Summary: | The case study aims at selecting optimal bivariate copula models of the
relationships between flood peaks and flood volumes from a regional perspective with a
particular focus on flood generation processes. Besides the traditional approach that deals
with the annual maxima of flood events, the current analysis also includes all independent
flood events. The target region is located in the northwest of Austria; it consists of 69 small
and mid-sized catchments. On the basis of the hourly runoff data from the period 1976–
2007, independent flood events were identified and assigned to one of the following three
types of flood categories: synoptic floods, flash floods and snowmelt floods. Flood events
in the given catchment are considered independent when they originate from different
synoptic situations. Nine commonly-used copula types were fitted to the flood peak –
flood volume pairs at each site. In this step, two databases were used: i) a process-based
selection of all the independent flood events (three data samples at each catchment) and
ii) the annual maxima of the flood peaks and the respective flood volumes regardless of
the flood processes (one data sample per catchment). The goodness-of-fit of the nine
copula types was examined on a regional basis throughout all the catchments. It was
concluded that (1) the copula models for the flood processes are discernible locally; (2)
the Clayton copula provides an unacceptable performance for all three processes as well as
in the case of the annual maxima; (3) the rejection of the other copula types depends on
the flood type and the sample size; (4) there are differences in the copulas with the best
fits: for synoptic and flash floods, the best performance is associated with the extreme
value copulas; for snowmelt floods, the Frank copula fits the best.
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