Crop Water Requirement and Evapotranspiration Calculator

How the Calculator Works

The calculator first obtains daily reference evapotranspiration (ETo). In quick mode, it converts your known ETo to millimeters per day. In weather mode, it applies the standardized daily FAO-56 Penman–Monteith equation using temperature, humidity, radiation, wind, latitude, elevation, and day of year.

Crop evapotranspiration is then estimated with the single crop coefficient method. The period total is reduced by effective rainfall to find net irrigation need. Net depth is divided by irrigation efficiency to estimate gross depth at the system inlet. Field area converts both depths to volumes.

The result is a planning water balance, not a complete irrigation schedule. It does not model root-zone capacity, readily available water, stress coefficient, capillary rise, leaching requirement, runoff, or the timing of individual rainfall and irrigation events.

Formulas and Assumptions

ETc = Kc × ETo

Crop water use for period = ETc × number of days

Net irrigation depth = max(0, crop water use − effective rainfall)

Gross irrigation depth = net irrigation depth ÷ irrigation efficiency

Water volume = depth × irrigated area

Daily FAO-56 Penman–Monteith mode uses:

ETo = [0.408Δ(Rn − G) + γ(900/(T + 273))u₂(es − ea)] ÷ [Δ + γ(1 + 0.34u₂)]

For the daily calculation, soil heat flux G is assumed to be zero. Grass-reference albedo is 0.23. Actual vapor pressure is estimated from mean relative humidity, and net radiation is derived from measured solar radiation using FAO-56 clear-sky and longwave relationships. The relative shortwave ratio used in the longwave term is constrained to the normal 0.30–1.00 range.

How to Estimate Crop Irrigation Water Requirement

1. Use reference ETo from a nearby agricultural weather network when available. Otherwise, enter quality-controlled daily weather observations in FAO-56 mode.
2. Choose a crop coefficient for the crop, growth stage, climate, and irrigation method. Kc normally changes through the season.
3. Set the period represented by the average daily ETo and Kc. A seven-day period is convenient for a weekly water budget.
4. Estimate effective rainfall—the portion stored in the crop root zone—and enter that period total.
5. Use an audited or defensible irrigation efficiency. Review gross depth and volume against pump capacity, infiltration, soil moisture, and forecast rainfall.

Worked Example

Suppose reference ETo averages 5.5 mm/day for seven days, Kc is 0.90, effective rainfall is 5 mm, irrigation efficiency is 85%, and the field is 10 ha.

FAQs

What is the difference between ETo and ETc?

ETo is reference evapotranspiration for a standardized, well-watered grass surface. ETc is estimated crop evapotranspiration and is calculated as ETc = Kc × ETo.

Which crop coefficient should I use?

Use a crop coefficient for the crop, growth stage, climate, and management method represented by the ETo data. Kc changes during establishment, development, mid-season, and late season. Prefer locally calibrated values or FAO-56 values adjusted for local conditions.

Why is irrigation efficiency included?

Not all applied water reaches the crop root zone. Dividing the net irrigation need by efficiency estimates the larger gross depth that the system must apply.

Should total rainfall or effective rainfall be entered?

Enter effective rainfall: the portion stored in the root zone and available to the crop. Runoff, deep drainage, interception, and rainfall timing can make effective rainfall lower than measured rainfall.

What weather data does FAO-56 Penman–Monteith need?

This daily implementation uses maximum and minimum temperature, mean relative humidity, wind speed at 2 m, solar radiation, latitude, elevation, and day of year. Weather data should be quality controlled and representative of the site.

Does this calculator create a complete irrigation schedule?

No. It estimates crop water use and water depth for a selected period. A field schedule should also consider root-zone storage, allowable depletion, irrigation capacity, leaching needs, runoff, soil moisture, and forecast rainfall.

Are weather and field inputs stored?

No. All calculations run locally in the browser, and the calculator does not send or store entered field or weather values.

Calculation Basis and References

• FAO Irrigation and Drainage Paper 56, Chapter 2 — standardized Penman–Monteith reference evapotranspiration equation and required variables.
• FAO-56, Chapter 3 — meteorological parameters, extraterrestrial radiation, clear-sky radiation, net shortwave radiation, and net longwave radiation.
• FAO-56, Chapter 5 — single crop coefficient relationship ETc = Kc × ETo.
• FAO CROPWAT — decision-support software for crop water and irrigation requirements based on FAO methods.

Planning Limits

This calculator provides an agronomic estimate under the selected assumptions. Use local crop coefficients, measured weather, soil-moisture observations, audited irrigation performance, water-quality requirements, pump and system limits, and applicable allocation or watering rules before operating an irrigation system. Weather-based ETo can differ from field water use when crops are stressed, diseased, sparse, saline, or otherwise outside standard conditions.