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

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A rather similar situation exists in many underdeveloped countries
now plagued by a lack of fresh surface waters. These countries
will have to rely increasingly on groundwater reservoirs
as a source of drinking water for human and animal consumption,
as well as for industrial or irrigation uses (7, 31).
Providing water in these arid areas requires drilling boreholes
or digging wells. It also means extensive use of strongly mineralized
water. Often, the only available water comes from underground
fresh water lenses floating over salt water.
As an example, consider the case of the Indus plain, sketched
in figure 1. The plain covers an area of 50 million acres out of
which 30 million acres are considered potentially cultivable (34).
W ater is diverted at the rate of 89 cm/year and unit area, from
the five rivers crossing the plain. O f the diverted water, 30 to
40 '% is lost by seepage from the canal distribution system.
Furthermore, the irrigated area is remarkably level which makes
it difficult to utilise surface drainage extensively for the return of
irrigation flows to the rivers. Additional water is thus allowed to
seep down to the water table. In some areas, this infiltration has
caused the water table to rise permanently to the root zone and
forced large agricultural areas to be abandonned. In other places,
the phreatic level is so close to the surface that the evaporation of
capillary water leaves a deposit of salt behind. This residual salt,
which is the cause of a tremendous setback in the agricultural
yield of the area, was removed from the deeper soil profiles (14).
Thus the development of irrigation has resulted in the widespread
salinization of the soils of Pakistan and the Indian
Punjab. The first record of salinization (11) dates as far back
as 1850. In that area, it is important to supplement insufficient
existing surface water supplies by recovering the fresh water
lost by seepage from the canal distribution system. This water
forms a fresh water lens floating over brackish water. The
study of the selective withdrawal of that water is the core of this
paper. Misguided exploitation of these lenses can only enhance
the salt-water encroachment.
It is then interesting to consider the problems involved in
the exploitation of these lenses against a background of quite
basic analytical models and theories, such as the origin of the
natural heavily mineralized groundwaters, the mathematical mo-