Regionalization based envelope curves forPMPestimation by Hershfield method

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Abstract
Cluster analysis based regionalization procedure is proposed to construct a regional envelope curve for use with Hershfield probable maximum precipitation (PMP) estimation method. The procedure suggests (1) delineation of the study area into homogeneousPMPzones (regions) by performing cluster analysis on attributes affectingPMP, and (2) construction of regional envelope curve for each of the zones, and use of the curve(s) as the basis to estimate regional frequency factor (RFF) andPMPfor site(s) in the study area. Conventional practice suggests construction of a single envelope curve for entire study area, which could be a political/physiographic area, or a river basin. This practice could yield unreliableRFFandPMPestimates if there is considerable variation in topography, landscape and/or climate in the study area. Effectiveness of the proposed regionalization procedure is demonstrated through a case study on 0.25° resolution rainfall data covering India. GlobalK‐means (GKM) and global fuzzyc‐means (GFCM) methods are considered for cluster analysis. The number of delineated homogeneousPMPzones is 20 withGKMmethod and 17 withGFCMmethod. ThePMPestimates obtained for 0.25° resolution grids using the proposed procedure are generally found to be lower than those obtained by considering (1) one/two envelope curves for the entire India determined based on zones considered in previous studies, and (2) envelope curves prepared for four major river basins (Mahanadi, Godavari, Krishna and Cauvery) in peninsular India. ThePMPestimates are often considered as the basis to arrive at probable maximum flood (PMF) for design and risk assessment of large‐scale hydrological structures in river basins. Hence,PMFestimates could be expected to be lower when based onPMPestimates obtained using the proposed procedure, which is likely to have economic implications in design of the structures.

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