Irrigation Scheduling

The questions to be answered...

How much water to apply
When it should be applied

We need to know...

The amount of water in the profile now
The current rate of depletion

We then need to calculate either...

The date when we will reach a given refill point

or..

The water content we will reach at a given date

The amount required to bring the water content back to the desired full point can then be calculated.

We cannot assume that the depletion rate will remain constant, but need to allow for the effect of weather and irrigations in the period up to the next irrigation.

The refill point and the full point will not necessarily be constant throughout the season. They could be varied during different stages in the season to control crop yield and quality.

Advantages of using soil water content to schedule...

Good precursor to the scheduling point - the water content falls linearly.
Repeatable
Accurate
Can be correlated with actual crop behaviour, such as infra-red, sap pressure, soil water tension.

Schedule depth and time graph

The latest profile is at about the third point between refill and full, and there is a wedge shaped change from the previous probe reading - the profiles are rotating about the rooting depth.

Assuming the weather remains constant the crop water use does not change for the next few days, the date at which the soil moisture will reach the refill point can be calculated, together with the amount of water required to bring the profile back to the full point.