Importance of Evapotranspiration in Agricultural Water Management

Water is the lifeblood of agriculture, and knowing exactly how much of it leaves a field – and when – is at the heart of smart irrigation management. Every day, water disappears from farmland through two simultaneous routes: it evaporates from the soil surface and it escapes through plant leaves. Together, these two processes are captured in a single, powerful concept: evapotranspiration (ET). For farmers and water managers, especially in regions where water is scarce, ET is not just a hydrological term – it is the primary guide for deciding how much to irrigate, when to irrigate, and how to avoid wasting a drop.

What is evapotranspiration? 🔗

According to the Food and Agriculture Organization of the United Nations (FAO), evapotranspiration is the combination of two separate processes: water lost from the soil surface by evaporation, and water lost from the crop by transpiration. Evaporation is the direct conversion of liquid water into water vapour from surfaces such as bare soil, water bodies, and wet vegetation. It is driven primarily by solar radiation, air temperature, humidity, and wind speed. Transpiration, on the other hand, is the vaporization of water contained within plant tissues. Crops lose water predominantly through stomata – tiny openings on plant leaves through which gases and water vapour pass. Water is taken up by the roots, transported through the plant, and vaporized from within the leaf before being exchanged with the atmosphere.

Because evaporation and transpiration occur simultaneously and cannot be easily separated, they are combined and measured together as ET. At the early stages of crop growth, nearly 100% of ET comes from soil evaporation, while at full crop canopy cover, more than 90% of ET comes from transpiration. This shift is important – it tells us that as crops mature, the plant itself becomes the dominant pathway for water loss.

Why ET matters in agricultural water management 🔗

Evapotranspiration is a major component of the water cycle and the agricultural water balance. Accurate estimation of ET is critical for agricultural water resources planning, management, and regulation – it helps establish a sustainable water balance, mitigates the impacts of water scarcity, and prevents the overuse and waste of precious water resources. In practical terms, ET defines how much water a crop actually consumes. Without knowing this figure, irrigation is guesswork – leading either to water stress and reduced yields, or to over-irrigation that wastes water and leaches nutrients from the soil.

Since a majority of the water used by a crop is transpired by the plant, crop water use is considered equal to the rate of evapotranspiration. ET is measured as the average depth of water that the crop uses, and this is a function of both the plant type and the prevailing weather conditions. A wheat field in a cool, humid climate will have a very different ET demand compared to the same crop grown in a hot, arid environment – and irrigation must reflect that difference.

Factors affecting evapotranspiration rates 🔗

ET is not a fixed value. It changes constantly based on a combination of climatic, crop, and soil conditions. Understanding these factors is essential for accurate water management.

Weather and climate 🔗

Solar radiation, air temperature, relative humidity, and wind speed all play crucial roles in determining ET rates. When temperatures rise and sunlight increases – providing more energy for evaporation – coupled with dry air, ET increases as more water is lost from the soil and plants. Strong winds also accelerate ET by blowing away the saturated layer of air that forms just above the crop canopy, maintaining a strong vapour pressure gradient between the leaf surface and the atmosphere.

Crop type and growth stage 🔗

Prevailing weather conditions, available water in the soil, crop species, and growth stage all influence crop water use. At full canopy cover, a crop reaches its maximum ET rate if soil water is not limited – that is, if the root zone is at field capacity. Different crops have different stomatal characteristics, canopy structures, and root depths, all of which affect how quickly they lose water. For instance, crops like grapes, beans, and grains typically require about one to two feet of water per growing season, while deeper-rooted crops like alfalfa can draw from greater soil depths and may have higher seasonal ET demands.

Soil water availability and other factors 🔗

Factors such as soil salinity, poor land fertility, limited fertilizer application, and poor soil management can limit crop development and reduce ET. Other factors include ground cover, plant density, and soil water content – and the effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. When soil moisture drops significantly below field capacity, plants begin to experience water stress, stomata close, and both transpiration and ET decline – along with crop yield potential.

Reference ET, crop coefficients, and crop water requirements 🔗

Because calculating ET individually for every crop under every condition would be impractical, agronomists use a standardized framework. A standardized value called reference evapotranspiration (ET₀) is used, calculated based on a reference crop – typically well-watered grass or alfalfa – and reflects the influence of weather conditions. To estimate actual crop water use, ET₀ is converted using a crop coefficient (Kc), which adjusts for differences in crop type and growth stage.

The formula is straightforward: ETc = Kc × ET₀, where ETc is the crop evapotranspiration under standard conditions. Crop water requirements (CWR) are defined as the depth of water in millimetres needed to meet the water consumed through evapotranspiration by a disease-free crop, growing under non-restricting soil conditions, and achieving full production potential. CWR is the sum of ETc over the entire growing period and is the foundation for designing irrigation systems, scheduling water delivery, and estimating seasonal water budgets.

Methods for measuring and estimating ET 🔗

Reliable ET data comes from a mix of direct measurement and mathematical estimation methods. Each has its own level of accuracy, cost, and practicality.

