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 



Fig. 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 



of 1911. 

 Fig. 14. — Graphs showing altitudinal decrease in rate of evaporation in the Santa Catalinas on 



south-facing slopes (heavy line) and on north-facing slopes (light hne) in humid mid-summer 



of 1911. 



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.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 



