Soil Moisture Deficit (SMD)

The 3-panel Soil Moisture Deficit (SMD) map shows the historical soil moisture deficit at the date shown (left), the soil moisture deficit at the same time last year (middle) and the current soil moisture deficit (right). The Soil Moisture Anomaly map shows the difference between the current and historical soil moisture deficits.

SMD is calculated based on incoming daily rainfall (mm), outgoing daily potential evapotranspiration (PET, mm), and a fixed available water capactity (the amount of water in the soil 'reservoir' that plants can use) of 150 mm. ET is assumed to continue at its potential rate until about half of the water available to plants is used up, whereupon it decreases, in the absence of rain, as further water extraction takes place. ET is assumed to cease if all the available water is used up.

Click on the maps for larger images.

Soil Moisture Deficit (SMD)


 

Soil Moisture Anomaly


SMD Drought Indicators

If the SMD is between -110 and -130 mm, then the soil is "severely dry". If the SMD is less than -130 mm, then the soil is "extremely dry". Many regions experience soil moisture deficits in these ranges every year (e.g. Central Otago), and farming systems are designed to cope. However:

  • If the SMD is less than -110 mm AND the Soil Moisture Anomaly is less than -20 mm, then soils are "severely drier than normal" and drought conditions (if not present already) may be imminent.
  • If the SMD is less than -110 mm AND the Soil Moisture Anomaly is less than -30 mm, then soils are "extremely drier than normal" and drought conditions are likely evident.