786 
Journal of Agricultural Research 
Vol. XXVIII, No. 8 
Those stations showing a rainfall between 3^ and 5 inches lie, for the most 
part in the Colorado desert, although two stations, Barstow and Mohave, lie 
in the portion known as the “Mohave desert,” Greenland Ranch in Death Valley 
and Keeler in the Owens Valley. These are subject to a drought lasting at least 
through May and June. At a number of the stations a drought approximately 
six months long occurs during winter. Most of these stations, especially those 
in Arizona, show a tendency toward a rainy period during July and August; 
in other words, the summer rainy period so characteristic of the southern shrub 
desert, especially of New Mexico and Arizona, carries this influence beyond 
the Colorado river. 
The stations showing a rainfall of between 5 and 9 inches are scattered through¬ 
out the area but usually occur at higher elevations than those of lower rainfall. 
In fact, there is rather strict correlation between elevation and rainfall in these 
stations. The vegetation shown in the region ranging from 2 to 9 inches is not 
markedly different in character but shows a correlation on the basis of amount or 
size of growth. The poorest growth, if washes and other locations which receive 
quantities of drainage water are eliminated, is found in the ranges which have a 
relatively low rainfall. At the stations showing from 5 to 9 inches of rainfall, 
the vegetation becomes much denser and more luxuriant, although the species 
may be the same as those showing a 2-inch rainfall. 
Stations showing a rainfall of between 9 and 15 inches are quite often char¬ 
acterized by a grass cover in addition to a scattered growth of creosote bush 
or mesquite, though Cabazon with a rainfall of 10i inches has a vegetation of 
chaparral, and Tucson and Florence with a rainfall of 12 inches have a particu¬ 
larly luxuriant type of desert shrub. Grasses are a prominent feature of the 
vegetation at practically all the other stations with a rainfall of 9 to 15 inches. 
In general, then, we may say that the rainfall throughout the area character¬ 
ized as southern desert varies from 2 to 15 inches and that where the rainfall is 
high, that is, approaching 15 inches, the vegetation is composed of large and 
luxuriant desert plants, or is characterized principally by grasses, but where the 
rainfall is less than 6 inches the vegetation is sparse and the plants are small. 
If we consider mean temperature for the 40 stations (fig. 15) we find a corre¬ 
lation, in a general way, in that places receiving a low rainfall have, likewise, 
a high temperature. 
The warmer stations are situated in the Colorado desert, the highest mean 
temperature, 77 degrees, being at Salton, Calif. Of the stations having a mean 
temperature above 70 degrees all are characterized by creosote bush, desert- 
sage, or salt desert vegetation. The other stations, which range from 60 degrees 
to 70 degrees in mean temperature are also characterized by a southern desert 
vegetation and, where moisture conditions are favorable, grasses often become 
a prominent feature. This is true at Congress, Sierra Blanca and, to some ex¬ 
tent, at Carlsbad, Douglas, and Lordsburg. At Cabazon the mean annual 
temperature is 65 degrees but the high rainfall has favored the development of 
chaparral. The few remaining stations have a temperature ranging from 56 to 
60 degrees. Willcox and Deming are characterized largely by grassland; St. 
George, Las Vegas, Socorro, and Lone Pine are stations characterized by both 
the northern and southern desert types of vegetation. 
If we now consider the termperature maxima (fig. 16) we find that those stations 
which have an average maximum temperature of 85 degrees or above are limited 
largely to the Colorado desert. Of these all but three stations have absolute 
maxima of 120 degrees or above and four above 125 degrees. The monthly mean 
maximum is above 100 degrees for the four months, June, July, August, and 
September at nine of the stations considered. The stations having annual mean 
maximum temperatures between 80 degrees and 85 degrees either lie near the 
