WSP01635 - Laserfiche WebLink

Home Browse Search

OJCl434 below freezing for nine months out of the year. and the other three months have temperatures only slightly above freezing. The capacity of the tran- sitory air to carry water away from these highest elevations is extremely limited and can he con- sidered as negligible throughout the entire year. It is easy to see from Table II how snowpack can build up at the higher elevations during the cold winter months. By contrast, at the 6000-foot level all months have temperatures above freezing. with the exception of December and January, and these two months are near the freezing level. The warmer months at the lower elevations have temperatures and dry air capable of accepting tremendous quan- titites of moisture either through direct evaporation or transpiration from plant life. The lower elevations of the watershed above Glen Canyon Reservoir are also characterized by being made up of generally flat sandy soil with tremendous capacity for absorbing large quantities of rainfall and preventing any direct runoff. The many dry washes are perennial evidence to this fundamental fact. Only in the instances of ex- tremely heavy local thunderstorms do these dry washes carry any water. and many times this water disappears long before it reaches the main stem of the Colorado River. Almost all of the water which does enter the soil returns in delayed evaporation 5 into the atmosphere before ever reaching the Colorado River. Little is known about actual rates of evapo- ration. However, some rough approximations can be made about the fractional portion of the ob- served precipitation which is lost to evapotrans- piration in this particular watershed. The entire watershed loses over 80 per cent. The area below 5000 feet loses over 90 per cent. The area above 11,000 feet loses less than 20 per cent. During the winter there is a much greater contrast between low elevations and high elevations. This is first due to the marked contrast in pre- cipitation amounts, the higher elevation stations recording nearly three times as much as the low elevation stations. Immediate evaporation at high elevations is negligibie, and the delayed evapora- tion tends to be consolidated in the amount of moisture entering the soil either at the beginning or end of the snowpack season. At the elevations above 10,000 feet, all the storms which occur from approximately early November through mid-April tend to accumulate as if they were one large storm, and the runoff from this accumulation also can be treated as if it were one large storm.