35.
It is not, of course, possible at present to calculate the quantity of water reaching
the Zone from any of the three above sources. The quantity obtained from direct rainfall
is limited by the average annual amount that falls in the area, which is probably of the
order of 4" but of which, of course, a large and variable proportion is lost by evapora tion. The incoming groundwater from Zone 4 may be considered as an underground overflow of
the surplus from that Zone, coming through and around the Faiyah-Azab range, and being
interrupted by fault zones. If the gravel plain of Zone 4 becomes highly developed it is
probable that the water reaching Zone 7 from it will be reduced in quantity. The flood
runoff from Wadi Semaini no doubt recharges the aquifer along the whole length of its
lower course and, since none of its water reaches the sea on the surface, all except what
is lost by evaporation and similar causes enters the aquifers. Runoff from Wadi Haremba
also must recharge the aquifers of the plain as it passes through but in that case some
of the water reaches the sea on the surface and is lost to the groundwater basin.
So far as can be seen it is not practically feasible to improve the groundwater
conditions of Zone 7 by increasing the re-charge from either (a) the direct rainfall, or
(b) the groundwater flow from Zone 4. It may, however, be possible to effect some im provements as regards the better utilisation of (c), the flood runoff, as explained in
the following paragraphs.
The quantity of flood water that enters the sea from Wadi Haremba has never, to our
knowledge, been measured directly, but during the course of this investigation we esti mated from flood marks that the rains of January 1965, recorded in Appendix 4, produced a
flood of about 200 cusecs, and this, we think may serve as a guide to the quantity of
water which reaches the sea in a not unusual season. Although this is not a large flow we
think it should be prevented from running to waste unless the cost of conservation works
to save it is excessive.
The flood water of Wadi Haremba could perhaps be prevented from reaching the sea by
a relatively low earth recharge dam in the general vicinity of Tawi Qaran (map reference
E 0374, N 2824) and in such a case the reservoir would spread the impounded water over a
large area and thus encourage it to sink into the ground and recharge the groundwater
basin. Because a recharge reservoir of this nature would not fill at regular intervals
and because its bed would not be sufficiently impervious to hold water for long periods,
the water could not be relied upon for other uses. The overflow from such a dam, in ex ceptional floods, could be made to pass down Bada-al-Ali, the eastern branch of the wadi,
which would thus become a natural spillway. A successful dam in this area would cause
local groundwater levels to rise and the salinity of the water to improve.
We consider that the above scheme is deserving of further investigations which would
consist of surveys to determine the size of the dam and the most favourable site as well
as an examination of the soil and sub-soil of the surrounding area to ascertain if it is
suitable for absorbing the impounded flood water.
The second large wadi bringing floodwater into Zone 7 is Wadi Semaini. Since this
wadi does not reach the sea all its water apart from what is lost in evaporation is already
absorbed into the groundwater and nothing can be done to increase the volume of water
entering the groundwater basin. However, not all the water is absorbed in the areas where
the demand for good water is greatest.
