Geology Reference
In-Depth Information
rill and gully erosion, sediment yield, soil mois-
ture and runoff at different spatial and temporal
scales; this strategy can also address the problem
of parameter equifinality (Ebel & Loague, 2006).
However, this approach requires an increase in
the data used for the calibration and validation
process, which contrasts with the generally poor
availability of data; hydrological data are often
only available for catchment outlets, while ero-
sion data are often not available at all, preventing
a calibration and validation study of this kind
in most catchments (Beven, 2000; Morgan &
Quinton, 2001). To overcome this problem a
qualitative evaluation of model performance can
be used when quantitative data are not available;
this approach consists of comparing model out-
puts with expected results in terms of process
knowledge to assert the rationality of model
behaviour, and therefore the model's capacity to
simulate responses to changes in environmental
conditions (Favis-Mortlock
et al
., 2001; Ebel &
Loague, 2006). In this approach, soft knowledge
of the impacts of climate change - from observa-
tions in different sites, laboratory experimenta-
tion or extrapolation from observations in
different climatic regions - can be useful to judge
model performance under changed climates.
plant residue generation and decomposition, or
soil consolidation. An additional model feature is
the capacity to simulate agricultural operations
and their impact on soil properties, making this a
good tool to simulate agricultural hillslopes
(although the model also simulates catchment-
scale processes operating in small watersheds
driven by infiltration-excess surface runoff). The
following applications of WEPP focused on the
model's strengths, namely the continuous simu-
lation of cultivated hillslopes.
Several publications have reported on climate
change impact assessment in the US using WEPP
(Table 15.1; for a partial review see Nearing
et al
.,
2004). A first approach was reported by Pruski and
Nearing (2002a); this comprised applying hypo-
thetical changes to annual rainfall (from −20% to
+20%) for three soils, three slopes, and four crops
in three locations in the US with different climate
characteristics. The results include a ratio of sen-
sitivity to climate change; WEPP indicates a 2.0
ratio of surface runoff increase to rainfall increase,
and a 1.7 ratio of erosion increase to rainfall
increase, showing the enhanced sensitivity of
these parameters to changes in climate.
Furthermore, this ratio also depends on the mode
in which rainfall changes; the ratios reported
above assume that half of rainfall changes are due
to changes in intensity, with the remainder due to
changes in the number of rain days. However,
when the model is applied with rainfall intensity
changes only, the sensitivity ratios increase to 2.5
and 2.4 for runoff and erosion respectively. This
points to the importance of knowing how climate
change may impact upon individual rainfall
events before estimating impacts on soil erosion.
Pruski and Nearing (2002b) also applied the
WEPP model with a climate change scenario
downscaled from the HadCM3 GCM. WEPP was
modified to take into account plant fertilization
by CO
2
, and applied to eight locations in the US
with the climate change scenario. The results
point to the complex interactions between differ-
ent erosion drivers, particularly rainfall and veg-
etation biomass production; rainfall changes were
often not the dominant impact on soil erosion.
The importance of different drivers changed with
15.4 Case Studies
This section presents a number of case studies to
demonstrate the application of different model-
ling strategies to different problems. Each case
study includes a number of different studies and
publications (listed in Table 15.1) grouped the-
matically, in order to illustrate how different
modelling approaches were used to answer differ-
ent questions.
15.4.1
Continuous modelling at the slope scale
This case study reviews work using the WEPP
model - Water Erosion Prediction Project
(Flanagan & Nearing, 1995). WEPP simulates
hillslope processes such as inter-rill and rill ero-
sion, sediment transport and deposition, as well
long-term processes such as vegetation growth,
