Summary Statement 
REED W. BAILEY 
Intermountain Forest and Range Experiment 
Station, United States Department of 
Agriculture, Ogden, Utah 
Not only have variability and predictability of water supply 
been discussed but also the broader subject of water supply in 
general. The major overall problem of arid lands is the need for 
more water. Lack of water has limited occupancy and develop- 
ment of arid lands since the beginning of history. It is important 
to understand the sources of water available to arid lands, the 
amounts, quality, variability, and predictability. 
Arid land economies must rely on three sources of water: pre- 
cipitation on the arid lands themselves, ground water, and stream 
flow from more humid areas. Each of these water supplies has 
distinct characteristics and presents different problems. 
Precipitation on Arid Lands 
Starting first with arid land precipitation, we can make several 
general observations. The precipitation on these areas is meager— 
usually much less than the evapotranspiration potential. The 
amount of precipitation varies greatly from year to year and from 
place to place in the same year. Droughts are common and some- 
times extend over several years at a time. Forage and crop pro- 
duction is highly uncertain. 
We are agreed, I think, that the major obstacle to greater and 
more efficient use of this limited and variable water supply is the 
lack of a reliable method of predicting the occurrence of favorable 
and unfavorable moisture conditions. Though much has been 
learned about the probable recurrence interval of wet and dry 
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