Evapotranspiration: What it is, how it is calculated and how to optimise irrigation

Evapotranspiration is composed of two processes that ultimately result in the loss of water from the soil.

The first process is evaporation, which mainly affects bare soil and in the first centimetres of depth and water bodies.

Depending on the phenological stage the Kc changes.

The atmosphere demands water due to the difference between the vapour pressure between the air and the evaporating surface, the lower this potential the less water evaporates. The variables that affect this are therefore radiation, temperature, atmospheric humidity and wind speed.

In addition, factors such as the orientation of the terrain, the vegetation cover and the hydraulic conductivity can modify the rate of evaporation.

The second process involved is transpiration. Transpiration is the loss of water from plants to the atmosphere. This loss occurs through the stomata, and thanks to the loss of water vapour the plant can exchange the CO₂ necessary for the construction of its tissues.

As with evaporation, the variables influencing transpiration are wind speed, radiation, air temperature and atmospheric humidity.

The water status of the crop also has an influence: when there is a water deficit, the stomata partially close, reducing transpiration and affecting photosynthetic efficiency.

The difference is that in order to know the crop evaporation it is also important to know the crop, the turgor level and the ability of the soil to conduct water to the plant due to texture and salinity.

This information allows irrigation to be adjusted appropriately using crop evapotranspiration (ETc) models, which increases water use efficiency.

• Reference Evapotranspiration

The general equations for the calculation of evapotranspiration result in the Evaporation of the reference crop (ETo). For this equation only the climatic parameters are relevant and the most commonly used equation is the Penman-Monteith FAP.

This model considers factors such as net radiation, temperature, relative humidity and wind speed, and is validated under all types of agro-climatic conditions.

• Crop evapotranspiration

It is aimed at calculating a more realistic evapotranspiration (ETc) than the reference one. It is based on empirical studies that obtain a coefficient that corrects the value to more closely resemble the reality of the crop (Kc).

This coefficient depends on the plant species, phenological stage, and the practices of agronomic managementThe water is essential for precision irrigation systems.

Healthy turfgrasses have an evapotranspiration very similar to the reference evapotranspiration, sometimes higher, with a weather station the most complicated decision in the field can be made much more easily.

Real-time monitoring of weather conditions allows automatic irrigation adjustments, reducing water stress and improving turf or crop quality.

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Frequently asked questions you have asked about evapotranspiration

• Why is it important to know evapotranspiration before irrigating?
  Because it allows to know how much water the crop actually loses, avoiding over- or under-irrigation and helping to optimise water use.
• What factors cause evapotranspiration to increase in summer?
  Increased solar radiation, higher temperatures, higher wind speed and low relative humidity significantly increase evapotranspiration.
• Can I use evapotranspiration data on a small farm?
  Yes, even on small farms you can use local data or nearby weather stations to estimate the necessary irrigation efficiently.
• How do I know if my crop has high or low evapotranspiration?
  It depends on the type of plant, its stage of development, local climate and soil characteristics. Fast growing crops with large leaf mass tend to have high ET.
• Which method is easier to calculate evapotranspiration without having all the data?
  The Hargreaves method is simpler and more useful when complete data are not available, as it uses mainly maximum and minimum temperature.
• Are there mobile applications to monitor evapotranspiration?
  Yes, there are now agricultural apps and online platforms that show real-time ET based on weather stations and satellite models.
• How does climate change affect evapotranspiration?
  Rising temperatures and altered rainfall patterns can increase ET in many regions, affecting water availability for crops.
• Is evapotranspiration the same as water demand?
  Not exactly. Water demand includes evapotranspiration but also considers the efficiency of the irrigation system and losses through percolation or runoff.
• Is evapotranspiration the same throughout Spain?
  No. It varies according to the region. For example, it is much higher in southern areas such as Andalusia than in humid northern areas such as Galicia.
• What instruments are used in the field to directly measure evapotranspiration?
  In addition to meteorological stations, lysimeters, tensiometers and humidity sensors to obtain more accurate data adapted to the local soil.