ET₀ Calculator — FAO-56 Penman-Monteith on Your Own Weather Data
Point the calculator at the temperature, humidity, radiation and wind channels of a station export and get a daily ET₀ series computed by FAO-56 Penman-Monteith, with every intermediate term shown so the arithmetic can be checked against the paper it came from.
Your data
Drop a logger CSV here
HOBOware and Postlad exports are recognised automatically. It is read on this machine — nothing is uploaded.
Synthetic 61-day daily record from an inland irrigation-district weather station at 38.54°N, 18 m, wind measured at 10 m — which are the site values the options are preset to. The pyranometer is dark from 4 to 9 May, so those six days fall back to the FAO-56 Eq. 50 temperature-range estimate and the figure says so. Not measured data.
SVG is vector — scale it to any size in Illustrator, Inkscape or LaTeX without losing quality, and edit the type. Use it unless a journal insists on raster.
Options
Channels
Daily Tmax, or a sub-daily temperature channel. °C.
Daily Tmin, or the same sub-daily channel again. °C.
Site
Decimal degrees, NEGATIVE south of the equator. Extraterrestrial radiation depends on it, so a sign error here is a large error in ET₀.
Metres above sea level. Sets atmospheric pressure and the clear-sky radiation ceiling.
Wind is corrected to 2 m by FAO-56 Eq. 47. The WMO standard height is 10 m, which is what most exports carry.
Method
Never inferred from the numbers. A W/m² channel is integrated over each day; a MJ/m²/day channel is taken as it stands.
FAO-56’s order is dewpoint, then RHmax+RHmin, then RHmax, then RHmean. The paper calls Eq. 19 less desirable than Eqs. 17 and 18.
Falls back per day, not per file: a station whose pyranometer failed for a week uses the estimate for that week only.
Coastal means a site where air masses come off a large water body. Choosing wrongly moves the estimate by about 15%.
Fraction of Ra reaching the ground on an overcast day. FAO-56’s default where no local calibration exists is 0.25.
FAO-56’s default is 0.50, so as + bs = 0.75 on a cloudless day.
FAO-56 offers 2 m/s, the worldwide average of u₂, as a temporary estimate. Any day that uses it is flagged on the figure.
Figure
All of that was worked out in this browser tab. Your file was not uploaded, and no request goes out while you work — open the network panel and watch, if you like.