Geology Reference
In-Depth Information
Table 12.4
Measured LISEM input dataset (plant and soil characteristics) for the 990720
event.
Crop
a
Fallow
Orchard
Shrub
Waste
Forest
Aggregate stability
6
5
6
6
8
7.25
(median drop no)
Cohesion (kg cm
−2
)
0.08
0.10
0.10
0.09
0.11
0.11
Random roughness (cm)
1.75
1.11
1.28
1.03
1.66
0.88
Manning's
n
SD
b
0.079
0.092
0.153
c
0.091
0.214
Leaf Area Index
0.06
0.12
1.46
1.25
0.54
1.63
Plant cover (fraction)
0.06
0.10
0.18
0.40
0.23
0.35
Plant height (m)
0.28
0.11
3.1
0.97
0.25
13.6
K
sat
-meas (cm day
−1
)
d
55.9
82.2
96.9
164
153
122
K
sat
-fitted (cm day
−1
)
d
1
1
25
10
5
13
Theta-init
Calculated with regression equation
a
Cropland was subdivided into five types. Here the values for foxtail millet are given. The other types were
pearl millet, potato, tall crops (maize, sorghum) and beans.
b
SD, slope dependent; see Hessel
et al
. (2003b).
c
Average of wasteland and forest.
d
K
sat
-meas,
K
sat
measured with constant head method;
K
sat
-fitted,
K
sat
value resulting from wind evaporation
method (Halbertsma & Veerman, 1997)
distance and lower flow velocity allow more time
for infiltration.
12.6.2
Evaluation of adaptations
The effects of the adaptations to LISEM were
evaluated. This section discusses the effects with-
out recalibration of the model, while Section
12.6.3 shows the results after recalibration.
(ii) Sine versus tangent
The effect of using sine
instead of tangent is small for simulations for the
entire catchment. Simulated total soil loss from
the catchment decreased from about 1050 tonnes
to 1000 tonnes. Because the slopes near the catch-
ment outlet are fairly gentle, transport capacity
close to the outlet will not have changed much
by using sine instead of tangent. It seems likely
that the effect would be larger when only steep
areas are simulated.
(i) Flow distance
The combined effect of the
slope correction and the use of sine in the
Manning equation proved to have a large effect
on simulated discharge; peak discharge decreased
by about 50% (Hessel, 2002). A decrease in dis-
charge was to be expected since:
●
On steep slopes the pixel areas have increased.
The same amount of rainfall is therefore spread
out over a larger area, so that the water layer
will be thinner. The hydraulic radius will also
be smaller, so that flow will be slower accord-
ing to the Manning equation.
●
The flow distance between pixels is larger.
●
Since on steep slopes the pixel area is larger,
infiltration will be greater as well.
Factors that affect flow distance or flow velocity
will also affect discharge because longer flow
(iii) Manning's
n
Using a slope-dependent
Manning's
n
retarded the peak discharge slightly.
The effect was much smaller than that of using a
slope correction.
(iv) Transport equations
Figure 12.4 shows
simulated concentrations at the catchment out-
let for different transport equations (Hessel,
2002). As can be seen from the figure, the differ-
ences are large. In the simulations the maximum
