lost from the soil while a crop grows. A plant is said to be an evapotranspirator when it contributes to evapotranspiration.

Estimating the evapotranspiration rates is therefore important when planning irrigation schemes. There are factors that affect the rate of evapotranspiration in plants which include the amount of solar radiation, temperature, soil factors, wind, and atmospheric vapor pressure. Soil factors like the available soil moisture, the depth of the water table, and the density of vegetation really have a great influence on ET. Factors like plant morphology, plant cover, crop geometry, and root depth of the plant are also factors affecting evapotranspiration in plants.

Since ET is characterized as the total losses of water from vegetation, it is difficult to determine evaporation separately from transpiration in the cropped areas. Hence, the evaporation process of water from the soil surface and the transpiration process from plants are combined in the single term evapotranspiration.

Water cycle

One of the processes involved in the water cycle is evapotranspiration. The water cycle involves the continuous circulation of water within the earth and the atmosphere. There are many processes involved in the water cycle and the most important include transpiration, evaporation, precipitation, condensation, and runoff. Within the cycle, the total amount of water remains essentially constants. Despite the total amount of water being constant, its distribution among the various processes changes continually.

Evaporation is one of the major processes in the water cycle. It involves the movement of water from the surface of the earth to the atmosphere. Through the process of evaporation, water in its liquid state converts to a gaseous or vapor state. This evaporation process occurs when some of the molecules in the water mass have gotten sufficient kinetic energy to exit themselves from the water surface. Measuring evaporation directly is quite difficult and only possible at point locations.

The primary factors that affect evaporation include wind speed, humidity, temperature, and solar radiation.  The ocean is the main source of water vapor, though evaporation also takes place in soils, ice and snow. When there is evaporation from snow and ice which is the direct conversion of solid to vapor, it is called sublimation.

Transpiration is also one of the main processes of the water cycle that involves the evaporation of water from plants through the minute pores or stomata in the plant’s leaves. Water is very crucial to plants but they take up only a small amount. The small amount of water that the roots of the plants take up is used for metabolism and growth while the remaining 97-99.5% of water is lost by guttation and transpiration.

Plants can regulate the rate of evaporation and transpiration by controlling the size of their stomatal apertures. Transpiration rate is also influenced by the evaporative demand of the atmosphere that surrounds the leaf such as humidity, wind, boundary layer conductance, and incident sunlight. Stomatal opening and transpiration rate can be influenced also by above-ground factors, soil moisture, and temperature.

The amount of water plant losses can also depend on the plant size and the amount of water it absorbs from the soil with its root. Transpiration accounts for most of the water loss by plants through their leaves and young stems. It serves to evaporatively cool plants because the evaporating water carries heat energy away due to the larger latent heat of vaporization of 2260 kJ per liter.

Hence, in the water cycle, the transpiration and evaporation of water from all water bodies, soils, vegetation, ice, snow, and other surfaces are summed together and called evapotranspiration or total evaporation.

Factors affecting evapotranspiration in the water cycle

The vegetation type and land use affect evapotranspiration rate and in turn affect the amount of water that leaves a drainage basin. Plants that possess deep-reaching roots can transpire water more because water that is transpired through leaves comes from the roots of the plants. This is why herbaceous plants generally transpire less than woody plants due to them having less extensive foliage. In the dormant and early spring season, conifer forests tend to have a higher rate of ET than deciduous forests. This is so because, during these periods, there is an enhanced amount of precipitation that is intercepted and evaporated by conifer foliage.

Due to isotope measurement, it is indicated that transpiration is the larger component of ET. Hence, the factors that influence evapotranspiration rates are:

  • Percentage of soil cover
  • Solar radiation
  • Humidity
  • The level of the plant’s growth or maturity
  • Temperature
  • Wind

Forests may reduce water yield through ET except in distinct ecosystems such as rainforests and cloud forests. The liquid water in fog or low clouds is collected by trees in cloud forests. The water then settles onto their surface which drips down to the ground. These trees usually collect more water than they evaporate or transpire though they still contribute to ET.

Water yield is increased in rain forests as evapotranspiration increases humidity within the forest. The density of the vegetation reduces wind speed and temperatures at ground level. As the rain forest is being preserved, the combined effects of the reduced wind speed and temperatures lead to an increase in the surface stream flows and a higher groundwater table. The frequent clearing of the rainforests can lead to desertification. This is because as the ground level temperatures increase the vegetation cover is lost. Then, soil moisture is reduced by wind and the soils are easily eroded by high wind and rainfall events.

Adaptation of plants to the factors

There are a number of adaptations that plants adapt to help reduce water loss from evaporation and transpiration. Usually, plants that inhabit areas with low humidity possess leaves with less surface area so that evaporation is limited. Whereas, plants inhabiting humid areas like those in low light conditions such as understory vegetation may possess large leaves. This is because the need for enough sunlight is heightened and there is a low risk of detrimental water loss.

During drought periods, there are several desert plants that possess minute leaves that are deciduous. These leaves during the dry season nearly eliminate water loss. Plants like the cacti even lack leaves. Leaf adaptations like waxy cuticles, sunken stomates, trichomes (leaf hairs), and others help to reduce evaporation and transpiration rate in the plant. This is done by protecting the surface of the leaf from air currents that increase evaporation or by keeping the surface of the leaf cool.

Furthermore, some plants in order to minimize transpiration losses have evolved alternative photosynthetic pathways e.g crassulacean acid metabolism (CAM). These plants as well as many succulents, at night, open their stomates to take in carbon dioxide and close them when the conditions are usually dry and hot during the day.

