Page 86 - Arabian Studies (V)

76 Arabian Studies V

cent is infiltrated and 30 per cent evaporated and recharge,
expressed as a percentage of storm rainfall, amounts to approxi­
mately 15 or 9 per cent of the total annual rainfall. On the basis of
these observations, coupled with isotope sampling and geomorpho-
logical evidence, estimates show that annual recharge has varied
from 5.5 x 106m.3 to 26.3 x 106m.3 in the northern part of Qatar and
from 3.3 x 10®m.3 to 30.1 x 106m.3 in southern Qatar over the past
five years. Long-term rainfall trends have a marked effect on
recharge to groundwater and individual short-term estimates of
recharge are of little value in assessing the probably long-term safe
yield of the aquifer. Ten-year moving averages of the 50-year rain­
fall record at nearby Bahrain exhibit a clear element of inter-annual
persistence—the so-called Hurst phenomena4—resulting in a ratio
of highest to lowest observed decade mean annual rainfalls of
about two. By relating the 18-year rainfall record at Doha (al-
Dawhah) to total rainfall over northern Qatar for the past five
years and approximating a probable historic sequence of recharge,
it is noted that since 1958 there have been three separate trends in
recharge; from 1958 to 1966 average recharge is estimated to have
been 18 x lO^m.3 per annum but from that year to 1972 declined to a
mean annual total of 9 x K^m.3 and thereafter recovered to an
annual average value of 18 x 106m.3.
There are two separate and distinct groundwater provinces in
Qatar. In the northern half of the peninsula groundwater occurs as
a freshwater ‘floating lens’ on brackish and saline water. This
!
freshwater is contained in limestones and dolomites of the
Dammam and Rus formation and is underlain by saline water
contained within the ‘Umm er Rhaduma’ (sic) formation. The
southern groundwater province is altogether different; there is no
extensive freshwater lens and water quality is generally brackish
with only a thin veneer of freshwater at the top of the water table.
Thus the major aquifer of Qatar is a shallow floating lens of fresh
water that has been developed as a result of local recharge and is a
minor, but distinct, hydrogeological feature of the extensive deep
regional aquifers of the northern part of the Arabian Peninsula
which, although underlying the Qatar peninsula, are in this region
too saline for any possible use. Southern Qatar does however
exhibit many hydrogeological features common to the Neogene
formations of eastern Saudi Arabia.
The northern groundwater province is the most important source
of reasonable quality groundwater in Qatar. It is limited in the
south by a V-shaped structure or boundary and which is now esti­
mated to contain some 2,500 x lC^m.3 of freshwater, with the base
of the lens or freshwater/saline water interface at a level of