Lysimeters 🔗

A lysimeter is a device that continuously measures the weight of a crop and its associated soil. Any water added through precipitation or irrigation is recorded, and changes in storage are used to calculate ET directly. When used properly, lysimeters allow precise ET measurement over small areas. Weighing lysimeters are considered the gold standard for validating ET estimation models, though they are expensive and not practical for routine farm use.

The FAO Penman-Monteith equation 🔗

For practical, wide-scale use, the FAO Penman-Monteith (FAO-PM) method is the globally recommended standard for estimating reference ET. The FAO Irrigation and Drainage Paper No. 56, published in 1998, advanced the accuracy and consistency of ET computation for agricultural and other land use types – and the FAO-PM equation is now adopted worldwide, not only in agricultural water management but also in hydrology, water resources assessment, and climate change studies. The equation combines energy balance and aerodynamic transfer principles, using inputs of solar radiation, air temperature, humidity, and wind speed to compute ET₀.

The FAO Penman-Monteith method uses the concept of a reference surface – a hypothetical grass crop with a fixed height of 0.12 m, a surface resistance of 70 s/m, and an albedo of 0.23 – removing the need to define parameters for each individual crop and stage of growth. ET rates for other crops are then derived using crop-specific coefficients.

Pan evaporation and remote sensing 🔗

Pan evaporation is a simpler, lower-cost method where water loss from a standardised open pan is measured and converted to ET₀ using pan coefficients. While less precise than the Penman-Monteith approach, it remains widely used in regions where weather station data are limited. At the other end of the spectrum, satellite-based algorithms like SEBAL and METRIC solve for the energy balance at the Earth’s surface using satellite imagery, allowing both actual and potential ET to be mapped on a pixel-by-pixel basis across large agricultural areas. These technologies are increasingly being paired with machine learning to improve spatial accuracy and operational efficiency.

ET-based irrigation scheduling in water-scarce agriculture 🔗

Translating ET data into practical irrigation decisions is where the concept delivers its most direct value. Knowing how much ET is occurring at a given time is essential for meeting a crop’s water needs. Weather data is plugged into the FAO-56 Penman-Monteith formula to calculate ET₀, which is then converted into actual crop water use using the crop coefficient. The difference between crop water requirement and effective rainfall equals the irrigation requirement – a simple but powerful calculation for scheduling irrigations and managing limited water supplies.

Crop evapotranspiration is an important parameter in hydrological, environmental, and agricultural studies, and plays a key role in designing and managing irrigation projects under both irrigated and rainfed agriculture. In semi-arid and arid regions – where water competition between agriculture, industry, and communities is intense – ET-based planning becomes even more critical. Well-grounded ET estimations are required to regulate the components of the irrigation system, including the sizing of canals, dams, and pumping capacity.

Modern tools are making ET-based irrigation more accessible than ever. Software such as FAO’s CROPWAT uses the Penman-Monteith equation to compute reference ET from standard meteorological inputs and generate irrigation schedules – all without requiring complex field measurements. Mobile apps and automated weather networks are extending these capabilities directly to farmers in the field, allowing real-time adjustments to irrigation based on live ET estimates.

ET and climate change: a growing challenge 🔗

Global warming has increased evapotranspiration over land, and this increased ET is one of the effects of climate change on the water cycle. As temperatures rise, the atmosphere can hold more water vapour, and more energy is available to drive evaporation from soils and transpiration from crops. This means that future crops may face higher water demands even under the same growing conditions. Accurate and timely ET information reduces the adverse consequences of climate change for efficient water management – and researchers recommend the FAO Penman-Monteith model as the global standard for estimating ET under changing climatic conditions.

The implications for agriculture are significant. Where rainfall remains unchanged or decreases, rising ET-driven demand means that existing irrigation infrastructure may become insufficient. Planning new irrigation schemes, recalibrating water allocation policies, and adopting deficit irrigation strategies all depend on a sound understanding of how ET will shift with the climate.

What do you think? With water scarcity becoming an increasing concern in many agricultural regions, do you think ET-based irrigation scheduling is realistically accessible to smallholder farmers who may lack weather station data or digital tools? And as climate change drives up evapotranspiration rates, how should water allocation policies in agriculture be redesigned to keep pace with changing crop water demands?

References

1. https://www.fao.org/4/x0490e/x0490e04.htm
2. https://www.mdpi.com/2306-5338/9/7/123
3. https://serc.carleton.edu/integrate/teaching_materials/food_supply/student_materials/1091
4. https://www.canr.msu.edu/news/what-is-evapotranspiration-why-is-it-important-in-irrigation
5. https://crops.extension.iastate.edu/encyclopedia/crop-water-use-or-evapotranspiration
6. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/crop-water-requirement
7. https://en.wikipedia.org/wiki/Evapotranspiration
8. https://www.fao.org/4/x0490e/x0490e06.htm
9. https://link.springer.com/article/10.1007/s00271-024-00975-x
10. https://www.hec.usace.army.mil/confluence/hmsdocs/hmstrm/evaporation-and-transpiration/penman-monteith-method
11. https://www.mdpi.com/2073-4441/10/4/405
12. https://www.fao.org/4/x0490e/x0490e08.htm