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<br />INTRODUCTION <br />In the study of flo requency relat' the underlying hypothesis is <br />that there is a relationship between peak discharges and basin area. In <br /> <br />general, there are two methods of determining flood frequency relations. One <br /> <br /> <br />consists of using stream flow-gaging station records and the other uses <br /> <br /> <br />rainfall-runoff relations. Within the state of Colorado there are four major <br /> <br /> <br />river basins; the South Platte, Arkansas, Rio Grande, and Colorado; and <br /> <br /> <br />estimates regarding the magnitude of hydrologic events have been prepared <br /> <br /> <br />using both general methods within each of the four basins. Jarrett and Costa <br /> <br /> <br />(1988) indicated that flood estimates based on streamflow records and <br /> <br /> <br />rainfall-runoff relations are sometimes different. For example, for Clear <br /> <br /> <br />Creek near Golden, Jarrett and Costa (1988) showed that there is between 381. <br /> <br /> <br />and 991. difference between flood discharge magnitudes for selected recurrence <br /> <br /> <br />intervals using weighted streamflow analysis and rainfall-runoff results (U.S. <br /> <br /> <br />Federal Emergency Management Agency, 1984) (Table 1). <br /> <br /> <br />There are over 1000 sites in Colorado where peak discharge magnitudes <br /> <br /> <br />have been estimated. Entering into the problem in Colorado are the different <br /> <br /> <br />applications by various investigators who have used the two general methods, <br /> <br />either the streamflow records or the rainfall runoff relations, to compute <br /> <br /> <br />estimates of flood-frequency magnitudes. When these sites are plotted on a <br /> <br /> <br />graph of area versus discharge there is unexplained randomness in the data <br /> <br /> <br />which cannot be explained solely because of being different hydrologic areas <br /> <br /> <br />(figs 1, 2, 3, and 4). Figure 1 shows the 100 year recurrence interval flood <br /> <br /> <br />estimates for 686 sites in the South Platte River Basin with hand-drawn upper <br /> <br /> <br />and lower maximum discharge envelope curves. For basins with a drainage area <br /> <br /> <br />of 10 square miles, the discharge varies from a lower envelope discharge <br /> <br />2 <br />