Why Evapotranspiration Is the Foundation of Modern Irrigation: the FAO-56 Paper Explained
What FAO Paper 56 (Allen et al., 1998) actually says, why Penman-Monteith became the standard for a garden's water needs, and what it means in practice.
The number almost nobody checks before watering
Anyone who deals with irrigation — professionals and ordinary homeowners alike — tends to fall back on rules of thumb: 'twice a week', 'early morning', 'a finger in the soil'. Those are useful heuristics, but none of them answers the underlying question: how much water did the garden actually lose in the last 24 hours? That question has a precise name — evapotranspiration — and a reference document that has been the standard used by agronomists, smart controller manufacturers and weather services worldwide for 27 years: the FAO Irrigation and Drainage Paper No. 56.
It's not a minor document: it's among the most cited publications in FAO's history, with over 11,500 citations in scientific articles. And yet outside agronomy circles almost nobody has actually read it — most online content that name-drops 'Penman-Monteith FAO-56' does so as a buzzword, without explaining what the document actually says or why it became the standard. This is an attempt to close that gap, in language readable by someone who tends a garden, not someone who studies one.
The paper at a glance
| Section | Content |
|---|---|
| 🎓 Paper | Allen, R.G., Pereira, L.S., Raes, D., Smith, M. (1998). Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper No. 56, FAO, Rome |
| ❓ Question | How can water loss from a crop (or a lawn) to evaporation and transpiration be calculated in a standard, reproducible way that's comparable worldwide? |
| 🔬 What they did | Compared existing methods (Blaney-Criddle, Hargreaves, pan evaporation, original Penman) against real lysimeter data across different climates, and standardised a single equation — the FAO-modified Penman-Monteith — as the global reference method |
| 📈 What they found | FAO-56 Penman-Monteith showed the best agreement with direct measurements in almost every climate tested, unlike simpler methods that systematically over- or under-estimate depending on local climate |
| 🌱 What it means for your garden | How much water your lawn loses each day depends on temperature, humidity, wind and solar radiation combined — not a fixed calendar rule. In the same spot, a windy July day can lose twice as much water as a cloudy May day |
| 🗺️ How it connects to SprinklerMap | The ET-based method confirms a principle the tool already applies on the hydraulic side: coverage must be uniform before ET calculations even matter — a perfect ET calculation is useless on a system that soaks half a zone twice as much as the other |
| 📚 References | See the References section at the end of the article |
The question: why a universal method was needed
Before 1998, anyone estimating a crop's or a lawn's water needs had roughly a dozen different methods to choose from — Blaney-Criddle, Hargreaves, Thornthwaite, Class A pan evaporation — each calibrated to a specific climate and often disagreeing with each other by 30-50% on the very same site. An agronomist in California and one in southern Italy could apply different formulas and get non-comparable results, even starting from the same raw weather data.
FAO had already tried to solve this in 1977 with the earlier Paper No. 24, but subsequent tests against lysimeter networks (tanks that directly measure water lost from soil, the most reliable instrument for validating any formula) showed that attempt at standardisation also lost accuracy outside the climate it had been calibrated on. The working group led by Richard Allen started over with a specific goal: a single, physically grounded equation, valid everywhere in the world, that required only easily measurable weather data.
What they did: from the original Penman to the FAO-56 version
The starting point is the Penman-Monteith equation, which combines the energy balance (how much solar energy reaches the surface) and aerodynamic transport (how effectively wind and humidity carry water vapour away) into a single physical formula. Unlike empirical methods, it isn't calibrated to a specific climate: it derives from thermodynamics and atmospheric physics, so in theory it works the same way everywhere.
FAO-56's specific contribution was standardising the parameters of a 'reference crop' — a hypothetical 12 cm-tall grass surface, with fixed surface resistance and albedo — so the result (called ET0, reference evapotranspiration) is the same regardless of who calculates it, as long as the same weather inputs are used: maximum and minimum temperature, relative humidity, wind speed at 2 metres, and solar radiation (or sunshine hours, from which it's estimated).
The working group then validated the resulting equation against available lysimeter networks across very different climates — from semi-arid Idaho to the Mediterranean — systematically comparing results against every previous method on the same datasets.
What they found: why Penman-Monteith won
The paper's central finding is that FAO-56 Penman-Monteith showed the best agreement with direct lysimeter measurements across nearly every climate tested — while simpler methods (which use only temperature, like Hargreaves or Thornthwaite) proved reliable only in climates similar to the one they were originally calibrated on, with significant systematic errors elsewhere: some overestimate in humid climates, others underestimate in windy or arid ones.
That's where the practical two-step formula now running under the hood of every modern smart controller (Rachio, Hunter Hydrawise, Orbit B-hyve) comes from: first calculate ET0 (the standardised reference lawn's water need) from weather data, then multiply by a crop coefficient Kc specific to the actual plant — a mature lawn typically has a Kc between 0.6 and 0.8 depending on the season, an established Mediterranean hedge often less. ETc (the real water need) = Kc × ET0. It's the same principle already covered, from the smart-controller side, in /enIrrigation automations: schedules, weather and sensors in practice.
A concrete figure to picture this: on a summer day in the 90s°F with moderate wind and clear skies, ET0 typically runs 6-8 mm; on a cloudy late-spring day it can drop to 2-3 mm. With the same Kc of 0.7, the lawn's real water need on those two days differs by a factor of three — a difference no fixed 'twice a week' rule can capture.
