Weatherwatch: soil moisture deficit – how dry are the UK’s thirsty soils?

When the ground is thoroughly wet, after excess rainwater has run off it is said to be at field capacity. Meteorologists describe drought in terms of soil moisture deficit (SMD) from field capacity. The SMD is a measure of how thirsty the soil is, expressed as how many millimetres of rain it would take to top it up to field capacity.

In southern England, the SMD tends to hover around zero in winter when fields are wet. In summer it may be about 100mm, going up to 150mm or more in a dry year. The extreme of dryness, when soil has no water accessible to plants, is known as the permanent wilting point or PWP.

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There are water storage deficits in much of England and Wales, according to the UK Centre for Ecology and Hydrology. The driest parts of southern England would require “unusually high rainfall” to make up the deficit by the end of this month, but it predicts “these deficits are likely to persist over the next six months.” Soils in western Scotland are a little wetter then average for the time of year.

SMD is a more useful measure than moisture content, which varies greatly depending on soil type. For example, sandy soil with a 20% water content might be at field capacity, while the same quantity of water in loam soil might be about right for crop cultivation. But heavy clay soil locks up a considerable amount of moisture, and 20% may be at PWP.

Extreme drought also produces what is known as a surge year. This is when clay soils dry out, the ground shifts and cracks open and buildings start to suffer damage, leading to a surge in subsidence claims.