Types of Evapotranspiration

  • Actual evapotranspiration
  • Potential evapotranspiration

There are two types of evapotranspiration which include:

Actual Evapotranspiration

The actual evapotranspiration (AET) or seasonal consumptive use (seasonal CU) is the water consumed in evapotranspiration and the one the plant used for metabolic activities. The term consumptive use is generally said to be equivalent to evapotranspiration because the water used in metabolic activities is insignificant. The water is about 1% of ET or less. However, when there is ample water, actual evapotranspiration is said to equal potential evapotranspiration.

Therefore, the total amount of water a crop uses in evaporation and transpiration throughout the entire growing season is referred to as the AET or seasonal CU. Actual evapotranspiration is the ET as governed by the availability of water to plants. When the available soil moisture is limited, AET remains less than maximum ET but when sufficient water is available to the crop the AET becomes equivalent to the maximum ET.

From research, the ratio of ET from well-watered corn to that of pan evaporation (Epan) varied. This was as a function of the growth stage. Most of the time during the year at which the experiment was conducted, the ratio ET/Epan remained about 0.9. Another experiment carried out by Pruitt and Lourence in 1968 reported that ET from grasses was 80% of the pan evaporation. However, there were exceptions when hot, dry, and strong winds were prevailing. The crop coefficient is the ratio of ET to Epan.

AET is normally no greater than precipitation in areas that are not irrigated. It is no greater with some buffer in time depending on the soil’s water retention capability. AET will usually be less because due to percolation or surface runoff, some water will be lost. However, there is an exception with areas that have high water tables where capillary action can make water from the groundwater to rise to the surface through the soil matrix.

Therefore, unless irrigation is used, the soil will dry out if potential evapotranspiration is greater than the actual precipitation. If there is not enough water to be evaporated or plants are not capable of transpiring easily, then ET can be lower than potential ET but can never be greater.

Potential evapotranspiration

The potential evapotranspiration (PET) is a representation of the water loss from a large area that is uniformly covered with a short green crop of uniform height and with adequate water status in the soil profile. This reflects the energy available to evaporate water. It also reflects the wind available to transport the water vapor from the ground up into the lower atmosphere.

PET is said to be the upper limit of ET for a crop in a given time. Under the same weather conditions, potential evapotranspiration cannot exceed evaporation but under humid conditions, this is applicable. PET is not affected by soil and plant factors but the rate of PET actually depends on the evaporative power of the air that is determined by factors like wind, radiation, temperature, and humidity.

PET is the quantity of water that a specific crop or ecosystem would transpire and evaporate if there is sufficient water available. This is expressed in terms of a depth of water and can be graphed. A value for the potential evapotranspiration called reference evapotranspiration (ETo) is usually calculated at a close climatic station on a reference surface, conventionally short grass. The reference evapotranspiration can be multiplied with a surface coefficient in order to be converted to potential ET. This is called a crop coefficient in agriculture and the difference between precipitation and potential evapotranspiration is used in irrigation scheduling.

The rate of potential evapotranspiration depends upon the evaporative power of the air as determined by temperature, humidity, wind, and radiation. Potential ET is not affected by soil and plant factors. In the summer and on less cloudy days, the potential evapotranspiration tends to be higher and closer to the equator. This is a result of the higher levels of solar radiation that provide the energy for evaporation. Also, PET is higher on windy days due to the evaporated moisture that can be quickly moved from the plant surface or ground to allow more evaporation to fill its place.

Evapotranspiration calculation method

There are various methods to measure evapotranspiration. These evapotranspiration calculation methods are grouped into direct and indirect methods.

Direct method

In this method, the measurements of soil moisture depletion are done. Generally, direct methods are expensive, time-consuming, and complicated. This method includes the following:

  • Soil moisture sampling
  • Equipment fixed in the soil
  • Atmometers
  • Pans
  • Lysimeter

Indirect Methods

In cases where direct data are not available, there is an actual procedure to determine ET under different soil and climatic conditions. For such empirical approaches, meteorological data has been used.

Most of these formulae are simple and are based on using the commonly recorded climatological data as input. To estimate evapotranspiration indirectly, there are 2 general approaches. They include:

  1. Catchment water balance
  2. Energy balance

Catchment water balance

To estimate ET, an equation of the water balance of a drainage basin is created. This equation balances the inputs and output with the water stored within the basin.

ΔS = P−ET−Q−D

where;

ΔS = change in storage

Input, P = Precipitation

Output, ET= Evapotranspiration

Q = Streamflow

D = Groundwater recharge

The missing flux, evapotranspiration ET, can be estimated if the change in storage (ΔS), streamflow, precipitation, and groundwater recharge are all estimated. This is done by rearranging the equation as:

ET = P−ΔS−Q−D

This gives us one of the evapotranspiration equations used to estimate ET.

Energy balance

This methodology involves the use of energy balance to estimate the actual evapotranspiration.

λE = Rn − G − H

Where;

λE= The energy required to change the phase of water from liquid to gas

Rn= net radiation

G= soil heat flux

H= sensible heat flux

The components of the energy balance can be calculated using instruments such as soil heat flux plates, scintillometer, or radiation meters. Therefore, the energy available for actual evapotranspiration can be solved by calculating the components of the energy balance.

Why is evapotranspiration important?

  • Evapotranspiration is one of the most important components of the water cycle.
  • In the agricultural sector, it is an important soil water balance component that plays a role in determining the potential yields.
  • Irrigators can use plant evapotranspiration information for more accurate irrigations schedule in order to help achieve top yields and improve water productivity.
  • In a farm situation, ET can help give a relatively objective and reliable estimate of the water needed for actively growing plants.

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