Crop Water Requirement Calculator

Calculate crop evapotranspiration (ETc), net and gross irrigation depth, and field water volume from local ETo or FAO-56 weather data.

Designed for field crops, orchards, vegetables, greenhouses, turf, irrigation planning, and water budgeting. All entered values stay in your browser.

Plan crop water

Calculation mode

Single period Seasonal water budget

Reference ET source A nearby agricultural weather network or extension service is usually the best source.

Known reference evapotranspiration

Average ETo Use the average for the selected period.

ETo unit

Crop and growth stage

Search or choose a crop

Growth stage

Selected Kc

Result units

FAO-56 Table 12 general value. Local climate, variety, planting density, wetting, and management can require adjustment.

Period and rainfall

Calculation period (days)

Rainfall/depth unit

Rainfall credit

Known effective rainfall

Seasonal stages

Enter each stage duration, average daily ETo, and measured rainfall. Kc is constant in the initial and mid stages and interpolated linearly through development and late season.

Stage ETo unit

Stage rainfall unit

Stage	Days	ETo/day	Rain
Initial
Development
Mid-season
Late-season

Effective rainfall is estimated stage by stage with the USDA-SCS relationship.

Irrigation system and area

System preset

Efficiency (%)Editable planning value; field audits are preferable.

Irrigated area

Area unit

Application rate (optional)Depth units per hour.

Pump flow (optional)L/s in metric; US gpm in US mode.

Available irrigation hours per period

Privacy: calculations run locally. Field size, weather, crop coefficient, and rainfall values are not transmitted or stored by this tool.

Water to apply

Gross irrigation requirement—Enter valid inputs to calculate.

Reference ET (ETo)—

Daily crop ET (ETc)—

Weekly crop ET—

Selected-period crop ET—

Net irrigation depth—

Gross applied depth—

Gross system volume—

Water per area—

Operational estimate—

Water balance

Crop use

—

Rain credit

—

System loss

—

Applied

—

Enter valid inputs to calculate the crop water requirement.

How the Calculator Works

The calculator obtains daily reference evapotranspiration (ETo), applies a crop- and stage-specific coefficient (Kc), credits effective rainfall, and divides the remaining root-zone need by system efficiency. Area converts the applied depth to volume. Seasonal mode totals four stages and linearly interpolates Kc during crop development and late season.

FAO-56 values are transferable starting points, not field prescriptions. Local coefficients can differ with climate, variety, planting density, canopy height, soil wetting, and management.

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 daily calculations, soil heat flux G is zero and grass-reference albedo is 0.23. Actual vapor pressure can use mean RH, RHmax/RHmin, or dew point. Sunshine duration uses the FAO-56 Angstrom relationship with default coefficients a_s = 0.25 and b_s = 0.50. Wind measured above or below 2 m is converted with FAO-56 Eq. 47.

Practical Planning Guidance

Where can I obtain local ETo?

Look first for agricultural weather networks, irrigation districts, university extension services, national meteorological agencies, or a quality-controlled on-farm station. Match the reference type and time step; this tool expects FAO grass-reference ETo.

How should I choose a period?

Use one to seven days for near-term operations when ETo, rain, and Kc are reasonably stable. Use seasonal mode when crop stage and weather change. A long-term average day can support preliminary design, but it should not stand in for a multi-day forecast or observed water balance.

How is effective rainfall estimated?

Known effective rainfall is preferred. The estimator normalizes rainfall to a 30-day amount, applies the USDA-SCS monthly relationship, then scales the credit back to the selected duration. Runoff, root-zone storage, soil condition, storm intensity, and deep percolation are not modeled, so the displayed credit is editable.

Typical irrigation-efficiency planning ranges

Drip 85–95%, subsurface drip 90–95%, center pivot 75–90%, sprinkler 65–85%, and surface/furrow 45–70%. These are defensible planning ranges, not guaranteed field performance; pressure, maintenance, wind, runoff, and distribution uniformity matter.

Loadable Worked Examples

Mid-season maize — center pivot

Metric, 10 ha. FAO-56 Table 12 Kc_mid = 1.20; ETo 5.5 mm/day; 7 days; 5 mm effective rain; 85% efficiency.

