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'"," , OG255~ Data-Reduction Techniques The CBOD in water samples collected during the 24-hour study on September 23-24, 1975, was analyzed using a procedure described by Hines, McKenzie, Rickert, and Renella (1977). Using this technique, DO concentrations in the sample bottles were measured initially and then remeasured 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, and 20 days after the initial measurement. These data were analyzed using a computer program developed by Jennings and Bauer (1976). Sample results of this form of analysis are shown on figure 11. By this technique, the ultimate CBOD (CBODU), CBODs, ,and decay- rate coefficient (Kj) are computed. In the example cited, CBODU=8.0 mg/L, CBODs=2.9 mg/L, and Kj=0.09 per day. An average-bottle decay-rate coeff,icient of about 0.1 per day for the entire study reach was computed using the average of the decay rates determined for the samples. The decay-rate coefficients for CBOD computed by the decrease in DO in the sample bottles with time are somewhat different from those occurring in a stream environment. A technique described by Velz (1970) was applied to the CBOD data determined in the laboratory for the stream reach downstream from site YM-4, which is 1.6 mi (2.6 km) downstream from the main wastewater- treatment plant at Steamboat Springs. The stream reach included sites between YM-4 and YM-8. The log of the percentage of CBOD remaining relative to some initial concentration was plotted versus the estimated stream traveltime down- stream from the starting point (fig. 12). By manually fitting a straight line through the data points, the arithmetic slope of the line will indicate the approximate decay rate, Kj, of CBOD in the stream reach. A value of 0.4 per day was determined for Kj by visual fit of the data points. A similar analysis also was made for the coliform-bacteria data. Site YM-4 again was used as the reference site with sites between YM-4and YM-8 considered. The results of this analysis are shown on figure 13. The die-off rates of coliform bacteria estimated by this visual fitting technique were 0.65 per day for fecal-coliform bacteria and 1.03 per day for total-coliform bacteria. CALIBRATION RESULTS The two water-quality models, U.S. Geological Survey and Pioneer-I, were calibrated using results of the data collected during September 23-24, 1975. In this section the calibration results are discussed and some variations in results of the two models are presented. To provide direct comparison, the same rate coefficients were used in both models. Water-quality data used in the model calibration included the following variables': CBODU; DO; organic nitrogen, as nitrogen; ammo,nia nitrogen, as nitrogen; nitrite-pIus-nitrate nitrogen, as nitrogen; total nitrogen, as ni- trogen; orthophosphate; fecal-coliform bacteria; and total-coliform bacteria. For modeling analysis, the study reach was divided into a number of subreaches defined by the 10 tributaries, diversion sites, discharge points of wastewater-treatment plants, and geometry of the study reach (fig. 6). 20