

64 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



throughout the day, except in the case of the instrument on the north 

 slope at 8,000 feet, which it was impossible to place in such a way as 

 to avoid a slight amount of light shade in the mid-morning and in the 

 mid-afternoon. 



Readings were secured at the stations from 3,000 to 7,000 feet from 

 April 25 to 27 until September 5 to 6, and at 8,000 feet from June 7 

 until September 5. The actual amounts of the readings are given in 

 table 9, in terms of the average loss per day in cubic centimeters from 

 a standard cup. 



TABLE 9. The average daily evaporation (in cubic centimeters), for the periods indicated, on 

 north and south exposures, at 6 elevations in the Santa Catalina Mountains. 



In order to ascertain the altitudinal gradient of evaporation rate 

 the readings from the north and south slopes at each altitude have 

 been averaged. The averaged total evaporation of the summer for 

 each station has been subdivided, so as to show the amount for the 

 arid fore-summer as shown by the first three series of readings, and 

 for the humid mid-summer as shown by the last six series. The curves 

 in figure 12 show the altitudinal fall in evaporation rate during the 

 two seasons, in terms of the average daily loss from the atmometer. 

 These curves bring out in striking manner the low evaporation rates 

 of the humid mid-summer as contrasted with the arid fore-summer, 

 the latter being nearly twice as great as the former. There is a strong 

 parallelism between the two curves, but the one for the humid season 

 is slightly flatter than the one for the arid season. This means that, 

 so far as concerns the evaporation conditions alone, there is a less 

 differentiation between Desert and Forest in the summer rainy season 

 than there is in the arid portion of the summer. The pronounced drop 

 in evaporation from 7,000 to 8,000 feet is particularly significant, as 

 the former elevation marks the lower edge of the Forest, while the 

 latter is in the midst of the best stands of pine. It is possible that the 

 forest itself interferes with air movements near the ground in such a 

 way as to be responsible for the sharp fall in evaporation. 



In order to exhibit the seasonal march of evaporation rate at the 

 several altitudes the curves of figure 11 have been drawn. These 



