Report on the Lands of the Arid Region of the United States, with a More Detailed Account of the Lands of Utah. With Maps. Page: 69
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WATER SUPPLY. 69
is part of an air current moving from west to east. The basin having no
outlet, the precipitation of rain and snow within its limits must be counterbalanced
by the evaporation. The air current must on the average absorb
the same quantity of moisture that it discharges. Part of the absorption is
from land surfaces and part from water, the latter being the more rapid.
If, now, the equilibrium be disturbed by an augmented humidity of
the inflowing air, two results ensue. On the one hand the precipitation is
increased, and on the other, the absorbent power of the air being less, the
rate of evaporation is diminished. In so dry a climate the precipitation is
increased in greater ratio than the humidity, and the rate of evaporation is
diminished in less ratio; while of the increased precipitation an increased
percentage gathers in streams and finds its way to the lake. That reservoir,
having its inflow augmented and its rate of evaporation decreased, gains in
volume and grows in breadth until the evaporation from the added expanse
is sufficient to restore the equilibrium. Giving attention to the fact that the
lake receives a greater percentage of the total downfall than before, and
to the fact that its rate of evaporation is at the same time diminished it is
evident that the resultant augmentation of the lake surface is more than
proportional to the augmentation of the precipitation.
We are therefore warranted in assuming that an increase of humidity
sufficient to account for the observed increase of 17 per cent. in the size of
the lake would modify the rainfall by less than 17 per cent. 'The actual
change of rainfall cannot be estimated with any degree of precision, but
from a review of such data as are at my command I -am led to the opinion
that an allowance of 10 per cent. would be as likely to exceed as to fall
short, while an allowance, of 7 per cent. would be at the verge of possibility.
The rainfall of some other portions of the continent has been recorded
with such a degree of thoroughness and for such a, period that a term of
comparison is afforded. In his discussion of the precipitation of the United
States, Mr. Schott has grouped the stations by climatic districts, and
deduced the annual means for the several districts. Making use of his
table on page 154 (Smithsonian Contributions, No. 222), and restricting
my attention to the results derived from five or more stations, I select the
following extreme cases of variation between ihe mean annual rainfalls of
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Powell, John Wesley, 1834-1902. Report on the Lands of the Arid Region of the United States, with a More Detailed Account of the Lands of Utah. With Maps., book, 1879; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc125/m1/87/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.