Geoscience Reference
In-Depth Information
uniquely defined as 'surface roughness'. It can be char-
acterised in a variety of ways, as, for example, devia-
tions from a plane, tortuosity or semi-variance (Kuipers,
1957; Boiffin, 1984; Linden and Van Doren, 1986). Even
studies using the term 'random roughness' to charac-
terise deviations from a plane employ it in a range of
different ways (Onstad, 1984). Moreover, it seems nec-
essary to distinguish between roughness as applied in
modelling studies, often as a calibration factor and a
property of the flow itself implicitly representing those
processes not directly included in any model including
some fluid boundary (Lane and Ferguson, 2005), and
a more field-based definition of roughness describing
the topographic form of a surface. More generally, all
processes operating at the Earth's surface interact with
surface roughness (at whatever scale) in a multitude of
ways. These processes often exhibit a complex relation-
ship with roughness, and so any attempt to represent sur-
face roughness must be sensitive to and informed by such
behaviour.
Field exploration has shown that flow resistance is
highly variable, even within a single plot (Smith, Cox
and Bracken, 2010). Simple relationships, as predicted by
conventional equations based on pipe-flow experiments,
were found to provide an inadequate description of over-
land flow processes. The complexity of the surface forms
demonstrated by natural soil surfaces and progressive in-
undation of roughness elements may explain the poor
performance of these equations, which represent surface
roughness using a single roughness measure (Smith, Cox
and Bracken, 2010).
is developing at an exponential rate (Bracken and Croke,
2007). Hydrological connectivity refers to 'the transfer
of water and related matter from one point in a catch-
ment to another' (Pringle, 2003; Tetzlaff
et al.
, 2007).
The development of hydrological connections via over-
land flows is a function of both water volume (supplied
by rainfall and runon, depleted by infiltration, evaporation
and transpiration) and rate of transfer (a function of flow
resistance). These processes interact with flow resistance
varying as a function of flow depth. This interaction es-
tablishes a feedback between rainfall, infiltration and flow
routing, which produces the nonlinearity seen in river hy-
drographs and scale dependence of runoff coefficients. Ex-
isting research has determined two elements to hydrologi-
cal connectivity: static/structural and dynamic/functional
connectivity (Bracken and Croke, 2007; Turnbull, Wain-
wright and Brazier, 2008). Structural connectivity refers
to spatial patterns in the landscape, such as the spatial
distribution of landscape units that influence water trans-
fer patterns and flow paths. Functional aspects of con-
nectivity refer to how these spatial patterns interact with
catchment processes to produce runoff, connected flow
and hence water transfer in catchments (Turnbull, Wain-
wright and Brazier, 2008). Research to date has been good
at describing the elements defining structural connectivity
(Lexartza-Artza and Wainwright, 2009; Kirkby, Bracken
and Reaney, 2002; Bull
et al.
, 2003). However, the el-
ements defining functional aspects of hydrological con-
nectivity are more important in understanding the con-
cept, but are more difficult to measure and quantify due to
their dynamic nature, complexity and variability (Bracken
and
Croke,
2007;
Lexartza-Artza
and
Wainwright,
2009).
Previous studies attempting to understand functional
aspects of hydrological connectivity have been conducted
in a range of environments selected from a continuum of
catchment types and hydrological behaviours. For exam-
ple, research in rangeland catchments in southeast Aus-
tralia and New Zealand demonstrated that patterns in shal-
low soil moisture can be used as an indication of saturated
excess processes that control the fluxes of water in their
catchments (Western
et al.
, 2004). However, results from
studies conducted in bedrock-controlled catchments in
the USA disagree and demonstrate that soil depth and
bedrock topography control the pattern of active flow
generated during storm events (Tromp van Meerveld and
McDonnell, 2006). At an intermediate point on the con-
tinuum between these two environments, work conducted
in temperate forest watersheds suggested a nonlinear re-
sponse in runoff for small variations in antecedent mois-
ture, but did not observe a significant change in geostatis-
11.4.3
Pipes and macropore flow
Soil pipes form under a range of conditions, but are most
common on clay-rich soils. More generally, macropores
develop as a result of soil moisture or temperature cycling,
plant-root decay and animal burrowing. The importance
of this type of flow is that it occurs in a non-Darcian
manner and thus is hydraulically very efficient. Farifteh
and Soeters (1999) demonstrated in southern Italy that
pipes can be extensive parts of the drainage pattern (Fig-
ure 11.10) and generally produced the first flows to reach
the channel from the slope system.
11.4.4
Scales of overland flow
An overarching framework for understanding runoff and
runon is the concept of hydrological connectivity. This
