Using cheque book irrigation scheduling

In order to schedule your irrigation properly you need to know the water requirements of your plants and how much water is in the soil profile. Once this has been determined you can:

  • ensure maximum crop production by maintaining soil water content,
  • control water distribution at deep and shallow levels using tensiometers,
  • vary water content of soil to allow for the influence of weather conditions i.e. rain,
  • manage optimal crop production through a process called regulated deficit irrigation


Figuratively speaking irrigation scheduling can be described as the accounting of soil water content. The quantity of water stored in the soil at the root zone can be seen as the account balance. A deposit is water that infiltrates the soil either through irrigation or rain while water removed from the soil through evaporation or usage by crops can be seen as a withdrawal. Therefore the aim of irrigation scheduling is to maintain a balanced soil water account.

When to irrigate can be determined by monitoring the soil water content and starting irrigation when soil water has been depleted to a predetermined quantity. For example the total water deficit over a period of time is measured as -50mm, this means that 50mm of water has been removed from the soil and an irrigation application of 50mm would fill the soil water profile again to its initial status. It is recommended that you “split” the total application into smaller applications for instance 4 x 12.5mm applications to eliminate water movement past the root zone.

To establish the soil water deficit there must be a starting point of reference. The logical point of reference can be determined directly after irrigation when the soil water profile has been filled up to field capacity.

Applying too little water will lead to unproductive crop stress while over irrigating will lead to leaching of valuable nutrients past the root zone of plants. Both have a negative impact on crop production. Therefore it can be concluded that for ideal conditions to prevail the irrigation quantity applied should reflect the crop water use, no more and no less.

In order to determine crop water requirements there are a couple of factors that need to consider such as soil texture and structure, type of crop, geographical area, climate and water quality.

Once the crop water requirement has been established it is possible to calculate your irrigation cycles more accurately.