Geoscience Reference
In-Depth Information
11
Runoff generation, overland flow
and erosion on hillslopes
John Wainwright and Louise J. Bracken
11.1
Introduction
Runoff is generated by three basic mechanisms. In the
first mechanism, precipitation arriving at the surface ex-
ceeds the capacity of the surface to absorb the precipitation
(Figure 11.1). The rate at which the surface can absorb
the precipitation is called the infiltration rate. The infil-
tration rate creates the threshold for runoff generation in
one of two ways. First, the rainfall intensity can exceed
the instantaneous infiltration rate of an unsaturated soil.
This mechanism produces infiltration excess or Horto-
nian overland flow (named after the Robert Horton, who
lived from 1875 to 1945 and made many significant ad-
vances to our understanding of hydrology and erosion in
the first half of the twentieth century). Second, saturation
of the soil can produce saturation overland flow (some-
times called Dunne overland flow after the geomorphol-
ogist/hydrologist Thomas Dunne) as precipitation arrives
on an already saturated surface. In reality, the distinction
between these two mechanisms is somewhat artificial, as
even saturated soils will enable infiltration at a low rate
(the general exception being in soils with high contents of
swelling clays) and so they can still be considered to be
producing runoff by infiltration excess. These conditions
may be relatively common in cases where storms follow
one another on successive days (Lange
et al.
, 2003).
The second basic mechanism of runoff generation is
by exfiltration, or return flow. Exfiltration occurs when
saturated soils receive lateral flows from upslope, which
causes them to exceed their capacity for soil-moisture
storage. Where slope angles get steeper in the downs-
lope direction, or where planform concavities occur, con-
vergence of subsurface flows can occur, producing con-
centrations of moisture that can lead to exfiltration. An
It may appear paradoxical to consider the role of water
flows in arid regions, yet they are one of the most important
landscape-forming processes in many drylands. As noted
in Chapter 1, there are a number of reasons why aridity
occurs. In the tropical and subtropical zones, low annual
rainfalls are a function of extended dry periods and a rel-
atively short rainy season. For example, in the US south-
west, over half the annual rainfall typically falls between
July and September (Osborn and Renard, 1969; Nielson,
1986; Wainwright, 2005); in the southern Mediterranean,
precipitation is concentrated over a few winter months,
while the northern shores tend to experience peaks in au-
tumn and again in spring (Wainwright and Thornes, 2003,
Bracken, Cox and Shannon, 2008). Rainfall in these en-
vironments is predominantly convective, producing large
pulses of rain in a matter of minutes or hours. High rain-
fall intensities lead to the crossing of process thresholds
and corresponding high energies produce the potential
for significant amounts of erosion. The position of these
thresholds is affected also by the nature of dry spells. Cold
ocean current deserts tend to have significant amounts of
their precipitation in the form of fog, which can con-
tribute to the formation of biological crusts (Belnap,
2006). The extent of periods of aridity leads to dis-
tinctively patchy vegetation characteristics (Wainwright,
2009a), which often provide extensive bare surface areas
that enhance runoff production. These periods and patches
also enhance aeolian activity (see Section 4), which can
itself feed back to thresholds of runoff production, e.g. by
the formation of mechanical crusts (e.g. Valentin, 1993).
