Actualité scientifique
Scientific news
N° 431
May 2013
Actualidad científica
Sub-Saharan water:
not as fossil as all that
© IRD/V. Simonneaux
The Sahara conceals large quantities
of water stored at depth and
inherited from ancient times. A recent
study by the IRD and its partners
has just shown that this groundwater
is not entirely fossil, but resupplied
every year. Using a method based
on data obtained by satellite,
scientists estimated the variations in
the volume of water lying under the
northern Sahara desert: the current
rate of recharge is on average
1.4 km3 per year, for the period
2003-2010. This represents 40% of
withdrawals, mainly for irrigation to
support the oasis economy. The
inputs therefore do not compensate
for the withdrawals, but their
existence means that these
transboundary aquifers, the main
water resource of semi-arid regions
in Algeria and Tunisia, could be
managed sustainably.
The Sahara Desert conceals large quantities of water, inherited from a ancient wetter period.
Good to know
The northern Sahara aquifer system extends over a
surface area nearly twice the size of mainland
France and, at depths of several hundreds or even
thousands of metres, holds more than 30 000 km3
of water, accumulated over wet periods that occurred
over the last 1 million years.
This reservoir of groundwater, one of the largest in
the world, has enabled the urban and agricultural
development of semi-arid regions in Tunisia,
Algeria and part of Libya over the last thirty years.
Glossary
Aquifer: Geological formation or rock containing
groundwater exploitable naturally or by pumping.
Evapotranspiration: the combined effect of
evaporation of water at the soil surface and
transpiration of plants.
Field of gravity: field of attractive force towards the
centre of the earth linked to the mass of the object.
Artesian well: well from which water spews
naturally due to pressure in the underground
reservoir.
Non negligible recharge
A new satellite approach
Until recently, groundwater in the northern Sahara
aquifer system was considered as “fossil”, i.e.
non-renewable, similarly to coal or oil. Precipitation in the region seemed too low and evapotranspiration too high to recharge deep aquifers.
But scientists have shown that, in reality, groundwater in the northern Sahara aquifer system, to
give it its exact name, is still being fed today.
Indeed, the recharging exists and has been quantified, as revealed in a study published in Geophysical Research Letters. Rainwater and runoff
bring an average of 1.4 km3 to the system per
year, or around 2 mm per year on the aquifer
recharge surface. From the period 2003 to 2010,
annual recharge even reached 4.4 km3 in some
years, or 6.5 mm per year.
The research team highlighted this recharge using
a new satellite measuring method. The scientists
analysed the data provided by the GRACE satellite
mission (Gravity Recovery and Climate Experiment) by NASA and the German aerospace centre.
In orbit since 2002, GRACE measures variations in
the earth’s field of gravity, enabling variations in the
water mass contained in the surface envelopes to
be deduced. These data were used by the scientists to estimate the change in the volume of water
stored and to deduce aquifer recharge, one withdrawals made on the aquifers were taken into
account. Among other things, this global approach
means that the uncertainties in the hydrogeological
models can be discounted, which are based on
local piezometric measurements, i.e. the level of
water noted in wells and boreholes.
FOR MORE INFORMATION
Withdrawals not compensated
3
The average recharge of 1.4 km per year corresponds to 40% of the 2.75 km3 in total withdrawn
every year in the region, according to data from
the Sahara and Sahel Observatory (OSS). As a
result, 60% of annual withdrawals are not
compensated. Despite significant recharge, the
Northern Sahara Aquifer System remains therefore overexploited.
Since the 1960’s, withdrawals have continued to
increase, to satisfy the growing need of various
social-economic sectors: industry, agriculture,
tourism, household use. Wells and boreholes
have multiplied and annual withdrawals have
risen from 0.5 km3 in 1960 to 2.75 km3 in 2010,
leading to a general drop in water levels, in some
places reaching 25 to 50 m. Numerous artesian
wells and natural springs, around which oases
have developed, have already run out.
Contacts
Pierre Deschamps, IRD scientist
Tel: + 33 (0)4 42 97 15 11
pierre.deschamps@ird.fr
The reduction in artesianism, i.e. the water pressure within groundwater, risks affecting the viability of the oasis economy. By quantifying current
recharge, this work will enable the development
of tools for to manage the resource responsibly,
while more economical systems of irrigation are
put in place. The challenge is considerable: these
groundwater resources will have to meet the
growing needs of a population which should
reach 8 million inhabitants by 2030 according to
the OSS.
Julio Gonçalvès, professor at AixMarseille University
Tel: + 33 (0)4 42 97 15 87
goncalves@cerege.fr
UMR Cerege (IRD / AMU / CNRS /
Collège de France)
Keywords
Water, Sahara ,Oasis, Irrigation
Photos: www.indigo.ird.fr
DID YOU KNOW?
The water extracted from groundwater under the Sahara sometimes spews forth at very high temperatures,
of up to 80°C! It needs to be cooled using cooling systems before it can be used for irrigation. Moreover,
with a salt content that can reach 1 to 5 grams per litre, this water is often too salty for drinking.
Coordination
Gaëlle Courcoux
Information and Culture Department
Tel: +33 (0)4 91 99 94 90
Fax: +33 (0)4 91 99 92 28
fichesactu@ird.fr
Partners
Aix-Marseille Université, CNRS and the Sahara and Sahel Observatory. This study is backed by the Labex OT-Med (AMU).
References
Gonçalvès J., Petersen J., Deschamps Pierre, Hamelin B., Baba-Sy O. Quantifying the modern recharge of the “fossil” Sahara
aquifers, Geophysical Research Letters, 2013, VOL. 40, 1–6. doi:10.1002/grl.50478
www.ird.fr/la-mediatheque
Indigo, IRD Photo Library
Daina RECHNER
Tel: +33 (0)4 91 99 94 81
indigo@ird.fr
A cooling system at left, used before the water can be used for oasis irrigation or cities water supply (here, in Tunisia).
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© IRD/DIC, Avril 2013 - Conception et réalisation graphique : L. CORSINI
© IRD / P. Deschamps
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© IRD / P. Deschamps
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