Download Soil erosion and climate change: the transect approach and the

Geomorphology 23 Ž1998. 219–227
Soil erosion and climate change: the transect approach and the
influence of scale
A.C. Imeson
a
a,)
, H. Lavee
b
Landscape and EnÕironmental Research Group, UniÕersity of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, Netherlands
b
Department of Geography, Bar-Ilan UniÕersity, 52900 Ramat-Gan, Israel
Received 15 December 1995; revised 8 July 1996; accepted 8 July 1996
Abstract
Studies of geo-ecological processes are being made along climatological transects on similar limestone rocks at different
locations across the Mediterranean. The main objectives of the research are firstly, to gain insight into the influence of
climate on key-geomorphological process–pattern relationships that characterise different locations along the transect; and
secondly, to obtain a better insight into the possible impact of climate change on ecosystem degradation. The paper begins
by considering the possible methodological approaches that can be applied to investigate the impact of climate change on
ecosystem degradation in complex ecosystems and explains why a nonlinear evolutionary modelling framework was chosen
for the transect studies. The conceptual basis of the methodology is presented and the transect approach described. The
research methodology takes into account the influence of climate at four different scales Žlandscape, slope, response unit and
patch.. Field research at these scales is oriented towards identifying and understanding the key processes and to identifying
key parameters that can be monitored to establish change. Some results are presented from the Judean Desert transect in
Israel to show how a key indicator, in this case aggregate stability, varies with temperature and to show how process–pattern
features vary along the limestone transect from the area having a Mediterranean climate to the desert. Both conceptual and
practical models of soil erosion need to address scale issues. In particular, as time and spatial scales change, so also does the
relative significance of different processes. It is concluded that process–pattern phenomena are useful in this context and that
they can be applied to the problem of up-scaling. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: soil erosion; climate change; transect approach; ecosystem degradation
1. Introduction
General relationships between climate and erosion
are well established ŽLangbein and Schumm, 1958.
and these have been applied by many researchers to
hypothesise about the probable impact of climate
)
Corresponding author. E-mail: [email protected]
change on soil erosion. Several global and regional
models have been developed for this purpose. These
have enabled the relative importance of different
factors on erosion to be investigated ŽKirkby and
Neale, 1987; Kirkby and Cox, 1995; Kirkby et al.,
1993; Kirkby, 1994.. One important outcome of such
research has been to reveal the very limited amount
of relevant field data available for establishing the
influence of climate on several key processes of soil
0169-555Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 1 6 9 - 5 5 5 X Ž 9 8 . 0 0 0 0 5 - 1
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A.C. Imeson, H. LaÕeer Geomorphology 23 (1998) 219–227
erosion at detailed scales of study. Another has been
to draw attention to temporal and spatial scales of
field data collection and modelling.
Parameters of soil erosion models that have constant values for periods of years become variable as
the temporal scale of interest increases. Over longer
time scales, thresholds and discontinuities occur as a
result of vegetation–soil interactions, human activity
and the adaptive evolution of the system under study.
This implies that as the temporal or spatial scale of
the research increases, new processes can emerge to
dominate soil erosion ŽImeson et al., 1995.. For
example, in the case of an ecosystem disturbed by a
wildfire, soil erosion over a period of 30–100 yr is
strongly dependent on the factors that influence the
recovery trajectory Žresilience. of the system. The
speed of this recovery determines the effectiveness
of erosive rainfall events ŽCerda` et al., in press a..
Similarly, as the spatial scale of measurement is
increased, the largest sediment transfer events are
separated by longer periods of time and the most
important erosion processes are those that emerge at
coarser scales. Coarser scale processes may be explained by the dynamics of finer scale processes but
they may be very different from them. A generalised
scheme showing how key soil erosion processes
change at different spatial and temporal scales on
semi-arid and subhumid slopes is shown in Fig. 1.
Fig. 1. Key soil erosion processes at different spatial and temporal
scales.
This figure only covers a scale relevant to soil
erosion and it describes the process–pattern phenomena at different scales along the transects described
in this paper. The correspondence between spatial
and temporal scales suggested in Fig. 1 is only
indicative. Relationships between process rate and
spatial scale are in reality more complex and ambiguous than the figure suggests.
Against the above background, a series of ‘transect studies’ was initiated in 1987, first in Israel, later
in Spain and Crete ŽLavee et al., 1991; Imeson et al.,
1994.. The initial objectives were firstly to obtain
information on relationships between climate conditions and soil erosion and secondly, to study the
changes that occur across major eco-geomorphological zones Ži.e., zones in which particular soil–
water–vegetation erosion relationships exist. on
limestone in a progression from humid Mediterranean to desert environments.
