Geology Reference
In-Depth Information
Table 12.10
Table 12.11
Percentage decrease for different scenarios
compared with scen0 results, catchment outlet,
980801 event.
Erosion (tonnes) for different land-use
scenarios.
Land use
Present
25°
20°
15°
scen0 scen0a scen0b scen1 scen2 scen3
Cropland
2949
1644
942
437
Peak
discharge
0.0
17.5
10.6
42.3
51.8
56.4
Orchard/cash tree
97
0
312
524
Wood/shrubland
173
198
173
140
Total
discharge
0.0
12.8
6.5
33.4
42.2
50.1
Wasteland
6833
5080
4881
4727
Vegetables
31
62
63
62
Total soil
loss
0.0
21.1
11.1
54.8
64.0
71.4
Fallow
870
499
360
130
Total
10,953
7483
6732
6019
scen0a
=
use of biological measures (mulching, improved fallow).
scen0b
=
use of mechanical measures (contour ridges).
moderate erosion rates, while the steeper parts of
the slopes had serious or severe erosion rates.
Apart from woodland, not much difference in ero-
sion is visible for the different land uses.
Figure 12.8 shows the results of scenario 3, using
the same classification scheme as used for Fig. 12.7.
It is evident that the erosion in the catchment has
decreased. The area with negligible erosion rates
has more than doubled in size, while the other ero-
sion classes have all decreased in area. Comparison
with Fig. 12.3 shows that the negligible erosion
rates again mainly occurred under woodland/
shrubland. The large decrease in predicted erosion
was therefore probably a direct consequence of the
increase in woodland/shrubland area.
Table 12.10 shows that erosion in woodland
decreased from present land use to that of the 15°
scenario, even though the woodland area increased
significantly (Table 12.1). Comparison of Tables 12.1
and 12.10 shows that erosion for cropland and fal-
low land steadily decreased with decreasing area
occupied by these land uses. Table 12.10 also shows
a decrease in erosion for wasteland/wild grassland
from the present land use to the 25° scenario.
However, from the present land use to the 15° sce-
nario, erosion in the catchment became increasingly
dominated by erosion in wasteland/wild grassland.
Apart from the erosion/deposition maps,
LISEM also generates time series of discharge
and erosion for the outlet of the catchment.
These time series showed that using conserva-
tion measures in the present land use (scenario 0)
decreased the peak discharge by 10% to 18%,
while the reduction was much larger when apply-
ing scenarios 1, 2 and 3 (40% to 60%). The sedi-
graphs showed much the same trend as the
hydrographs.
Table 12.11 shows that the decrease in total
discharge was smaller than the decrease in peak
discharge and total soil loss. Total soil loss showed
the largest decrease. Relatively small decreases
are shown for the conservation measures (ranging
from 2% to 21%), whereas large decreases (rang-
ing from 33% to 71%) are shown for the alterna-
tive land-use scenarios compared with scenario 0.
The differences between scenarios 1, 2 and 3 were
much smaller than those between scenario 1 and
scenario 0. These differences between scenarios
1, 2 and 3 were caused by conversion of cropland
to orchard/cash tree, while the larger difference
between scenario 1 and scenario 0 was caused by
a major redistribution of land uses.
12.7 Discussion
12.7.1
Adaptation of LISEM
The adaptations to LISEM that were made to
make it able to deal with the Loess Plateau char-
acteristics of steep slopes, high sediment concen-
trations and the presence of gullies, influenced
the outcome of the model considerably. It was
found that especially the choice of transport equa-
tion, and simple improvements like using the
