Geology Reference
In-Depth Information
requirements of the need to account for vegeta-
tion explicitly and to have been applied in
Europe with the criteria listed above, only WEPP,
EUROSEM and MFF were selected for more
detailed consideration. A further ten criteria
were now added to the original criteria in order
to aid selection between these. The extra crite-
ria were selected based on what was perceived
to be useful in providing guidance and support-
ing decisions on the design and placement of
vegetated buffers in the field. It can be seen from
Table 13.3 that the MMF model (Morgan &
Duzant (2008) version) scores highly against
these new criteria. It is therefore selected as the
preferred model.
ity, flow velocity, flow depth and slope length.
The detachment, transport and deposition of the
soil particles are also calculated separately for the
clay, silt and sand fractions of the soil.
When operated over small catchments, the
MMF model requires the landscape to be divided
into elements of reasonably uniform soil type,
slope and land cover. The user needs to determine
the pathways that the runoff follows in moving
from one element to another. Each element is
therefore assigned a number (e.g. element 1,
element 2, etc.) and then, for each element, deter-
mining the number of the element from which it
receives water and sediment and the number of
the element to which it discharges water and sed-
iment. For example, Fig. 13.2 shows the processes
represented by the model as operating on a single
element. If this was element number 2, it would
receive runoff and sediment from an element
upslope (e.g. element 1) and contribute runoff and
sediment to an element downslope (e.g. element 3).
The model calculates the runoff and sediment
budgets for each element, taking account of the
runoff and soil particle detachment from raindrop
impact generated on the element itself, and the
runoff and sediment inputs from upslope.
Together these determine the total runoff on the
element, soil particle detachment by runoff, and
the sediment transport capacity.
Previous experience with the model has shown
that better results are generally achieved when
measured data rather than guide values are used
for the input parameters (Morgan
et al
., 1984) and
that care needs to be taken when determining
the element structure. A particular problem is that
very different results can be obtained according to
how the landscape is described. For example, for a
100 m long uniform slope, the model produces dif-
ferent results if this is treated as a single element
compared with describing it by two 50-m long ele-
ments, or routing the water and sediment over five
20-m long elements. In order to keep these differ-
ences within acceptable limits, it is recommended
that element lengths should normally be about
10 m and no shorter than 5 m or longer than 50 m.
Table 13.4 lists the input parameters required
to operate the model and the sources of informa-
13.4 Model Application
The MMF model simulates the movement of
water and sediment over the landscape from
source to delivery to the river system. The origi-
nal version of the model was developed to predict
mean annual soil loss from field-sized areas on
hillslopes (Morgan
et al
., 1984), but with the addi-
tion of simple routing procedures, it could also
be applied to small catchments (Morgan, 2001).
The model operates by separating the erosion
into two phases: a water phase in which the
energy of the rainfall and the volume of runoff are
estimated, and a sediment phase, which consid-
ers the detachment of soil particles by raindrop
impact and runoff and their transport over the
land surface by overland flow. The predictions of
total particle detachment and transport capacity
of the runoff are compared and the lower of the
two values represents the annual soil loss, thereby
indicating whether detachment or transport proc-
esses are the limiting factor. In the Morgan and
Duzant (2008) version, the effects of vegetation
are described explicitly by the parameters of
percentage canopy cover, percentage ground
cover, plant height, effective hydrological depth,
density of the plant stems and stem diameter. In
addition to detachment and transport, sediment
deposition is modelled through a particle fall
number which considers particle settling veloc-
