Description of hydrogeological systems traditionally relies on a few factors, such as porosity, rock type and the hydraulic parameters, transmissivity and storativity. Some studies include terms for rainfall, recharge and aquifer boundaries. Inconsistency in reporting makes it hard to compare hydrogeological units and reliance on hydraulic parameters fails to consider the assumptions in hydraulic testing theory, which are often not met.
South Africa has a stratigraphic record spanning 3.5Ga, including world class examples of Archaean granite-greenstones, Archaean volcano-sedimentary sequences, Proterozoic metamorphic belts, the world's largest layered igneous complex, the Pan-African orogeny, a Permo-Triassic fold belt, a Phanerozoic continental foreland basin, a continental flood basalt, and extensive inland and coastal Mesozoic to Cenozoic deposits. The landscape ranges from sea level to 3500m elevation, with plains, fold mountains, the Southern-African Great Escarpment, canyons, inselbergs and other landforms. The climate ranges from Mediterranean near Cape Town with snow on the mountains in winter, through sub-tropical to tropical, with seasonal monsoon style thunderstorms or perennial advective rainfall, to desert. Annual rainfall ranges from zero often in the deserts of the north-west to over 3m in the Cape Mountains. Temperatures range from winter minimums of <0 C to summer maximums of >40 C.
Hydrogeologists working in South Africa face this complexity when describing a hydrostratigraphic unit. Multilinguilism (11 official languages) and developing world constraints on time, money and education provide additional challenges. This paper refreshes knowledge of technical terms, promotes correct use of this terminology, highlights similarities and differences between hydrogeological units countrywide, and suggests the level of information needed to make management decisions. Terms such as soil, regolith and saprolite are defined; grading and sorting and their influence on porosity are explained; representative elementary volume, especially in context of fracture networks is also explained. The distribution and flow of water is dealt with, especially complex vadoze zone flows such as imbibition, interflow and percolation. The above factors are integrated by relating these to 5 fundamental drivers: geology, geomorphology, climate, water state and human effects. The first three of these are seen as earth system constants, unchanging on a human time scale, whereas the last two are the temporal behaviours that modify the earth system condition.
A fixed hydrostratigraphic classification system is not proposed, as the complexity and level of uncertainty precludes this. However, 10 examples of hydrogeological settings are given, spanning the diversity found in South Africa, and applying the terminology presented. This shows how reporting of typical hydrogeological investigation results (hydraulic aquifer tests, water quality analyses) needs to be augmented with information from the 5 fundamental knowledge areas to achieve a more meaningful description of the hydrogeological units, creating more consistent descriptions and ultimately allowing hydrostratigraphic classification countrywide.