Potential evapotranspiration (PET) is the maximum amount of water that would evaporate from soil and transpire through plants if water supply were unlimited. It represents the atmosphere’s “thirst,” a theoretical ceiling driven entirely by weather conditions like heat, sunlight, humidity, and wind. In reality, water is almost always limited to some degree, so the amount that actually leaves the surface is called actual evapotranspiration, which is always less than or equal to PET. The gap between the two is one of the most important numbers in hydrology, agriculture, and climate science.
PET vs. Actual Evapotranspiration
The distinction matters because PET isolates atmospheric demand from water supply. Imagine a hot, dry, windy day over a field. The atmosphere could pull enormous amounts of moisture from the ground, but if the soil is parched and no rain has fallen, very little water actually leaves. PET describes what the atmosphere wants; actual evapotranspiration describes what it gets.
Because actual evapotranspiration can never exceed PET, scientists use PET as the demand side of drought equations. Hotter temperatures push PET higher, meaning more precipitation is needed just to keep up. When rainfall falls short of rising PET for extended periods, drought intensifies even without a change in precipitation.
What Drives PET Higher or Lower
Four weather variables control PET: air temperature, solar radiation, humidity, and wind speed. Temperature is the dominant factor. Research in the Yanhe River Basin in China found that rising air temperatures alone accounted for about 1.09% of the observed increase in PET rates, with solar radiation contributing another 0.55%. Wind speed has a positive effect, meaning faster winds pull moisture away from surfaces more quickly. Humidity works in the opposite direction: higher relative humidity reduces PET because air that already holds moisture has less capacity to absorb more. The sensitivity of PET to humidity was roughly twice as strong as its sensitivity to wind speed in that study.
How PET Is Calculated
The global standard is the FAO Penman-Monteith equation, adopted by the Food and Agriculture Organization of the United Nations. It models PET as the water loss from a hypothetical reference surface: a uniform, well-watered grass lawn 12 centimeters tall. The equation requires daily records of solar radiation, maximum and minimum air temperature, humidity (expressed as vapor pressure), wind speed measured 2 meters above the ground, plus the station’s altitude and latitude.
Not every weather station collects all of those variables. In many parts of Africa and other data-scarce regions, only temperature and rainfall are reliably recorded. For those situations, simpler formulas exist. The Thornthwaite equation and the Hargreaves-Samani equation both estimate PET using temperature data alone. A comparison study in South Africa found that the Hargreaves-Samani model consistently outperformed the Thornthwaite equation, producing results within acceptable accuracy ranges even without wind or humidity data.
Measuring PET Directly
Equations estimate PET, but physical measurement is also possible. A weighing lysimeter is essentially a large container of soil and plants embedded in a field and placed on a scale. As water evaporates from the soil and transpires through the vegetation, the container gets lighter. By tracking weight changes hour by hour, researchers convert kilograms of lost water into millimeters of depth, the same unit used for rainfall. Lysimeters have been used for decades to calibrate and ground-truth the mathematical models, ensuring that equations match real-world water loss.
Why Farmers Rely on PET
Irrigation scheduling depends on PET. The standard approach, developed in the late 1970s and still widely used, multiplies the reference PET by a crop coefficient (abbreviated Kc) to get the water need of a specific crop. Corn, cotton, and tomatoes all pull water at different rates depending on their growth stage, leaf area, and canopy cover. A young corn plant with little leaf surface might have a Kc of 0.3, meaning it needs only 30% of the reference PET. At peak growth with a full canopy, that coefficient can exceed 1.0.
This two-step method lets farmers use a single weather-based PET number from a local station and tailor it to whatever they’re growing. Over-irrigating wastes water and can waterlog roots; under-irrigating stunts growth. Getting the PET estimate right is the foundation of both.
PET in Drought and Climate Classification
PET is half the equation that defines how dry a region is. The Aridity Index, used by the United Nations Environment Programme, divides mean annual precipitation by mean annual PET. An index below 0.03 classifies a region as hyper-arid, 0.03 to 0.2 is arid, 0.2 to 0.5 is semi-arid, 0.5 to 0.65 is dry sub-humid, and anything above 0.65 is humid. A place that receives 400 mm of rain per year might be semi-arid or dry sub-humid depending on how high its PET is, which is why temperature and sunlight matter as much as rainfall when assessing water stress.
In watershed management, PET and precipitation are the two essential inputs for hydrological models that simulate streamflow and estimate groundwater recharge. Because precipitation and actual evapotranspiration are the two largest components of any basin’s water balance, even small biases in PET estimates can cascade into significant errors in water supply forecasts.
How Climate Change Is Shifting PET
Rising global temperatures are increasing PET across most of the world, effectively making the atmosphere thirstier. A 2025 study published in Communications Earth & Environment found that evaporative demand has increased worldwide, with one notable exception: South Asia. There, widespread irrigation has added so much moisture to the air and soil that cloud cover has increased, blocking sunlight and raising humidity. The result is that PET in South Asia has actually declined, even as global surface temperatures outside the region rose from an average of 5.4°C in 1979 to 6.2°C in 2023.
This connects to a broader puzzle known as the “evaporation paradox.” Despite decades of warming, evaporation rates measured from open water pans have declined in the United States, China, Australia, Canada, and India. The leading explanations include declining wind speeds in some regions and reduced solar radiation caused by aerosol pollution dimming the sky. South Asia illustrates how human activity, in this case massive irrigation, can locally reshape the water cycle enough to override the global warming signal. Outside South Asia, 95% of global land areas showed stable or increasing wind speeds, and cloud cover has generally declined, reinforcing the upward trend in PET.
For water planners, the takeaway is that a warming climate does not simply reduce water supply through less rain. It also increases demand through higher PET, squeezing both sides of the equation at once.