741 
May 24,1924 Significance of the Southwestern Desert Vegetation 
In order to appreciate more fully the moisture conditions the reader should 
consult figure 8 which gives the daily rainfall in the Gila Valley during the 
years 1914 and 1915. The distribution of rainfall is much more uniform through¬ 
out this season than in Coachella Valley. A comparison on a monthly basis is 
shown in figure 9. A comparison of figures 7 and 8 with figure 9 in which the 
mean monthly record is shown will illustrate the great uncertainty as to 
quantity and distribution in these desert areas. 
Salinity 
Wherever there is a good growth of the creosote bush the salt content of the 
soil is very low. Even a poor growth indicates a stony infertile or shallow rather 
Fig. 8 —The daily rainfall in the Gila Valley, Ariz., during 1914 and 1915 illustrating not only the character 
of the rains but the long drought periods during the years when soil moisture studies were made. 
than a saline soil (Table VIII). Individual plants of the creosote bush are 
sometimes found on soil containing considerable salt, but a good uniform growth 
has never been found on such soil. Those plants growing on saline soil are 
stunted and quite unlike the normal growth of the creosote bush. Comparing 
the creosote bush land of the two valleys (Tables VI and VII), the salt content 
is found to be greater in the Gila Valley. The quantity, while two or three 
times as great as that found in creosote bush land in Coachella Valley, is, never¬ 
theless, small and, as far as farm crops are concerned, negligible. 
