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Estimate the garden's water need

Work out how much irrigation water the garden needs at peak season, so the water module can size storage and pumping. Use the FAO method: crop water use (ETc) equals reference evapotranspiration (ETo) multiplied by a crop coefficient (Kc).

How

1. Find ETo for your site in the hottest month from a national weather service or agricultural agency. FAO gives typical ranges from about 1 to 2 mm/day in cool temperate climates to 6 to 9 mm/day in warm arid ones, but use local data where available.
2. Pick a crop coefficient. FAO Table 12 gives mid-season Kc of about 1.05 for small vegetables, 1.00 for lettuce, 1.15 for tomato and potato, and 1.20 for field maize.
3. Multiply: ETc (mm/day) = Kc x ETo. One mm over one square metre is one litre (FAO: 1 mm over a hectare is 10 cubic metres).
4. Multiply by the planted area and divide by irrigation efficiency. WSU Extension rates drip at about 90 to 95% efficient and sprinklers at about 70%.
5. Use the Garden water need calculator in this module, then give the peak daily and monthly figures to the water module.
6. Repeat for spring and autumn: WSU notes plant water needs in April and October are less than half those of midsummer in its region, so storage can be sized to the season profile.

Do these first

• water-1

Sources

1. Crop evapotranspiration (FAO Irrigation and Drainage Paper 56), Chapter 6: ETc single crop coefficient (Kc). FAO, accessed 2026-11
2. Crop evapotranspiration (FAO Irrigation and Drainage Paper 56), Chapter 1: Introduction to evapotranspiration. FAO, accessed 2026-11
3. Drip Irrigation for the Yard and Garden. Washington State University Extension, accessed 2026-11

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