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<br />. <br /> <br />roughness coefficient ('n' value) of 0,035 is considered appropriate for the Los Piiios River channel. <br />An 'n' value of 0,050 represents the undeveloped overbank areas in the hydrologic model, and an 'n' <br />value of 0,10 represents the developed overbank area in the model in the reach through Ignacio, <br />Colorado, <br /> <br />Frequency Rainfall <br /> <br />The Albuquerque District calculated the frequency rainfall based on the National Oceanic and <br />Atmospheric Administration (NOAA) Atlas II, Volume III (1973), for Colorado, To imitate a <br />general storm centered over the entire watershed, the point rainfall was reduced for the 300-square- <br />mile contributing drainage area using the hypothetical storm record in the HEC-I program, The <br />duration depths required for the HEC-I analysis were computed using the methodology published in <br />NOAA Atlas II, Volume III. The 24-hour rainfall was applied uniformly to each subarea in the <br />watershed using a triangular (critical) distribution pattern which places the peak intensity at the center <br />of the storm and the remaining increments so that they ascend in magnitude to the peak and descend <br />in magnitude to the end of the storm, <br /> <br />Infiltration Rate Calibration <br /> <br />. <br /> <br />The Los Piiios River watershed between Vallecito Reservoir and the New Mexico state line is <br />divided into two physiographically different regions, The watershed upstream of Bayfield is <br />characterized by a hilly foothill terrain whereas the watershed downstream ofBayfield is distinguished <br />by a more gently sloped plains terrain, The difference in physiography is reflected in the different <br />infiltration rates used in the watershed model. <br /> <br />To determine infiltration rates for the two regions, the Albuquerque District calibrated two <br />separate HEC-I watershed models to frequency-discharge peaks calculated from regional equations <br />presented in U.S, Geological Survey (USGS) Water-Resources Investigations Report 96-4112, <br />Analysis of the Magnitude and Frequency of Peak Discharge and Maximum Observed Peak <br />Discharge in New Mexico (1996), The following regional equations presented in Report 96-4112 <br />were developed for the northwest plateau region of New Mexico including the San Juan River <br />watershed in southwestern Colorado: <br /> <br />Q(50.0%) = 8.47 X 10' x A 0,47 <br />Q(200%) = 1,97 X 102x A 0.46 <br />Q(lo.o%) = 3,06 X 102x A 0,46 <br />Q(4,O%) = 4,86 X 102x A 0.45 <br />Q(2,O%) = 6,54 X 102x A 0.45 <br />Q 5 2 A 0.45 <br />(1.0%) = 8, 3 x 10 x <br />Q 5 I 3 A 0.45 <br />(0.2%) = 1.4 x 0 x <br /> <br />. <br /> <br />4 <br />