Average soil moisture deficit onset and termination maps

Daily soil moisture content (in mm) for the whole country is calculated using a simple water balance model, assuming a non-varying available water capacity (AWC) of 150mm and daily inputs of rainfall and potential evapotranspiration (PET).  If the soil moisture content falls to less than half of the AWC (i.e. 75mm) then a "Day of Soil Moisture Deficit" (DSMD) is recorded.  Using daily gridded Virtual Climate Station data, the long-term average first and last SMD dates have been calculated.  Maps of these dates are shown here, and can be used to identify the "typical" SMD onset and termination dates, and hence the typical soil moisture defcit "season", for anywhere in the country.  The maps can also be used as a basis for comparison from year-to-year (i.e. are soils in moisture deficit earlier or later than the long-term average?). Some areas of the country are shown as "No Data" as there are insufficient days of SMD to calculate a long-term average.

Information about the Virtual Climate Station data here

The number of days between the typical SMD onset and termination dates does not represent the typical number of days of SMD per year. Periodic rainfalls during the soil moisture deficit season will add moisture to the topsoil, such that the soil moisture content may exceed half of the AWC for a number of days (i.e. those days will not be recorded as a DSMD), before potentially falling below the 75mm threshold once again.

Long term average first soil moisture deficit date. [NIWA]
Long-term average last soil moisture deficit date. [NIWA]

Research subject: Drought