The moisture deficit reported on Farmwest is the difference between the measured evapotranspiration and effective precipitation.
Moisture Deficit = ET - Effective Precipitation
When the weather is cool and wet the moisture deficit reported on Farmwest may be too high the Total Precipitation instead of the effective precipitation should be used to calculate the moisture deficit.
What does moisture deficit mean?
A moisture deficit of 10 mm over a time period means 10 mm of moisture was removed from the soil that was not replaced by precipitation during this period.
A moisture deficit of -10 mm means that over the time period there was 10 mm more precipitation than evapotranspiration during the period.
How does the moisture deficit relate to my soil moisture?
To relate the climatic moisture deficit to the amount of moisture left in the soil there must be a starting point of reference. The easiest reference point is after an irrigation when the soil water profile has been filled up to field capacity (full).
Use the date of irrigation as the start date in the time period, and the current day as the end date.
If the total moisture deficit over the time period is 50mm, then 50 mm of moisture has been removed from the soil and an irrigation of 50 mm would fill the soil water profile again to where it was at the beginning of the period.
Factsheets from the Ministry of Agriculture, Food and Fisheries that provide additional information are: