108 THE FUTURE OF ARID LANDS 
in regions where the water comes near enough to the soil surface 
to be evaporated from the soil or transpired by the vegetation. 
Evaporation, whether from the soil or by plants, results in soil 
salinity. 
In some regions, the ground water bodies lie below closed 
topographical basins whose bottoms are occupied by salt lakes, 
without surface water, for the better part of the year. These are 
the chotts and the sebkhas of the south Mediterranean regions. 
They are filled by very fine silt. Their flat surface is covered with 
salt. When soundings are made, water saturated with salt is 
found under the salty crust, but if deeper soundings are made, 
it is not uncommon to encounter layers of less salty water. The 
existence of underground circulations through the silt is proved 
in Chott Djerid by the existence of little springs and of natural 
chimneys whose appearance from the air is shown in Figure 4. 
A volume of water is also lost by defective catchments and 
springs more or less choked by sand. These waters feed phreatic 
aquifers, often salty, and join those evaporated by the halophile 
plants. 
It is a well-known fact that old defective wells, whose casings 
are corroded, must be sealed off. The sealing operation can gen- 
erally take place with more or less difficulty. But certain types of 
leakage are much more difficult to avoid. This is notably true 
when imprudent operations have caused the collapse of the roof 
of an artesian aquifer. 
The evaluation of all these water losses is quite difficult. It 1s, 
however, indispensable for a correct study of underground water 
resources development. 
Circulation Reserves. Underground formations sometimes oc- 
cur in desert regions, in great basins with huge reserves. The 
flow of the springs of the Djerid (Tunisia) is so constant that 
all the measurements taken for 50 years gave results whose 
differences are comparable to errors of measurement. The same 
statement has been made in the Mzab (Sahara) (4). Given what 
we know of the irregularity of the supply of these aquifers, that 
signifies that the free water table has a very considerable extent. 
Its position is unfortunately not well known on account of lack 