2. Approaches in projecting the impact of climate
change on ecosystem degradation
Recent reviews of the impact of climatic change
on environmental processes can be found in Jeftic et
al. Ž1992. and in Troen Ž1994.. In a review of the
impact on land degradation in the Mediterranean,
Imeson and Emmer Ž1992. described different approaches used to study the impact of climate change
on soil erosion. One approach is using temporal or
spatial analogues, for example by looking at historical data or by studying existing conditions along
climatological gradients ŽLavee et al., 1991.. Analogue studies give insight into trends but they cannot
be used as a basis for prediction due to the complexity of ecosystem processes and a range of historic
and socio-economic factors that affect the underlying
processes. Lavee et al. Ž1991. emphasised that relationships are highly nonlinear and that threshold
conditions are very important. Another approach is
modelling. Modelling the impact of future climate
change on soil erosion is a very complicated task
requiring an integrated methodology. A first step
proposed and being developed by the GCTE Soil
Erosion Network ŽIngram et al., 1996. is to calibrate
existing soil erosion models such as EPIC ŽWilliams
et al., 1990. and MEDALUS ŽKirkby et al., 1993.,
A.C. Imeson, H. LaÕeer Geomorphology 23 (1998) 219–227
and run these with inputs from weather generation
models ŽNicks et al., 1990.. This work will improve
understanding of relationships between climate and
erosion but there are many future tasks that are
required before projections can be made. New dynamic models might be developed as a second step
that consider dynamic interactions between biological and physical processes and incorporate land use
change projections that can be made with socio-economic models.
A lack of information about processes affecting
soil erosion and how these are related to climate is
another problem. The transect approach aims to identify and understand relationships between climate
and erosion at different scales. To this end soil
erosion measurements are made under different climatological conditions found along the transect. The
Judean Desert transect reported by Lavee et al. Ž1991,
1995. and the transects in Crete and SE Spain ŽImeson et al., 1994; Boix-Fayos et al., 1994a,b, 1995.
provide examples of this approach.
Long term investigations of soil erosion require
information about the way in which climate change
influences the resilience and stability of geo-ecosystems ŽWestman, 1992.. It is, therefore, necessary to
identify and monitor indicators of ecosystem resilience and stability. An important element of climate change—soil erosion relationships is establishing how rapidly such key indicators respond to
change. This can be done, for example, by following
changes occurring after fire, grazing or land abandonment on slopes having different aspects.
3. The transect approach methodology
A major component of the transect approach is
monitoring. This requires establishing base-line stations where long-term measurements can be made.
Monitoring climate is straight forward but soil erosion is more difficult to monitor, partly because
temporal and spatial scales require consideration.
The transect studies of Lavee et al. Ž1991, 1995.,
Imeson et al. Ž1998., Boix-Fayos et al. Ž1994a,b.
involved establishing measurements and experiments
at representative reference sites along climatological
and topographical gradients. The geology, land use
and geomorphic conditions of the reference sites
221
were kept as uniform as possible. The transects were
located on similar lithologies and vegetation in Israel, Crete and SE Spain. At each site, climatological, geomorphological and ecological measurements
were made at different scales of study, and soil–
vegetation–erosion relationships were analysed.
Soil aggregation characteristics were found to be
sensitive indicators of ecosystem resilience, stability
or health ŽCammeraat and Imeson, 1998., as well as
of soil erodibility. Cammeraat and Imeson Žin press.
and Cerda` et al. Žin press b. have established changes
in key erosion indicators on abandoned cultivated
land in the Guadalentin catchment over a period of
about 25 yr. Lavee and Pariente Ž1995, 1996. established seasonal variations in aggregation processes
and controlling factors along the Judean Desert climatological gradient.
The methodological approach applied in the transect studies was developed to overcome scale problems ŽImeson et al., 1994, 1995. that are usually
inherent to the methodologies of field experiments
and modelling, traditionally used in geomorphological and soil erosion studies. For this purpose an
integrated evolutionary modelling framework ŽIEF.,
drawn from complex systems and hierarchy theory
ŽO’Neill et al., 1986; Salthe, 1985., was applied in
our transect studies as a demonstration of how concepts from nonlinear systems dynamics can be applied in geomorphology. It is a conceptual methodology that views the complex response of erosional
systems, in terms of the hierarchy of processes that
drive its dynamics, at different spatial and temporal
scales. These processes can be affected by both
socio-economic and bioticrabiotic factors.
The IEF firstly uses field measurements to identify and study key processes of erosion at different
scales, in order to assess the effect of these on
structures and flows observed at different spatial
scales Žplot, slope, catchment.. A second stage involves monitoring key parameters that can be used
either Ža. to monitor the rate of change in patterns or
structures as a result of the key process or Žb. as
model input parameters. Finally, dynamic models are
developed to explore how process–pattern phenomena are affected by factors such as climate, grazing
and fire Žsee for example Imeson et al., 1995.. The
dynamic models can be validated by comparing how
well they can predict changes in patterns, or differ-
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A.C. Imeson, H. LaÕeer Geomorphology 23 (1998) 219–227
ences in patterns under different climatological or
other site conditions. Consequently, it is not necessary to make long-term measurements of sediment or
water fluxes that are highly sensitive to initial conditions.
