POTASH SALTS AND OTHER 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 ^ 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 



I Water-Supply Paper No. 294. 



