Method
Sunburn
Because monitoring the temperature dynamics of all fruits throughout the entire growing season is challenging, models that simulate fruit surface temperature (FST) are developed and applied. One such experimentally based FST model (Li et al., 2014) calculates the maximum temperature of the fruit surface when facing directly to the sun at midday on a sunny day, without any shading of the fruit surface. It then calculates the difference between FST and air temperature, which is on average ~ 12°C. This value which corresponds well with experimentally obtained values in South African and Australian apple orchards.
In this study, we assumed that sunburn browning (FST ≥ 48°C) would occur when daily maximum air temperature (Tmax) was between 34.0 and 38.9°C. Sunburn was thus modelled for the study region using the following scenario:
Number of days with FST ≥48.0°C and daily Tmax 34.0 – 38.9°C
We then analysed the total number of days per month (November to April) where daily Tmax crossed the threshold for potential sunburn damage.
The historical climate risk (days per month meeting the criteria), the projected intermediate future climate risk, and the difference (days per month) between the two were mapped using the average outcomes from four CMIP3 GCMs.
Maps
Map Information
The average number of days per month when the conditions were historically conducive to sunburn of apples for the months of November to April are presented in the maps. These are accompanies by maps showing the results for intermediate future conditions (2050s), and the differences between the two time periods.
The risk of sunburn browning in apples under historical climatic conditions is low in November, and the increased risk in future is small (up to two additional days) in the apple production regions. Historical sunburn browning risk increases from December onwards, peaks in January/February and declines from March. In December and January, the risk in future (mid-century) increases by up to five days per month in the warmer production areas. The additional risk then decreases from February onwards, with the exception of parts of the Berg River valley which could experience five additional risk days in March. The south-western coastal production regions e.g. EGVV, show a lower increase in risk (up to two additional days per month), with the risk increasing across a wider area until March. The Langkloof is projected to experience up to two additional risk days per month throughout the summer.