Evapotranspiration Calculator

Reference ET (ET₀) models a specific, standardized surface — conceptually a well-watered, actively growing short grass — rather than your actual crop, so it isolates the effect of weather (temperature, humidity, wind, and radiation) from the effect of the crop itself. To estimate actual crop water use, irrigation planners multiply ET₀ by a crop coefficient (Kc) that varies by crop type and growth stage — a young seedling and a mature, full-canopy crop of the same species have very different Kc values even under identical weather.

Evapotranspiration is driven by the vapor pressure deficit — the gap between how much moisture the air could hold at saturation and how much it actually holds. Drier air has a larger gap, so it can absorb more water vapor before reaching saturation, which pulls more moisture from the soil and plant surfaces through evaporation and transpiration; this is why arid, low-humidity climates generally show higher ET₀ than humid ones at the same temperature.

It's based on the same FAO Penman-Monteith standard equation used in official agricultural water management (FAO-56), but this calculator uses a simplified net-radiation estimate and a fixed sea-level psychrometric constant rather than the full method's more detailed radiation balance and elevation-adjusted parameters. For casual irrigation planning this tracks the real relationship well; for anything requiring a defensible, standards-compliant ET₀ figure, use the full FAO-56 method with your site's actual elevation and measured or properly interpolated radiation data.

Hargreaves' original 1975 radiation-based formula is a simpler alternative ET₀ equation that needs only temperature and solar radiation, making it useful as an independent cross-check when you're uncertain about your humidity or wind speed inputs. (This is a different, earlier formula than the better-known 1985 Hargreaves-Samani equation, which instead uses the daily temperature range and extraterrestrial radiation.) If the two methods diverge substantially, it's often worth double-checking your input values, since Penman-Monteith is generally considered the more physically complete and accurate of the two when good weather data is available.

No — this calculator has no location, latitude, or date input, so the monthly chart applies a generic sinusoidal seasonal shape (peaking in July, troughing in January) to the single daily ET value calculated from your inputs, purely to illustrate what a typical Northern Hemisphere seasonal pattern looks like. It is backwards for the Southern Hemisphere and not meaningful if your inputs represent a single point-in-time reading rather than a full year's typical conditions. The Daily, Weekly, Monthly, and Annual ET₀ figures above the chart are the actual calculated results from your entered values and don't carry this assumption.