Geology Reference
In-Depth Information
Rainfall
Land cover
Slope
Soil
topographic conditions. Since the relationships
on which the model is based are essentially
statistical, it is an empirical model. Meyer and
Wischmeier were interested in developing a more
process-based approach in which the actual proc-
esses of erosion were described mathematically.
Their objective was not predictive in terms of
erosion rates, but rather to demonstrate that such
an approach was feasible and that it could simu-
late the patterns of erosion and deposition
observed in the landscape.
Based on research by Ellison (1947), Meyer and
Wischmeier (1969) conceptualized erosion as a
two-phase process comprising the detachment
and transport of soil particles by rainfall and
runoff. They therefore described erosion as
a result of (a) the detachment of soil particles by
rainfall; (b) the detachment of soil particles by
runoff; (c) the transport of soil particles by rainfall;
and (d) the transport of soil particles by runoff.
Empirical equations were selected, derived largely
from the results of laboratory experiments, to
describe each of these processes. The landscape
was visualized in simple terms as a single slope
profile from hilltop to valley bottom which could
be divided into a series of segments. Erosion was
simulated by calculating for each segment the
amount of soil detached and the capacity to trans-
port it out of the segment in a downslope direc-
tion. The amount of soil supplied by detachment
on each segment was that detached on the
segment and that transported into the segment
from upslope. A simple arithmetical calculation
compared the amount of sediment available from
detachment with that which could be carried in
transport. If the amount of detachment was less
than the capacity for transport, all the sediment
was removed downslope; if the amount of
detachment was greater than the capacity for
transport, only the amount which could be trans-
ported was carried downslope and the rest was
deposited on the segment. Since the transport
capacity depended on the steepness of the slope,
the model simulated a net loss of soil by erosion
on the upper and mid-slope segments, and the
deposition of soil on gentler concave segments at
the foot of the slope.
Interception
Soil
surface
condition
Vegetation
storage
Throughfall
Surface
depression
storage
Leaf drainage
Stemflow
Surface
water
depth
Net rainfall
Infiltration
Infiltration-
excess
overland
flow
Detachment
by raindrop
impact
Detachment
by flow
Flow transport
capacity
Total detachment
Sediment
transport /deposition
Soil loss
Fig. 2.3
The structure of EUROSEM: one of many
models produced as erosion modellers aspire towards
a white-box model.
the foundation for many subsequent erosion
models. In the 1960s, the only widely-used
approach to erosion prediction was the USLE
developed by Wischmeier and Smith (1965) as a
design tool for soil conservation workers in the
US, particularly in the Corn Belt. As noted above,
it is a grey-box model which predicts the mean
annual rate of soil erosion at a field scale under
different cropping systems and management
practices for a given set of rainfall, soil and
