Using geological domains to characterise regional to continental scale aquifers across eastern Africa

We have developed a geological domains-approach to characterise the spatial variability of key aspects of the geology that directly impact the hydrogeology of aquifers at different scales. This approach utilises a range of datasets, including geological maps, terrain maps, satellite imagery, research papers and thematic datasets such as fault maps or climatic zones to develop a set of spatial geological domains. The specific variables and subsurface depth represented by the domains will depend on the objective of the specific mapping exercise and/or the type of hydrogeological modelling being undertaken. The quality of geological and geomorphological spatial data can be highly variable on a regional to continental-scale and this be qualitatively represented with a a description of the uncertainty of each individual domain. The domains approach can either data-driven or conceptual model driven, typically when working at a site to catchment scale. Where data is readily available a data-driven approach can be implemented. However, at a continent-scale, where spatial data is less likely to be available, a conceptual model-driven approach is needed. This requires a hypothesis of key geological and landscape processes to be developed, which can be used to create a domains map based on a continental-scale geological maps and terrain data. To illustrate this process, we will demonstrate the use of the domains approach to characterise groundwater properties of the shallow aquifer across a range of geological settings in northern Kenya, Somalia and Somaliland. The geology of the region included Archean basement, the East African Rift and several major Cenozoic basins, with a variable Quaternary cover. The domains focused on the spatial variability of regional-scale faults, bedrock lithology and potential weathering profiles (regolith).