💦 Crop Water Requirement Calculator

Start with the water balance for one period

A crop loses water through evaporation from the soil and transpiration through its leaves. Weather data describe the demand from a reference surface. The crop coefficient adjusts that weather demand for a particular crop and growth stage. Rainfall may supply part or all of the result. If irrigation is still needed, the system must deliver more than the root-zone amount when application efficiency is below 100%.

This calculator keeps those steps in order. It calculates crop water use, subtracts effective rainfall, accounts for the efficiency you enter, and converts the final depth into a field volume. Use either one day or one seven-day week. Every water-depth input must describe that same period.

How to use the calculator

How the calculation works

The first formula is crop evapotranspiration (ETc) = reference evapotranspiration (ETo) × crop coefficient (Kc). ETc is the crop water use under the assumptions behind those values. The calculator then finds net irrigation depth = max(ETc − effective rainfall, 0). The maximum function prevents rainfall from producing a negative irrigation requirement.

Application losses are handled with gross irrigation depth = net irrigation depth ÷ (efficiency ÷ 100). At 75% efficiency, every 3 units stored in the target root zone require 4 units delivered. The difference is shown as a system-loss allowance. It describes the arithmetic effect of efficiency; it does not identify whether water was lost to drift, runoff, evaporation, deep drainage or uneven application.

For metric inputs, gross volume in m³ = gross depth in mm × area in hectares × 10. One millimetre over one hectare is 10 m³ or 10,000 litres. For US inputs, gross acre-inches = gross depth in inches × area in acres, then gallons = acre-inches × 27,154.2857. Calculations retain full precision, while the page normally displays up to four decimal places.

Example for a daily field plan

Suppose local ETo is 5 mm for the day and the crop-stage Kc is 0.8. Crop water use is 5 × 0.8 = 4 mm. If 1 mm of effective rain is available, the net irrigation depth is 3 mm. At 75% application efficiency, the system must deliver 3 ÷ 0.75 = 4 mm.

Across 2 hectares, 4 mm is 80 m³, or 80,000 litres, of gross irrigation water. Of that total, 60 m³ is the calculated root-zone need and 20 m³ is the allowance implied by the entered efficiency. These figures do not choose the ETo, Kc, rainfall effectiveness or efficiency; they show what the entered values mean together.

Keep ETo and Kc compatible

ETo reflects weather conditions such as radiation, temperature, humidity and wind. Kc represents how the crop differs from the reference surface. It commonly changes as canopy cover and growth stage change. A coefficient for an initial stage should not be carried through mid-season without checking it.

Use ETo and Kc from compatible methods and time periods. If the source reports daily ETo, add the appropriate daily values before using the weekly option. Do not multiply a weekly ETo total by seven again. This calculator does not look up weather or choose a coefficient because local conditions and crop stage matter more than a generic preset.

Use effective rainfall, not automatically the gauge total

Rain that runs off, evaporates quickly or drains below the active roots may not help meet crop water use. Effective rainfall is the portion expected to remain useful in the root zone during the planning period. Entering the full gauge amount when only part is useful will understate irrigation demand.

If effective rainfall equals or exceeds ETc, the calculator reports zero irrigation for that period. It does not save surplus rain for the next week or estimate stored soil water. A separate field water balance is needed when soil storage, groundwater contribution or carryover matters.

Read net and gross water correctly

Net irrigation is the useful water still needed in the root zone after rainfall. Gross irrigation is what must leave the water supply or enter the application system after accounting for the entered efficiency. Equipment capacity should be compared with the gross volume. Crop need should be checked against the net depth and the assumptions used to calculate it.

A weekly result also shows a daily average so you can understand the scale of demand. That average is not a command to irrigate every day. Irrigation timing depends on soil water-holding capacity, rooting depth, allowable depletion, intake rate, weather changes and the system's practical operating limits.

What this result leaves out

The calculation is a short-period planning balance. It does not measure soil moisture, forecast rain, calculate ETo from weather observations, select Kc, or model crop stress. It also excludes stored soil water, capillary rise, leaching fractions, paddy land preparation, flooding water and special salinity management.

It does not calculate pump or zone runtime. Runtime requires the active flow serving the same area. Before irrigation, confirm that the system can apply the gross depth without runoff or excessive deep drainage, and compare the plan with current field conditions.

Common mistakes to avoid

• Mixing time periods. Use daily values together or weekly totals together.
• Using the wrong crop stage. Kc can change through the growing season.
• Entering all rainfall as effective. Count only the portion useful in the active root zone.
• Multiplying by efficiency. Gross delivery is net water divided by efficiency as a decimal.
• Using whole-farm area for one zone. Area, water demand and system delivery must describe the same field or zone.
• Treating the daily average as a schedule. It is the weekly volume divided by seven, not a timing recommendation.

Frequently asked questions