Evapotranspiration terminology for agronomists and mesonets

Quantifying crop water use, generally referred to as evapotranspiration, or ET, can be very important to agricultural water management. While the soil acts as a reservoir, the crop uses (or “loses”) water through ET and is replenished via rainfall and/or irrigation. In arid or semi-arid areas such as the western U.S., which have little rainfall, estimating ET can especially guide efficient irrigation applications. Because many tools and reports now provide estimates of ET, having consistent terminology and methodology is essential so that everyone is talking about and using the same thing. 

Essentially, ET is determined by evaporative demand attributed to weather conditions and the status of the crop. These two ingredients—weather and crop status—are the foundation of modern ET estimation methods. Regarding weather, the hotter, drier, windier, and sunnier it is, the more ET demand there is. Regarding crop status, tall, leafy, healthy crops generally use more water through ET than short crops or stressed crops. 

For decades, scientists have refined methods to estimate ET for crops based on these ideas. Many early methods used the idea of “potential ET” as a sort of maximum level; but over time, this idea of “potential ET” evolved into several different meanings. For example, in some cases, “potential ET” was used to mean ET from a reference grass surface, whereas to others it meant the maximum ET from a specific crop under non-stressed conditions. This confusion is a problem because different meanings for the same term or how it is used can result in different answers. Additionally, many different methods were used in the past, which makes it hard to compare apples to apples in terms of ET estimates.

Standardizing ET: Reference ET and crop coefficients

To address this concern, ET experts decided on a standardized methodology that would be both accurate and consistent. This methodology essentially separates the weather and the crop status into two parts, the reference ET (ET_ref) and the crop coefficient (K_c), by using the equation for crop ET:

ET_c = ET_ref × K_c

The reference ET represents the evaporative demand from a reference surface, and the crop coefficient (K_c) can be used to scale to the crop ET. For most crops, the K_c is small early in the season, increases as the plant grows, levels off during peak growth, and then reduces again with crop senescence, thus creating a trapezoid-shaped curve over time.

The reference ET is an estimate of the ET on a reference (hypothetical) surface that is easy to maintain and keep consistent. One of the main surfaces is grass, which grows all the time and can be cut to a specific height. Using a weather station on a big grass field, the temperature, humidity, solar radiation (sunlight), and wind speeds will determine grass reference ET, referred to in equations as ET_o. Another reference surface often used is an alfalfa crop, where the reference ET is referred to as ET_r. These ideas were most notably standardized in a manual called FAO-56 (FAO, 1998, 2025) for grass only and defined for both grass and alfalfa in a simplified equation (Allen et al., 2005). 

A few interesting facts about reference ET:

• ET_o and ET_r are not the same. Alfalfa ET (ET_r) is typically 10–25% higher than grass ET (ET_o), depending on climate. Because ET_o and ET_r are different, the crop coefficients (K_c values) are also different. Grass-based crop coefficients are therefore higher than alfalfa-based crop coefficients. 
   
• Most countries and states adopt a preferred reference surface. Alfalfa (ET_r) is a common standard in many western U.S. states while grass (ET_o) is more common internationally.
   
• Alfalfa is a little more challenging to maintain since cuttings bring the height of the canopy closer to the ground. Grass cuttings may be barely noticeable if done frequently. 
   
• Alfalfa reference ET (ET_r) can be determined off a grass reference station, and vice-versa. The results are all wrapped up in the math of the equation.
   
• While many different methods were used in the past, the American Society of Civil Engineers (ASCE) recommends the Standardized ASCE or FAO Penman-Monteith equation as the sole method for the calculation of ET_ref. 

Many states host networks of weather stations, called mesonets, that are very valuable to informing local weather conditions, watching long-term trends, forecasting, and other uses. If sited properly, these mesonets can accurately estimate ET_ref (ET_r and/or ET_o). By knowing the crop and planting date, crop coefficient curves (known from previous experimental research) can then be multiplied by ET_ref to estimate crop ET (ET_c). Many mesonets have online interfaces where a user can choose a local station, provide cropping information, and receive estimates of ET_c. A few networks that may be familiar to readers include Oklahoma Mesonet, CoAgMET, High Plains Automated Weather Data Network, CIMIS, AgriMET, etc.

Applying ET in practice: Mesonets, methods, and tools

Proper mesonet siting is important for accurate ET estimation but can be challenging to implement in practice. An ideal station includes a large homogeneous grass or alfalfa reference that is well watered (not water stressed). Other ideal conditions are sites that a long distance from trees or tall buildings that can affect wind, are surrounded by irrigated agriculture, and have sensors that are appropriately configured and frequently calibrated (Figure 1). Many weather stations are situated in or near urban areas (for example, airports commonly have weather stations) or perhaps in arid environments. However, these stations may not be appropriate for use in ET_ref calculation. In fact, ET_ref calculated from most non-reference sites often can be hotter or drier than what a reference site would be (think about a parking lot vs. a golf course nearby). In these cases, ET_ref would be overestimated, potentially suggesting that farmers irrigate more than they need to. So, choosing a well-watered reference site is very important.  

Combined with ET_ref estimation, there are two similar but different approaches to use crop coefficients: the single crop coefficient and the dual coefficient. The single crop coefficient is more simple because it combines the effects of both evaporation from the soil and transpiration from the crop into a single term. Because of its simplicity, this coefficient method is what is commonly used in public-facing mesonet outputs and irrigation-scheduling applications. While the single crop coefficient method is easier to use, it may not be as accurate as the dual crop coefficient method, which calculates evaporation and transpiration separately and can consider the effects of water stress. Whereas ET_c represents a well-watered (non-stressed) crop, the term ET_a (actual evapotranspiration) is more encompassing and can include the idea of both ET_c but also applications where the crop is under water stress.

There has been significant progress in making new products available, which may help users estimate ET. First, as previously mentioned, mesonets can give ground-truthed estimates of ET_ref and even ET_c through use of crop coefficients. On a spatial basis, the Open ET platform is an ensemble of models that are used to predict actual ET (ET_a) for every field across the lower 48 states. Finally a new publicly available model called “pyfao56”, a Python code version of the FAO-56 standardized ET methods (FAO, 1998, 2025), can be customized for water balance modeling and irrigation management (Thorp et al., 2025). 

In conclusion, confident use of ET information is a precursor to good irrigation and agronomic management, and consistency in measurements, definitions, and terminology are key for practitioners to ensure. The principles and best practices in this article are summarized from a much more detailed technical note called “Evapotranspiration Terminology and Definitions,” (DeJonge et al., 2025) which was recently been published by ASCE with support from more than 50 stakeholders across the U.S. and globally. This is a valuable resource for agronomists, irrigation managers, researchers, students, and various other water professionals.