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Figure 8.3. Impact of reservoir
operation, apparent from April 1990
onwards, on the amplitude of runoff
fluctuations (0: low fluctuation, 1:
high fluctuation) for the Salzach
River at Wald, Austria. (Left)
Monthly runoff, (right) daily average
fluctuations of hourly runoff.
Fluctuations are calculated as the
mean absolute differences of hourly
runoff scaled by the mean runoff for
each day.
Other anthropogenic effects on the low flow regime
include land use changes, such as deforestation, afforest-
ation or urbanisation. Van Lanen et al.( 2004b ) reviewed
the relationship between hydrological processes and low
flows and the impact of human influences on droughts.
Land use effects have been studied by temporal analyses of
runoff before and after the human interferences and com-
parative analyses such as paired catchment studies (e.g.,
Brown et al., 2005 ; Holko and Kostka, 2008 ; Schumann
et al., 2009 ). Land use/land cover change, typically, is a
local phenomenon, so its impact is likely to strongly
decrease with catchment size. The position in the land-
scape will modulate the scale effects. In contrast, climate
impacts may occur at larger scales so one would expect
them to be apparent in both small and large catchments and
be consistent in a region (Blöschl et al., 2007 ).
The combined effect of the various catchment and cli-
mate related processes discussed above can thus be seen to
control the observed patterns of low flows over space and
time. Understanding their relative roles is useful for inter-
preting differences and similarities of low flow regimes
between places,
low flow processes discussed above. Measures of similar-
ity may relate to low flow runoff itself, to climate charac-
teristics and to catchment characteristics.
Runoff similarity
One group of similarity measures extracts properties of the
hydrograph that reflect the effects of climate forcing and
catchment controls on the low flow regime in a collective
way. Most importantly, they describe the timing of low
flows (seasonality and the delay in the response to precipi-
tation). In circumstances where these runoff properties can
be related to climate or catchment characteristics, or to
geographic location, these properties can be profitably
used to identify catchments exhibiting a similar low flow
regime. In particular, similarity measures that reflect sea-
sonality, the response time or flashiness, baseflows and the
recession behaviour may be useful
in regionalisation
studies.
Seasonality of low flows is an indicator of the regime
type and may help identify characteristic processes as a
basis for regionalisation (Laaha and Blöschl, 2006b ). This
is most important in regions where both summer and
winter low flows occur, as these two types are governed
by different processes and need to be treated separately.
Seasonality is mainly related to climate. In a region with
uniform climate, differences in seasonality may arise from
the different timing of runoff recessions, governed by
geology and other catchment characteristics. A number of
similarity measures for characterising low flow seasonality
have been proposed that differ in terms of complexity and
information content. The most common measure is the low
to assist
in estimating low flows in
ungauged basins.
8.2.2 Similarity measures
This section discusses measures of hydrological similarity
relevant to low flows. Catchments that are similar in terms
of their shape, topography, climate, hydrogeology and land
cover can be expected to be similar in terms of their low
flow regime as well, if these characteristics relate to the
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