How Reliable Is the FAO-56 Method? The Original Authors Answer, 17 Years Later
In 2015 the original FAO-56 authors published a review 17 years on: what held up against later research, what didn't, and why the gap between standard and actual crop coefficients is the most common mistake in applying the method.
Who better than the original authors to take stock
In the first article of this series we covered what the FAO Irrigation and Drainage Paper No. 56 (1998) actually says: why the Penman-Monteith equation became the world standard for calculating evapotranspiration. A natural question at this point is: nearly three decades on, does the method still hold up? The most authoritative possible answer doesn't come from an outside commentator, but from the four original FAO-56 authors themselves — Luis Pereira, Richard Allen, Martin Smith and Dirk Raes — who in 2015, 17 years after the original publication, wrote a review dedicated to exactly that: what worked, what didn't, and what's still needed.
It's a rare case in scientific literature: the same researchers who created a standard come back to assess it against the research accumulated in the meantime, rather than leaving that job to others. Here's what they wrote, in language readable by non-researchers too.
The paper at a glance
| Section | Content |
|---|---|
| 🎓 Paper | Pereira, L.S., Allen, R.G., Smith, M., Raes, D. (2015). Crop evapotranspiration estimation with FAO56: Past and future. Agricultural Water Management, 147, 4-20 |
| ❓ Question | 17 years after FAO-56's publication, how valid are its guidelines in light of the research, data, and modelling capabilities that have become available since? |
| 🔬 What they did | Systematic review of FAO-56's four main pillars (reference ET, single crop coefficient, dual crop coefficient, non-standard conditions) checked against hundreds of validation studies published between 1998 and 2014 across very different climates |
| 📈 What they found | The Penman-Monteith reference ET method holds up everywhere it was tested; the dual crop coefficient proved to be a genuine improvement; but the distinction between 'standard' Kc (transferable) and 'adjusted/actual' Kc (NOT transferable) is systematically misunderstood by users, and is the most common cause of disappointing results |
| 🌱 What it means for your garden | A Kc value found online for 'lawn' or 'hedge' only applies if it represents optimal conditions (well-watered, unstressed) — if the source measured it on stressed plants or in a different climate from yours, applying it as-is produces wrong estimates |
| 🗺️ How it connects to SprinklerMap | Further confirms why the tool focuses on what it CAN guarantee with certainty regardless of climate or plant — uniform coverage and correct pressure per zone — leaving the water-need calculation (which needs plant- and context-specific judgment) to whoever manages irrigation with real local data |
| 📚 References | See the References section at the end of the article |
The question: does a 1998 method still hold up today?
When a scientific method becomes a global standard, the risk is that it stops being tested — everyone uses it because 'it's the standard', not because someone recently verified it still deserves to be. Pereira and colleagues address this risk head-on: FAO-56 had been validated mainly against data available through the late 1990s. Since then, decades of additional lysimeter networks, tens of thousands of weather stations, and the development of satellite remote sensing for estimating evapotranspiration have arrived — tools that in 1998 simply didn't exist in their current form.
So the question isn't rhetorical: with all this new data, has the Penman-Monteith method standardised in FAO-56 held up to scrutiny, or has later research found cracks that couldn't be seen back then?
What they did: a systematic check on four fronts
The authors organised the review around FAO-56's four main contributions, checking each against later literature. First: reference ET (ET0) calculated with Penman-Monteith, verified against comparative studies across dozens of climates — from semi-arid Spain (Berengena and Gavilán, 2005) to China, where a single study analysed 41 years of daily data from 150 national weather stations (Gong et al., 2006), to southern Italy (Steduto et al., 2003).
Second: the single crop coefficient (Kc), checked against new studies on crops not present in the original edition — from quinoa to teff, from tree crops like peach and citrus to natural vegetation in semi-arid regions. Third: the dual crop coefficient approach (Kc = Kcb + Ke, which separates plant transpiration from direct soil evaporation), checked against applications on annual crops, orchards and vineyards. Fourth: procedures for non-standard conditions — water stress, salinity, mulching, intercropping.
What they found: what held up, and the mistake almost everyone makes
The most solid finding concerns the very heart of the method: the Penman-Monteith equation for reference ET showed excellent agreement with direct measurements in virtually every climate tested in the 17 years after publication. The original philosophy — 'physics are physics everywhere', so a physically grounded equation should work regardless of local climate — turned out to be correct, not just a starting hypothesis.
The dual crop coefficient (Kcb + Ke) proved a real step forward over the single Kc, especially for tree crops and vineyards, where canopy variability made the older approach imprecise — a problem later addressed by a follow-up method from the same Allen and Pereira (2009), based on the fraction of ground shaded by the canopy.
