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reported by Faulkner (1974), De Ploey (1989),
Kemp (1990), and Robinson and Hanson (1994).
Merz and Bryan (1993) and Stein and Julien (1993)
described more refined approaches for nickpoints
at incipient rilling and headcut migration in gul-
lies and rills. Hanson
et al
. (2001) proposed a
detailed model based on failure at headcuts. They
used the geometry of the plunge pool and water-
fall combined with the hypothesis that erosion
rate is proportional to the excess of effective
stress on the critical stress needed to cause fail-
ure. Alonso
et al
. (2002) described the headcut
migration as a self-similar propagation process.
Central to their model is the description of the
erosive action of the jet that enters and scours the
gully plunge pool. Flores-Cervantes
et al
. (2006)
extended their approach and implemented it in
the three-dimensional landscape evolution model
CHILD (Tucker
et al
., 2001). Prasad and Römkens
(2003) presented a holistic and energy-based
conceptual framework for modelling headcut
dynamics.
Although several attempts have been made to
develop empirical and process-based models for
predicting either gully subprocesses or gully ero-
sion rates in a range of environments, there are
still no reliable (i.e. validated) models available
allowing one to predict impacts of environmental
change on gully erosion rates at various temporal
and spatial scales, and their impacts on sediment
yield, hydrological processes and landscape
evolution.
by Esteves and Lapetite (2003) in Niger. Such
water transmission losses have also been reported
to occur in smaller erosion channels (i.e. rills, see
Poesen & Bryan, 1989; Parsons
et al
., 1999) as well
as in larger (ephemeral) river channels (for a review,
see Beven, 2002). Recent studies (e.g. Leduc
et al
.,
2001; Avni, 2004) indicate that gully development
in semi-arid areas may therefore lead to significant
groundwater recharge. On the other hand, if gul-
lies develop into hillslopes with temporary water
tables, they will cause an enhanced drainage and a
rapid water table lowering, which results in a sig-
nificant drying-out of the soil profiles in the inter-
gully areas, as observed by Moeyersons (2000) in
Africa. In addition, Okagbue and Uma (1987)
reported that gullies located at the seepage areas of
groundwater systems in southeastern Nigeria may
become very active during the peak recharge times
of the rainy season, because high porewater pres-
sures reduce the effective strength of the uncon-
solidated materials along the seepage faces. The
seepage forces caused by exit hydraulic gradients
at the levels of seepage on the gully walls produce
boiling conditions, piping and tunnelling that
undermine the gully walls and activate their
retreat. Most erosion models are driven by hydro-
logical models (runoff). The previous discussion
clearly indicates that there are also important
feedback mechanisms - gully erosion may in turn
also control the intensity of some hydrological
processes (water transmission losses or groundwa-
ter depletion) at the hillslope scale. These interac-
tions deserve more attention.
How does gully erosion interact with other
soil erosion processes? Once gullies develop they
often trigger other soil degradation processes such
as piping, soil fall or soil topple (driven by gravity)
after tension crack development and undercut-
ting. Furthermore, gully channels enhance the
export of sediment produced at the intergully
areas (sheet and rill erosion) by increasing the
connectivity in the landscape (e.g. Stall, 1985;
Poesen
et al
., 2002, 2006), which leads to an
increased risk of sediment deposition in the lower
parts of the landscape. If no gully control meas-
ures are taken, gully growth rates usually decline
exponentially (e.g. Graf, 1977; Rutherford
et al
.,
19.4 Interaction Between Gully Erosion,
Hydrological and Other Erosion Processes
What is the impact of gully erosion on hillslope
hydrological processes such as infiltration and
drainage? Once gullies develop, water infiltration
rates through the gully bottom may be signifi-
cantly greater compared with that of the soil sur-
face in the intergully areas, if the gully channel
develops into more permeable horizons. Through
the gully bed and banks, significant runoff water
transmission losses can then take place, particu-
larly in semi-arid and arid environments as shown