What it actually means for your garden
The most direct practical consequence is that a fixed-schedule timer is, by design, almost always wrong: it delivers the same amount of water regardless of whether yesterday was 95°F, dry and windy or 72°F and overcast. ET-based (weather-based) controllers solve exactly this, pulling local or nearby-station weather data and recalculating ET0 daily to automatically adjust cycle length — a documented water saving of 20-40% versus a fixed programme, with equal lawn health.
For anyone without (or unwilling to buy) a smart controller, the principle is still useful manually: on hot, windy, dry days the lawn loses more water and needs longer or more frequent watering; on cool, humid days the need drops sharply, and sticking to the same schedule just wastes water and encourages root rot.
How this connects to SprinklerMap: what it confirms, and what it does NOT do
Worth being precise here: SprinklerMap's design tool does not calculate evapotranspiration — that's not what it's built for, and we don't want to imply otherwise. The tool handles the hydraulic side: sprinkler placement, coverage, available pressure per zone, material lists.
The connection to FAO-56 is conceptual but real: the paper itself, in its chapters on practical application, makes clear that an accurate ET calculation is pointless if water isn't distributed uniformly across the ground — a poorly placed sprinkler that soaks one spot twice as much as another defeats any precision in the theoretical water-need calculation. Correct hydraulic design (the work SprinklerMap does) is the precondition for any ET calculation, with any method, to be worth applying. For the soil-based run-time calculation side, the practical guide is /enHow to set irrigation run times by soil type.
The short version
FAO-56 isn't an academic footnote: it's the reason 'how much to water' finally has a calculable answer instead of a rule of thumb. Penman-Monteith beat earlier methods because it's physically grounded, not calibrated to a single climate — and every smart controller today promising to 'save water by adapting to the weather' is, under the hood, applying exactly this 1998 paper. Understanding the principle makes it easier to interpret whatever number your smart controller's app shows you.
References
Allen, R.G., Pereira, L.S., Raes, D., Smith, M. (1998). Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper No. 56. Food and Agriculture Organization of the United Nations, Rome. Open access: openknowledge.fao.org/handle/20.500.14283/cd6621en
Pereira, L.S. et al. (2015). Crop evapotranspiration estimation with FAO56: Past and future. Agricultural Water Management, 147, 4-20. doi.org/10.1016/j.agwat.2014.07.031
Frequently asked questions
Are ET0 and ETc the same thing?
No, and mixing them up is the most common mistake when reading an evapotranspiration figure. ET0 is reference evapotranspiration: that of a standardised lawn, well watered and unstressed, which for the same weather is identical anywhere in the world. It exists precisely as a common unit of measure, and it is how FAO-56 makes different climates comparable. ETc is the actual need of the plant you really have in the garden, and you get it by multiplying ET0 by the crop coefficient Kc, which depends on the species and its growth stage. In practice: controllers and weather services give you ET0, but the water to give your lawn or hedge is ETc. Using ET0 as if it were the requirement means irrigating on a number that does not describe your plants.
Do I need a weather station in my garden to use these calculations?
No, and in most cases it is not worth it. Smart controllers on the market pull data from nearby public weather stations or from services like Open-Meteo, and for calculating evapotranspiration that is almost always enough: the variables that matter, meaning temperature, humidity, wind and radiation, change little within a few kilometres. A personal station such as an Ecowitt improves precision and makes sense if your garden has a marked microclimate: very windy, heavily shaded, at the bottom of a valley or unusually exposed. It stays a refinement rather than a prerequisite: the FAO-56 method was designed to work with standard weather data available anywhere, not with dedicated instruments installed on site.
Is the FAO-56 method still considered valid in 2026?
Yes, and not out of inertia: it remains the internationally recommended reference method, and the original authors re-examined it in 2015, seventeen years on, against hundreds of validation studies published in the meantime. The conclusion of that review, which we cover in the next article in this series, is that the 1998 framework held: the Penman-Monteith equation for reference ET proved solid everywhere it was tested. Real refinements did arrive, mostly on the dual crop coefficient and on the distinction between transferable and non-transferable Kc values, but none of them replaces the method, they sharpen it. For garden practice, then, a calculation based on FAO-56 is not an outdated reference.
Why not just use temperature, which is easier to measure?
Because temperature on its own ignores two variables that weigh as much as it does: wind and air humidity. At the same temperature, a windy dry day can make the lawn lose twice the water of a calm humid one, because moving air carries the vapour away from the leaf surface and the process starts again. This is exactly the limitation the paper documents for simplified methods: measured against lysimeters, meaning against direct measurements of the water actually lost, methods such as Blaney-Criddle, Hargreaves and the pan evaporimeter systematically over- or under-estimate depending on the local climate, while Penman-Monteith showed the best agreement in almost every climate tested. Temperature is the easiest figure to measure, not the one that explains the phenomenon on its own.
Technical note: This article explains, in plain language, the findings of a published scientific study, necessarily simplifying the original methodology to make it readable. For the full statistical analysis, the limitations the authors themselves note, and the experimental details, always refer to the original paper linked in the references at the end of the article: a study's results shouldn't be generalised automatically to your specific case.