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002,551 Profiles of computed versus observed DO concentrations are shown on figure 15. The DO profiles computed by the two models are almost identical (fig. 15). To compare the DO concentrations computed by the two models required some consideration to the methods by which the data for mean depth and velocity of the stream are processed by each model. The U.S. Geological Survey model uses mean-depth and velocity data entered into the computer directly in the reaeration-rate computations; whereas, the Pioneer-I model computes the mean depth and velocity using the following equations: and b V=aQ , b D=alQ 1, (3) (4 ) where Q=mean flow in reach, in cubic feet per second; and a, b, aI' bi=regression coefficients. In the water-quality comparisons, the hydraulic coefficients a, b, ai' and bi in equations 3 and 4 for the Pioneer-I model were adjusted until the computed values of mean depth and velocity approximately equaled those used by the U. S. Geological Survey model. In earlier computational comparisons, of the two models, somewhat larger differences in the computed DO profiles were due to discrepancies in the traveltime computations in the, Pioneer-I model. As indicated by the model computations (fig. 15), the DO, sag in the study reach is barely discernible. The computed DO values remained within 95,percent of saturated DO concentrations. Relatively larger diel variations in DO concen- trations were observed at several sites, especially in that part of the study reach downstream from the main wastewater-treatment plant at Steamboat Springs (sites YM-3 to YM-7, fig. 15) and downstream from Hayden (site YM-14, fig. 15). In addition to total nitrogen, the following nitrogen components were modeled: Organic nitrogen (nonfiltered form), ammonia nitrogen, and nitrite- plus-nitrate nitrogen. The temperature-corrected rate coefficients using first-order exponential decay algorithms from the model calibrations were organic nitrogen, 0.2 per day; ammonia nitrogen, 0.3 per day; nitrite nitro- gen, 1.0 per day; and nitrate nitrogen, 1.7 per day. Each nitrogen-rate coef- ficient was corrected for water-temperature variation by an empirical coeffi- cient of 1.047 using an expression similar to equation 2. The rate coefficient of 1.7 per day for nitrate nitrogen applies only to ,the U.S. Geological Survey model. The Pioneer-I model accumulates the nitrate nitrogen as the final step in its nitrogen-cycle computations. The model-calibration results give estimates for nitrite and nitrate concentrations separately. These values then were added together and compared to the observed data. Total nitrogen, which was treated in both models as a conservative constituent, was modeled to provide some indication of where the nitrogen was being used or supplied in the study reach , other than from point sources. 26 ~ -