The UK Centre for Ecology and Hydrology reports water storage deficits across much of England and Wales, with the driest parts of southern England requiring "unusually high rainfall" to recover by the end of this month. The agency predicts these deficits are likely to persist over the next six months.
Understanding soil moisture deficit
Soil moisture deficit (SMD) measures how much rain is needed to bring soil to field capacity, the point where excess water has drained away after thorough wetting. Meteorologists use SMD to describe drought conditions, expressed in millimetres of water required to saturate the soil.
In southern England, SMD hovers near zero in winter when fields are wet, but rises to about 100mm in summer, and can reach 150mm or more in dry years. The extreme dryness threshold, where plants cannot access water, is called the permanent wilting point (PWP).
Regional variations and soil types
Western Scotland's soils are slightly wetter than average for this time of year, contrasting with the dry conditions elsewhere. SMD is considered more useful than moisture content, which varies by soil type. Sandy soil at 20% water content may be at field capacity, while loam with the same water content could be suitable for crops. Heavy clay soil, however, locks moisture tightly, and 20% may indicate PWP.
Impacts of extreme dryness
Extreme drought can trigger a "surge year," when clay soils dry out, shift, and crack, causing building damage and a spike in subsidence claims. This phenomenon highlights the practical consequences of soil moisture deficits for infrastructure and agriculture.
