110 THE FUTURE OF ARID LANDS 
to increase as much as possible the precision of the measurements 
because of the slowness of their variation. Also one should make 
use of a precise survey system, generally absent in desert regions. 
One must not lose sight of the fact that desert oases support 
an extremely dense population. In the oasis of the Djerid a 
population of 45,000 lives on 40 square kilometers of irrigated 
land. One must therefore be very prudent in the execution of 
work capable of modifying the flow of water. 
When working on a virgin or little-used aquifer, it is often pos- 
sible to oye spectacular improvements without preparing 
very precise bases, but beyond a certain degree of use the ameliora- 
tion of yield requires greater and greater efforts and increasingly 
precise scientific information. 
For good use of the reserves, it is naturally better to take out 
the water at the lowest possible part of the formations. The 
usable reserve is thus increased, the losses diminished, and at 
the same time the instantaneous flow is increased. 
But the lowering of the takeoff level by pumping raises difficult 
social and economic problems. It can lead to the elimination of 
small owners and the concentration of property. In areas of 
great population density, divided into small parcels of land, 
increases in agricultural production should yield priority to drink- 
ing needs. These needs satisfied, landowners may lack the re- 
sources to pay additional pumping expense. Hence the interest of 
utilizing local sources of energy and especially wind energy. 
Pant eullesky advantageous is the case where it is possible to 
lower the takeoff level of the water without the expense of pump- 
ing. This is the case in Chott Chergui in Algeria (1) where utiliza- 
tion of the waters is projected by taking them below the level of 
the Chott, situated on a plateau 1,000 meters high, to use them 
in the valley of Cheliff, several hundred meters below. 
From our point of view this experience is of utmost interest. 
It gives the opportunity for a program of studies now in progress 
which will not fail to better our knowledge on all the points 
covered in the present study. ; 
Another advantageous case, because it is relatively easy to 
study, is that of the very important water-bearing limestone 
formations in the eastern Mediterranean basin. It is easier in 
