POTASH SALTS AND OTHEE SALINES IN THE GREAT BASIN REGION. 11 
Evaporation from water surfaces varies with the seasons. It is greatest in the 
summer and fall months. In the Owens River region 73 per cent of the annual evapo- 
ration takes place in the six summer and fall months, and the remaining 27 per cent 
in the winter and spring months. We have not sufficient data to strike an average 
for the whole basin region, but it is believed that an annual evaporation of 60 inches 
would fairly represent that which takes place from the surface of the lakes in the 
basin region. 
From an intensive study made of conditions in the Owens River basin a the follow- 
ing figures for the evaporation of water from ground surfaces are taken. The annual 
ground-surface evaporation depends largely upon the depth of ground water. Where 
the ground water exceeds 10 feet in depth practically no water is lost from the surface. 
Where ground water and ground surface coincide the maximum of 42.3 inches per 
year is found; with ground water at a depth of 1.34 feet from the surface 39.95 inches 
is found; and with ground water 4.98 feet below the surface 7.9 inches is found. Of 
the annual evaporation the summer and fall months account for 79 per cent of the 
total. 
Observations in the same locality established the fact that even in a wet season 
percolating water does not penetrate to depths exceeding 2\ feet unless more than 1 
inch falls within a short period on moist soil. Even then it does not appear to reach 
depths greater than 4 feet. We would conclude from these observations that on 
detrital fills, on levels, or on low slopes, much of the rainfall is retained close to the 
surface and seldom penetrates to depths reaching 10 feet. It can not, therefore, form 
any permanent addition to the ground water, but must be lost by capillarity and 
evaporation. On steeper slopes the water penetrates slowly downward and in the 
lower portions of such slopes may be expected to accumulate sufficiently to reach 
the 10-foot level, and thus a part escapes loss by joining the permanent ground water. 
Streams debouching upon outwash slopes raise a ridge in the ground-water level and 
contribute a part of their seepage loss to permanent ground water. We would also 
conclude that ground water 10 -feet or more from the surface would be permanent, 
and that ground water reaching the 10-foot level or less would be reduced in amount 
by evaporation. It should be noted that this limit of 10 feet can not be applied to all 
conditions, for in very fine silts capillarity would no doubt extend to a greater depth 
than 10 feet. It does, however, establish a limit under what we might term average 
conditions within which capillarity becomes effective. We would expect in all 
regions of the basin where ground water reached within the 10-foot level that a slow 
upward movement of moisture would follow. In this manner soluble salts would be 
brought to the surface or close to it and would appear as incrustations or be deposited 
within the surface soil. We should be safe in concluding that where surface incrusta- 
tions are found in quantity ground-water levels are apt to be within the 10-foot limit. 
This is, of course, not an entirely accurate criterion, for surface waters may penetrate 
to depths of several feet and be returned by capillarity, carrying with them dissolved 
salts to the surface, where they would crystallize and form efflorescences. 
Still another fact should not escape our attention. If we assume a soil void space 
of 25 per cent of volume, a depth of 30 inches of water would be necessary in order to 
saturate the soil to a depth of 10 feet. In a loose coarse soil but a small fraction of 
these 30 inches would be required for water to penetrate and reach to a depth of 10 
feet. In a mixed soil with much fine silt and clay probably a large proportion of this 
would be retained, if it penetrated at all, in the upper 10 feet. The low average rain- 
fall of the desert region, together with the observed facts concerning the penetrations 
of soil by rainfall in the Owens region and the fact noted above, would lead us to con- 
clude that were it not for the concentration of part of the rainfall into stream flows 
the ground water of the basin region would be a negligible quantity and would be 
present only in those places where subterranean supplies could act as feeders or in 
places occupying the lowest depressions of the surface. While these conclusions may 
be accurate for present climatic conditions it must be kept in mind that the basin 
region has been subjected to many climatic changes. Humid periods have alternated 
with arid. We may be not greatly in error when we say that the underground water 
supply of the basin region is perhaps consequent upon the greater rainfall of the 
Quaternary period and not upon present climatic conditions. 
The surface of the Great Basin is covered with sparse vegetation. One is apt to get 
the idea from reading maps that vegetation is extremely scarce in the basin region, 
but we find some kind of vegetation over the whole area, with the exception of the 
playas and areas occupied by alkali incrustations. Many of the mountain ranges of 
the basin are thickly covered with grass, and sagebrush dominates over vast areas of 
plain and mountain slope. We have no accurate determinations of the transpiration 
loss of desert plants. Such work as has been done on this question has concerned 
1 Water-Supply Paper No. 294. 
