Method
Chill - Month Achieved
The Daily Positive Chill Unit (or PCU), also known as the Modified Utah Chill Unit model or the Infruitec model, was used to produce maps of mean seasonal or monthly chill units. The optimum temperature range for chilling accumulation is between 7.2 and 9.1°C, with temperatures >15°C negating chilling. In contrast to the Utah model, the PCU model assumes that high temperatures can only negate the chilling received on the day of occurrence and do not affect the chill units accumulated previously.
The techniques by which PCUs were computed from hourly temperature values are outlined in detail in Schulze (2011). From the hourly PCU calculations, daily PCUs were accumulated, from which monthly and seasonal totals of PCUs could be computed for the period April to August.
The modelling for the intermediate future (2050s) was conducted using five different CMIP3 GCMs (Schulze, 2011).
In addition, the month in which a certain level of chilling has accumulated was computed and mapped for the historical climate and the intermediate future. The levels were set at 250 (low), 500 (medium) and 700 (high) PCUs.
Maps
Map Information
Large parts of the KwaZulu-Natal interior have historically reached 250 PCUs by May (Drakensberg and Escarpment) or June, but this area contracts significantly into the intermediate future, mostly to the Drakensberg foothills / Midlands and the Escarpment. The remainder of the interior achieves this threshold by July or August. The medium chilling threshold (500 PCUs) has been historically reached by June or July, shifting to July, August or later in the Drakensberg foothills / Midlands, while the northern interior does not reach this threshold except on the Escarpment. Only small areas of this province are cold enough to receive 700 PCUs by June or July, historically. By the mid-century a very small colder area south of Estcourt-Greytown reaches this threshold by July-August, with the remainder of the Midlands and Escarpment areas achieving 700 PCUs only by September or October, or never.