A rainfall-runoff modeling procedure for improving estimates of T-year (annual) floods for small drainage basins

Maps depicting the influence of a climatic factor, C, on the magnitude of synthetic T-year (annual) floods were prepared for a large portion of the eastern United States. The climatic factors were developed by regression analysis of flood data using a parametric rainfall-runoff model and long-term rainfall records. Map estimates of C values and calibrated values of rainfall-runoff model parameters were used as variables in a synthetic T-year flood relation to compute ' map-model ' flood estimates for 98 small drainage basins in a six-state study area. Improved estimates of T-year floods were computed as a weighted average of the map-model estimate and an observed estimate, with the weights proportional to the relative accuracies of the two estimates. The accuracy of the map-model estimates was appraised by decomposing components of variance into average time-sampling error associated with the observed estimates and average map-model error. Map-model estimates have an accuracy, in terms of equivalent length of observed record, that ranges from 6 years for the 1.25-year flood up to 30 years for the 50- and 100-year flood. (Woodard-USGS)