Crop Irrigation Calculator
Rain Doesn't Erase the Watering Job, It Just Shrinks It
A crop's water need — its evapotranspiration — doesn't disappear when rain falls; effective rainfall simply offsets part of it, leaving the remainder for irrigation to supply. And because no irrigation system delivers water with perfect efficiency, the volume actually pumped or piped has to be larger than the crop's net requirement to account for losses in the delivery system.
The Formula
Gross Irrigation (mm) = Net Irrigation ÷ (Efficiency ÷ 100)
Volume (liters) = Gross Irrigation (mm) × Area (m²)
The volume formula relies on a direct physical equivalence: 1 millimeter of water depth spread over 1 square meter of area equals exactly 1 liter of water.
Where This Calculation Matters
- Scheduling irrigation runs — converting a crop's water deficit into an actual volume to pump tells you how long to run a system or how much water to order.
- Comparing irrigation system efficiency — drip irrigation (often 85–95% efficient) requires meaningfully less gross water than flood irrigation (often 50–60% efficient) to deliver the same net amount to the crop.
- Water rights and allocation planning — in regions with metered or allocated water rights, converting crop need to gross volume is necessary to plan within an allocation.
- Drought and deficit irrigation decisions — when water supply is constrained, this calculation shows exactly how much of the crop's need can be met at a reduced volume.
Worked Example
A crop with 6.5mm of daily evapotranspiration and 1.5mm of effective rainfall has a net irrigation need of 5.0mm. At 75% system efficiency, gross irrigation required is 6.667mm — over a 5,000 m² field, that's 33,333 liters (33.33 m³). At a hypothetical 100% efficient system, the same net need would require exactly 25,000 liters, illustrating how much extra volume system losses add.
How to Use This Calculator
- Enter the crop evapotranspiration in millimeters.
- Enter effective rainfall in millimeters (defaults to 0 if none).
- Enter the field area in square meters.
- Enter irrigation system efficiency as a percentage (defaults to 100%).
- Select Calculate to get gross irrigation depth and total water volume.
Related Calculations
Confirm the field area first with the Land Area Calculator, or plan the nutrient side of the crop with the Fertilizer Calculator.