Method
Mean Annual Precipitation
The mean annual precipitation (MAP, in mm) characterises the long-term quantity of water available to a region for hydrological and agricultural purposes, and is one of the most used statistics in crop production.
Note that while simple to calculate and attractive to use, the concept of MAP nevertheless has its weaknesses, in that in this region (and elsewhere in South Africa) years with lower than average rainfall are more numerous than years with higher than average rainfall. This is because MAPs are frequently inflated by a few very high annual totals from very wet years, especially in areas of low rainfall.
MAP was obtained using methods described in the section Background: Climate Change Modelling. This resulted in a unique 50-year daily rainfall record for each of the Quinaries covering the region (Schulze et al., 2010).
Future rainfall projections remain challenging and are not presented here. This is because:
- Rainfall is a derived rather than a direct output from GCMs.
- Complex rainfall-generating processes such as cloud formation and land surface-atmosphere interactions are not yet fully understood and resolved in climate models.
- Rainfall is an event based variable, and not continuous, as is temperature.
As the climate model developers address the weaknesses and inconsistencies in the older GCMs, the new set of GCMs can alter the picture somewhat. The future rainfall maps can be presented once there is a greater model agreement and confidence in the results.
Maps
Map Information
In the following maps, mean annual precipitation (mm) and inter-annual coefficient of variation of rainfall (CV%) are presented under historical climatic conditions.
MAP in the region is highly spatially variable with a range from < 150 mm to > 1 000 and even 2 000 mm. The lowest MAPs are in the arid north-west and central interior of the Karoo, while the highest MAPs are in the south-west mountain areas, the linear fold mountain belts and the south-east coastal area. Major variations in MAP frequently occur in locations within close proximity to one another.
The inter-annual variability of rainfall in the region is high at between 20% and 60%, with the highest variability in the more arid regions of the north-west and the Karoo.