COMPARISON OF ANNUAL MAXIMUM SERIES AND PARTIAL DURATION SERIES METHODS FOR MODELING EXTREME HYDROLOGIC EVENTS .2. REGIONAL MODELING

Two regional estimation schemes, based on, respectively, partial durat ion series (PDS) and annual maximum series (AMS), are compared. The PD S model assumes a generalized Pareto (GP) distribution for modeling th reshold exceedances corresponding to a generalized extreme value (GEV) distribution for annual maxima. First, the accuracy of PDS/GP and AMS /GEV regional index-flood T-year event estimators are compared using M onte Carlo simulations. For estimation in typical regions assuming a r ealistic degree of heterogeneity, the PDS/GP index-flood model is more efficient. The regional PDS and AMS procedures are subsequently appli ed to flood records from 48 catchments in New Zealand. To identify hom ogeneous groupings of catchments, a split-sample regionalization appro ach based on catchment characteristics is adopted. The defined groups are more homogeneous for PDS data than for AMS data; a two-way groupin g based on annual average rainfall is sufficient to attain homogeneity for PDS, whereas a further partitioning is necessary for AMS. In dete rmination of the regional parent distribution using L-moment ratio dia grams, PDS data, in contrast to AMS data, provide an unambiguous inter pretation, supporting a GP distribution.