Reference evapotranspiration (ET0) is the climate-driven baseline that irrigation scheduling, water budgeting, and crop planning all build on. This calculator uses the Hargreaves-Samani equation, a simplified method that needs only daily temperature — no humidity, wind, or solar radiation sensors required — making it usable anywhere a temperature record exists.
How the Evapotranspiration Calculator works
The calculator applies ET0 = 0.0023 × (Tavg + 17.8) × √(Tmax − Tmin) × Ra, the Hargreaves-Samani (1985) equation. Tavg, Tmax, and Tmin are the day's average, maximum, and minimum temperature in °C. Ra, extraterrestrial radiation, is the solar radiation that would reach the top of the atmosphere at your latitude and time of year — it's computed from latitude and day-of-year using the FAO-56 astronomical formula (dr, δ, and ωs intermediate terms), then converted from MJ/m²/day to an equivalent mm/day evaporation rate by multiplying by 0.408.
Hargreaves is a deliberate simplification of the more rigorous FAO-56 Penman-Monteith method, which additionally requires humidity, wind speed, and measured solar radiation. Where that weather station data isn't available — which is common for many fields and most historical records — Hargreaves gives a reasonable ET0 estimate from temperature alone, at the cost of somewhat lower precision in windy or very humid climates.
Inputs and what they mean
Average, max, and min temperature drive the equation directly — a wider diurnal range (Tmax − Tmin) generally signals clearer, drier conditions and higher ET0, since it appears under a square root in the formula. Average temperature shifts the baseline up or down.
Latitude and day of year determine Ra astronomically: lower latitudes and summer months get more extraterrestrial radiation, pushing ET0 higher for the same temperatures. If you already have a measured or tabulated Ra (from a weather station, agricultural extension table, or another source), switch to Manual entry and skip the latitude/day-of-year calculation entirely.
Crop coefficient (Kc) on the Irrigation Estimate tab scales ET0 to a specific crop's actual water use (ETc = ET0 × Kc). It defaults to 1.0 (the reference crop itself) but should be set per crop and growth stage for a real irrigation plan — young plants and bare soil use less water than a full, actively growing canopy.
Limits and edge cases
Hargreaves-Samani trades some accuracy for simplicity. It tends to overestimate ET0 in very humid, low-wind climates and can underestimate it in very windy, arid ones, because it has no direct humidity or wind term — those effects are only captured indirectly through the temperature range. Where full weather-station data (humidity, wind speed, measured solar radiation) is available, FAO-56 Penman-Monteith is the more accurate standard method and should be preferred for high-stakes irrigation decisions.
The crop coefficients on the Irrigation Estimate tab are mid-season midpoints, not growth-stage-specific values — actual Kc for a given crop typically starts lower during establishment, peaks mid-season, and declines again at maturity. For precise scheduling, consult a crop-specific Kc curve (e.g. from FAO-56 or a local agricultural extension) rather than a single flat number. Near the poles in winter or summer, the sunset-hour-angle term can hit its mathematical limits (polar day/night); the calculator clamps this case rather than producing an invalid result, but Ra estimates at extreme latitudes should be treated cautiously.