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<br />3.0 Orographic impacts on Mason RElserloir precipitation <br /> <br />Mason Reservoir is located along the higher southeastern slopes of the high <br />alpine zone surrounding Pikes Peak at an elevation of about 11,600 feet. <br />The basins which drain into Mason Reservoir cover an area of approximately 6 <br />to 10 square miles along a roughly northwest to southeast axis from the higl1est <br />to the lowest points. The highest point in the basins is about 13,500 feet while <br />the lowest is 11,100 feet. The average elevation of the basins was calculated by <br />HMS to be 11,600 feet. This very high basin average elevation should be a <br />major consideration in the calculation of the site-specific PMP, A more detailed <br />view of the basin and associated topography is shown in Figure 6. <br /> <br />Pikes Peak, the dominating near-by topographic feature, rises abruptly to an <br />elevation of 14,110 feet off the eastern Colorado plains from an elEwation of <br />about 4,500 feet near Pueblo 45 mileS to the south and from an elevation of <br />6,000 feet near Colorado Springs 5 to 10 miles to the east. Thus southeasterly <br />winds are forced to rise upslope about 9,600 feet to reach the peal< while <br />easterly winds rise about 8,000 feet. Moist air masses approaching from the <br />south or east quickly reach their condensation point due to the forced upslope <br />motions caused by the terrain, <br /> <br />Palmer Ridge extends from west to east as a spur of the Rampart Range onto <br />the eastern Colorado plains to almost the Kansas border. Immediately west of <br />Pikes Peak is South Park a broad intermountain plateau noted for its windy and <br />dry climate. Further west, the Collegiate Mountain Range looms. Eoach of these <br />terrain features exerts an influence of varyin!~ degree on the formation of <br />precipitation in the Mason I~eservoir basin, <br /> <br />HMS conducted a site visit to the basin to assist in the evaluation of the local <br />orographic effects on general storm precipitation and, more importantly, on local <br />storm rainfall. This visit was delayed until mid-July 1995. The delay was caused <br />by the persistence of a record snow pack over the entire basin, This record <br />snow pack was reported throughout Colorado and produced river flooding in <br />many portions of the state. In addition to the high elevation of the reservoir and <br />its basins, there are several orographic features which were judged to havEl a <br />controlling impact on local precipitation. Figure 7 shows the three I<ey features <br />which impact orographic precipitation distribution and occurrence in the Mason <br />Reservoir drainage basin, These are the Palmer Divide and its associated <br />low level circulation features, the zones of primary orographic lift to the <br />south through east, and Pikes Peak. Available atmospheric moisture tends to <br />"rain out" or to be spent over these features thus reducing the rainfall potential in <br />the Mason Reservoir basin and enhancing it over topographic features to the <br />north through east. <br /> <br />19 <br />