WSP01635 - Laserfiche WebLink

Home Browse Search

00u479 26 IV. MOISTURE SOURCES FOR PRECIPITATION IN THE UPPER COLORADO RIVER BASIN A. OBJECTIVES The inland location of the catchment area of the Upper Colorado River Basin receives its mois- ture from air masses which have been modified by travel over a considerable distance of land. The objective of this special study of mois- ture source was to determine whether precipitation falling in the Upper Basin has originated from (a) the Pacific Ocean, (b) the Gulf of Mexico, or (c) repeat precipitation from nearby evapotrans- piration. B. PROCEDURE The method of study has been examination of the weather map sequence related to all storms which occurred in a 46 -year period. By moving backward in ti me from the periods when precipita- tion has been measured, it is possible to estimate the original source region for the moisture. Only broad generalizations could be made, since any air mass picks up moisture over a long period of time, and it is not possible to fix any small source region. For instance, the air which moves from east to west over the Gulf of Mexico previously has been moving over the Central Atlantic Ocean, and part of the moisture which it contains as it arrives over Mexico may have been picked up through the evapo- ration process sever! thousand miles upwind. Following preliminary investigation, it was decided that source regions could be bet~er clas- sified into three general categories. These were (1) Gulf of Mexico, (2) Pacific Ocean, with a trajectory south of the high Sierras, and (3) mod- ified Pacific air mass which moved from west to east crossing mountainous terrain at some point north of the south end of the high Sierras. C. RESULTS Figure 34 shows the general areas of source regions for precipitation collected in the Upper Basin of the Colorado River. 1. Summer Summer shower activity occurs mainly in July and August. The source region is primarily the Gulf of Mexico, and some local evapotranspira- tion brought about by collection of moisture through <:!vapotranspiration within one day's travel time from the south and southwest. The typical trajec- tory of warm and moist air luoves over northern Mexico and then to the north over Utah and Colo- rado. The high mountainous terrain experiences more showers and has a greater reliability for precipitation during this period than low elevations. The north end of the basin in Wyoming is at a maximum distance from the Gulf of Mexico I and consequently receives a smaller amount of rainfall from summer showers. 2. Fall During the fall period when general rains can occasionally occur I there is still a general source region from the Gulf of Mexico, but an important alternate source region comes from the warm Pacific south of the high Sierras. Most of the major storms- .which have less than an annual frequency of occurrence--come from this source region in the period between September and Decem- ber. A few of the most notable storms of this period have actually been remnants of a storm which was a hurricane of tropical origin in the Pacific Ocean south and west of Mexico. The move- ment of such a storm carries tremendous quantities of moisture as it moves from near the mouth of the Colorado River up to the upper catchment basin. Such storms are particularly important in produc- ing precipitation in the south half of Utah and the southern slopes of the mountains in Colorado. 3. Winter Nearly all of the wintertime precipitation comes from air masses which have moved from west to east across the mountainous terrain, ex- tending from the south end of the high Sierras to the Canadian Border. The actual trajectory of some of this air moves eastward into Montana and then southward into the Upper Colorado River Catch- ment Basin. Such trajectory produces the greatest amount of precipitation on the northern and north- western slopes of mountainous terrain. Precipitation activity is accentuated greatly at the higher elevations, since a large amount of lifting and cooling is required to produce precipita- tion from this air after its passage over the moun- tainous terrain upwind. An extreme example of such an influence of the upwind mountains can be illustrated from a trajectory moving toward Colo~ rado across the high Sierras of California. Such an air mass would lose a very high fraction of its moisture as it moved upward over the mountain barrier in California. A$ this air mass moves downslope on the east side of the Sierras, it is heated and can continue to carryall available mois- ture in vapor form until it is again lifted and cooled moving against the very high terrain in the Rocky