Environmental Engineering Reference
In-Depth Information
provide a 'new generation of water erosion prediction
technology
hydraulic conductivity, porosity and wetting front poten-
tial. Of these parameters, the hydraulic conductivity is
the most sensitive (Nearing
et al
., 1990) and is calcu-
lated on a daily timestep, being constantly altered due
to changes in plant growth, management practices and
dynamic soil characteristics (Risse, 1995). Once rainfall
exceeds infiltration, the excess is routed downslope as
overland flow, in interrill areas as unconcentrated flow or
as concentrated flow in rills. This routing is controlled by
the kinematic wave equations as described below for flow
on a plane (Flanagan and Nearing, 1995):
' (Nearing
et al
., 1989). The model has
been applied extensively throughout the US (Risse
et al
.,
1995; Savabi
et al
., 1995; Zhang
et al
., 1996) and alsoon the
South Downs, in the UK (Favis-Mortlock, 1994) as well
as the Woburn Experimental Plots, Bedfordshire (Brazier
et al
., 2000) and elsewhere (Morgan and Nearing, 2011).
A summary of the equations behind theWEPP hillslope
model are found below and can be supplemented with
reference to Lane and Nearing (1989), Flanagan and
Nearing (1995) and Flanagan and Livingstone (1995).
The WEPP is essentially built from four main groups of
data: climate, slope, soil and management. These data
provide the backbone for model runs, with varying levels
of complexity as options should the user desire them.
Climate data can be input in the form of breakpoint
rainfall data, accompanied by daily parameters to describe
the rainfall hyetograph (duration, time to peak and peak
intensity) amongst other variables.
The infiltration component of the hillslope model
is based on the Green and Ampt equation assuming
homogeneous soil characteristics for each overland flow
element within the hillslope, to calculate infiltration for
unsteady rainfall (see Chu 1978):
...
∂
h
t
+
∂
q
x
=
v
(15.3)
∂
∂
and a depth discharge relationship:
h
m
q
=
α
(15.4)
where:
h
=
depth of flow (m);
q
=
discharge per unit width of the plane
(m
3
m
−
1
s
−
1
);
α
=
depth/discharge coefficient;
m
=
depth/discharge exponent;
x
=
distance from top of plane (m).
K
e
1
N
s
F
F
=
+
(15.1)
The Chezy relationship is used for overland flow routing
in WEPP, therefore
CS
0
.
5
o
where C is the Chezy
α
=
where:
coefficient (m
0
.
5
s
−
1
).
The erosion submodel is based on the steady-state con-
tinuity equation. The WEPP calculates soil loss according
to the rill-interrill concept (Foster and Lane, 1981) on
a per rill area basis. A crucial way in which WEPP dif-
fers from earlier soil-erosion models is that the equation
relating to sediment continuity does not rely on the use of
uniform flow hydraulics but is applied separately within
the rills (Risse
et al
., 1995). Interrill erosion or detachment
(
D
i
)isgivenby:
D
i
=
K
i
I
e
σ
ir
SDR
RR
F
nozzle
R
s
W
K
e
=
effective hydraulic conductivity (mmh
−
1
);
F
=
cumulative infiltration depth (mm);
N
s
=
effective matric potential (mm) calculated
from:
N
s
=
(
η
e
−
θ
i
)
(15.2)
where:
η
e
=
effective porosity (cm
3
cm
−
3
);
θ
i
=
initial soil water content (cm
3
cm
−
3
);
=
average wetting-front capillary potential
(mm).
(15.5)
Wetting-front capillary potential is then estimated
from readily measurable soil properties (Rawls
et al
.,
1989) as is
Ke
(Flanagan and Livingston, 1995).
To begin with, infiltration rate will equal rainfall inten-
sity until saturation or ponding occurs, at which point
infiltration starts to decrease towards a constant rate or
'final infiltration rate' (Risse
et al
., 1995). Key parameters
required to drive the infiltration sub-model are therefore:
where:
K
i
=
interrill erodibility (kg s m
−
4
);
I
e
=
effective rainfall intensity (m s
−
1
);
σ
ir
=
interrill runoff rate (m s
−
1
);
SDR
RR
=
sediment delivery ratio;
F
nozzle
=
adjustment factor for sprinkler impact
energy variation;
Search WWH ::
Custom Search