Skip to content

Crop ET and Irrigation Water Budget Calculator

Estimate crop ET from entered ETo and Kc, then compare a separate effective-rain and efficiency budget in metric or US units.

Use this result well

Inputs that matter
ET and area units, Reference ET per day, Crop coefficient (Kc), Crop area, and 3 more
Output to expect
Crop evapotranspiration, ET and effective-rain budget
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
Was this tool helpful?

Related Tools

Tools you might need next

Irrigation Flow, Runtime and Catch-Test Calculator

Calculate drip emitter delivery, measured emitter flow, sprinkler precipitation and sampled uniformity. Convert a chosen water amount into runtime.

Gardening Water
Rain Event and Tank Capture Calculator

Model one rain event with catchment area, runoff assumptions, first-flush diversion and available tank space. Inspect retained inflow and overflow.

Gardening Water
Measured Water Use and Comparison

Calculate water use from meter readings or measured fixture activity, compare daily averages and estimate variable charges using your actual rate.

Green Living

Reference & details

How it works

Crop coefficient method

FAO 56 describes ETc = Kc × ETo. Use matching reference surface, crop stage and growing conditions. The supplied Kc example is illustrative.

Period ET depth = ETo per day × Kc × days

Depth to volume

Metric uses 1 mm over 1 m² = 1 L. US inputs use inches and square feet, converted through exact length/area definitions and US liquid gallons.

Volume = depth × represented crop area

Simplified replacement budget

The model holds soil-water storage constant and excludes capillary contribution, stress and additional leaching requirements. It does not generate an irrigation calendar.

Net = max(0, period ET − effective rain); gross = net ÷ (efficiency / 100)

Updated: September 2026

Example Scenarios

0.25 in/day ETo, Kc 1, 500 ft², seven days.

→ 1.75 inches crop ET, about 545.455 US gallons of crop water use.

5 mm/day ETo, Kc 0.8, seven days, 100 m², 8 mm effective rain and 80% efficiency.

→ 28 mm crop ET; 20 mm net; 25 mm gross or 2,500 L.

Effective rain at least equals the modeled period ET.

→ Zero simplified replacement budget, not negative irrigation or a runoff estimate.

Common Mistakes to Avoid

Applying ET volume directly

Review rainfall, soil moisture and delivery losses before choosing an application.

Switching units without converting inputs

Selection reinterprets the numbers. Convert and re-enter every affected depth and area value.

FAQ

No. Enter reference ETo from an appropriate local source or model. The calculator multiplies it by your matched crop coefficient and period.

No. Crop ET is modeled water use. The separate budget subtracts entered effective rainfall and divides net replacement by entered efficiency, holding soil-water storage constant and excluding other water contributions.

Enter the effective portion for the crop over the whole period. The tool does not derive runoff, infiltration or rain effectiveness from a rain-gauge total.

A generic outdoor ETo/Kc pair may not represent a greenhouse crop and media. Use greenhouse-specific measurements and an appropriate method; this tool supplies no fixed watering frequency.

About Crop ET and Irrigation Water Budget Calculator

Crop evapotranspiration retains the Kc × entered ETo method. ET and effective-rain budget adds a simplified application calculation. Use local crop and weather evidence, matched units and a clear period; no weather-based ETo model or default greenhouse multiplier is supplied.