126 THE FUTURE OF ARID LANDS 
beyond the reach of those early people. The modern inhabitants, 
like the Masai of Tanganyika, or the Somali of northern Kenya, 
merely use, often without even troubling to maintain, the wells, 
hafirs, and catchments that were discovered and constructed by 
their long-vanished predecessors. 
In the present-day search for water it is necessary to use every 
modern aid. A thorough study of the geology is required, with 
special reference to the absorption, retention, and transmission 
of ground water, together with an investigation of all aspects of 
the hydrogeological cycle, even though studied under more humid 
conditions. In addition, the application of geophysical methods is 
needed to determine the presence of dykes, faults, shears, fissures, 
and other structures, and the depth to solid or impervious rocks, 
and of aerial photography to assist in the determination of 
geological, typographical, hydrogeological and vegetational fea- 
tures. A study must also be made of the vegetation itself. 
Where highlands of higher rainfall rise out of the deserts, the 
runoff on the windward side often increases the deeper ground 
water flow even at a considerable distance from the foot of the 
mountains. 
Accumulated local geological experience becomes of special 
importance in these circumstances and sometimes leads to an 
almost intuitive appreciation of the possible presence of ground 
water. Experience shows that in any program of development of 
water supplies in arid regions it is most important, and far more 
economical, that water should be sought in places where it is 
most likely to occur rather than where it would be most con- 
venient, and drilling merely on a grid or interval basis is likely 
to be very wasteful of boreholes. In some territories, as in much 
of eastern Africa, this is already leading to an increasing difficulty 
in finding good supplies, in that the more promising sites are 
being taken up to an increasing extent. 
Well Yields 
The question of the maintenance of yield of boreholes in arid 
places is a very important one, for even when the pumping pro- 
ceeds at only 50 or 60% of the tested yield, the yields sometimes 
