The all-season, winter, and spring models are compared in Figure 1. The all-season and winter models produce essentially the same results due to the fact that the annual maximum 3-day precipitation accumulation tends to occur in the winter season. However, multiple storm types affect the Willamette River watershed. When multiple storm types are combined within an annual maximum series, the assumption of identical distribution can become tenuous, which affects the ability to make inferences about rare floods. Additionally, the behavior of the right-hand tail typically used in dam and levee safety studies may be dominated by a storm type that is less dominant in the more-frequent part of the accepted all-season precipitation-frequency model. Treating multiple storm types as separate, IID samples allows for examination of the frequency of storms per type of storm. A mixed population analysis can then be used to combine the models fit to each storm type into a single probability distribution that reflects the occurrence of more than one type of storm in a year.
