Soil Moisture Deficit (SMD) refers to the amount of water needed to bring the soil back to its field capacity (the maximum water content after drainage). In simple terms, it measures how dry the soil is—a higher SMD means drier soil.
The UK Met Office provides SMD data for different land use and soil types. This analysis focuses on two specific scenarios:
Loamy soils typically have an AWC of 100–150 mm/m. For a grass root depth of 0.5m, this means:
Total available water = 125 mm/m × 0.5 m = ~62.5 mm
At full saturation, SMD = 0 mm. At wilting point, SMD = ~62.5 mm for loamy soils and ~87.5 mm for clay soils.
With deep loamy or clay-rich soils (AWC ~175 mm/m), and a 50:50 grass-to-tree split, we calculate:
Total AWC = (0.5 × 175 × 0.5) + (0.5 × 175 × 2.0) = ~218 mm
In tree-only areas, this could be as high as ~350 mm. Higher SMD values are expected with greater tree cover.
Most subsidence in clay soils is caused by moisture withdrawal by trees. Thus, the SMD values for the Grass + Deciduous Trees model are a good proxy for when buildings may be at risk due to clay shrinkage.
Soil Moisture data
This chart shows sustained SMD levels above 320mm during 2018 and 2022 – both known surge years for subsidence insurance claims. In contrast, the wet year of 2024 shows SMD staying below the 218mm trigger.
By reviewing past years, we can correlate the number of weeks above 320mm against surge events and compare that to grass-only SMD.
Current SMD levels for 2025 are tracking between those seen in 2018 and 2022. If these high values are sustained, it may suggest a growing likelihood of a subsidence surge year ahead.