BENEFICIAL USE OF WATER IN ARID LANDS lpe7, 
Our studies developed the following facts with regard to the 
four questions we asked: 
What Is the Average Annual Precipitation? 
The 18-inch-maximum zones comprise some 335,600 square 
miles or 80% of the total surface area of the four states. The 
average annual precipitation over this zone is about 13 inches. 
This represents some 225,800,000 acre-feet of water and is the 
total water supply or precipitation occurring over the arid zone of 
these states. 
What Part of Original Supply Is Available for Irrigation, Domes- 
tic, and Other Uses? 
Map measurements of the various water yield zones indicate 
that some 5,200,000 acre-feet of water subsequently becomes 
available for other than natural watershed uses. Conversely, about 
220,600,000 acre-feet, or 98%, of the water that falls in the form 
of precipitation is either consumed where it falls or is lost in 
transit to points of downstream use. In checking other informa- 
tion and data concerning water yields of the various states, it 
appears that the yields shown on the maps are, if anything, 
generous. Therefore, the estimates of water available from arid 
zones for downstream use are probably high. 
What Watershed or Climatic Factors Are Predominant in Pro- 
duction of Water Yields2 
During the course of our studies, notes were made of the physi- 
ographic, vegetational, and other watershed characteristics that 
might influence the relation between precipitation, the runoff in 
surface streams, and ground water accretions. Extensive tables 
were prepared showing the soils, vegetation, topography, geologic 
conditions, and numerous other factors for each watershed that 
had been gaged. An analysis of these data was then made to 
attempt to delineate the factors or combinations of factors that 
profoundly influence water supplies. In certain instances the 
presence or absence of deep soils with high water holding capaci- 
ties was found to exert a strong influence on water yields. Other 
watershed characteristics, such as topography (which may or may 