ETc = 5.5 × 1.20 = 6.60 mm/day
ETc₇ = 6.60 × 7 = 46.20 mm
Net = 46.20 − 5 = 41.20 mm
Gross = 41.20 ÷ 0.85 = 48.47 mm
Volume = 0.04847 × 100,000 = 4,847 m³

Mid-season vegetables — drip

US customary, 12 acres. FAO-56 Table 12 fresh-market tomato Kc_mid = 1.15; ETo 0.24 in/day; 5 days; 0.20 in effective rain; 90% efficiency.

ETc = 0.24 × 1.15 = 0.276 in/day
ETc₅ = 0.276 × 5 = 1.38 in
Net = 1.38 − 0.20 = 1.18 in
Gross = 1.18 ÷ 0.90 = 1.311 in
Volume = 1.311 × 27,154 × 12 ≈ 427,200 US gal

Complete FAO-56 weather example

45.72°N, 200 m, 15 July; Tmax 26.6°C, Tmin 14.8°C, RH 60%, Rs 22 MJ/m²/day, and u₂ 2 m/s. The tool derives day 196 and calculates every radiation and vapor-pressure term before ETc.

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

FAO-56 Crop Coefficient Reference

Crop	Initial	Mid	Late/end
Maize (grain)	0.30	1.20	0.35
Wheat	0.30	1.15	0.25
Tomato, fresh market	0.60	1.15	0.80
Potato	0.50	1.15	0.75
Alfalfa hay	0.40	1.20	1.15
Apple/cherry/pear, active ground cover	0.50	1.20	0.95
Grapes	0.30	0.85	0.45
Turf, cool-season	0.90	0.95	0.95

General single-coefficient values from FAO-56 Table 12. Development is interpolated from Kc_ini to Kc_mid; late-season values move from Kc_mid to Kc_end.

FAO-56 Symbols and Units

Symbol	Meaning	Unit
ETo	Reference evapotranspiration	mm/day
ETc	Crop evapotranspiration	mm/day or period
Kc	Crop coefficient	dimensionless
Δ	Slope of saturation vapor-pressure curve	kPa/°C
Rn	Net radiation at crop surface	MJ/m²/day
G	Soil heat flux density	MJ/m²/day
γ	Psychrometric constant	kPa/°C
T	Mean daily air temperature at 2 m	°C
u₂	Wind speed adjusted to 2 m	m/s
e_s	Saturation vapor pressure	kPa
e_a	Actual vapor pressure	kPa
R_a	Extraterrestrial radiation	MJ/m²/day
R_so	Clear-sky radiation	MJ/m²/day
R_s	Incoming solar radiation	MJ/m²/day
n / N	Actual / maximum daylight hours	hours / hours

FAQs

What is the difference between ETo and ETc?

ETo describes a standardized, well-watered grass reference. ETc = Kc × ETo estimates water used by a particular well-watered crop. Actual ET can be different when water stress, salinity, disease, incomplete cover, or field conditions depart from standard assumptions.

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, humidity or dew point, wind speed adjusted to 2 m, measured solar radiation or sunshine duration, latitude, elevation, and calendar date. 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.

Methodology, Review, and Validation

Calculation version 2.0 Last substantive method review: 31 August 2026. Method maintained by Starlight Robotics; no independent technical reviewer is claimed.

The daily weather route implements FAO-56 Chapter 2 Eq. 6; Chapter 3 Eqs. 11–13, 17, 21, 35–39, and 47; and Chapter 4 Eqs. 48 and 50. Crop ET uses Chapter 5 Eq. 58 and Table 12. Seasonal development and late-season Kc are linearly interpolated following the FAO stage curve. Simplifications: daily G = 0; no climate adjustment to Kc_mid or Kc_end; no soil-water stress coefficient, dual Kc, capillary rise, leaching, or carry-forward rain storage.

Validation case	Expected	Calculator check
FAO-56 Example 17, Bangkok monthly mean daily ETo	4.56 mm/day	Agreement target within 0.1 mm/day when the published inputs and monthly radiation terms are used; daily rounding and radiation-path choices can differ.
Maize example on this page	48.47 mm gross; about 4,847 m³	Internal arithmetic check to within 0.02 mm and 2 m³ after display rounding.
Unit conversion	25.4 mm = 1 in; 1 acre-ft = 1,233.4818 m³	Exact conversion constants used internally, with display rounding only.

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.
USDA NRCS National Engineering Handbook, Part 623, Chapter 2 — SCS effective-rainfall planning method.

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.