Crop Water Requirement Calculator: ETc and Irrigation

Calculate crop evapotranspiration (ETc = ETP × Kc), soil readily available water (RAW), irrigation scheduling, and pumping volumes.

Crop water requirement and irrigation dose: the calculation

A crop's daily water need is its evapotranspiration ETc = ET0 × Kc, where the crop coefficient Kc depends on the species and growth stage (FAO-56 method). The tool then derives the soil's readily available water, a gross dose per pass and the pumping time: maize at mid-season with 5.5 mm of reference evapotranspiration uses 6.6 mm a day, i.e. 66 m³ per hectare.

Calculation steps

  1. ETc (mm/day) = ET0 × Kc. The tool's Kc, initial / mid-season / late: maize 0.40 / 1.20 / 0.60, wheat 0.35 / 1.15 / 0.30, soybean 0.40 / 1.15 / 0.50, sunflower 0.35 / 1.15 / 0.35, potato 0.50 / 1.15 / 0.75, tomato 0.60 / 1.15 / 0.80, grapevine 0.30 / 0.70 / 0.45, orchard 0.50 / 0.95 / 0.75.
  2. Available water (AWC) = rooting depth × soil AWC (sand 80 mm/m, sandy loam 120, deep loam 180, clay loam 190, heavy clay 160); readily available water (RAW) = 60% of AWC.
  3. Daily deficit = ETc − effective rain ÷ 7 (the rain entered is spread over a week).
  4. Net dose = RAW capped at 30 mm; interval = net dose ÷ deficit; gross dose = net dose ÷ efficiency; 1 mm = 10 m³/ha; pumping time = volume ÷ flow.

Worked example (default values)

Maize at mid-season, deep loam, ET0 5.5 mm/day, no rain, 80% efficiency, 15 ha, 50 m³/h pump. ETc = 5.5 × 1.20 = 6.6 mm/day. AWC = 1.0 × 180 = 180 mm; RAW = 108 mm. Net dose 30 mm, gross 37.5 mm, i.e. 375 m³/ha and 5,625 m³ for the field, in 112.5 h of pumping. Interval shown: 30 ÷ 6.6 = 4.5 days.

Limits

  • The interval is the number of days the net dose (30 mm at most) covers; it does not account for upcoming rain. The full RAW, shown under the result, would last longer at the same deficit (108 mm, i.e. 16.4 days in the example).
  • The 60% threshold and the reserves per soil texture are agronomic reference figures, not verified; a soil analysis refines them.
  • Mid-season and late Kc match Table 12 of FAO-56; the tool's initial Kc are average values, whereas FAO adjusts them to how often the soil is wetted.

Sources and dates: FAO, Irrigation and Drainage Paper 56 (1998), chapter 6, Table 12 (checked 14/10/2026)

Frequently Asked Questions

How do you calculate a crop's water requirement?

Multiply the reference evapotranspiration (ET0, in mm per day) by the crop coefficient Kc for the growth stage. For maize at mid-season (Kc 1.20) and an ET0 of 5.5 mm, the need is 6.6 mm a day, i.e. 66 m³ per hectare.

How many m³ per hectare is 1 mm of irrigation?

1 mm of water over one hectare is 10 m³. A gross dose of 37.5 mm is therefore 375 m³ per hectare, and 5,625 m³ for 15 ha.

What is readily available water (RAW) in a soil?

It is the share of available water the plant can take up without stress; the tool sets it at 60% of the available water capacity. A deep loam at 180 mm/m with 1 m of roots gives 108 mm of RAW.

Which efficiency should I use for an irrigation system?

The tool starts at 80% by default; the code mentions 90% for drip, 80% for sprinklers and 70% for hose reels. Lower efficiency raises the gross dose: 30 mm net becomes 42.9 mm at 70%.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.