Estimating a regional flood frequency distribution

When floods at different sites are assumed to arise from the same distribution except for scale (U.S. Geological Survey's Index Flood Method), one can attempt to identify the dimensionless flood‐flow frequency distribution by normalizing small samples by each sample's sample mean. Unfortunately, the resultant curve may be a poor description of the true dimensionless flood distribution. This problem can be overcome by working with the logarithms of the peak flow values and using unbiased moment or probability weighted moment estimators. However, the correlation among concurrent flood flows in a region is shown to place severe limits the accuracy with which a distribution's moments can be estimated with many such correlated records.