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RAINFALL-RUNOFF MODELING AND PRELIMINARY REGIONAL FLOOD CHARACTERISTICS OF SMALL RURAL WATERSHEDS IN THE ARKANSAS RIVER BASIN IN COLORADO By Russell K. Livingston ABSTRACT Both recorded and synthetic rainfall-runoff and annual peak-discharge data for 17 rural watersheds were analyzed to evaluate the magnitude, frequency, and volume of floods in the plains region of the Arkansas River basin in Colorado. Flood-frequency relations from analysis of recorded data were weighted or combin- ed with flood-frequency relations from analysis of synthetic data to provide im- proved estimates of selected flood characteristics for 15 of these watersheds. The 10-, 25-, 50-, and 100-year peak discharges were regionalized using multiple- regression and station-year methods. Regression relations were developed to determine peak discharge from effective drainage area (standard error of estimate 30 to 50 percent) and flood volume from peak discharge (standard error of esti- mate 62 percent) for ungaged basins between 0.5 and 15 square miles in size. Using these two flood characteristics, a dimensionless hydrograph method provides synthetic hydrographs very similar in shape to recorded flood hydrographs. INTRODUCTION . The magnitude and frequency of flood discharges and volumes are major con- siderations in the design of highway bridges and culverts. Extensive streamflow data available for large perennial streams have generally provided the flood in- formation necessary for the design of major drainage structures. Previous reports on the estimation of the flood characteristics of Colorado streams include Patterson {1964, 1965j, Patterson and Somers (1966j, Matthai (1968j, Livingston (1970), and Hedman, Moore, and Livingston (1972j. The methods described by these reports, however, do not generally apply to very small watersheds, particularly those having ephemeral streams. McCain and Jarrett (1976j presented regression equations applicable to basins within the plains region of Colorado with drainage areas greater than 1 square mile. These equations, however. were based only on limited data for small watersheds; only 2 of 36 watersheds studied had areas less than 30 mi2. Procedures described by the U.S. Soil Conservation Service (1975, 1977) apply to small watersheds but are based primarily on theoretical rainfall- runoff relationships developed for regions extending over many States rather than local hydrologic areas within any particular State such as Colorado. 1