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<br />METHODS OF STUDY <br /> <br />Study Plan <br /> <br />In every investigation of infiltration rates using manmade devices, the <br />question is raised as to whether the results obtained are comparable to events <br />occurring in natural rainstorms. The logical method of answering this question <br />is to measure the results of both artificial and natural rainfall on the same <br />site. To determine applicability of the study plot data to larger areas, the <br />plots should be located within a larger gaged watershed. The study plan was <br />to instrument such a watershed to measure precipitation, runoff, and sediment <br />yield from rainfall-simulation plots, subwatersheds, and the total watershed, <br />for a period long enough to obtain information on natural rainsto~. Simu- <br />lated rainfall would be applied to the plots to obtain data for comparison <br />with natural events. <br /> <br />Site Selection and Location <br /> <br />~ <br /> <br />In 1978, a search was begun for a suitable watershed to instrument for <br />study. Several criteria were considered for selection of a watershed: (1) <br />Proximity to Denver (50 to 100 mi); (2) proper size (less than 5 mi2); (3) <br />necessary water supply; (4) reasonable homogeneity: (5) accessibility to <br />equipment: (6) access provided by landowner; (7) reasonable occurrence of <br />natural runoff; and (8) some historical record (preferably). Several areas <br />were visited over the next 2 years, before a site was chosen in 1980. <br /> <br />The study watershed is at the headwaters of Willow Gulch, a tributary of <br />Middle Fork Bijou Creek, about 20 mi south of Byers, Colorado,. and about 50 mi <br />east of Denver. A miscellaneous record station was operated at this site by <br />the U.S. Geological Survey, Water Resources Division, Colorado District, from <br />1970 to 1979. <br /> <br />Description of the Study Watershed <br /> <br />The area is 1.7 mi2, about 3 mi long and about 1 mi wide (maximum width) <br />(fig. 1). The upper half of the watershed has many steep slopes with deeply <br />incised channels. This part of the watershed contains numerous ponderosa pine <br />interspersed with open areas of grass cover. <br /> <br />The lower half of the watershed has much gentler relief and nondescript <br />drainage boundaries. Vegetation consists mostly of sod-forming grasses with <br />numerous areas of yucca. The main drainage channel contains deep, coarse <br />sand in its entire length. Altitude of the basin ranges from about 5,640 <br />to 6,040 ft. <br /> <br />The entire watershed is underlain by the Dawson Arkose of Late Cretaceous <br />and early Tertiary age (Bryant, 1981). The upper part of the watershed, gen- <br />erally the area containing the ponderosa pine, is underlain by arkosic sand- <br /> <br />3 <br />