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The Deep FullStop Always Activates Before The Shallow One

If a deep detector responds before the shallow one then there is a lot of variability in the way the wetting patterns are moving.

Reasons include:

  • Poor uniformity of irrigation; more water is being applied near the deep detector
  • Poor plant growth around the deep detector. The soil is wetter before irrigation and so the wetting front moves deeper
  • Water is moving across the soil surface, away from the shallow detector and towards a deeper detector. It is important to ensure that the soil around the detectors is level.
  • Cracks or soil pores are dominating the movement of water

There are cases where the quick response of a deep detector is telling us something important. One example is where crops are grown on ridges or raised beds under sprinkler irrigation. We have observed that water is often shed into the furrows, either by the leaves of the crop or by water running off a crusted soil surface. In such cases it is common for a deep detector, below the depth of the furrow, to respond before a shallow FullStop in the centre of the bed. Sometimes the deep FullStop may be activated but not the shallow. This is because wetting of the soil is primarily taking place via the furrows. In such cases the FullStop should be positioned towards the edge of the beds.

Another example of deep detectors responding before shallow can occur when the deep detector is positioned within or just above a clay layer. If the layer has a much lower conductivity than the soil above, a water table can form just above the clay. This is called a perched water-table. Irrigation water infiltrating quickly to depth in 'wet' spots can cause the perched water-tables to rise, thus activating deep detectors several metres away.

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