10 
BULLETIN 61, U. S. DEPARTMENT OF AGRICULTUBE. 
that the lowest part of a basin often receives less than 5 inches of annual rainfall and 
the surrounding mountains a much larger amount. The disposal of the rainfall is 
illustrated in the following summary: 
Distribution of rainfall. 
Region. 
Percent- Mean 
age of annual 
whole preeipita- 
area. tion. 
Mountain area 
Outwasfc area 
Silt and playa area 
Inches. 
Of the run-off from the mountain area given in the above table probably more than 
one-half is lost by seepage in the outwash area. This would leave only 20 per cent 
of the mountain rainfall as run-off. and of the 10 per cent run-off of the outwash area 
we might well say that all is lost by seepage. Twenty per cent of 13 inches is 2.6 
inches. This comes from one-half of the entire area and would be equivalent to 1.3 
inches over the whole area. A large part of the seepage water is brought to the sur- 
face by capillarity and lost by evaporation. It is admitted that the proportions esti- 
mated for evaporation and seepage in the above table are more or less arbitrary. 
Still, we may qualify some of these figures by comparison. The run-off factors for 
streams in the Sierra Nevada Mountains vary considerably. The average for the 
Kings. Merced. Tuolumne. Tule, Kern, Carson, Walker, and Truckee is 42.7 per 
cent. This would justify the 50 per cent run-off figure estimated. For the outwash 
slopes a percolation figure of 80 per cent is not unreasonable and for the silt areas 
100 per cent. 
From three lines of inquiry are obtained 0.71. 1.19. and 1.30 inches as the run-off 
for the basin region. The mean of these is 1.06 inches, or 10.3 per cent of the mean 
annual precipitation of the Great Basin. 
It should be noted, however, that the southern portion of the basin region is char- 
acterized by a scanty and irregular run-off, only a fraction of that indicated above, 
while the run-off for the area contiguous to the Sierra Xevada and Wasatch Moun- 
tains is. no doubt, much higher than the above. 
EVAPORATION. 
Practically all of the rainfall of the basin region is lost by evaporation. During 
periods of excessive precipitation there is undoubtedly an increase and during periods 
of aridity a decrease in the amount of ground water. Evaporation from the surface 
of lakes, from the surface of the ground, and the transpiration of plants are the three 
ways by which the water is taken back into the atmosphere. How important each 
of these factors is in the basin region is the subject of our inquiry. 
Many experiments to determine the amount of evaporation from surface waters 
have been made and variable results have been obtained. Some of these results, 
such as more particularly apply to this region, are given in the following table: 
Evaporation from icater surfaces. 
Locality. 
Conditions. 
Annual 
evapora- 
tion. 
Owens Vallev region, Cal. 1 
Evaporation from pan in water, 1909 and 1910 
Inches. 
/ 69. 05 
\ 65.44 
/ 69. 01 
\ 80.00 
60.00 
Owens Vallev region, Cal., Owens Lake. 
Bishop 
Fallon. Xev.2 
Lake Tahoe. Cal 
4-inch pan floatine in canal 
42.21 
Salton Sea. Cal 
106.45 
Pvramid Lake, Nev.s 
60.00 
Great Salt Lake * 
90.00 
i Bui. No. 294, Water-Supply Papers. 
1 American Civil Engineering Pocket Book. 
s Bui. No. 52, Nevada Exp. Sta. 
* C". S. Geol. Survey Report No. 11. 
