Process realism: flow paths and storage - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 55

system response observed in the catchment and tries to
interpret it in terms of the controls that may have caused
that response (Sivapalan et al., 2003b ). The collective
system response may be the runoff from the catchment or
it could also be environmental tracers that give additional
information on the sources, flow paths, travel times and
storage. The main strength of the top-down approach is
that it captures the functional behaviour of the catchment in
an integral way. Because of the wealth of information that
tracers can provide they are particularly appealing for
understanding the activation of flow paths and storage.
There are artificial tracers (i.e., applied externally into the
system) and environmental tracers where the naturally
occurring chemical and isotopic signatures of the waters
are exploited (Leibundgut et al., 2009 ). In ungauged
basins, very often tracer data are unavailable. However,
there is potential for transposing relationships between
flow path or storage indices (such as proportion of event/
pre-event water and mean transit times) and bio-geo-
physical catchment characteristics that have been
developed in gauged catchments. These relationships can
subsequently be used to estimate the respective flow path/
storage indices in ungauged basins.
The bottom-up approach utilises knowledge of compon-
ent processes that operate within the catchment. These are
controlled by the bio-geo-physical catchment structure

relatively gentle slopes and predominantly convex land-
forms, runoff is usually generated in a narrow area close to
the streams where there is good subsurface connectivity
(Kirkby, 2005 ). The distance water travels in the subsur-
face therefore tends to be shorter than in arid climates,
where the most significant flows can occur at depths of
hundreds of metres (Möller et al., 2007 ). In a similar way,
flow paths can vary significantly between seasons. For
example, in Mediterranean and monsoonal climates flow
paths tend to be shorter during the wet part of the year than
during the dry part. During the dry part of the year slow
shallow subsurface movement of water through hillslopes
maintains low flows. However, the steep topography of the
north-west USA and western Canada means that preferen-
tial flow paths dominate runoff generation, despite the
Mediterranean climate. Second, climate controls flow
paths indirectly through soil moisture. High soil moisture
states tend to produce faster and shallower flow paths (e.g.,
Grayson et al., 1997 ; Western et al., 2004 ) than lower soil
moisture states. However, in hydrophobic soils, the oppos-
ite can be the case when infiltration increases with increas-
ing soil moisture (Zehe et al., 2007 ). Snowmelt typically
causes surface saturation and therefore surface or shallow
flow paths, and similar effects can occur with frozen
ground (Carey and Woo, 1998 ). Third, climate controls
flow paths through the co-evolution of landscapes, vegeta-
tion and soils where flow paths reflect the dynamic
equilibrium between drainage and storage functions of a
catchment (Savenije, 2010 ; Zehe et al., 2010 ).
In arid environments with limited vegetation cover but
high rainfall intensities, infiltration excess overland flow
tends to occur (Smith and Goodrich, 2005 ). In humid
environments with well-developed vegetation cover and
frequent occurrence of frontal rainfall systems, saturation
excess runoff generation is a more likely mechanism. Also,
in humid catchments efficient subsurface drainage features
may have developed that are conducive to fast subsurface
response (McGlynn et al., 2002 ). In arid regions the inter-
play of landscape, geology and rainfall tends to result in
highly non-linear runoff generation processes. In the
coastal mountains of Oman ( Figure 4.2b ), for example,
most of the surface runoff is generated during high inten-
sity storms on bare rock. Some of it may re-infiltrate into
debris fans resulting in the recharge of local aquifers (right
valley side of Figure 4.2b ). The date plantations in the
photograph are an indication of the existence of a shallow
aquifer. The surface runoff during the flash floods is then
routed out to the coastal plain ( Figure 4.2a ), where much of
the water may infiltrate into the groundwater below the
Wadi, or discharge into the sea (Al-Rawas and Valeo,
2009 , 2010 ).
Topography and landscape characteristics control flow
paths at different

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soils, geology and topography and climate, among other

controls. Much effort has been spent on understanding how
structural features of catchments control these component
processes in research catchments (e.g., Zehe et al., 2001 ).
The main strength of the bottom-up approach is that the
component processes can be connected to the characteris-
tics of the flow paths (e.g., location of the flow paths,
residence times), i.e., to the perceptual model of the
catchment. For estimating runoff signatures in ungauged
basins, the bottom-up approach can be assisted by terrain
analyses or reconnaissance field trips where, say, erosion
marks are used to judge the presence of surface flow paths.
This chapter specifically focuses on flow paths and
storage. We review top-down and bottom-up approaches
of understanding flow paths and relate integral flow path/
storage characteristics to the process controls, so as to
provide a link between the top-down and bottom-up
approaches. Finally, we provide some guidance as to how
predictions of runoff signatures in ungauged basins can be
informed by an understanding of flow paths and storage.

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4.2 Process controls on flow paths and storage
Climate and landforms control flow paths in a number of
ways. First, the most direct way is through the water and
energy input. For example, in regions with humid climates, time scales. Surface and bedrock