Method
Chill Units
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
In the Western Cape, low PCUs (250) have historically been reached by May in the cooler parts, and by June in the remainder of the fruit production areas. By the projected intermediate future of the 2050s, 250 PCUs are generally reached one month later, but in some areas up to two months later.
On the other hand, 500 PCUs are achieved historically largely by May-June (cooler areas) or July (warmer areas) in the fruit growing areas. By the intermediate future of the 2050s, the southern parts of the fruit growing areas only achieve 500 PCUs by August or September or later, whereas the cooler northern regions (with the exception of Wolseley-Tulbagh) and Langkloof mostly still achieve 500 PCUs by June or July.
Under historical climatic conditions, high PCUs (700) are reached by June-July (cooler northern fruit areas and western Langkloof) or July-August (warmer southern areas and Little Karoo). Into the intermediate future, this shifts to mostly July in the north and western Langkloof, and to September to October (cooler south) or never (warmer south e.g. Berg and Breede River valleys).