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I I HISTORY AND OVERVIEW OF FLOOD REGIONALlZATION METHODS By W.O. Thomas, Jr. INTRODUCTION The USGS has been involved in the development of flood-regionalization procedures for over 40 years. These regionalization procedures are used to transfer flood characteristics, such as the IOO-year flood-peak discharge, from gaged to ungaged sites. The USGS has traditionally used regionalization procedures that relate flood characteristics to watershed and climatic charac- teristics through the use of correlation or regression techniques. Herein, flood characteristics are defined as flood-peak discharges for a selected T-year recurrence interval (such as the IOO.year flood). Because these flood characteristics may vary substantially between regions due to differences in climate, topography, and geology, tests of regional homogeneity form an integral part of flood regionalization procedures, The evolution of flood-peak discharge regional- ization procedures within USGS is described by dis- cussing the following three procedures: (1) index- flood procedure used from the late 1940's to the 1960's, (2) ordinary-least-squares regression procedure used in the 1970's and 1980's and (3) generalized-least-squares regression procedure that is being used today (1990's), Index-Flood Procedures Dalrymple (1949) states "The method of com- puting flood frequencies that is presented in this paper reflects the latest developments based on a continuing study of the subject by engineers of the Water Resources Division of the United States Geological Survey. The method has been revised several times in the last few years and probably will be again in the future." This statement indicates that the index-flood procedure was being used by the USGS in the 194O's. The index-flood procedure consisted of two major parts. The first was the development of basic, dimensionless frequency curves representing the ratio of flood discharges at selected recurrence intervals to an index flood (mean annual flood). The second part was the development of a relation between watershed and climatic characteristics and the mean annual flood, to enable the mean annual flood to be predicted at any point in the region. The combination of the mean annual flood with the basic frequency curve, expressed as a ratio of the mean annual flood, provided a fre- quency curve for any location (Dalrymple, 1960). The determination of the dimensionless fre- quency curve involved: (1) graphical determination of the frequency curve for each station using the Weibull plotting position, (2) determination of homogeneous regions using a homogeneity test on the slopes of the frequency curves, and (3) computation of the regional dimensionless frequency curve based on the median flood ratios for each recurrence interval for each station in the region, The homogeneity test used the ratio of the IO-year flood to the mean annual flood to determine whether the differences in slopes of frequency curves for all stations in a given region are greater than those attributed to chance. The IO-year flood discharge was first estimated from the regional dimensionless fre- quency curve. The 95-percent confidence interval for the recurrence interval of this discharge, as determined from the individual station frequency curves, was then determined as a function of record length. If the recur- rence interval for a given station was within the 95- percent confidence bands, then this station was consid- ered part of the homogeneous region. Otherwise, the station was assumed to be in another region. The mean annual flood, as used in the index- flood procedure, was determined from the graphical frequency curve to have a recurrence interval of 2.33 years. The mean annual flood for an ungaged location was estimated from a relation that was determined by relating the mean annual flood at gaging stations to measurable watershed characteristics such as drainage area, area of lakes and swamps, and mean altitude. The index-flood procedure described above was used to develop a nationwide series of flood-frequency reports entitled "Magnitude and Frequency of Roods in HISTORY AND OVERVEW OF FLOOD REGIONAUZAll0N METHODS 3