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
The coldest areas of Mpumalanga, suitable for deciduous fruit production, are the high-lying areas west of the Escarpment. Historically, this area has achieved 250 PCUs by May-June, but this shifts to June-July into the intermediate future, similar to the map for the historical achievement of 500 PCUs. By mid-century, 500 PCUs are expected to be reached by July in the coldest areas, and by August or September over most of the remaining area. Most of the area has historically reached 700 PCUs by July, but into the intermediate future this threshold may only be reached by August (coldest area), September or October (other cooler areas) or not at all.
- Month Achieved (250 PCU) - Mpumalanga
- Month Achieved (500 PCU) - Mpumalanga
- Month Achieved (700 PCU) - Mpumalanga