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II-4 I I I I I I I I I I I I I I I I I I I I 2. Climatology The climate in the Project drainage area ranges from arid to wet and pronounced climatic variations occur within relatively short distances due to dramatically varying topography. Within Garfield County the climate is characterized by cool sUllllllers and moderately severe winters, especially in the mountainous regions. Characteristically, great temperature changes occur from sUllllller to wint-eI", and local weather conditions may change rapidly due to move- ment of storm systems from west to east through the region. Temperatures are o often in the range of 90 F during the stunmer and below freezing in the o winter. Winter temper at ures may drop as low as -20 F in the colder areas. A verage annual precipitation varies considerably with elevation, ranging frem 9 inches at Palisade, 10 to 15 inches in the river valleys of Garfield County, and 30 to 40 inches annually in the mountainous headwater regions. Most wintertime pre'~ipitation occurs as snow and a deep snowpack normally accumu- lates. Average snowfall varies considerably within the river valley with approximately 34 inches falling at Grand Junction, 40 inches at Rifle and 70 inches at Glen..ood Springs. The upper mountainous regions may have over 400 inches of snowfall in a wet year. Snowpack in the upper basin normally begins to accumulate in late October, and snownelt begins in late April and continues into June or early July. The typical water equivalent snowpack on the ground (21) above approximately El 9000 by April 1, based on seven snow courses . con- si dered repres.mtati ve of the drainage basin, is 16 inches and ranges frem 9 inches to 24 inches in dry and wet years, res pecti vely. General rainfall may occur over large areas from late spring through late fall and convective-type cloudburst storms of small areal extent occur frequently in summer. 3. Average Runoff The main stem of the Colorado River is well gaged and of the avail- (27)(28) able gage records, two primary gaging stations operated by the U.S. Geological Survey (USGS) have been utilized to estimate the long-term runoff at the dam sitE' after numerous diversions occur upstream. These two USGS gag- ing stations are as follows: