Perhaps the greatest
objection to the use of the reservoirs is not their relatively high
cost, but the fact that since they are usually small and the water
shallow, too large a proportion of the water, even under favorable
conditions, is lost by evaporation. It is ordinarily assumed that
one half of the water stored in small reservoirs throughout the year
is lost by direct evaporation.
Available subterranean water
Where surface waters are not readily available, the subterranean
water is of first importance. It is generally known that, underlying
the earth's surface at various depths, there is a large quantity of
free water. Those living in humid climates often overestimate the
amount of water so held in the earth's crust, and it is probably
true that those living in arid regions underestimate the quantity of
water so found. The fact of the matter seems to be that free water
is found everywhere under the earth's surface. Those familiar with
the arid West have frequently been surprised by the frequency with
which water has been found at comparatively shallow depths in the
most desert locations.
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