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
For the Western Cape and Langkloof, the means of accumulated PCUs from April to August under historical climatic conditions range from < 600 in the north-west, to > 2 000 PCUs in the high-altitude mountain areas. PCU values of 800-2000 are seen in the pome fruit production regions, with values of 600-800 also common in the stone fruit production regions.
For the intermediate future of the mid-century, a sharp decline in PCUs is very clear. Reductions of 250 to 500 or more PCUs are likely to be experienced across large areas. Accumulated seasonal PCUs are projected to range between < 200 and 2000 across the region. In the pome fruit regions, the PCUs are projected to be between 200 and 1600, while values of 200-600 could become common in the stone fruit regions.
Spatial patterns of PCUs across the region are preserved under historical climatic conditions in the monthly maps, compared to the seasonal map. PCU accumulation starts very slowly in April and increases through May to reach maximum levels in June, July and August. The maps for projections into the intermediate future (2050s), derived from multiple GCMs and the RCP8.5 scenario, indicate significant losses of PCUs in April and May, especially in the warmer production regions of the south. In future, most PCUs in these regions could be accumulated in the core months of June, July and August compared to the longer period of accumulation in the cooler regions. In some regions, this will leave chill accumulation too late to reach medium or higher PCU thresholds.