Geology Reference
In-Depth Information
cropland - the coefficient
a
for non-cultivated
land is significantly larger than that for cropland.
From a comparison of six critical
S
-
A
datasets cor-
responding to various Mediterranean study areas
in Europe and collected using the same methodol-
ogy, Vandekerckhove
et al
. (2000) found that veg-
etation type and cover were far more important
than climatic conditions in explaining differences
in topographic thresholds for different areas. In
cultivated fields, topsoil structure and soil mois-
ture conditions, as controlled by the antecedent
rainfall distribution, are crucial factors affecting
the
S
-
A
relationships, rather than daily rain for the
gully initiating events. For rangelands, vegetation
cover and type (annuals and perennials) at the
time of gully head development appears to be the
most important factor differentiating between
topographic thresholds. The importance of vege-
tation biomass in concentrated flow zones for
reducing gully initiation risk in semi-arid envi-
ronments was also stressed by Graf (1979) and
Nogueras
et al
. (2000). Graf (1979) observed criti-
cal flow shear force (
F
c
, dynes) for gully cutting
into valley floors in Colorado to depend on the
valley-floor biomass (
B
v
, kg m
−2
) following the
relation
F
c
and therefore gully density will increase, as
pointed out by Kirkby (1987).
Several studies have applied the topographical
threshold concept in combination with a hydrau-
lic threshold to predict areas at risk of gullying
(e.g. Dietrich
et al
. 1993; Prosser & Abernethy,
1996; Jetten
et al
., 2006). Desmet and Govers
(1997) and Desmet
et al
. (1999) investigated the
relative importance of slope gradient (
S
) and
drainage area (
A
) for the optimal prediction of the
initiation and trajectory of ephemeral gullies. In
the latter study, a striking discrepancy was found
between the high
A
-exponent (i.e. 0.7-1.5)
required to predict optimally the trajectory of the
gullies, and the low
A
-exponent (i.e. 0.2) required
to identify spots in the landscape where ephem-
eral gullies begin.
Where do (ephemeral) gullies end?
Gullies usu-
ally end where the transporting capacity of the
concentrated runoff drops and/or where the ero-
sion resistance of the topsoil increases sharply.
A sudden change from one land use to another
might trigger sediment deposition instead of
channel entrenchment (vegetation-controlled
sediment deposition, e.g. Takken
et al
., 1999;
Beuselinck
et al
., 2000; Steegen
et al
., 2000). In
many field conditions, a lowering of the slope
gradient with increasing drainage area causes a
drop in transporting capacity, and hence a decrease
in gully channel depth (slope-controlled sediment
deposition). In contrast with critical
S
-
A
relations
established for the location of gully heads, few
S
-
A
relations have been established for the loca-
tion of sites where (ephemeral) gullies end (e.g.
Poesen
et al
., 1998; Vandekerckhove
et al
., 2000;
Nachtergaele
et al
., 2001a,b). Field measurements
in different cropland areas of northern Europe
reveal that topographically-induced sediment
deposition at the lower end of ephemeral gully
channels, which developed in loamy to loamy
sand soils, usually occurs in a narrow range of
local slope gradient under cropland of 2-4%.
However, when the rock fragment content of
the topsoil increases, slope-controlled sediment
deposition occurs on steeper slopes of up to
25-30% (Poesen
et al
., 2002).
0.07 (
B
v
)
2
. Along the same lines,
Prosser and Slade (1994) demonstrated through
flume experiments on an unincised valley floor
near Canberra, Australia, the crucial role that
vegetation cover plays in decreasing the suscepti-
bility of valley floors to gully formation. In addi-
tion to the above-ground biomass, plant roots
significantly contribute to the increase of the site
resistance to concentrated flow erosion. De Baets
et al
. (2006, 2007) demonstrated that fibrous
(grass) roots provide a larger increase in soil cohe-
sion and hence in resistance to concentrated flow
erosion compared with tap-rooted plant species.
From these observations it becomes clear that
any land-use change implying a vegetation bio-
mass decrease (either above- or below-ground),
as well as a lowering of the erosion resistance of
the topsoil by tillage operations in concentrated
flow zones, will decrease the topographic thresh-
old for incipient gullying. This implies that for a
given slope gradient (
S
), the critical drainage area
(
A
) for gully head development will decrease,
=
