WATER RESOURCES IN ARID REGIONS 109 
Figure 4. Aerial view of Chott Djerid. 
of sufficient explorations and because geological outcrops are 
often hidden under aeolian formations. 
Because of the importance of the reserves, a long period of 
time is necessary to appreciate the effects produced by new water 
developments and to determine the moment past which the 
safe yield of the aquifer will be exceeded. 
We saw that for the rainfall and the runoff, it is better to make 
numerous observations than very precise ones because of the 
extreme irregularity of these phenomena. For the study of water 
levels and the flow of aquifers, it is necessary, on the contrary, 
