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
Wolseley and Tulbagh, 250 PCUs are historically reached in May and June, respectively. This could shift to June (Wolseley) and July (Tulbagh) under climate model projections for the intermediate future (mid-century). 500 PCUs are historically reached in June (Wolseley) and July (Tulbagh), which could shift to August (Wolseley) but this threshold may not be reached in Tulbagh. 700 PCUs are historically reached in July and August, respectively, which could shift to October in future (Wolseley) but will not be reached in Tulbagh.