The largest source of uncertainty in a flow frequency analysis often comes from not having enough data. Reducing the uncertainty in the estimated annual exceedance probability of a flood requires incorporation of as much information into the flow frequency analysis as reasonably practical. Opportunities to reduce uncertainty include spatial (trading space for time), temporal (historical information), and causal (rainfall-runoff modeling) information expansion. The full spectrum of information includes at-site systematic and historic data, regional skew information, estimates of flood quantiles derived from a regional regression analysis, and estimates of flood quantiles derived from a rainfall-runoff model using regional precipitation-frequency. Bulletin 17C can accommodate much of the available information, but the current procedures cannot seamlessly combine all types of information into a single flow frequency analysis. These limitations can be overcome by adopting a Bayesian analysis framework. In collaboration with the Engineer Research and Development Center (ERDC), the U.S. Army Corps of Engineers Dam and Levee Safety Programs have adopted the Bayesian framework and developed supporting software. The Bayesian framework effortlessly combines all types of information in a single and more formal probabilistic model. Bayesian estimation methods have less bias and greater efficiency than the traditional method of moments. This leads to greater confidence in flow frequency curves. Bayesian estimation methods provide more flexibility by supporting additional distributions other than Log-Pearson Type III, such as the Generalized Extreme Value and Generalized Pareto distributions. This allows a hydrologist to adapt their analysis to accommodate unique situations. Complementing a flow frequency analysis with all the available temporal, spatial, and causal information is the gold standard. Adopting a Bayesian framework provides a pathway to achieve the recommendations in Bulletin 17C and advance the state of practice in flow frequency analysis.
