A probabilistic view of Hershfield's method for estimating probable maximum precipitation

A simple alternative formulation of Hershfield's statistical method for est imating probable maximum precipitation (PMP) is proposed. Specifically, it is shown that the published Hershfield data do not support the hypothesis t hat there exists a PMP as a physical upper limit, and therefore a purely pr obabilistic treatment of the data is more consistent. In addition, using th e same data set, it is shown that Hershfield's estimate of PMP may be obtai ned using the generalized extreme value (GEV) distribution with shape param eter given as a specified linear function of the average value of annual ma ximum precipitation series and for return period of about 60,000 years. Thi s formulation substitutes completely the standard empirical nomograph that is used for the application of the method. The application of the method ca n be improved when long series of local rainfall data are available that su pport an accurate estimation of the shape parameter of the GEV distribution .