Geology Reference
In-Depth Information
C
systems, it is recognised that information on soil
erodibility is but
one
basic desirable input.
Where direct experience is lacking, soil char-
acteristics, soil type, expected soil management
and cropping regimes, landscape factors, and
the climatic/hydrological context can all be
combined to provide broad guidelines on the
danger of soil erosion, and thus the emphasis
required in implementing soil-conserving man-
agement strategies. Of the wide range of such
strategies, the agronomically-based experimen-
tation reviewed in this chapter support the
effectiveness of soil contact cover (Fig. 11.5)
which can be implemented by all types of
mulching strategies. The use of vegetative bar-
riers has also been found to be most effective in
reducing soil loss, and their implementation in
steeper tropical lands, for example, can be
encouraged by recognition of their potential role
in adding to soil nutrition and productivity. More
recent research, based on the same theory as
employed in GUEST, has provided a physically-
based understanding of the effectiveness of veg-
etative barriers in reducing soil loss (Rose
et al
.,
2002; Ghadiri
et al
., 2002; Hussein
et al
.,
2007a,b).
Ongoing work is designed to build on the
GUEST approach in ways that address the spa-
tial complexity of natural landscapes (e.g. Fentie
et al
., 2004b).
The fraction of bare soil surface
in which soil aggregates are not
fully immersed by overland flowing
water
C
f
Surface fractional coverage by con-
tact cover
d
Rate of sediment deposition
(kg m
−2
s
−1
)
d
i
Sediment deposition rate of sedi-
mentary particles of size range
i
(kg m
−2
s
−1
)
D
Depth of overland flow (m)
e
i
Rainfall detachment rate for particles of
size range
i
(kg m
−2
s
−1
)
e
ri
Rate of rainfall re-detachment for
particles of size range
i
(kg m
−2
s
−1
)
F
Fraction of streampower effective in
erosive processes of erosion event
i
as a subscript refers to a particular
sediment size range
¯
Spatial average value of infiltration
rate (m s
−1
)
Maximum possible value of
¯
for a
plot (m s
−1
)
I
m
J
Specific energy of entrainment
(J kg
−1
)
k
Defined by Equation (11.14)
(kg m
−3.4
s
0.4
)
K
Defined by Equation (11.20)
L
Length of plane
n
Manning's roughness coefficient
P
Rainfall rate (m s
−1
)
q
Symbols List
Volumetric water flux per unit width
of plane (m
3
m
−1
s
−1
)
Q
Symbol
Description
Runoff rate per unit area of plane
(m s
−1
)
A
Cross-sectional area of flow (m
2
)
c
Q
e
Sediment concentration (kg m
−3
)
Effective steady-state runoff rate for a
rainfall event (m s
−1
)
¯
Average sediment concentration for an
erosion event (kg m
−3
)
r
i
Rate of flow entrainment (kg m
−2
s
−1
)
c
t
r
ri
Sediment concentration at the trans-
port limit (kg m
−3
)
Rate of flow re-entrainment (kg
m
−2
s
−1
)
¯
t
R
Average transport limit sediment
concentration for an erosion event
(kg m
−3
)
Hydraulic radius of a rill (m)
S
Slope of the land surface (the sine of
the slope angle)
c
b
t
Bare soil sediment concentration
(kg m
−3
)
Time (s)
V
Velocity of overland flow (m s
−1
)
