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The influence of groundwater abstraction on interpreting climate controls and extreme recharge events from well hydrographs in semi-arid South Africa
Authors:Sorensen  James P R  Davies  Jeff  Ebrahim  Girma Y  Lindle  John  Marchant  Ben P  Ascott  Matthew J  Bloomfield  John P  Cuthbert  Mark O  Holland  Martin  Jensen  K H  Shamsudduha  M  Villholth  Karen G  MacDonald  Alan M  Taylor  Richard G
Institution:1.British Geological Survey, Maclean Building, Wallingford, OX10 8BB, UK
;2.Department of Geography, University College London, London, WC1E 6BT, UK
;3.International Water Management Institute, P.O. Box 5689, Addis Ababa, Ethiopia
;4.Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
;5.School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK
;6.School of Civil and Environmental Engineering, The University of New South Wales (UNSW), Sydney, Australia
;7.Delta-H Water Systems Modelling PTY Ltd, Faerie Glen, Pretoria, South Africa
;8.Department of Geography, University of Sussex, Brighton, UK
;9.International Water Management Institute, Cresswell Road, Pretoria, South Africa
;10.British Geological Survey, Lyell Centre, Edinburgh, EH14 4AP, UK
;
Abstract:

There is a scarcity of long-term groundwater hydrographs from sub-Saharan Africa to investigate groundwater sustainability, processes and controls. This paper presents an analysis of 21 hydrographs from semi-arid South Africa. Hydrographs from 1980 to 2000 were converted to standardised groundwater level indices and rationalised into four types (C1–C4) using hierarchical cluster analysis. Mean hydrographs for each type were cross-correlated with standardised precipitation and streamflow indices. Relationships with the El Niño–Southern Oscillation (ENSO) were also investigated. The four hydrograph types show a transition of autocorrelation over increasing timescales and increasingly subdued responses to rainfall. Type C1 strongly relates to rainfall, responding in most years, whereas C4 notably responds to only a single extreme event in 2000 and has limited relationship with rainfall. Types C2, C3 and C4 have stronger statistical relationships with standardised streamflow than standardised rainfall. C3 and C4 changes are significantly (p <?0.05) correlated to the mean wet season ENSO anomaly, indicating a tendency for substantial or minimal recharge to occur during extreme negative and positive ENSO years, respectively. The range of different hydrograph types, sometimes within only a few kilometres of each other, appears to be a result of abstraction interference and cannot be confidently attributed to variations in climate or hydrogeological setting. It is possible that high groundwater abstraction near C3/C4 sites masks frequent small-scale recharge events observed at C1/C2 sites, resulting in extreme events associated with negative ENSO years being more visible in the time series.

Keywords:
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