The study of process–pattern phenomena at different scales can be illustrated on semi-arid hillslopes. Usually runoff and sediment transport in
these regions are spatially discontinuous ŽYair and
Lavee, 1985.. Rain that falls relatively uniform results in a heterogeneous but nonrandom pattern of
infiltration and sediment accumulation on the slope
which results in the emergence of relatively arid and
humid soil–vegetation subsystems. In these subsystems, soil aggregation and infiltration characteristics
are totally different; those in the humid subsystem
Fig. 2. Water redistribution and relevant processes at different scales. ŽA s aggregation; B s biological activity; ET q evapotranspiration;
I s infiltration; R s runoff; SA s structure of aggregates and SM s soil moisture..
A.C. Imeson, H. LaÕeer Geomorphology 23 (1998) 219–227
Fig. 3. Process–pattern phenomena at four positions along the Judean Desert transect. A: GIV, Mediterranean site Žannual rainfall 620 mm.; B: MAL, Semi-arid site Žannual
rainfall 320 mm.; C: MIS, Arid site Žannual rainfall 260 mm. and D: KAL, Extreme arid site Žannual rainfall 120 mm..
223
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A.C. Imeson, H. LaÕeer Geomorphology 23 (1998) 219–227
are dependent on the supply of water from the dry
subsystem. The process–pattern structures resulting
from water and sediment redistribution reflect both
relatively static Že.g., rock fragments. and dynamic
Že.g., soil aggregation. factors. Positive feedback
loops occur when water redistribution reinforces differences in plant growth and soil aggregation development ŽFig. 2.. Examples of process–pattern phenomena produced by water redistribution on the
Judean Desert transect are shown in Fig. 3. Fig. 2
also illustrates how process–pattern phenomena at
one scale influence the response of the system at
other scales. The effects of the most rapid Žfine
scale. processes may be usually considered as being
incorporated ŽO’Neill et al., 1986. into the long-term
effects of slower Žcoarse scale. processes. The relevance of incorporation for the degradation of
Mediterranean ecosystems is described by Bergkamp
Ž1995. and Imeson et al. Ž1995.. Imeson et al. Ž1995.
observed process–pattern phenomena at three scales
Žpatch, response unit, and slope.. Differentiation at
the patch scale was described as being the result of
first-order processes Žsuch as soil aggregation dynamics and surface sealing.. Second order processes
were those that resulted in process–pattern phenomena used to characterise the second level of scale
called the response unit. The key process at this scale
was the effect of local water redistribution on vegetation–soil patterning. At the hillslope scale, third-order
processes were those that resulted in the removal of
sediment. The emergence of such processes Žslope
scale movement of sediment and runoff. depends on
the dynamics of the response unit process–pattern
phenomena produced by the second order processes.
Bergkamp Ž1995. stressed the need to consider
the connectivity of processes occurring at different
scales because in practice this is an important criteria
determining the degree to which processes at different hierarchical levels interact. Under field conditions, lithological and land use controls and boundaries restrict the degree of hierarchical organisation.
4. The effects of climate on erosion at different
spatial scales
The effect of parameters such as temperature and
soil moisture can be considered with respect to the
different process–pattern phenomena present in the
landscape at different scales. A point measurement
has a value which is influenced by Ža. the general
position along the transect Žlandscape scale., Žb.
slope characteristics, such as aspect and hillslope
shading Žslope scale., Žc. plot characteristics, such as
shading and cover Žresponse unit scale., and Žd. local
cover and soil conditions Žpatch scale.. Comparisons
of data at these different scales enable these differences to be observed. For example, Fig. 4 shows the
relationship between soil temperature, soil moisture
and aggregate stability Žerosion indicator. to be compared at the landscape scale. In this case, the effects
of differences at other scales are kept constant by
considering sites that have the same characteristics
of cover, aspect and slope position.
At the landscape scale, it was found that at different positions along the transect, key processes varied
in affecting how temperature influenced erosion. Below about 320 mm of rainfall, soil aggregate stability
was uniformly low because of the influence of high
levels of water soluble salts; above 320 mm aggregate stability varied according to the way in which
temperature influenced organic matter dynamics. In
some places, specific soil–animal–vegetation interactions resulted in relationships that were specific to
a particular site ŽLavee et al., 1996.. In general,
‘domains’, within which conditions are uniform, are
present along the transect.
