Generalized Flood Skew: Map versus Watershed Skew

The skew coefficient is an important statistic in flood estimation at gaged sites, but sample values are subject to considerable sampling variation, and the map skew lacks both a conceptual basis and acceptable accuracy. While weighting the sample skew with the Bulletin 17B map skew can temper the variation inherent to sample skew coefficients, the inaccuracy of the map suggests that the weighted value of skew may still not have either the desired accuracy or the accuracy that would be possible if a more realistic generalized skew were used in place of the map skew. A new generalized skew coefficient is proposed herein, namely watershed skew. It has a conceptual basis in its relationship to the skew of the rainfall that generates the floods and the watershed and channel storage that transform rainfall skew to the skew of the runoff. Weighting the sample skew with the watershed skew will provide an estimate of the skew coefficient that incorporates the effects of the physical processes inherent to measured flood data both with rainfall frequency effects and with watershed characteristics that reflect watershed and channel storage. The resulting weighted skew should be more accurate than the current approach. The proposed procedure is applied to four moderately sized watersheds.