Geology Reference
In-Depth Information
(a)
(b)
9
8
7
6
5
4
3
2
1
0
1:1
3.5
1:1
3
equation 2
equation 3b
equation 2
equation 3b
2.5
2
1.5
1
0.5
0
0
1
2
3
4
5
6
7
8
9
0
0.5
1
1.5
2
2.5
3
3.5
Observed sediment kg
Observed sediment kg
(c)
(d)
1
2.5
100
90
80
70
60
50
40
30
20
10
0
1:1
1:1
0.9
2
0.8
equation 2
equation 3b
1.5
equation 2
equation 3b
0.7
1
equation 2
equation 3b
0.6
0.5
0.5
0
0.4
0
0.5
1
1.5
2
2.5
Observed sediment kg
1:1
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
5
10
15
20
25
30
Observed sediment kg
Observed sediment kg
Fig. 6.4
Summary of erosion-model results versus observed sediment yield for the monitored plot events at
Walnut Gulch using optimized sediment-transport parameters for hydraulics relationships (equation (2) and
equation (3b) in Wainwright
et al
., 2008b). The four plots are (a) Abbott - 14.48 m long; (b) Dud - 18.95 m long;
(c) Laurel - 4.12 m long; and (d) Wise - 22.78 m long. The inset shows details of events with small yields.
erosion models. As system dynamics and feed-
backs between components change (even assum-
ing an invariant climate, which is demonstrably
not the case), measurements of erosion made over
short timescales may not be valid when upscaled
(or may show problems with different bases of
measurement, as demonstrated by Parsons
et al
.,
2004). Extrapolating from short to long term ero-
sion rates thus needs to be approached with care
for a number of reasons.
Over longer timescales, changes in system
state (or the pattern of erosion processes within
a hillslope or catchment) may occur because of
internal system feedbacks that change process
domination, or changes in system variables
that are dependent on other extrinsic factors
(Le Bissonnais
et al
., 2005). Modelling studies
have allowed us to analyze the relationship
between contributing area and sediment yield
(Birkenshaw & Bathurst, 2006) for different sys-
tem states. If we are considering decadal or longer
time-periods, the operation of these processes
will give rise to long-term deficits
or
gains in sed-
iment yield with increasing area. This may alter
system state (via changes in topography, vegeta-
tion or surface state) to feed back with process
operation and to change dominant erosion proc-
esses, thus altering catchment sediment yield rela-
tionships. In erosion models, land use, crops,
vegetation or even topography may be assumed
static for short time-periods (although at the
expense of greater model complexity and number
