Rainwater Infiltration Rate and Groundwater Sustainable Management in the Dakar Region

Sylvestre Dasylva, Claude Cosandey, Didier Orange, Soussou Sambou

Abstract


The aim of this study is to contribute to the solution of recent problems in the Dakar region
due both to surface water (runoff hazards and waterlogging) and to groundwater recharge
deficit. The rainwater infiltrating volume to the groundwater is a key-element for the
understanding and the solution of these problems. Given that rain is the only refill parameter
(Béture-Sétame, 1988), the groundwater dynamics (especially, the refill and the static level
annual balance) is studied in relation to rainfall variability, in order to propose solutions
adapted to the current pluviometric context. Piezometric data of the NSQ (Nappe des Sables
Quaternaires) from 1953 to 1990, are used to evaluate the groundwater dynamics. The method
is based on the addition of groundwater top-level monthly height changes in the aquifer –in
others words, on the addition of the “relative” static level changes between two consecutive
months. During one hydrological year, the refill and the static level balance of the
groundwater are calculated for three annual pluviometric scenarios: 1/ yearly rainfall (529
mm), close to the regional average (484 mm); 2/ rainfall excess (10 years recurrence: 712
mm); and 3/ rainfall deficit (20 years recurrence: 220 mm).
Compared with precipitation heights, results allow to determine the natural annual values (in
rainwater height equivalent) of: 1/ the refill (= 107 mm/year) in the current pluviometric
context, corresponding to the refill in average rainfall periods, 2/ the refill generating a
positive evolution of the groundwater static level (= 217 mm/year), which is equivalent to the
recharge observed in rainfall excess periods. By comparing these two results, we estimate 3/
the refill deficit (= 110 mm/year) in today’s pluviometric context, and 4/ the infiltration rate
needed for a sustainable management of water resources (50% of total annual rainfall). To
obtain this rate, the infiltrating rainwater volume -height equivalent- feeding the groundwater
should be increased from 107mm to 217mm by artificial recharge.

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