Geography Reference
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consists of learning from the similarities and differences of
catchments in terms of their climate, catchment character-
istics and runoff signatures. The underlying assumption is
that catchments that are similar with respect to climate and
catchment characteristics will also behave similarly in a
hydrological sense. This assumption can be tested in
gauged catchments, where one can learn from relating the
runoff signatures to climate and catchment characteristics.
One can then use the concept of similarity to transpose
what one has learned in gauged catchments in order to
predict runoff in ungauged basins.
words, the value of mean annual precipitation as a climate
similarity index for floods goes beyond the event scale
causality, and reflects the net effects of co-evolutionary
processes.
Catchment similarity
Catchment similarity, in the context of this topic, entails
similarity in those catchment characteristics that control
runoff processes (McDonnell and Woods,
2004
). From a
catchment functioning perspective these are processes
that control the partitioning, transmission, storage and
release of water, so similarity relates to similarity in
one or more of these functions. Catchment characteristics
that relate to partitioning include the infiltration proper-
ties of soils, such as hydraulic conductivity, which is
often estimated with the use of pedo-transfer functions
from soil texture. They also include vegetation indices,
often as a proxy of evaporation at seasonal or annual
time scales. Catchment characteristics that relate to trans-
mission are those that represent flow paths in some way.
One example is the topographic wetness index (upslope
contributing area divided by the local surface topo-
graphic slope) that provides similarity of the competition
between hillslope recharge and drainage (Kirkby,
1978
).
Catchment characteristics that relate to storage are geol-
ogy and soil properties such as soil depth. Also, area is
sometimes used as an indicator of catchment storage, as
larger catchments tend to be more groundwater domin-
ated with deeper flow paths and more active storage
availability.
Many of the catchment processes occur below the sur-
face, so similarity is difficult to quantify unambiguously.
Co-evolutionary indices related to interacting catchment
processes are therefore particularly important. The classic
index is stream network density (stream length per area).
The rationale behind the use of stream network density as a
similarity index is that the stream network is itself a result
of the co-evolution of the landscape, soil and vegetation,
subject to the climate and geology in a particular region
(Abrahams,
1984
; Wang andWu,
2012
). Drainage densities
tend to be the result of water availability (precipitation
Climate similarity
Climate similarity, in the context of this topic, entails
similarity in climate characteristics that are relevant for
hydrology. Climate classification schemes such as those
by Köppen (
1936
) and Thornthwaite (
1931
) define regions
through a combination of mean annual precipitation, air
temperature and their seasonal variability. Budyko (
1974
)
and L
'
vovich (
1979
) developed long-term average rela-
tionships between measures of water and energy availabil-
ity in various regions. A typical index of this kind is the
aridity index, which is the ratio of annual potential evapor-
ation and annual precipitation. Those catchments with
aridity indices larger than unity are deemed water-limited,
and those with an aridity index smaller than unity are
energy-limited. If the aridity indices are similar, the catch-
ments are deemed similar with respect to the relative avail-
ability of water and energy. Catchment characteristics,
such as soils, topography and vegetation, puzzlingly only
play a secondary role in this partitioning, which is suggest-
ive of their co-evolution. Climate similarity can also be
defined as similarity in the inter-annual variability of pre-
cipitation if one is interested in the long-term fluctuations
of runoff. Climate similarity can further be defined as
similarity in the extreme rainfall and its seasonality if one
is interested in floods, and in terms of dry spells and of
their seasonality if one is interested in droughts and low
flows. The relative importance of snow processes can be
very relevant for hydrological similarity, and these can be
indexed by air temperature and/or catchment elevation.
Comparative hydrology sometimes discovers similarity
indices and predictors that contradict or defy process
interpretations. This may be because they represent sev-
eral, not one, factors that contribute to the explanation of
a variable of interest, and so mask the process interpret-
ation. An example is mean annual precipitation, which
happens to be a powerful similarity index for flood peak,
for example. It becomes a useful similarity index not only
because of its direct effect on runoff generation at the
event scale but also through its indirect effect on longer-
term soil moisture availability and still longer term land-
scape, soil and vegetation evolution processes. In other
−
evaporation), infiltration characteristics of the surface soils
and the drainage characteristics of the underlying geology,
which together determine howmuch runoff is generated and
the fraction of surface runoff, and the armouring provided
by the presence of vegetation. In this way, the drainage
density is a holistic index combining a range of processes
at a multitude of time scales, and thus reflects the
overall catchment functioning. Hydrology clearly exhibits
many similarities with geomorphology (de Boer,
1992
). In a
review of predictive modelling in geomorphology, Haff
(
1996
) states, inter alia:
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