10 



BULLETIN 61, U. S. DEPARTMENT OF AGEICULTUEE. 



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. 





Percent- 

 age of 

 whole 

 area. 



Mean 

 annual 

 precipita- 

 tion. 



Percentage. 



Region. 



Evapora- 

 tion. 



Seepage. 



Run-off. 



Mountain area 



50 

 20 

 30 



Inches. 

 13 

 10 



5 



50 

 50 

 100 



10 

 40 



40 



Outwasb area 



10 



Silt and playa area 











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 figares 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 Nevada 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 water surfaces. 



Locality. 



Owens Valley region, Cal.' 



Owens Valley region, Cal., Owens Lake. 



Bishop 



Fallon, Nev.s 



Lake Tahoe, Cal 



Salton Sea, Cal 



Pyramid Lake, Nev.' 



Great Salt Lake < 



Conditions. 



Evaporation from pan in water, 1909 and 1910. 



Deep tank in soil, 1910 



Pan in irrigation ditch 



4-inch pan float ing in canal 



4-inch pan 2 inches above water 



4-lneh pan 7,500 foet from sea 



Estimated from mean flow of Truckee. 



Annual 

 evapora- 

 tion. 



Inches. 



69.05 

 65.44 

 69.01 

 80.00 

 fiO.OO 

 53. 65 

 42.21 

 106. 4S 

 60.00 

 80.00 



» Bui. No. 294, Water-Supply Papers. 



' American Civil Engineering Pocket Book. 



» Bui. No. 52, Nevada Exp. Sta. 

 * V. S. Geol. Survey Report No. 11. 



