Feb. 16, 1914 
Indicator Significance of Vegetation 
369 
the electrical resistance. Therefore, in the case of soils containing gypsum the elec¬ 
trical-resistance method may be considered to be more reliable than the excess-solvent 
method. The probable error of determinations by the electrical-resistance method 
is approximately 10 per cent of the actual salt content. 
CLIMATE OF TOOELE VALLEY 
Tooele Valley is dry, having a mean annual precipitation of 16 inches. 1 
The average monthly distribution of the precipitation at Tooele is shown 
in figure 2. No precipitation records are available for other parts of the 
valley, save fragmentary records at Grantsville for two years, which 
indicate that the western side of the valley receives decidedly less prer 
cipitation than the eastern slope. During the first nine months of 1912, 
the total precipitation recorded at Grantsville was 7.6 inches, as com¬ 
pared with 13 inches 
at Tooele. The con- 
dition of the native 
vegetation and of the 
crops grown without 
irrigation also indi¬ 
cates that the western 
side of the valley is 
much drier than the 
eastern side. 
In view of the im¬ 
portance of the soil- 
moisture conditions in 
explaining the distri¬ 
bution of the different 
types of vegetation in 
Tooele Valley, it is in¬ 
teresting to consider 
the precipitation of the period immediately prior to that during which 
the field work was carried on. The precipitation during the months 
from October to May, inclusive, probably furnishes all of the stored 
soil moisture available for the growth of plants during the following 
summer. The total precipitation at Tooele during the period from 
October, 1911, to May, 1912, was 13.5 inches, or 0.9 inch above the normal 
(12.6 inches) for the locality. Hence, it may be assumed that at least the 
normal quantity of moisture was present in the soil on the date when 
field operations were begun in the valley (May 28). As regards the 
season of active growth in 1912, the precipitation of the month of May 
was about 0.5 inch below the normal for Tooele, while that of June was 
very nearly twice the normal. For the remaining summer months the 
precipitation was about normal. 
-Monthly distribution of precipitation at Tooele, Utah (mean 
for is years). 
1 Based upon 15 years’ measurements at the town of Tooele. From data furnished by the U. S. Weather 
Bureau, through the courtesy of Mr. A. H. Thiessen, Section Director. 
