Environmental Engineering Reference
In-Depth Information
soil erosion and vegetation cover. The chapter starts by
examining this interaction in a complex systems context,
developing the argument that soil erosion results from
the instability between soil production and removal rates,
which in turn is a function of the stability of the vegetation
cover. This interaction in turn leads to the concept of
competition between soil and plant cover that leads to
erosional outcomes and to a deeper discussion of the
concept of complexity and how it can be identified.
Since the great drought crisis of the Sahel in the 1960s
and 1970s, desertification has been strongly identified
with climate change. The third part of the chapter
attempts to demonstrate how change on climatic gra-
dients can lead to critical instabilities at different places
along the gradient, and how to conceptualize the recovery
trajectories both in space and time. This emphasis on
climate change belies the fact that plant productivity is
strongly affected both by human and animal impacts on
the plant cover and by the intrinsic changes within the
plant cover itself through time. These issues form the
final section of the chapter, where we emphasize that the
emergence of instability in the plant cover itself results
not only from climatic and anthropic variation but also
from genetic drift among the hillside plant communities.
reticular between the scattered and concentrated vegeta-
tion cover. Rills form and lead to higher erosion rates.
The second basic proposition is that erosion is caused
by either Hortonian or saturated overland flow. The
former occurs when the rainfall intensity is greater than
the soil-infiltration rate. The latter occurs when the soil
is already so full of water that no more can enter. Soil-
water storage is thus crucial to the occurrence of erosion.
Soil storage is the product of the soil moisture per unit
depth and the depth over which the soil moisture is stored.
Deep soils with lowmoisture content have greater storage.
Conversely, shallow soils with high moisture content are
close to saturation. According to Musgrave's equation,
erosion rate ( E ,MT 1 ) is a function of the power of unit
discharge and slope in the form (Musgrave, 1947):
Kq m s n
E
=
(24.1)
where K is an erodibility parameter, q is the discharge
per unit width (L T 1 ), s is the slope angle (dimension-
less), and m and n are exponents shown empirically and
theoretically for sheet wash to be 2 and 1.66, respectively
(Carson and Kirkby, 1972). This relationship means that
thin soils with high soil-moisture content have much ero-
sion, while deep soils with low soil-moisture content have
low erosion rates and that intense rainfall produces more
runoff and erosion than less intense rainfall. K in the
Musgrave equation is the soil erodibility, as measured by
the shear strength and cohesion of the soil. Most activities
that change the soil erodibility, water-holding capacity or
infiltrability of the soil also thereby change the likelihood
of erosion.
Since 1976, the role of the stone content of soils has also
been identified as being involved in runoff production,
though the evidence is not unequivocal (Poesen and
van Waesemael, 1995). When particles are lying on the
surface, infiltration tends to increase as the fraction of
stone fragments increases. When particles are embedded
within the soil, the infiltration and storage capacities
decrease. Surface stones reduce runoff and protect soil
against erosion. Embedded stones reduce storage, increase
runoff andmay lead after erosion to an armoured layer. In
the models described below, stone content is considered
an important component of erosion control.
The third basic proposition needed is that, after vege-
tation has been cleared, for example by grubbing up, by
deliberate removal by machinery, or perhaps by fire, the
recovery rate is assumed to be logistic. That is, the rate
of regrowth takes place slowly at first, then very quickly,
then it converges on the carrying capacity of the envi-
ronment. This process is illustrated in Figure 24.2, where
24.2 Basic propositions
The fundamental core of what follows is the proposition
that plant cover reduces the rate of erosion. Although
hypothesized for over 100 years, this was first empirically
demonstrated in a simple but important experiment by
Elwell and Stocking, published in 1976 and reaffirmed
several times since (e.g. Shaxson et al ., 1989). They showed
that there is a steep decay in the rate of soil erosion,
compared to that on bare soil, as the plant cover increases.
The erosion rate is scaled so that the erosion rate for a
bare soil (i.e. no vegetation)
100% and vegetation is
scaled between 0 and 100% (Figure 24.1a). Notice that the
curve is not linear but negative exponential. With a 30%
cover, the erosion rate has reduced to 30% of the bare soil
value. Francis and Thornes (1990) further showed that
this relationship depends on the intensity of rainfall. With
higher mean intensity of rainfall, the curve is steeper than
with lower mean rainfall intensity (Figure 24.1b).
Another variant of the exponential decay (Figure 24.1c)
is that the erosion rate increases slowly above zero as the
cover increases. This pattern is thought to represent the
effect of shrubs or bunch grasses breaking up sheet flow
into more concentrated rill flows as the flow becomes
=
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