174 THE FUTURE OF ARID LANDS 
one step farther and say that in some localities underground water 
is being used at a much more rapid rate than it is being replen- 
ished. 
We now have at our disposal highly efficient sounding devices 
for locating ground water supplies. We also have drills that can 
tap those sources and pumps to extract the water. However, there 
are three obstacles standing in the way of utilizing new water 
supplies: (1) a lack of information about ground water origin; 
(2) inadequate knowledge of ground water recharge rates; (3) lack 
of knowledge about the usability of impure water and economical 
ways of making it more usable. Research in these fields is already 
under way, but there is need for more. 
Water from Humid Areas 
A third source of water for arid lands is from streams that origi- 
nate elsewhere. Like most of the ground water of great conse- 
quence, this inflow is from more humid areas. The origins of the 
streams in some places are humid islands within the arid zone and 
in others they are situated at great distances. 
The streams that flow from these humid areas in many arid 
areas are the principal source of water for irrigation, power, and 
manufacturing. The Rio Grande Valley is truly arid, and the 
scale of occupancy and development as we find it today would not 
have been possible except for the water—both ground and surface 
—that comes to it from the humid mountains in the headwaters 
of the Rio Grande. The highly productive Imperial Valley in 
Southern California would still be a desert if it were not for the 
availability of water from the Colorado River. 
There has been a great expansion in the development of moun- 
tain-born waters for use in arid zones in recent years. Great 
storage dams, diversion works, and canals are being constructed 
on most of the great rivers of the world and still more are planned. 
Therein lies the big opportunity of more water for developing the 
arid regions. 
Several ideas presented in the discussion are worth repeating. 
This outside or inflow water supply has some important limitations. 
It is subject to annual and seasonal fluctuations. Periods of flow 
