(1973) n°3 - Royal Academy for Overseas Sciences

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flows recently infiltrated fresh water. This is a common observation
in many arid areas.
K u n i n (15) defined two types of freshwater lenses in the
deserts by their condition of replenishment. The first type is
associated with sand areas without surface drainage and is
characterized by the presence of a dissicated layer in the zone
of aeration. Large lenses of that type, containing water volumes
measured in cubic kilometers, are known to exist in the Russian
deserts and in North Africa. It appears that microdistillation
plays a considerable role in the processes of recharge and
'migration of these waters and moisture, such as was suggested
for the case of the Grand Erg area in the Sahara desert (5).
The second type of replenishment is by surface water. It is
associated with at least minor drainage systems and a certain
amount of rain, or with ephemeral water courses of local nature.
An interesting case is given in the Kalahari desert, sketched
in figure 6, by the Swakop and Kuiseb rivers which rise
in the semi-arid interior and flow westward through country
which becomes increasingly arid, until desert conditions are
reached at Walvis Bay on the Atlantic Coast. Both rivers are
characterized by the marked degree to which flood waters are
lost by seepage through the sandy river channels, so that appreciable
discharge at the outlet occurs only in exceptional