But the paper's most interesting point, the one with the most direct practical payoff, is the distinction between 'standard' and 'adjusted' (or 'actual') Kc. Standard Kc represents a crop under optimal conditions — well-watered, unstressed — and is the one transferable from one climate to another with the adjustments the method provides. 'Adjusted' Kc, on the other hand, represents the real plant, often under water or salinity stress or managed sub-optimally — and by definition is NOT transferable: it depends on site-specific conditions that don't repeat elsewhere. The authors are explicit: much of the criticism about 'poor transferability' of Kc stems from critics comparing adjusted Kc values measured in different contexts, not standard Kc — a conceptual error, not a limitation of the method.
What it actually means for your garden
The practical consequence is simple to state but easy to ignore: if you find a Kc value online for 'lawn' or a specific hedge, that number only applies if it describes a plant under optimal conditions — not a plant that, in the source's own photo, was clearly drought-stressed or grown in a climate with very different humidity and wind from yours. Using an 'adjusted' Kc measured elsewhere as if it were a transferable standard is, according to the method's own authors' review, the most common cause of wrong water-need estimates.
The second practical consequence concerns the quality of input weather data: the review devotes an entire section to the need for weather data quality control (QA/QC), because a physically accurate equation fed poor weather data still produces poor results — garbage in, garbage out applies to Penman-Monteith too. A smart controller pulling data from a distant or poorly sited weather station inherits this same limitation.
How this connects to SprinklerMap
The same honesty from the first article applies here: SprinklerMap doesn't calculate ET or apply crop coefficients — that's not what it's for. But this review reinforces a point already made: the method's own authors warn that crop coefficients require local judgment and aren't a magic number to copy from a table without context. That's exactly why the tool focuses on what it CAN guarantee with certainty regardless of climate or plant — uniform coverage and correct pressure per zone — leaving the water-need calculation (which needs plant- and context-specific judgment) to whoever manages irrigation with real local data. For the practical run-time calculation side, see /enHow to set irrigation run times by soil type.
The short version
17 years after publication, the FAO-56 authors themselves put their own method under review, and the conclusion is that it held up: Penman-Monteith remains valid everywhere it was tested, the dual crop coefficient proved a real improvement, and the weak points identified are more about application (data quality, confusing standard with actual Kc) than the method itself. For anyone watering a garden, the most useful practical takeaway is exactly this: a crop coefficient found online only applies if it represents optimal conditions, not the stressed plant in the photo next to it.
References
Pereira, L.S., Allen, R.G., Smith, M., Raes, D. (2015). Crop evapotranspiration estimation with FAO56: Past and future. Agricultural Water Management, 147, 4-20. doi.org/10.1016/j.agwat.2014.07.031
Allen, R.G., Pereira, L.S. (2009). Estimating crop coefficients from fraction of ground cover and height. Irrigation Science, 28(1), 17-34. doi.org/10.1007/s00271-009-0182-z
First article in this series: Why Evapotranspiration Is the Foundation of Modern Irrigation — /en/blog/evapotranspiration-modern-irrigation-fao56-explained
Frequently asked questions
Did the authors change their minds compared to 1998?
No, and that is what makes the review interesting: the people who created the standard went back to assess it instead of leaving the job to others, and they confirmed the original framework. Pereira, Allen, Smith and Raes took the four pillars of FAO-56, meaning reference ET, single crop coefficient, dual crop coefficient and non-standard conditions, and compared them against hundreds of validation studies published between 1998 and 2015. The Penman-Monteith method for reference ET proved solid everywhere it was tested. So it is not a retraction but a sharpening of focus: the part that needs most care is not the equation, it is the crop coefficients, where the review introduces a distinction that had stayed implicit in 1998.
What did they flag as real limitations?
Three above all, all practical rather than theoretical. The first is the quality of the input weather data: the equation is reliable, but if the humidity or wind figures are poor or come from an unrepresentative station, the error enters there and the method cannot correct it. The second is the limited availability of crop coefficients for non-traditional crops: the tables cover well what has been studied most, much less the rest. The third concerns fruit trees and vineyards, where the canopy structure and partially shaded soil make application harder than on a uniform crop. For a domestic garden it is the first limitation that really counts: the calculation is worth exactly as much as the weather data you feed it.
Does the standard/adjusted Kc distinction apply only to farming, or to home gardens too?
It applies to the garden as well, through the same mechanism. The distinction is between a standard Kc, measured under optimal conditions on well-watered, unstressed plants, and an actual Kc, measured on plants in less than ideal conditions. Only the first is transferable, because it is designed to be comparable across places and climates; the second describes one specific situation and cannot be moved elsewhere. In practice a well-watered lawn has a different Kc from a lawn stressed by drought, and if you take a value found online without knowing the conditions it was measured under, you risk applying to your garden a number that described another plant in another climate. That is why the authors insist on this label.
Are there even more recent reviews?
Yes: the same group, joined by further co-authors, published another update in 2021 devoted to crop coefficients for field crops, a possible subject for a future article in this series. The direction of that work is consistent with the 2015 review: the reference equation is left alone, and what gets refined is the coefficients, which is where real-world variability is greatest and where picking the wrong value has the largest effect on the final result. For anyone designing garden irrigation the practical message does not change: the stable part of the calculation is reference ET, while with Kc values it always pays to know where the number came from.
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.