How Reliable Is the FAO-56 Method? The Original Authors Answer, 17 Years Later

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

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.

Frequently asked questions about this FAO-56 review

Did the authors change their minds compared to 1998? No: the review confirms the original framework rather than contradicting it. The central message is that the method held up to 17 years of new research, with additions (dual Kc, non-standard conditions) progressively integrated by the same authors.

What did they flag as real limitations? Mainly the quality of input weather data, limited availability of Kc for non-traditional crops, and application difficulty for orchards and vineyards due to variable canopy (later addressed by a follow-up method).

Does the standard/adjusted Kc distinction apply only to farming, or to home gardens too? The principle is identical: a well-watered lawn has a different Kc than a lawn stressed by drought, and only the former value is comparable to tables built for climates other than yours.

Are there even more recent reviews? Yes, the same group (with additional co-authors) published a further update in 2021 specifically on crop coefficients for field crops — a possible topic for a future article in this series.

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.