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
In the following maps, reference potential evaporation (mm) is presented under historical climatic conditions, and as the projected change from the historical climatic conditions to the immediate future climatic conditions (2030s).
In the immediate future, annual reference potential evaporation could increase from as little as 20 mm in the eastern mountains to > 140 mm in the eastern Karoo. Over the pome and stone fruit production regions the historical annual values are in the range 1200-1900 mm, and the projected annual increases in the immediate future are in the range 70-120 mm.
Reference potential evaporation is highly seasonal. Projected changes into the immediate future display the largest increases in spring and summer at 30-40 mm. The projected increases are 10-30 mm in autumn and only 10-20 mm in winter. However, each season shows insignificant increases in some of the high-lying areas (mainly in the east) and hotspots of large increases in the semi-arid zones of the east.
Projected increases in reference potential evaporation will impact on water availability by leading to greater losses from dams, wetlands and riparian zones. Additionally, soils are anticipated to dry out more rapidly in future, leading to potential negative implications for runoff production. Irrigation water demands will be higher than at present.