How much water a landscape actually needs
Applying crop coefficient to reference evapotranspiration, then correcting for how efficiently water is delivered.
The engineering question
Irrigation is either under-watering or wasting water. How much does the landscape need on the hottest week?
Worked example
The figures below were produced by running the Irrigation Demand Calculator with the same inputs — during the build, from its authoritative engine.
| Daily water requirement | 1333 L/day |
|---|---|
| Weekly requirement | 3.1111 m³ |
| Crop water requirement | 4 mm/day |
| Indicative emitter count | 167 |
| Assessment | Crop requirement 4.0 mm/day → 1333 L/day over 250 m² (≈ 167 emitters running 2 h/day. Rotate zones against the 3-day/week schedule to hold storage. |
- Crop requirement is reference evapotranspiration multiplied by the crop coefficient.
- Volume over the area is divided by the delivery efficiency.
- Spread the weekly requirement across the chosen irrigation days.
Sanity check
Turf at 5 mm/day over 250 m² is roughly 1,250 L before losses. Use that as a sanity check on any irrigation proposal.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- FAO Irrigation and Drainage Paper 56 — crop evapotranspiration
- CIBSE Guide G — irrigation
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