106 THE FUTURE OF ARID LANDS 
Legend 
¢ Observed depth 
7 Observed movement _ 
--~ Supposed movement 
Depth of water table in meters 
1945 71946" 1947" 1948 ~911949)) 1950) 19510 = 1952S SSeoas 
Figure 3. Movement of the water table, Larue Well. 
aquifers by stream floods. These can occur several times per 
year. It is clear that saturation and percolation can frequently 
occur locally along stream beds and on the flooded adjoining 
areas. 
In Figure 3, we realize that current annual supplies are not 
sufficient to offset pumping in the dry season. There must be 
added to these the supply of exceptional sort which coincides 
with an intense runoff. The efficiency of this supply process is 
certain (22). The computation of the safe yield of one aquifer 
cannot therefore be established without observations covering 
long periods. 
It is possible to obtain an idea quite rapidly of the relative 
participation of these two processes of supply in semi-arid regions, 
but for the great desert artesian basins one is reduced to mere 
suppositions. 
The methods that can be considered to increase the recharge 
of aquifers differ according to the process involved. To obtain 
scientific bases it would be necessary to follow over many years 
the fluctuations in water levels by sinking piezometric tubes in 
the zones of recharge, and mainly in the zones of privileged supply 
which are neighboring to water courses and their alluvial fans. 
But the increase of recharge can be faced in a more practical 
way by utilization of runoff waters. Two processes have been 
employed: the first consists in stopping the water in infiltration 
basins, which leads to taking special caution for waters contain- 
ing sediment to avoid clogging; the second consists in slowing 
down the runoff in such a way as to increase the time spent in 
passing over the recharge areas. 
