What is the Hargreaves equation?

The Hargreaves method estimates reference evapotranspiration as 0.0023 times extraterrestrial radiation times the square root of the daily temperature range times mean temperature plus 17.8. It needs only temperature data, making it useful where humidity, wind, and radiation are unavailable.

What is a crop coefficient (Kc)?

Kc scales reference ET to a specific crop and growth stage. It is near 0.3 to 0.4 at emergence, rises to 1.05 to 1.2 at mid-season full canopy, then falls toward 0.5 to 0.9 at maturity. Multiplying reference ET by Kc gives the actual crop water use.

How do I find extraterrestrial radiation?

Extraterrestrial radiation depends only on latitude and day of year and is tabulated in FAO Irrigation and Drainage Paper 56 in megajoules per square metre per day. The tool converts it internally so reference ET comes out in millimetres per day.

What is effective rainfall?

Effective rainfall is the portion of rain that is actually stored in the root zone and used by the crop, after runoff and deep percolation losses. The net irrigation requirement is crop ET minus effective rainfall over the period.

Is Hargreaves as accurate as Penman-Monteith?

Penman-Monteith is the FAO standard and is more accurate when full weather data is available. Hargreaves is a temperature-only approximation that performs well at monthly timescales and in data-sparse settings, but it can be biased in very humid or very windy locations.

Crop Water Use (ET) Estimator

Knowing how much water a crop uses lets you schedule irrigation precisely instead of guessing. This tool estimates reference evapotranspiration from temperature using the Hargreaves method, applies a crop coefficient to get actual crop water use, scales it over a growth stage, and subtracts effective rainfall to give the net irrigation requirement.

How it works

Reference ET is estimated from temperature and extraterrestrial radiation, then scaled by the crop coefficient and the stage length:

ETo (mm/day) = 0.0023 × Ra × (Tmax − Tmin)^0.5 × (Tmean + 17.8)
ETc (mm/day) = ETo × Kc
seasonal ETc = ETc × stage days
net irrigation = seasonal ETc − effective rainfall

Ra is extraterrestrial radiation in millimetres of water equivalent per day (the tool converts your MJ/m²/day input by dividing by 2.45). The crop coefficient Kc rises from emergence to a mid-season peak and then declines, so the same weather gives very different water use depending on growth stage.

Tips and example

With Tmax 30 °C, Tmin 16 °C, Tmean 23 °C and Ra of about 16.5 mm/day, reference ET is roughly 6.0 mm/day. For maize at mid-season (Kc ≈ 1.15) that is about 6.9 mm of crop water use per day, or 207 mm over a 30-day stage. Subtract, say, 60 mm of effective rainfall and you need about 147 mm of irrigation for the stage. Use a Kc that matches the actual canopy you see in the field, and re-run the estimate as temperatures and stage change through the season.