WATER RESOURCES IN ARID REGIONS 105 
deserts, the hydrology of underground waters is rather a deep 
hydrology. 
Whatever their nature, the study of underground water can 
begin only by ropes cal and geological recognition of water- 
bearing formations, by the survey of springs, their depth (level), 
flow, and all the hydrological indications on the soil surface. 
Among these it is particularly useful to observe the deposits left 
by evaporation of subterranean waters. These deposits are linked 
with the circulation whose conditions could have been modified in 
more or less recent times. 
Geological, geophysical, and hydrological prospecting does 
not dispense with boring. At present the system most employed 
is drilling with rotary equipment, but it consumes a great amount 
of water, and this increases costs. 
For deep drillings with a rotary drill, it is dificult to identify 
and localize the different layers encountered. Systematic use of 
electric logging permits the determination of precise correlations 
between the formations encountered in several borings. It is 
hoped that this type of process will be more and more improved. 
Water Balance 
Recharge. Everything that has been said about the irregular 
and discontinuous supply of runoff is true for the supply of under- 
ground waters. The phenomena of saturation of alimentary out- 
crops over a vast extent are particularly rare. However, we have 
seen that they could take place, at variable intervals, according 
to the aridity of the region under consideration, for instance at 
intervals of the order of a century. 
In semi-arid zones, the discontinuity of the supply is already 
very obvious, as shown in Figure 3, which gives the fluctuations 
of a well of the Menzel Bou Zelfa aquifer, where average rainfall 
IS 450 mm per year. 
This graph allows the identification of two very clear cases of 
recharge. 
In desert regions, rarer still are rains capable of saturating the 
soil. There is another supply process which can take place at 
much more frequent intervals. It is the supplying of underground 
