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The precipitation values were obtained from the Precipitation Frequency Atlas <br />of the Western United States, Volume 3, Colorado, (NOAA, 1973). <br />Hydrograph Response Shape. A unit hydrograph was chosen for each <br />drainage area or sub-area model to predict the runoff response. The <br />hydrograph responses available in the SEDCAD+ model are stow, medium, and <br />fast. A slow response corresponds to a forested area, or an area with a <br />number of obstructions, whereas a fast response corresponds to an <br />unvegetated or poorly protected area. For each of the areas modeled, the <br />appropriate runoff hydrograph was chosen. <br />The internal convolution increment is 0.05 hours and values are saved at the <br />user specified interval of 0.1 hours or greater. (A convolution increment of 0.1 <br />was specified for all pit areas). It should be noted a time of concentration less <br />than 0.125 hours bypasses the unit hydrograph technique and instantaneous <br />runoff is assumed. As explained in the SEDCAD+ User's Manual, page 113, <br />the time of concentration restriction of 0.125 hours is somewhat mandated due <br />to a combination of array size restrictions, minimum internal convolution interval, <br />and the user-specked time increment for saving convoluted values. <br />Drainage Basin Area. The drainage areas were determined by digitizing <br />directly from a 1"=400' scale map. <br />Time of Concentration, Tc. Time of concentration was calculated by using the <br />SCS Upland Method as presented in Barfield, et al., (1981) page 100. All <br />hydraulic lengths, drainage heights, and slopes were measured directly from <br />1'=400' scale maps. <br />The land use corresponding to short grass pasture (overland flow) was utilized <br />to simulate both pre and postmine conditions. <br />3 <br />