Comparison of Flood-Frequency Estimates Based on Observed and Model-Generated Peak Flows

Comparisons were made of flood-frequency estimates from a rainfall/runoff model and from observed annual peak data to determine if these estimates exhibited the same statistical variability. The analysis was based on data at 173 stations in 10 States where there were 20 or more rainfall/runoff model. Paired t-tests and analysis-of-variance techniques indicated that there were statistically significant differences between the two sets of flood-frequency estimates. These differences existed for different recurrence interval floods (2-, 10-, 50-, and 100-years), different watershed sizes, and different States. These differences appeared to be primarily related to three infiltration parameters in the rainfall/runoff model and to a lesser extent the observed record length at the station. Furthermore, the ratio of the model-generated 100-year flood discharge to the 2-year flood discharge was 3.98 whereas the same ratio based on observed peak flows was 5.72. Additional research is needed to identify what factors causes flood estimates from the rainfall/runoff model appear to exhibit less variability than flood estimates based on observed peak flow records.