SMD is the moisture the soil must make up until it is at capacity, with very dry conditions over most of England and Wales
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.
