Step 3. Calculate your crop’s water requirement

ETo x Kc = crop water requirement

An example – growing tomatoes:

ETo for the day is 4.4 mm (0.17 in). It is mid-season, therefore the Kc is 1.15:

  • ETo x Kc = water requirement
  • 4.4 x 1.15 = 5.06 mm (0.17 x 1.15 = 0.20 in) water requirement


However, what if you did not irrigate the two days before (no rainfall)?

In that case, you sum the ETo data from the last irrigation event. For the example, let’s presume two days:

  • (ETo x days since last irrigation event) x Kc = water requirement
  • 5.06 mm (0.20 in) x 2 days = 11.2 mm (0.44 in) required



  • How to convert mm to cubic meters per hectare

Multiply the mm water application rate by 10.
This number is the m3 of water per hectare that you need to apply.

E.g. 5.06 x 10 = 50.6 m3 water per hectare to apply


  • How to convert inches of precipitation to gallons, cubic feet, or acre-feet per acre

Multiply the inches water application rate by 27,154 to obtain gallons per acre.
Multiply the inches water application rate by 3,360 to obtain cubic feet per acre.
Divide the inches water application rate by 12 to obtain acre-feet per acre.

E.g. 0.2 inches x 27,154 = 5,430 gallons per acre
E.g. 0.2 inches x 3,630 = 726 cubic feet per acre
E.g. 0.2 inches / 12 = 0.017 acre-feet per acre

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