The pooling-group method is a flexible tool allowing new and updated data to be used as they become available through HiFlows-UK. Consequently, the pooling-group method was retained in this project.
The improved method presented in this report distinguishes between a pooling-group formed for a gauged or an ungauged catchment and defines the weights given to the L- moment ratios for each catchment accordingly. Further model development could be undertaken to account for the intermediate case where a pooling-group is created for an ungauged catchment but data from a nearby gauged catchment are available.
These could be used in a manner similar to the donor transfer scheme defined for estimation of QMED at ungauged catchments. Further research would be needed to identify and quantify the underlying structure of the model errors arising from use of the pooling-group method.
As in the FEH, the formation of pooling-groups in the improved method is based on the the concept of hydrological similarity as defined by a set of catchment descriptors. However, the actual definition of hydrological similarity has been changed in this study, with the substitution of BFIHOST with FARL and FPEXT (retaining ln[AREA] and ln[SAAR]). This was a reasonable choice based on a thorough investigation of
predictive ability of the catchment descriptors, but it does leave the method without any special attention being paid to growth curve estimation on permeable catchments -- the effect in FEH might have been largely illusory. It has been suggested that flood peak data from permeable catchment can exhibit what could be realisations from two distinct flood-generating mechanisms. It is recommended that further research should be undertaken to investigate the existence of such multiple mechanisms and, if confirmed, to determine how to incorporate such effects into the current procedures.
The use of pooling-groups (or regional methods in general) for enhancing single-site estimation is often referred to as “substituting space for time”. Among applied
hydrologists there is a very reasonable attraction in trying to extend the flood data series extracted from systematic flow records back in time with more anecdotal evidence of large flood events that occurred before the systematic recordings were initiated and often pre-dating living memory. To support such activities, efforts have been made to compile and make available, through an on-line archive
(http://www.dundee.ac.uk/geography/cbhe/), information on historical flood events on UK rivers (Black and Law, 2004). Unfortunately, these endeavours have not been matched by associated methodological developments for incorporating such
information into flood frequency analyses based on data from systematic records. A further development of the existing FEH methodology would to combine both types of information in an overall flood frequency analysis. This would provide a significant development and effectively bring together long-standing efforts made by the gauging authorities, the British Hydrological Society, the academic community and a myriad of individuals to provide an improved methodology.
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