Geology Reference
In-Depth Information
land use and climate change on soil erosion and
basin sediment yield. Experience in the use of
SHETRAN to represent different land uses and
climates, including scenario simulation of possi-
ble future basin conditions, is therefore summa-
rized here. Full details of the applications are in
the cited references, and only the most pertinent
points are presented below.
It should be noted that model simulations are
only one contribution to the process of decision-
making, which is also likely to include political,
social and economic dimensions. The particular
contribution from the modelling is the prediction
of basin response (e.g. runoff and sediment yield)
for different scenarios of land use and climate
change, so enabling optimum solutions from the
point of view of the basin environment to be
identified. It is important therefore to establish
the extent to which the model can correctly rep-
resent basin responses to different land use and
climate conditions.
with SHE, SHETRAN and ANSWERS suggest
that the same model parameters can be applied at
both plot (1-100 m
2
) and microbasin (order of 1 ha)
scales, using small grid spacings (20 m or less) and
with a good availability of field data (Wicks
et al
.,
1988; Connolly & Silburn, 1995; Figueiredo,
1998; Figueiredo & Bathurst, 2002). For larger
basins, scale effects in evaluating soil saturated
conductivity appear not to be significant, or at
least to be masked by uncertainty in parameter
evaluation, as long as basin topography is sub-
dued and there is a general homogeneity of land
use, soil characteristics and hydrological response
within the basin. Thus Bathurst
et al
. (1996)
found that conductivities evaluated at the plot
scale could be successfully applied with a model
grid spacing of 2 km for the 701 km
2
Cobres basin
in Portugal. Even in dissected badlands terrain,
Bathurst
et al
. (1998b) found that any scale effects
which may distinguish simulations at the scales
of 0.133 and 86 ha were small enough to be
masked by uncertainty in parameter evaluation.
However, for basins with hilly terrain, experience
is that model conductivity values are relatively
high compared with point measurements (e.g.
Bathurst
et al
., 2002, for the 1532 km
2
Agri basin
in Italy; Bathurst
et al
., 2005, for the 160 km
2
Valsassina basin in Italy; Bathurst
et al
., 2007, for
the 45 km
2
Ijuez basin in Spain).
Table 14.3 shows the SHE/SHETRAN erodi-
bility coefficients calibrated for a number of plots.
The results are not fully comparable as the basic
rainfall erosion equation was changed slightly
between the SHE and SHETRAN versions. Also,
there is a mixture of natural and artificial rainfall
application. However, the results indicate an
excellent ability to represent the effects of signifi-
cantly different soil conditions. The coefficients
calibrated by Wicks
et al
. (1992) clearly distin-
guish between tilled ground and natural brush
and grass cover grazed by cattle. Bathurst
et al
.
(1996) found a need to vary the coefficients
between moderate rainfall events and extreme
events, with the suggestion that rilling occurred
under the extreme conditions. Likewise Adams
and Elliott (2006) calculated different coefficient
values for the more and less erodible conditions
14.7.1
Use of erosion plots for parameter
evaluation
A requirement common to all the physically-
based erosion models is for the erodibility coeffi-
cients to be calibrated, as currently they cannot
be determined directly from measurable soil
properties. Erosion plots are an excellent basis for
such calibration as they are relatively simple to
model, and measurements often represent a range
of rainfall and land use conditions. Bathurst
et al
.
(1996) noted that plots allow the calibration of
not only the soil erodibility coefficients but also
the overland flow resistance coefficient and the
soil saturated conductivity, provided that, for the
latter, overland flow is generated by rainfall
excess over infiltration rate rather than upward
saturation and that runoff is a significant percent-
age of the rainfall. Furthermore, plot calibrated
values already represent a spatially integrated
response, albeit at a relatively small scale of tens
to hundreds of square metres, and may form a
sounder basis for extrapolation to larger scales
than values determined at a point scale (e.g. con-
ductivity measured using an auger hole). Studies
