REFERENCE POTENTIAL EVAPORATION (ETo) – Somerset West

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.