Prediction of floods in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 201

Figure 9.9. Flood frequency plots with the process types indicated for two streams in Austria: (a) Krumbach at Krumbach (43 km² catchment
area); (b) Kleine Mühl at Obermühl (200 km²). From Merz and Blöschl ( 2008b ). Photos: (a) Steindy (b) D. Stancin.

close to each other are characterised by similar climate,
topography, geology, soils and landuse, which gives rise to
similar catchment hydrological response and therefore to
similar floods. The grouping is usually found by combin-
ing maps of the catchment characteristics or by geograph-
ical boundaries (Beable and McKerchar, 1982 ), sometimes
supported by the mapping of the residuals from a regres-
sion model (Wandle, 1977 ; Tasker, 1982 ; Choquette,
1988 ; Jingyi and Hall, 2004 ; see Section 8.2.3).
The assumption of contiguous regions can be relaxed to
allow the catchment groups to be non-contiguous. The
group of catchments is then selected on the basis of climate
and catchment characteristics alone, without using spatial
proximity as a similarity measure. Multivariate statistical
methods, and in particular cluster analysis, are common
methods to perform the grouping (see e.g., Acreman and

Sinclair, 1986 ; Burn and Goel, 2000 ). The grouping can
also involve flood characteristics and climate characteris-
tics, in particular the seasonality of the floods (Castellarin
et al., 2001 ; Piock-Ellena et al., 1999 ). Since the groups
are non-contiguous, an allocation rule is needed to associ-
ate ungauged basins to a particular group.

Different group for each target catchment
An alternative is the region of influence (ROI) approach
(Burn, 1990a ), which assigns a different pooling group to
each catchment of interest. This pooling group is the region
of influence, i.e., a set of gauged basins that are similar to
the ungauged basin of interest. Similarity between catch-
ments is usually measured by the root mean square differ-
ence of all the catchment and climate characteristics in a
pair of catchments. The characteristics are usually