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By sensitivity we mean 'Can water move past a FullStop without the float being activated?'

The answer is yes.

How does this happen? We might expect that once water enters into the funnel it is trapped. Gravity will cause the water to move to the base of the funnel where the water will collect, thus activating the float.

Unfortunately it's not that simple! Convergence of water in the funnel caused by the narrowing sides means that soil inside the funnel is wetter than soil outside the funnel. Water in the soil moves from wetter zones to drier zones, be that upwards, downwards or sideways.

Even though irrigation is taking place, the soil inside the funnel is wetter than the soil outside. The slightly drier soil outside 'wicks' some of the water out of the funnel. The ability of drier soil to pull water away from wetter zones is called capillarity.

Convergence fills the funnel, but capillarity tries to empty it. At a certain soil 'wetness', convergences wins over capillarity and enough water is collected at the base of the funnel to activate the float.

Scientists describe the level of wetness as the suction with which water is held by the soil. The soil suction at which a wetting front will be activated occurs between 2 and 3 kPa (or 20 to 30 cm suction).

A lot of research went into picking the right level of sensitivity, but ultimately there is a compromise between the sensitivity of a device and its cost and simplicity.

When a FullStop is activated, a front with a strength of 2 to 3 kPa suction has passed. In the vast majority of cases, wetting fronts produced by irrigation will fall inside the sensitivity limits of a FullStop.

Wetting fronts continue to move downwards through the soil after irrigation is stopped. As the fronts move deeper they get weaker and weaker until they fall below the detection limit of a FullStop. So frequent light irrigation by sprinkler may fail to activate a FullStop, even though some water may move below the depth it is buried.

Note that if you are have continuous soil water monitoring equipment close to a FullStop, you will at times see the soil getting wetter without the FullStop at a similar depth being activated. In such cases the soil is being wet up by weak fronts, and the 2 kPa sensitivity limit has not been reached.

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