Laserfiche WebLink
<br />OOJIJ5 <br /> <br />the past 50 years [3]. Very large increases in runoff are <br />predicted to occur for moderate increases in precipitation. A <br />station recording 24 inches of annual precipitation receives <br />only three times as much precipitation as a station recording <br />8 inches. But the runoff from the area surrounding the <br />24-inch location is 24 times greater (19.2 vs. 0.8 inches) <br />than the area around the 8-inch precipitation location. <br />The increased runoff value for an extra 2 or 4 inches of <br />precipitation in the broad areas of the Colorado River water- <br />shed that ordinarily receive 8 to 14 inches is measured by <br />tremendous percentage differences. For instance, the runoff <br />from 14 inches of precipitation (4.2 inches) is 280 percent of <br />what it is from 10 inches (1.5 inches). Here a 40 percent <br />increase in precipitation yields a 280 percent increase in <br />runoff. A 30 percent precipitation increase (20 to 25 inches) <br />yields a 195 percent runoff increase (12.0 to 23.4 inches) [4]. <br /> <br />"The arid-semiarid upper Colorado River Basin region features <br />notable variations in precipitation, affected primarily by <br />moisture supply and topography. Most of the precipitation is <br />provided by northern Pacific air masses [5] that move inland <br />from the west. In summer, thunderstorms sweep into the <br />southern portion of the region fed by moisture from the Gulf <br />of Mexico. Canadian arctic air can occupy the northern <br />portion of the region during winter months. Since the region <br />is distant from major sources of moisture and the air masses <br />cross numerous mountain ranges en route to the area, precipi- <br />tation is sparse except in high mountain areas. Average <br />annual precipitation varies from less than 5 inches in the <br />lowest valleys to more than 50 inches in the higher mountains. <br />Average precipitation in the valleys and agricultural areas is <br />from 10 to 20 inches per year. Precipitation from late <br />October through mid-April consists primarily of snow, particu- <br />larly at higher elevations. Annual snowfall ranges from <br />about 5 inches in the lower valleys and plateaus to 200 to <br />300 inches in the higher mountains. Snow accumulations <br />occasionally exceed 100 inches at the higher elevations and do <br />not completely melt until late summer. The frost-free period <br />varies greatly with elevation from 20 days or less at eleva- <br />tions above 8,500 feet to more than 180 days at elevations <br />below 5,000 feet [5]. <br /> <br />"In a given storm, snow often occurs first in the western part <br />of the Colorado basin, and gradually moves eastward. At the <br />time snow is occurring over the main massif of the Rockies, <br />the Great Plains to the east are normally not experiencing <br />precipitation and may even have clear skies. However, when <br />the low-pressure center passes onto the plains the storm may <br />regenerate by drawing in warm moist air from the south, i.e., <br />air from the Gulf of Mexico [7]. This transition to a new <br />moisture source as the storm moves out to the plains has very <br /> <br />8 <br />