SUMMARY STATEMENT 173 
years over a period of time, we have yet to learn how to predict 
conditions for specific years and seasons. Solution of this problem 
calls for further knowledge about the causes of climatic fluctua- 
tions. More research on the physics of the air, and especially on 
air-mass movements, is needed if we are to obtain that knowledge. 
What runoff there is from arid lands occurs mainly as overland 
flow during brief but torrential rains. These storm flows com- 
monly are of short duration. They sometimes develop into violent 
floods and almost always carry great quantities of sediment. It 
has been pointed out that the water in these discharges is therefore 
of limited usefulness unless it can be impounded and desilted. 
Furthermore, though the arid lands yield very little runoff, they 
are a major source of damaging sediment. I should like to under- 
line this point with some figures of my own. About 85% of the 
lands in the Upper Colorado River drainage basin are arid. They 
contribute less than 10% of the water to the flow of the river, but 
they are the source of more than 85 % of the sediment that is being 
deposited in Lake Mead behind Hoover Dam. 
We may conclude from what has been said that the prospects 
are not very promising for increasing usable amounts of runoff 
from our arid lands. The main problems are to get better control 
of the runoff, reduce erosion and sedimentation rates, and to con- 
serve the meager supplies that are available. 
Ground Water 
The second source of water in arid regions is ground water. 
Springs and wells have been one of the more important sources of 
water supplies in arid regions down through the ages. They have 
been the principal and the most reliable source of water for domes- 
tic needs, for irrigation, and for stock-watering purposes. The big 
question seems to be what are the possibilities of getting more 
sustained and usable yield from ground water sources? 
It seems to be agreed that virtually all the readily accessible 
ground water supplies have already been located and put to use. 
New supplies must therefore come from new discoveries. The con- 
sensus is also that we are not likely greatly to increase the water 
supply by new discoveries of underground sources. I might go 
