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
In the Free State, 250 PCUs have historically been reached by May in the south-eastern half, and by June in the north-western half, while in the intermediate future this is expected to shift to June and July, respectively. Historically, the 500 PCU threshold has been achieved by June (south-east) or July (north-west), but by the mid-century this threshold may only be reached in July (in the coldest areas), August (other parts of the south-east), or September-October or not at all (north-west). Large parts of the province historically achieved 700 PCUs by July, but into the intermediate future only a small area could achieve this threshold by August, with other parts of the south-east reaching 700 PCUs by September or October.
- Month Achieved (250 PCU) - Free State
- Month Achieved (500 PCU) - Free State
- Month Achieved (700 PCU) - Free State