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stone and claystone facies, coarse to fine-grained, locally conglomeratic. According to the soil survey of Elbert County, Colorado, western part (Larsen, 1980), the soil in this area is Elbeth-Kettle Complex; these soils are formed in material from arkosic deposits. The lower part of the watershed is under- lain by fine-grained sandstone, carbonaceous shale, and lignite facies (Bryant, 1981). Much of the north side of the lower part of the basin is Renohill-Louviers Complex soils. Renohill weathered from interbedded sandstone and shale; Louviers formed in material weathered from noncalcareous shale. The south side of the lower watershed is largely eolian material, Wiley-Baca loams, both soils formed in calcareous silty eolian material. ,/'/ ,-Gl~mate at the Willow Gulch study area generally is characterized by dry, windy conditions. Average precipitation at Byers, about 20 mi north of Willow Gulch, is 15.4 in. Of this amount, about 56 percent occurs from April through July. .1 December has the least precipitation (0.37 in); May has the most prec~p~- tation (2.64 in). Temperatures during May and June are usually warm during the day and cool at night. During July and August, daytime temperatures are often between 900 and 100oF. The study area is fairly typical of the High Plains region of eastern Colorado, as far as wind movement is concerned: prevailing wind in the winter is from the northwest and blows a large part of the time; during summer months, air is often calm at night, but usually windy in the afternoon because of solar heating. Vegetation at Willow Gulch is basically of two types: grassed and wooded. Within each type are numerous different kinds of grasses, forbs, and shrubs. The wooded area is at the upper end of the watershed, located on arkosic sand- stone. This area is suitable for the growth of ponderosa pine; some grass grows beneath the pine trees, but grass is generally sparse. Small open areas con- taining good grass cover are scattered through the trees; the primary grass in most of these areas is blue gramma. A few shrubs grow'along incised stream channels, where some water is availab"le in deep sand. A few sites with sparse vegetation and steep slopes are included in this area. The lower half of the watershed is almost entirely grass covered. Most of the area has a well-developed sod largely made up of blue gramma; however, numerous other grasses are present (figs. 2 and 3). These grasses include . western wheatgrass, green needlegrass, bluegrass, Indian ricegrass, prairie junegrass, and sideoats gramma (Larsen, 1980). Clumps of large yucca are scattered throughout the area. Delineation of Hydrologic-Response Units and Development of Test Plots Reconnaissance of the Willow Gulch watershed indicates three major types of hydrologic-response units (HRUs). The upstream 40 percent of the watershed contains ponderosa pine and has steep slopes and deeply incised main channels. The lower part of the watershed has much gentler slopes and vegetative cover 5