May 24, 1924 
Significance of the Southwestern Desert Vegetation 
795 
between 300 and 500 gm. calories per sq. cm. per minute, while at Akron it is 
between 600 and 650. The evaporation from a 6-foot tank of water at Bard for 
January to March, inclusive, for the nine-year period, 1911-1919, averaged 12 
inches, while at Akron for May to July, inclusive, for the thirteen-year period, 
1908-1920, averaged 24 inches. A comparison of the water requirement of 
small grains at Akron and at Bard shows the requirements at Akron to be about 
25 per cent higher than at Bard. 
It is therefore evident that, in the Colorado desert, during the most active 
growing period, the conditions favoring a rapid transpiration are not so extreme 
as they are during the most active growing period in the central high plains. 
CORRELATION BETWEEN THE TYPES OF VEGETATION AND THE 
CHARACTER AND PRODUCTIVITY OF THE LAND 
CORRELATION WITH PHYSICAL CONDITIONS 
While each of the valleys of the Southwestern desert region wherein studies 
were made differ somewhat in vegetation, the most important types are common 
to all of them. These types indicate fairly definite soil conditions. There is, 
however, considerable variation and the correct interpretation of the types of 
vegetation is often difficult. The wide variation in soil conditions where some 
of the types occur makes it necessary to know more than merely the fact that a 
certain type of vegetation is present on a piece of land. For instance, creosote 
bush throughout the southwestern desert region indicates well-drained, nonsaline 
land with a low water table. Whether the soil is of good depth or shallow is 
understood much better if it is known whether the growth of the creosote bush 
is tall and vigorous or scrubby and poor. Likewise, if the creosote bush is mixed 
with the bur-sage or if bur-sage occurs near by, the soil is likely to be looser, 
stonier and less fertile. If the creosote bush is mixed with desert-sage or occurs 
near the desert-sage area the soil conditions indicated are a finer, more fertile 
soil not so permeable and possibly with a trace of salinity in the subsoil. Creo¬ 
sote bush in regions of higher rainfall as in eastern Arizona is often characteristic 
of land with caliche near the surface. 
The physical condition of the soil characterized by each type of vegetation is 
summarized in Tables XXXI and XXXII. The results of study of these tables 
are given in Table XXXIII which summarizes the conditions under each type 
of vegetation. In Table XXXIV a summary is given of salinity of the soils at 
different depths under the different types of vegetation and the different stations. 
The results of a study of these tables were included in the following general 
summary of the conditions under each type. 
CREOSOTE BUSH (COVILLEA GLUTINOSA (ENGELM.) RYDB.) 
While creosote bush land is usually porous, nonsaline and high above the water 
table, it is not always good farming land. That which is covered by a good 
growth of creosote bush, about 5 feet or higher, is good farming land having a 
good depth of porous nonsaline and well-drained soil. That which is covered 
with a poor, scrubby growth, usually less than 4 feet high, is land either too 
stony or else too shallow, due to underlying hardpan or rock layers, to be farmed. 
At the present time much of the creosote bush land is uncultivated, for, as a 
rule, it lies above the highest canals that supply the irrigation water. These 
lands, composing a» they do the warmest slopes of the valleys, are probably best 
adapted to fruit culture. 
