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I;' 2SSG ified probability level, was implemented only for April IS-May 14 and only for the Arkansas River basin upstream from Pueblo Reservoir. Implementation of ESP for all of April and Mayor for the basin down- stream from the reservoir was not necessary for com- pletion of the PRlUP study. Description of Study Area The Arkansas River basin has an area of 4,669 mi2 upstream from Pueblo Dam and 1,658 mi2 from the dam downstream to the near Avondale station. The downstream area includes 926 mi2 for the Foun- tain Creek basin and 482 mi2 for the SI. Charles River basin. The study area varies from plains and rolling hills in the eastern part of the basin to rugged moun- tains in the western part. Elevation ranges from about 4,500 to 6,500 It in the plains to more than 14,000 It at severa] mountain peaks near Leadville and Buena Vista. The transition from plains to mountains is in the vicinity of Canon City and Colorado Springs (fig. I). The variation in topography has a pronounced effect on precipitation. Average annual precipitation, which generally increases with elevation, ranges from about 10 in. in the plains to about 40 in. in the higher mountains (Colorado Climate Center, 1984). Precipi- tation in the plains is distributed unevenly in time and most precipitation is from summer storms. Because the storms usually are localized and of short duration, most of the precipitation in the plains results in little stream- flow. Precipitation in the mountains is distributed more evenly in time, but because of the higher eleva- tions, much of the precipitation is snow. The snowfall accumulates during the winter, producing deep snow- packs that have 10 to 25 in. of water equivalent. Melt- ing of the snowpacks during late spring and early summer provides about 50 to 80 percent of the annual streamflow over a period of about 3 months. The per- centage generally is largest for streams in the moun- tains and decreases as distance from the mountains increases. Hydrographs of RDM discharge for selected sta- tions in the vicinity of Pueblo Reservoir are shown in figure 3. The hydrographs for water year 1944 are typ- ical of an average year in which discharge generally reaches a maximum during June. By contrast, the hydrographs for water year 1942 are for an extremely wet year in which discharge during April and May gen- erally was much larger than during the remainder of the water year. For the stations presented in figure 3, many of the maximum RDM discharges during April and May were recorded during water year 1942. Discharge peaks during July-October (fig. 3) primarily are the result of summer storms; the rapid rise and fall of the discharge illustrates the short duration of the precipita- tion. Although the peak discharges from the summer storms can be large, the contribution to annual volume usually is small. Report Terminology To avoid the repeated use of some words and phrases, acronyms and abbreviated terminology will be used in this report. Several acronyms have been intro- duced in the previous sections and a few more will be introduced in subsequent sections. For easy reference, a list of all acronyms used herein is presented at the beginning of this report after the table of contents. Much of the discussion herein will focus on the streamflow-gaging stations (table 1) used in the fre- quency analysis. To aid in easy identification of a sta- tion when mentioned in this report, stations on the Arkansas River will be referred to in reference to their location. For example, station 07099500 Arkansas River near Pueblo (table I) will be referred to as the "near-Pueblo station," or station 07097000 Arkansas River at Portland will be referred to as the "at-Portland station." Station 07106500 Fountain Creek at Pueblo (table I) will be referred toas merely "Fountain Creek" and the four stations on the SI. Charles River (table I) will be referred to collectively as "St. Charles River." Acknowledgments Completion of the PRlUP study was greatly facilitated by the assistance of the National Weather Service. During the initial stages of the study, imple- mentation of the ESP procedure was determined to be achieved best through use of the National Weather Ser- vice River Forecast System (NWSRFS) model, of which the ESP procedure is an integral part. The National Weather Service cooperated with the U.S. Geological Survey to apply the operational capability of the NWSRFS model for the Arkansas River basin upstream from Pueblo Reservoir during the PRlUP study. The assistance of the National Weather Service personnel, E.A. Anderson, lV. Bowman, a.N. Day, and S.c. Van Demark, in developing the operational capability of the NWSRFS model, includ- ing implementation of the ESP procedure, for applica- tion to the PRJUP study, therefore, is gratefully acknowledged. INTRODUCTION 5