How do I use light to trigger my irrigation starts?

How do I use light to trigger my irrigation starts?


Light intensity

The intensity of light is measured in watts/m2. This is an instant reading of the brightness of light at the moment of measurement.
 

Light sum

The light sum is measured in joules/cm2. This is the total amount of light received over the period. So over a period of one hour the total amount of light received in joules/cm2 will depend on the intensity of the light during this period.
 

Light triggered irrigation starts

In glasshouses plant water use is measured per m2. A general figure used in Northern European conditions is 2.5 to 3.0 mls per joule per square meter as a setting for the computer with a target figure of 3 mls per joule for the total on a bright day.This is based on a full crop canopy.

Plants use more water per joule of light at low light intensities which is why it is important to have an irrigation trigger on time and not just light.
 

The best day of the year would be about 3,000 joules

  • 3 cc per joule on 3,000 joules = 9 lts/m2 per day

The highest light intensity in Northern European conditions would be around 1,000watts/m2

  • 1,000 watts for 1 hour = 360 joules/cm2
  • 3 cc per joule on 360 joules = 1.08 lts/m2 per hour


The dose size per irrigation round is calculated by the dose size calculator in relation to the volume of the substrate.


For example with a calculated dose size of 100ml with 3 drippers per m2

  • 3 cc per joule on 3,000 joules = 9 lts/m2 per day
  • 1 irrigation round is 3 x 100mls = 300 cc per m2
  • 9 lts/m2 per day / 300 cc per round = 30 irrigations per day
  • 3 cc per joule on 360 joules = 1.08 lts/m2 per hour
  • 1 irrigation round is 3 x 100 cc = 300 cc per m2
  • 1.08 lts/m2 per hour / 300 cc per round = 3.6 irrigations per hour which is every 16 minutes.


In hotter countries especially where air humidity is low (or HD is high) the cc per joule requirement can be 4 or higher. This would mean that the time between irrigations would need to be less.

This is why the volume of the substrate per m2 and its relation to the dose size and the necessary frequency of the subsequent irrigations during the brightest part of the day need to be match to the limitations of the nursery irrigation system.

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