1910] ROBBINS—VEGETATION IN COLORADO 261 
greater amounts. The higher mountainous parts receive over 20 
inches annually. North, Middle, and San Luis parks are very dry, 
the annual precipitation being less than 10 inches; South Park is 
drier than localities at the same altitude on the eastern slope. On the 
western slope there is a gradual decrease westward in the annual 
rainfall, which becomes on the western border less than 10 inches. 
SEASONAL. DISTRIBUTION.—The precipitation of Colorado shows 
great variability, not only in amount but also in seasonal distribution. 
Jan. Feb. Mar April. May. June. July. Aug. Sept. Oct. Nov Dec 
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Fic. 4.—Mean annual precipitation curves: Meeker (6182 feet) and Grand 
Junction ses feet) are west of the continental divide; Pike’s Peak (14,108 feet), 
Denver (5275 feet), and Pueblo (4672 feet) are east of the continental divide; the 
curves for Meeker and Grand Junction in heavy lines are very regular, showing an 
€qual distribution of precipitation throughout the year; on the other hand, the curves 
for Denver, Pike’s P eak, and Pueblo show for those stations a very unequal seasonal 
distribution; the season of maximum rainfall for each station cha be noted. 
From table IT and fig. 4, it is seen that at stations east of the conti- 
hental divide the greater part (average 74 per cent.) of the precipitation 
occurs during the growing season, when vegetation needs it, and this 
usually in the months of April, May, and June. On the other hand, 
West of the continental divide only about one-half (average 52 per 
cent.) occurs during the growing season, the most rainy months being 
July, August, and September. For this reason, so far as plant life 
is concerned, localities on the western slope are drier than localities at 
