BENEFICIAL USE OF WATER IN ARID LANDS 159 
tation during those times of year when precipitation may be high 
and evapotranspiration low, or vice versa, the situation in the 
arid zone would undergo no major change were this factor con- 
sidered. Recognizing this normal preponderance of evapotrans- 
piration potentials over precipitation, it is self-evident that any 
precipitated water that does not leave an area through surface 
runoff or through infiltration to ground water will ultimately be 
consumed. 
This fact brings to light a popular misconception concerning the 
importance of the relative evapotranspiration potentials of various 
types of vegetation in the yield of water from arid zones. It has 
been commonly believed that if one plant uses more water than 
another then there will be less water available for downstream 
use. It is not usually recognized, however, that whenever evapo- 
transpiration potentials exceed the supply, or precipitation, it makes 
no difference what type of plant occupies an area, the water will be 
removed. Such being the case, the replacement of non-beneficial 
plants with forage producing plants may be accomplished with 
little or no loss of water. 
Another result of the normally high evapotranspiration poten- 
tials in arid zones is the tremendous amount of water evaporated 
or transpired before reaching a point of downstream use. In the 
San Simon Valley of Arizona, for example, only some 20% of the 
indicated water yield from small experimental watersheds ever 
reaches the mouth of the valley; the remainder, for the most part, 
is consumed by evaporation or transpiration. 
What Might Be Done to Reduce Losses of Water to Non-benefi- 
cial Use? 
At this point it might be well to review several facts brought 
out in the previous discussion. (1) There are some 220,600,000 
acre-feet of water, or about 98% of the total precipitation, being 
consumed naturally on our arid watersheds before reaching a 
point of downstream use. (2) In the arid portions of our four-state 
area, evapotranspiration potentials are usually sufficient to con- 
sume any water not flowing out of a watershed. This is without 
regard to the particular type or amount of vegetation that may be 
present on the area. (3) Tremendous water losses occur through 
