Method
Red Colour
The criteria used to determine conditions conducive to peel red colour development were broadly based on experimental research in South African apple orchards, and modelled for mid- to late-season blushed/bi-colour cultivars for the months March and April. The criteria used were one day of daily minimum temperature below 12°C (for the induction of anthocyanin synthesis), followed by daytime maximum temperatures in the range 20-28°C for three consecutive days (for strong anthocyanin synthesis), conditional upon solar radiation on the qualifying days being at least 60% of those days’ maximum solar radiation potential.
The number of occasions during March and April that the above conditions were met in each of the 1 401 Quinaries were counted for each year of the historical record (1950-1999), the average determined, and the outcome mapped separately for March, April, and the combined March-April period. Using daily outputs from the ECHAM GCM, considered to be a representative GCM for the region, the same criteria were isolated, and the results mapped separately for the intermediate future period of the 2050s. Finally, the differences (in days per month) between the historical and future climate conditions were calculated and mapped for these months.
Maps
Map Information
In Piketberg, March historically provides 4-5 days of optimal temperatures for red colour in apples, which decreases by up to 3 days under climate model projections for the intermediate future (mid-century). April provides generally around 10-12 days, decreasing by 3-6 days in future. Both months combined generally provide around 12-15 days of optimal temperatures historically, decreasing by 6-9 days in future.