Method
Reference Potential Evaporation
Evaporation is controlled by three atmospheric conditions:
- The capacity of air to take up water vapour. This increases rapidly at higher temperatures and at lower relative humidity (RH) of the air.
- The amount of energy available for the process of evaporation. This energy is provided mainly by solar radiation.
- The degree of turbulence (related to wind) in the lower atmosphere.
These three factors create an atmospheric demand, and when this demand can be met fully, e.g. when soils are wet and actively growing vegetation covers the ground completely, then potential evaporation (Ep) takes place.
There are many methods of estimating Ep, all giving slightly different answers under different climatic conditions, and a reference potential evaporation (Er) must therefore be selected. The choice of Er has inherent advantages and defects and these should be understood.
Here, the Penman-Monteith method of estimating evaporation (Epm) is used (Penman, 1948; Monteith, 1981). The Penman-Monteith equation uses solar radiation, humidity, temperature and wind as its climate inputs. A version for South Africa can be found in Schulze (2012). Daily values of Epm were then multiplied by a local factor varying from 1.19 (summer) to 1.27 (winter) to convert Epm to an A-pan equivalent reference potential evaporation (mm), which is commonly used in South Africa.
The mean annual reference potential evaporation was calculated from daily weather data (see section Background: Climate Change Modelling). In addition, the seasonal monthly reference potential evaporation was calculated for spring (September-November), summer (December-February), autumn (March-May) and winter (June-August).
The modelling for the immediate future (2030s) was conducted using five different CMIP5 GCMs.
Maps
Map Information
Stellenbosch-Berg has a historical annual evaporation ranging widely from around 1400-1500 mm (Franschhoek, Simonsberg), 1500-1600 mm (Stellenbosch, Windmeul), 1600-1700 mm (Simondium-Paarl, Riebeeck Kasteel), to 1700-1800 mm (Wellington). Climate model projections for the intermediate future (mid-century) show an increase of around 90-100 mm per annum (Simonsberg, Windmeul), 100-110 mm (Franschhoek, Stellenbosch, Paarl) and 110-120 mm (Wellington, Riebeeck Kasteel). Seasonal future changes in spring are around 33-36 mm (cooler areas) to over 36 mm (the largest part of the region), and in summer the changes are around 30-33 mm (cooler areas) and 33-36 mm (most of the region), with higher increases possible in Riebeeck Kasteel. Future changes in autumn are around 18-21 mm across the region, and generally 15-18 mm in winter, with larger increases possible in Riebeeck Kasteel.