Geology Reference
In-Depth Information
soil loss and measured soil loss was unsatisfac-
tory, the LISEM model was calibrated on sedi-
ment yield (Hessel, 2002). To do this, cohesion,
aggregate stability and median grain-size can be
used because these parameters affect only sedi-
ment yield and have no influence on predicted
discharge. In this chapter, results are only shown
for calibration on peak discharge.
In accordance with Hessel (2005), pixel size
and timestep length were chosen before calibra-
tion started. For all simulations, LISEM was used
with a pixel size of 10 m and a timestep length of
15 s. Since the upper few decimetres of the soil
are crucial for infiltration during a storm, ten cal-
culation layers were used in the finite difference
solution of the Richards equation, with node
spacing increasing with depth. A single median
grain-size (
D
50
) of 35
the quantified rill erosion map to obtain a field
erosion map. The resulting map was then com-
pared with the erosion map produced by LISEM.
12.5.4
Land use scenarios
LISEM was used to evaluate the effects of different
land use scenarios. The scenarios were developed
based on a biophysical resource inventory (Messing
et al
., 2003a,b), farmer's perceptions (Messing &
Hoang Fagerström, 2001; Hoang Fagerström
et al
.,
2003) and the plans of the authorities to re-green
the Loess Plateau. These plans include the gradual
restriction of cropland to slopes of less than 15°,
and the prohibition of grazing.
Four scenarios were developed, one using the
1998 land-use map (scenario 0), and three using
respectively 25°, 20° and 15° as the upper slope
limit for cropland (scenarios 1 to 3). The 15° limit
scenario (already proposed by Fu & Gulinck, 1994)
is considered a long-term scenario and the 25° and
20° limits are therefore short-term intermediate
scenarios (Chen
et al
., 2003). These scenarios had
subscenarios that considered the effects of biolo-
gical measures (such as cropland mulching and
improved fallow) and mechanical measures (e.g.
contour ridges on cropland). These measures are
relatively simple and inexpensive, but labour-
intensive.
Compared with the present land use (Fig. 12.1),
the scenario land-use maps (15° map in Fig. 12.3)
all have much more woodland/shrubland, while
the cropland area is decreased according to the
specified slope limits. Table 12.1 shows that for
the 25°, 20° and 15° land-use maps there is a grad-
ual decrease in cropland and fallow land and a
gradual increase in orchard/cash trees. By defini-
tion, the change was limited to areas below 25°.
The other land uses (including all slopes of more
than 25°) remained unaffected for these scenario
groups, so for these land uses and slopes, only the
present land-use scenario differed. The economic
consequences of the large changes in land use
specified in Table 12.1 were discussed by Chen
et al
. (2003) and Hoang Fagerström
et al
. (2003).
To simulate the effect of land-use scenarios, a
real storm event was used, because the use of a
m was used in all cases in
the sediment transport equations of LISEM.
At the end of each rainy season (September)
the occurrence and intensity of rilling was
mapped throughout the 3.5 km
2
catchment. Rill
intensity was classified in three classes: slight
rill erosion, moderate rill erosion, and severe rill
erosion. Quantification of the amount of erosion
for each class was possible due to a number of
measurements of rill frequency, width and depth
that were conducted for each rill erosion class.
For most years such mapping will give an aggre-
gated result for all events, but in 1999 only a sin-
gle rill-producing event occurred, so that the rill
erosion map made in that year can be used
directly to evaluate the performance of LISEM.
A single erosion plot was installed in 1999 to
determine the amount of erosion occurring in
arable fields. The plot was assumed to be repre-
sentative of the cropland area in the Danangou
catchment. Its dimensions were about 34 × 6.5 m,
while slope steepness ranged from 15% at the
top to 55% at the bottom. The total amount of
water and sediment was measured on an event
basis using a divisor and barrels. Since rill meas-
urements were also conducted on the plot, it was
possible to calculate the total sheet erosion by
subtracting rill erosion from total plot erosion.
This estimate is expressed as an amount of ero-
sion per unit area of cropland, and was added to
μ
