GROUND-WATER INVESTIGATIONS—-SAYRE 221 
areas would prevent excessive lowering of the ground-water levels. 
Artificial recharge of ground-water reservoirs by spreading flood- 
waters and by other means has been successful in several areas. 
Abatement of pollution in streams and ground-water reservoirs, 
retention of floodwaters in reservoirs for later use, control of reservoir 
stages by forecasting normal and flood flows of streams, control of 
silt and sedimentation, and other measures are being carried out to 
increase the supply available for perennial beneficial use. The 
continued growth and prosperity of the Nation will depend to a large 
degree upon the success with which these problems are attacked and 
solved. 
Unlike most mineral resources, water is not exhaustible, in the strict 
sense, because it is replenished from time to time by precipitation. A 
surface reservoir may be dangerously low and be refilled in the nick 
of time by heavy rains. Heavy pumping may cause ground-water 
levels to decline progressively until pumping is no longer economically 
feasible, but when pumping is temporarily or permanently reduced 
the water levels usually recover. Likewise, a period of heavy rainfall 
following a drought period may induce recharge sufficient to restore 
water levels essentially to predrought levels. 
Only a part of the water taken from streams or pumped from wells 
is actually consumed. In many manufacturing processes water is 
merely a washing agent and is essentially unreduced in volume by 
its use. Water used in boilers or in quenching hot metal is partly 
evaporated, but the remainder is discharged or re-used. Water used 
in irrigation is partly evaporated and partly transpired by plants, but 
there is always an excess which is discharged and which carries away 
undesirable salts. The excess water from all these uses returns to 
the stream or to the ground altered by the concentration of minerals 
contained in it, or by the addition of dissolved constituents, or of 
color, or sediment, or simply by the addition of heat. It may be 
re-used for the same or other purposes with or without the addition 
of new water. For example, the water of the Pecos River, in Texas 
and New Mexico, is used and re-used for irrigation and domestic 
supply seven or eight times between its source and Girvin, Tex. Al- 
though water is added from tributary areas along its course, each 
time the water from the river is used the mineral concentration 
increases, and a few miles above Girvin it is so highly mineralized 
that even the most resistant crops are unable to survive its application. 
Even so, it may still have potential use, because water also possesses 
the energy of position and in its journey from the mountains to the 
sea it may be used many times over for generating hydroelectric 
power. 
Another characteristic peculiar to water results largely from the 
vagaries of precipitation. Many places are faced successively with 
