Estimation of flood frequencies for ungaged catchments

Accurate flood frequency curves are difficult to obtain for ungaged catchments. A typical procedure uses a simulation model to compute peak discharges where the recurrence interval is assumed to be equal to that of the design storm. Derived flood frequency distributions provide an alternative to this approach. The Diaz-Granados derived flood frequency distribution is modified to use the Soil Conservation Service (SCS) curve number method instead of the Philip equation because the required data are more readily available. The derived approach, the methods of Hebson and Wood and Diaz-Granados, and a HEC-1 simulation model are evaluated for four catchments in Texas with different climate, geomorphology, soil-type, and land-use characteristics. The procedure derived in this work is an improvement of the previous approach but none of these methods provided consistently better results when compared to Log-Pearson type III distributions of historic data for all four catchments. Improvement in the parameter estimation procedure to provide more reproducible parameters may yield consistently better results.