Class A Pan Evaporation Calculator

Class A pan evaporation and reference ET

The Class A pan, standardised by the US Weather Bureau, is a galvanised cylinder 1.21 m in diameter and 25.5 cm deep, mounted on a wooden grid 15 cm above the ground. You read daily evaporation (Epan, mm/day) from how far the water level dropped. Reference evapotranspiration then follows ETo = Kp · Epan, where the pan coefficient Kp usually falls between 0.7 and 0.8, the exact value depending on upwind fetch, humidity and wind. During Brazilian summers Epan tends to sit at 5–10 mm/day, dropping to 2–4 mm/day in Southeast winters.

Applications

It shows up in operational hydrology (reservoir water balance, and a reference for the ANA / National Water Agency), in irrigation scheduling (EMBRAPA bulletins turn Epan into crop water demand through Kp and Kc), in climate research (long Epan series help flag aridity trends) and in aquaculture (estimating losses from fish ponds).

FAQ

Why not measure ETo directly? Direct methods like Penman-Monteith or a lysimeter require radiation, wind, humidity and temperature data. A Class A pan is cheap by comparison, and it comes with decades of historical records to lean on.

How do I pick Kp? The FAO-56 tables match Kp to the fetch (the upwind grass border), the mean wind speed and the relative humidity. At humid, low-wind sites Kp lands closer to 0.85; at arid, windy ones it can fall to 0.55.

What if it rained? You'll need to adjust Epan. Subtract the rain recorded by a nearby pluviometer from the water-level drop, or add it back if the level actually rose.

Is the pan being phased out? Plenty of automatic weather stations now compute ETo with Penman-Monteith, yet the Class A pan is still the official standard at many Brazilian stations and ANA reservoirs.