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capacity. Soil depth is another determinant of runoff gen-
eration, including its distribution in space, especially in
relation to annual precipitation, or typical event rainfall
depth. Therefore the depth to bedrock or to an imperme-
able stratum would be indicative of the likelihood of satur-
ation excess and/or subsurface stormflow within the
catchment. Vegetation type and cover play important roles
in the water balance and catchment co-evolution. For these
reasons geology, soil texture and vegetation cover are
potential similarity indicators at the catchment scale. The
hypsometric curve expresses how the area of the catchment
is distributed according to elevation, and thus governs the
distribution of topographic gradients that drive the flow,
and is
a)
b)
Figure 10.8. Two types of catchment similarity: (a) similarity
between two different catchments considered as holistic functional
units; and (b) similarity between two different functional units within
the same large catchment.
therefore
another
catchment-scale
similarity
indicator.
The second type of similarity is between different land-
scape units as represented by computational pixels, hill-
slopes or subcatchments within a catchment ( Figure
10.8b ). Measures of this type of similarity can be used in
spatially distributed modelling to reduce the dimensional-
ity of the parameter estimation problem ( Blöschl et al.,
1995 ; Grayson and Blöschl, 2000 ). The similarity meas-
ures are similar to those for the whole catchments; how-
ever, they relate to smaller landscape units. Often, index
methods are used for this purpose ( Section 4.4.2 ). Topo-
graphic indices such as that of Beven and Kirkby ( 1979 )
can be used to delineate similar regions within a catch-
ment. Figure 10.9 presents an example of the delineation of
a catchment into functional landscape units. These are
wetlands, hillslopes and plateaus, corresponding to three
dominant runoff generation mechanisms: saturation excess
overland flow, storage excess subsurface flow, and deep
percolation. In the hydrological response unit (HRU)
concept (Leavesley, 1973 ; Flügel, 1995 ) the catchment is
partitioned on the basis of slope aspect, vegetation type and
soil type. Each resulting subunit is considered homoge-
neous with respect to its hydrological response. The char-
acteristic of the vertical soil profile is often essential and
therefore a useful similarity parameter. An example is the
HOST classification, where the interaction between soils,
geology and topography has been used to identify seven
classes, each of which is considered homogeneous with
respect to local soil response. More generally, measures of
similarity could include surface infiltration capacity (per-
meable vs. impermeable soils), soil depth (deep vs. shallow
soils), topographic slope and lateral saturated hydraulic
conductivity (indicating well drained vs. poorly drained
soils). Of course, many of these features could exhibit
organised heterogeneity within a catchment, such as soil
and vegetation catenas, and these can be additional indica-
tors of catchment similarity.
Similarity measures (such as aridity index, topographic
wetness index, runoff coefficient, bifurcation ratio and
likely flash flooding, whereas more uniform, widespread
precipitation produces runoff fields that cause more wide-
spread and persistent flooding (Viglione et al., 2010ba , b ;
Zoccatelli et al., 2011 ). These differences in precipitation
regimes can have different impacts not only on runoff
fields but also on vegetation patterns that develop in
response to precipitation, and on soil erosion and channel
morphology. Thus, in the long term, through such co-
evolution processes, they may lead to differences in catch-
ment characteristics that are attributable to the climate as
well, and give rise to fundamental differences in runoff
behaviour. For example, the dominant control on runoff
variability in northern Queensland, Australia, is stormi-
ness, whereas in south-west Australia the dominant feature
is the seasonality of precipitation (Jothityangkoon and
Sivapalan, 2009 ; Samuel and Sivapalan, 2008 ). These
characteristics have implications for the type of models
that are needed in different regions.
Catchment similarity
Catchment similarity can be defined in two different ways
( Figure 10.8 ). The first is the similarity of two catchments
as a whole, based on catchment-scale indicators ( Figure
10.8a ). This type of similarity measure can be used to
transfer model structures or calibrated model parameters
from gauged to ungauged catchments. The most important
control on the way catchments transform climate inputs
into runoff variability, and therefore the most important
similarity index for catchments as a whole, is catchment
area, because of its aggregation effects and the fact that
bigger storage is commonly associated with catchment size
( Nester et al., 2011 ). With respect to runoff generation and
partitioning, the factors that govern similarity are those
relating to the soils and geology. The saturated hydraulic
conductivity of surface soils is the key to determining if
infiltration excess runoff is dominant, especially in relation
to typical precipitation intensities, and quite often soil
texture can provide a first
indication of
infiltration
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