How Much Water?

Calculating water requirement

As an example, assume we have a sprinkler system operating uniformly and applying 7 mm per hour. (Note that sprinkler output needs to be confirmed during sprinkler testing.) We have a block of carrots in which the root diameter has reached about 18 mm, and looking at Figure 1, we see the crop factor is 1.4. If the weather station tells us the evaporation the previous day was 6.8 mm:

Figure 1. Crop factor curve for irrigating carrots on sand ( McKay, Calder & Deyl, 2000)

Then the water requirement for our crop is:

Irrigation required (mm) = Crop factor x Evaporation (mm)
= 1.4 x 6.8 mm
= 9.5 mm

For a system output of 7 mm per hour, the total system runtime for the day will be:

Runtime (minutes) = Irrigation required (mm) / output (mm/h) x 60 minutes
= (9.5 / 7) x 60 minutes
= 81.4 minutes