How much irrigation water does the project need?
Peak water demand is estimated from reference evapotranspiration for the site, a crop coefficient for the crop and growth stage, the irrigated area and the system application efficiency — the FAO-56 method. Peak demand, not the annual average, sizes the source, pump and pipework. The result is an indicative planning figure: local ETo data, real cropping calendar, soil or substrate behaviour, leaching requirement and water quality must be confirmed by an irrigation designer before equipment is specified.
What actually drives the answer
Use local meteorological ETo where available. Regional averages are adequate for a first pass and inadequate for procurement.
Crop coefficients change through the season. Sizing on a mid-season peak, not a whole-season mean, is what prevents an undersized source.
Saline water adds a leaching requirement, which increases the volume that must be delivered above crop demand.
Protected cultivation changes demand relative to open field because radiation, ventilation and humidity inside the structure differ from outside conditions.
Variables that change the answer
- Reference evapotranspiration at the site
- Crop and growth stage coefficient
- Irrigated area
- System efficiency and uniformity
- Water salinity and leaching requirement
- Open field versus protected
Evidence a supplier needs before quoting
- Local ETo or climate station data
- Crop and cropping calendar
- Area
- Water analysis (EC in particular)
What this answer does not settle
- Any figure produced without site ETo data and a water analysis is indicative only and must not be used for permitting or final equipment sizing.
- It does not establish that the source can legally or physically supply that volume.
Responsibility
- Professional review:
- Irrigation designer or agricultural water engineer
- Delivered by:
- Irrigation designer, then irrigation supplier
- Feeds RFQ section:
- Water source and demand
Sources
- FAO Irrigation and Drainage Paper 56 — Crop evapotranspiration (guidelines for computing crop water requirements)Food and Agriculture Organization of the United Nations (FAO) · 1998 · Primary source · accessed 2026-08-21
- AQUASTAT — FAO's global information system on water resources and agricultural water managementFood and Agriculture Organization of the United Nations (FAO) · Primary source · accessed 2026-08-21
- FAO Irrigation and Drainage Paper 29 — Water quality for agricultureFood and Agriculture Organization of the United Nations (FAO) · 1985 (rev. 1) · Primary source · accessed 2026-08-21
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