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• <br />as <br />6.3.1 Snowmelt Runoff <br />r <br />• <br />The method used to estimate the snowmelt runoff volumes, as stated in the <br />Preliminary Data Base Report, is based on the snowmelt model wATBAL developed <br />by Leaf and Brink (1973). This model vas used to develop baseline hydrographs <br />of the annual water excess distribution for the Rocky Mountain/Inland Inter- <br />mountain hydrologic region. These hydrographs are presented in the Environ- _ <br />mental Protection Agenry publication "An Approach to water Resources Evaluation <br />of Hon-Point Silvicultural Souroes• along with the methodology for estimating <br />the runoff due to snowmelt. The EPA publication uses the methods to determine <br />the effects due to Silvicultural activities. However, within the methodology <br />is a procedure for estimating baseline conditions which was used for this <br />application. The required information to estimate the snowmelt runoff is the <br />condition (baseline), energy aspect (high, east), Silvicultural prescription <br />(baseline, no Silvicultural activities considered), seasonal precipitation <br />(Winter - 6.62, Spring - 8.1, based on climatological data for Canon City), <br />cover density (maximum - s0 percent), snw redistribution factor (1.0), and <br />evapotranspiration modifier (1.0). The result is defined as water available <br />for stream flow in inches. This value is then applied to the baseline hydzo- <br />graph shown in Figure 6. The resulting eater available for stream flow for <br />Magpie Creek drainage based on the 10-year return frequency is 4.72 inches for <br />the winter season and 1.7 for the spring season. The winter season will result <br />in a larger volume and peak discharge. Table 12 gives the discharge hydrographs <br />and total wlume of runoff to each sediment pond except •D• and •E• for the <br />winter season. Again, the main consideration for snowmelt runoff is the volume <br />produced. <br />S_neci`ic assumptions of the model and characteristics of the study area <br />make the snowmelt runoff estimates, using the described methods, very conservative. <br />The major assumption made is that snow accumulates through the Winter to some <br />estimated date until snowmelt runoff begins and continues until the snow is all <br />melted. This is the most conservative assumption because the characteristics <br />of the study area are such that the snowfall will accumulate for a short period <br />and then melt off intermittently throughout the Winter with the snow depth even <br />going to zero at times. Another conservative aspect is that the developers of <br />. the model applied it to a subalpine watershed for verification which had a low <br />elevation of 9300 feet mean sea level. Whereas, the Magpie drainage area has <br />