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<br />Each of the major drainage basins studied was divided into preliminary geographic/hydrologic <br />subregions, Subregions in the eastern Colorado basins (Arkansas River, South Platte River, and <br />Republican River) were divided using the generalized divisions of plains, foothills, and mountains, <br />as appropriate. Basin divides, elevation limits, topographic breaks, stream confluences, and other <br />features were used to define preliminary subregions. Subregions in the western Colorado are <br />generally defmed by major watershed boundaries. Parameters such as channel slope, geologic <br />conditions, vegetative conditions, precipitation, and other factors were assumed to have already <br />been considered in the development of the previously published flow values. <br /> <br />During the data collection process, published 100-year flow values for studied streams were <br />compiled along with their associated drainage areas for each of Colorado's major drainage basins. <br />Those data pairs were entered into a spreadsheet, and then grouped into preliminary <br />geographic/hydrologic subregions, The 100-year flow values and respective drainage areas were <br />plotted for each subregion to determine trends and outliers, <br /> <br />Regression equations were then developed using a best-fit regression technique within a statistical <br />software package entitled "GB-Stat" (Refei:ence 1). The statistical routine fits values of X (i.e. <br />drainage area) and Y (i.e. 100-year flow value) to several different equations and identifies the <br />regression correlation for each equation. An acceptable correlation was assumed to be achieved <br />when the standard error of estimate was 50% or less, or when the R squared for a regression was <br />0.80 or greater. For simplification purposes, the final equation selected was not necessarily the <br />best-fit equation, but rather the simplest and most logical equation that still produced a reasonable <br />correlation, <br /> <br />The subregional boundaries and data sets were refined during the course of the regional analysis. <br />The subregional boundaries were also dictated by the availability of hydrology information from <br />the detailed floodplain studies, Some preliminary subregions had to be split up or combined with <br />other subregions because of data trends or other factors. Some subregions were deemed to be areas <br />of "No Regression" due to lack of data points or lack of sufficient correlation of the data, The final <br />data points used in the regression analyses are presented in graphical format in the technical <br />addendum at the end of the report, <br /> <br />Due to limitations of the available data, the use of the regression equations is restricted by drainage <br />basin size in each of the subregions. Extrapolation beyond the limits of the data set cannot be <br />justified without a means of checking the accuracy. <br /> <br />The methodology and the draft equations were presented to the Committee for review and <br />comment. Endorsement of the methodology was given by a majority of the Committee members, <br />provided that the limitations and disclaimers were identified in this document. Recommendations <br />made by the Committee were incorporated into these Guidelines to the extent possible. <br /> <br />5 <br />