Geography Reference
In-Depth Information
transferred from similar gauged catchments ( Chapter 10 )
one would of course also hope that the flow paths in those
gauged catchments are represented in realistic ways, i.e.,
that the runoff model is not obtained through hydrograph
fitting but is right for the right reasons. The information on
flow paths and storage can therefore also be useful for
setting model structure and model parameters in neigh-
bouring catchments that are gauged. Again, methods are
available where flow path information, e.g., from tracers,
can be used to choose more realistic model structures over
less realistic structures (Vaché and McDonnell, 2006 ; Son
and Sivapalan 2007 ; Birkel et al., 2010 ).
available, but relationships between flow path/storage
indices and bio-geo-physical catchment characteristics
could be obtained from studies in gauged catchments
and then used to extrapolate to ungauged catchments on
the basis of similarity.
The bottom-up approach reasons on the basis of com-
ponent processes within catchments, which are con-
trolled by the bio-geo-physical catchment structure.
Findings in research catchments can help to understand
how structural features of catchments control these flow
paths. For estimating runoff signatures in ungauged
basins, the bottom-up approach can be assisted by
index-based terrain analyses that help generate the dis-
tribution of available storage across the catchment, as
well as partitioning surface and subsurface flow paths.
Other indices are based on the concept of co-evolution,
and account for diverse feedback processes between
hydrology, climate, geomorphology and ecology. The
idea here is that different landscape units within a catch-
ment may have different functions and are typically
formed by different processes, and can therefore be
easily distinguished.
4.6 Summary of key points
Flow paths and storage are catchment dynamic charac-
teristics that may not be visible on the surface, but have
a key role in the nature of hydrological processes occur-
ring in catchments, and are strongly reflected in their
runoff signatures. The intricate patterns of intercon-
nected surface and subsurface flow paths occurring at a
multitude of scales are a result of the co-evolution of
climate, vegetation and other landscape features through
the interaction of several earth system processes, includ-
ing flow processes.
Information on flow paths and storage is beneficial for
predicting runoff signatures in ungauged basins as it
underpins the perceptual model of the flow system. In
process-based models, understanding of the flow system
helps define the model structure and/or parameter
values. In statistical models, such as regressions, it pro-
vides guidance on the selection of catchment character-
istics and interpretation of the coefficients.
In order that a model can be reliably extrapolated to
ungauged basins, it has to predict accurately for the right
reasons, i.e., it must reflect essential hydrological pro-
cesses within the catchment, and must correctly repre-
sent the flow paths in the subsurface.
Flow path estimation in ungauged basins can be
achieved by both top-down and bottom-up approaches.
The top-down approach examines the collective system
response observed in the catchment, such as runoff or
tracers, and attempts to infer the functional behaviour in
an integrated way. Tracers are particularly appealing for
understanding the activation of flow paths and storage.
In ungauged basins, very often, no tracer data are
Reading the landscape in terms of geomorphic features,
vegetation, soils, rock outcrops and land use will assist
in choosing model structure and setting model param-
eters. Reconnaissance field trips documented by photo-
graphs are an essential way to achieve this. It is always
beneficial to take extra measurements, particularly
measurements of runoff through spot gauging or install-
ing a stream gauge.
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