Geology Reference
In-Depth Information
Rainfall
Water flux
Sediment flux
Control link
Interception
Infiltration
Surface
storage
Splash
erosion
Flow
erosion
Overland
flow
Transport
deposition
Water
discharge
Sediment
discharge
Fig. 12.2
Simplified flowchart of the
LISEM soil erosion model.
well as the physically-based Richards equation
(using the SWATRE submodel: Belmans
et al
.,
1983). Surface storage will result in surface runoff
once a certain threshold is exceeded. Flow veloc-
ity is calculated with the Manning equation and
surface runoff is routed over the landscape with
the kinematic wave equation. The user can spec-
ify a separate channel network. Overland flow
can flow into the channel and is then routed to
the catchment outlet as channel flow.
Overland flow and channel flow are both
routed with the kinematic wave, which is solved
by a four-point finite difference solution using an
implicit method (Chow
et al
., 1988). LISEM routes
the sediment explicitly using water fluxes at the
beginning and end of each time step to determine
the sediment concentration in each pixel.
LISEM simulates erosion by rainfall and ero-
sion by overland flow and channel flow. Rainsplash
erosion is calculated as a function of rainfall and
throughfall kinetic energy and depth of the sur-
face water layer. Sediment transport only occurs
by overland flow and channel flow. For both over-
land flow and channel flow, LISEM uses the trans-
port equation developed by Govers (1990) for
slopes of up to 12°. The equation is based on a
stream-power approach:
TC
=
c
(
SV
−
SV
cr
)
d
· r
s
(12.1)
where
TC
is the transport capacity (g l
−1
)
, S
is the
slope (m m
−1
),
V
is the mean velocity (m s
−1
),
SV
cr
is
the critical unit stream power (m s
−1
),
r
s
is the den-
sity of solids (kg m
−3
), and
c
and
d
are coefficients.
According to Govers (1990) the critical unit
stream power is 0.004 m s
−1
. The coefficients
c
and
d
depend on the median of the grain-size distribu-
tion and can be calculated with equations given
by Morgan
et al
. (1998). Net flow detachment and
deposition are calculated with two equations
based on the EUROSEM model (Morgan
et al
.,
1998), but reformulated for use with pixels:
