66 VEGETATION OF A DESERT MOUNTAIN RANGE. 
taken in the heart of the Forest stand apart in three loosely defined 
groups in close parallelism to the zonation of the vegetation itself. 
During the arid fore-summer the evaporation at 5,000 feet is similar 
to that at 4,000 feet, while during the humid mid-summer it is more 
nearly like that of the 6,000-foot station. In other words, the advent 
of the rains causes the evaporation conditions of the Upper Encinal 
and lower Forest region to extend downward into the Lower Encinal. 
The significance of slope exposure in determining evaporation rate 
is indicated in figures 13 and 14. In these graphs the vertical gradients 
of evaporation at the six elevations are shown separately for the instru- 
ments on the south slopes and the north slopes at each station. The 
gradients for the arid fore-summer and for the humid mid-summer 
are shown, as well as the curves for the entire summer. In the arid 
season there is even a slightly greater evaporation on north slopes at 
4,000 and 5,000 feet than there is on south slopes, but this condition 
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Fic. 13.—Graphs showing altitudinal decrease in rate of evaporation in the Santa Catalinas on 
south-facing slopes (heavy line) and on north-facing slopes (light line) in arid fore-summer 
Fie. Ge ect showing altitudinal decrease in rate of evaporation in the Santa Catalinas on 
ae slopes (heavy line) and on north-facing slopes (light line) in humid mid-summer 
is reversed at the higher elevations. In the humid season there is also 
a slightly greater rate of evaporation on the north slope at 4,000 feet, 
while all of the higher stations show an almost uniformly greater rate 
on the south slopes. It is impossible to explain the cases in which the 
evaporation was greater on north slopes than on south ones. It is 
possible, of course, that they require no explanation but are typical of 
the extremely arid conditions of the lowest elevations at the driest 
time of the year. They are at least accordant with the fact that the 
soil moisture is sometimes greater on the south slopes. 
The summer averages show a difference of from 5 to 10 ¢.c. per day 
between the evaporation on opposed slopes, in readings of 35 to 45 c.c. 
per day or less. As the actual amounts of evaporation fall with increas- 
ing altitude, the difference between the opposed slopes becomes pro- 
portionately greater. 
As in the case of all climatological data, it would be impossible to 
state the normal conditions of evaporation at the various altitudes 
