Publication record details

Title An initial estimate of depth to groundwater across Africa
Ref no OR/11/067
Author Bonsor, H.C.; MacDonald, A.M.
Year of publication 2011
Abstract This work aimed to provide an initial estimate of depth to groundwater across Africa, to help with considerations of the applicability of different hand pump technologies in Africa. The main findings and outputs from the work are summarised below: ï‚· Developing an estimate of depth to groundwater across Africa - In the absence of much observed groundwater-level data in Africa, several modelling approaches were used to estimate depth to groundwater in GIS (refer to section 2.4) using continent-scale datasets of geology, geomorphology and rainfall. Based on comparison to the available observed data, the most successful method was an empirical rule based approach, where depth to groundwater was assigned according to rainfall and aquifer type. ï‚· Depth to groundwater - Across much of central, western and eastern Africa, where the climate is wet or seasonally wet and basement geology predominates, natural groundwater-levels are generally shallow - approximately 0-25 mbgl. Shallowest groundwater-levels (<7 mgbl) are estimated to be adjacent to perennial rivers. Deepest groundwater levels (>250 mbgl) are mapped in the major sedimentary basins in north Africa where average annual rainfall is low and aquifers are generally hundreds of metres thick. ï‚· Expected drawdown - At a pumping rate of 5 m3/d, which is equivalent to that of a hand pump, drawdown is small (<5 m) in aquifers with transmissivity >1 m2/d. However, where transmissivity is <1 m2/d, drawdown rapidly becomes limiting. This is as a result of the inverse power relationship between transmissivity and drawdown. Transmissivity of <1m2/d is most likely in basement aquifers, therefore significant effort should be made to site boreholes in the most productive parts of these aquifers. Higher abstractions from submersible pumps (e.g. 100 m3/d) will lead to much greater drawdowns, which may become limiting in aquifers with a transmissivity of less than 10 m2/d. ï‚· Population affected - The majority (85%) of Africa's population lives in regions where depth to groundwater is 0-50 mbgl, and hand pumps may be used to abstract water. A significant minority however (8%; 80 million) of Africa's population live in regions where depth to groundwater is between 50 and 100 mbgl and common hand pump technologies such as India Mark II are generally inoperable. These areas are mainly within northern and southern Africa, and to a lesser extent the Sahel. ï‚· Lack of observed groundwater-level data - The need for observed groundwater-level data in Africa cannot be over-estimated; observed data would not only enable a much higher level of validation of the outputs from this work, but enhance many other aspects of hydrogeological work in Africa.
Publisher British Geological Survey
Series Open Reports
View publication View online   View on NORA