THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



69 



in evaporation. The differences in this factor, though not strikingly 

 great, are constant throughout the year, the Adenostoma chaparral 

 being the highest and the Arctostaphylos chaparral intermediate. 

 This conclusion is drawn from the values obtained at the top of the 

 vegetation. The rate at the surface of the ground does not show 

 differences of import to the problem in hand. 



It may be objected that the evaporation values used here are 

 affected to an unknown degree by the presence of abundant vegeta- 

 tion. In order to discover something of the actual comparative 



50 



ioo Meters 



JULY 

 7 16 23 29 



AUG. 

 12 19 26 



JULY 

 7 16 23 29 



AUG. 

 7 12 19 26 



Sta.I (S-facing) 

 Sta.lI(N-facing) 



Sta. IV (S- facing) 

 Sta.lII (N-facing) 



Fig. 18. — Evaporation during the dry season of 1915 on opposite 

 north- and south-facing bare slopes. Foothills of the 

 Santa Cruz Mountains, near Palo Alto. 



values upon opposing north and south slopes, uncomplicated by 

 reaction, a short study was made in the hills near Stanford Uni- 

 versity in the summer of 1915, an account of which is inserted here. 

 Evaporation was measured upon the contrasting north- and south- 

 facing slopes of two ravines, distant about 5 km. from Jasper Ridge. 

 The profiles of these are shown in figure 18. Stations 1 and 2 

 were 15 meters, and stations 3 and 4 were 18 and 15 meters, re- 

 spectively, above the valley floors. The slopes were bare of shrubs 

 and trees, the vegetation being largely wild oats, which, with the 

 other herbaceous plants, were dead and brown at the time of the 



