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<br />3.1.1 Precipitation <br />The IOO-year storm was modeled as a 24-hour event with an NRCS <br />type II distribution. Rainfall depths for the 100-year recurrence <br />interval were estimated from the NOAA Atlas 2, Volume III. The <br />values used are provided in the hydrologic supporting computations. <br /> <br />3.1.2 Watershed Delineation <br />Existing condition drainage areas and subwatershed boundaries were <br />drawn using USGS IO-feet and 20-feet contour mapping. The areas <br />were drawn and measured using ArcGIS Version 9.2 (Reference 2). <br />Due to the size of the drainage areas, a small scale drainage area map <br />is provided with the hydrologic supporting computations. See the <br />attached disk for digital GIS data for each drainage basin. <br /> <br />3.1.3 Soils Information <br />Soil types for the watersheds were determined using NRCS spatial <br />soils data. Digital soils mapping is provided with the GIS data on the <br />attached disk. The data shows the soils in the drainage areas to be <br />predominately types A and B for Running Creek, Kiowa Creek, and <br />Comanche Creek. The West Bijou Creek basin consists predominately <br />of soil types A and D. <br /> <br />3.1.4 Land Uses and Curve Numbers <br />The NRCS Curve Number (CN) is used to estimate runoff volume <br />from a watershed. The CN can be estimated as a function of land use <br />and soil type. The Hydrologic Soil Groups were determined from the <br />information described above. <br /> <br />Land uses for the models were determined from aerial photography <br />obtained from the USDA. CNs were assigned based on a land use of <br />"pasture, grassland, or range." The soil hydrologic condition was <br />estimated to be "fair," indicating a ground cover of 50- to 75-percent. <br />The CNs are detailed further in the hydrologic supporting <br />computations. <br /> <br />3.1.5 Times of Concentration <br />The time of concentration is the time it takes runoff to travel from the <br />hydraulically most remote point on the watershed to the watershed <br />outlet. The time of concentration is used to determine the magnitude of <br />the runoff peak. The times of concentration were determined as <br />detailed in the hydrologic supporting computations. <br /> <br />3 <br />