Reference Evapotranspiration ET0 (Hargreaves / FAO-56)

Reference evapotranspiration (ET0) from air temperature alone.

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Details, formula, and sources

by the Hargreaves method FAO-56 recommends when only temperature is reliable: ET0 = 0.0023 (Tmean + 17.8) sqrt(Tmax - Tmin) Ra. Enter latitude, month, and the daily high/low (F); the extraterrestrial radiation Ra comes from the FAO-56 astronomical equations by latitude and mid-month day, so no radiation table is needed. A 45 deg N site in July at 86/59 F reads ET0 = 0.234 in/day (5.93 mm). ET0 is the baseline every crop's demand scales from (crop ET = Kc x ET0), feeding acre-foot and irrigation-scheduling math. The daily temperature range stands in for cloudiness/humidity, so a clear dry site reads higher than a humid one at the same mean. A planning aid; a local station ET0 (CIMIS, Mesonet) and the full Penman-Monteith method govern when full climate data exist.

Hargreaves: ET0 = 0.0023 (Tmean + 17.8) sqrt(Tmax - Tmin) Ra (Ra as equivalent evaporation, mm/day = Ra_MJ/2.45). Ra (FAO-56 Annex 2): dr = 1 + 0.033 cos(2pi J/365); dec = 0.409 sin(2pi J/365 - 1.39); ws = arccos(-tan(phi) tan(dec)); Ra = (24*60/pi) 0.0820 dr (ws sin(phi) sin(dec) + cos(phi) cos(dec) sin(ws)). J = mid-month day; temperatures converted from F to C.

Hargreaves & Samani (1985) reference-ET equation as presented in FAO Irrigation & Drainage Paper 56 (Allen, Pereira, Raes, Smith 1998), Annex 2 for the extraterrestrial-radiation equations, by name.

FAO-56 is a free public PDF at fao.org; the Hargreaves equation and the FAO-56 astronomical Ra equations are public.

Estimate. AHJ and licensed professional govern.

Field names used by the API: latitude_deg, month, tmax_f, tmin_f, et0_mm_day, ra_mj_m2_day

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