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
Historically, low PCUs (250) are reached by May in the mountains of the Western Cape, along the Escarpment, the Drakensberg Mountains and their foothills, and parts of the high-lying interior. This covers most of the deciduous fruit production regions, with some regions reaching this threshold in June. My mid-century the areas still reaching this threshold by May may contract to only the highest mountainous areas, with most of the fruit production regions reaching 250 PCUs by June or July.
The medium PCU threshold of 500 units has historically been reached between May (mountains) and July, whereas in the intermediate future, this shifts to a patchwork of June (mountains), July, August, and in some areas September.
High PCUs (700) have historically been reached by June or July in most of the pome and stone fruit regions, and sometimes August in stone fruit regions. The projected changes by mid-century suggest that only the coldest areas will achieve 700 PCUs by July, with other production regions reaching this threshold by August, September and October (or never, in the warmest regions).
- Month Achieved (250 PCU) - South Africa
- Month Achieved (500 PCU) - South Africa
- Month Achieved (700 PCU) - South Africa