50 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



it fell to 11.7 per cent. Neither does the percentage of summer rain 

 fluctuate in relation to the occurrence of very wet or very dry years. 

 In 8 of the wettest years since 1876 (14 inches or above) the summer 

 rain was 53.1 per cent of the total, and in 11 of the driest years (9 inches 

 or less) the summer yielded 47.6 per cent of the total (see table 2). 



The impossibility of securing figures for the winter precipitation in 

 the Santa Catalina Mountains makes it necessary to estimate the 

 annual totals of rainfall at different altitudes from the known figures 



Fig. 3. — Graphs showing percentage of winter rainfall to annual total 

 (light line) , and of summer rainfall to annual total (heavy line) , for 

 a chain of 13 stations from the Pacific to the Rio Grande. 



for the sununer rain. The average rainfall at the stations at 7,600 feet 

 and 8,000 feet for the years 1907 to 1914 is 17.45 inches (443 mm.), 

 from which it may be assumed that the annual average is approximately 

 35 inches (889 mm.). The summer rain at Tucson during 1907 to 1914 

 was 54.7 per cent of the annual total. If the seasonal distribution of 

 rain is the same on the mountain that it is at Tucson, the above esti- 

 mate of the annual total for the mountain is correct within 1 or 2 inches. 

 The influence of altitude on the seasonal distribution of rainfall in 

 Arizona is a matter which can not be determined without further data 