Considering the slope and response unit scales
and the relationships of climate to aggregate stability, the results showed that the important factor
influencing aggregate stability was the degree of
shading. Under vegetated or litter covered areas high
values of aggregate stability were found. Just as at
the humid sites at the landscape scale, favourable
organic matter dynamics resulting from a large
turnover of organic matter and production of soil
stabilising substances and roots, gave uniformly high
levels of aggregate stability. Conversely, areas of
soil not covered, had a uniformly low level of aggregate stability, sometimes as a result of dispersive
conditions resulting from small amounts of water
soluble salts. Aspect seemed to be important not at
the patch scale but at the hillslope and response unit
scales. This can be explained by differences in the
amount of vegetated cover on north and south facing
slopes. Irrespective of the slope aspect, soil aggrega-
A.C. Imeson, H. LaÕeer Geomorphology 23 (1998) 219–227
225
Fig. 4. Aggregate stability and soil temperature at three sites. Ž1. GIV, annual rainfall 620 mm; Ž2. MIS, annual rainfall 260 mm; and Ž3.
KAL, annual rainfall 120 mm, along the Judean Desert climatological gradient Žafter Lavee et al., 1996..
tion properties are similar under vegetated or bare
surfaces ŽCerda` et al., in press b.. Fig. 2 illustrates
how the more favourable conditions on north-facing
slopes allow the evolution of plot scale process–pattern features that trap water on the hillslope. These
are absent from the south facing slopes because of
the relatively high amount of erosion and runoff that
occur when the soils are in a period of post disturbance recovery. The redistribution of water on the
south facing slope is at a hillslope scale.
At the finest scale of observation, the spatial and
temporal variabilities in aggregate stability are impossible to explain unless they are considered within
the context of the multi-scaled process–pattern phenomena within which the sampling point is located.
In other words, any point selected for monitoring soil
aggregation, occupies a geographic position within
the dynamic process–pattern elements present at different hierarchical levels.
5. Climate and soil erosion at different hierarchical scales
Information from the transect studies can be used
to demonstrate the relative importance of processes
at different hierarchical levels. In the response unit
methodology mentioned above, the terms first, sec-
ond and third order processes were introduced, corresponding partially to the patch, response unit and
slope scales.
The key first-order processes studied on the transects were those related to soil structure evolution.
Soil aggregate stability was used as an indicator. It
was found that the general level of aggregate stability was controlled by the climatological conditions of
the soil, thus determining the time biological activity
was possible. It was high, when the production of
soil stabilising substances and mechanisms exceeded
the rate of that were lost; low, when the opposite was
the case or variable where a limited food supply was
available and where systems were disturbed by recent fires, trampling or drought.
When soil erosion is considered along the transect
in Israel, there is a fairly close correspondence with
the soil aggregation expressing the effectiveness of
the first order processes ŽFig. 4.. At the transects in
SE Spain and Crete, this was not the case ŽBoix-Fayos
et al., 1995; Imeson et al., 1994.. The reason for this
is that the second order processes, dependent on the
dynamics of first order processes, override the effects of organic matter dynamics on soil erosion. The
most eroded sites were those that were climatologically favourable for biological activity. In Crete,
grazing was intensive at all of the study sites. The
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A.C. Imeson, H. LaÕeer Geomorphology 23 (1998) 219–227
effect of trampling was greatest on soils that produced the most biomass. Consequently, most erosion
seemed to be on west facing slopes and not on south
facing ones where relatively less biomass was produced. In SE Spain, high biomass production at sites
with favourable soil aggregation dynamics resulted
in very frequent wild fires. The high rainfall of the
areas most affected by fire and grazing is an important factor. Seen at the scale of the transect, erosion
processes associated with grazing and fire reverse
the general relationships with climate and erosion
that are seen in Israel.
As soon as the temporal scale is considered, soil
aggregation dynamics become important in explaining the longer term erosional impact of fire and
grazing. This is because they are a key factor in
explaining the resilience of the sites affected. This is
why the areas of low biological activity are the most
vulnerable to erosion under intensive disturbance
regimes. The more resilient areas are subject to the
most disturbance.
The relationship of climate to erosion will therefore depend on the effect of climate at different
scales. Over a period of months or a few years
erosion will be related to the actual amount of
rainfall that occurs along the transects and to the
aggregation Žerodibility. status of the soil. Over a
period of 15–30 yr it will be dependent on the
stability and resilience of the soil and vegetation
Žindicated by process-pattern structures. and the frequency and severity of disturbance Žfire and grazing.
in combination with the amount of erosive rainfall.
6. Conclusions
Transect studies can be used to collect information about relationships between climate and soil
erosion. The evolutionary modelling framework was
found to provide a useful methodology for addressing the scale problem. Although still under development several conclusions can be made from the
results analysed so far.
Firstly, it is possible to consider hierarchies of
processes operating in time–space domains. According to the spatial or temporal scale of interest different time–space domains will form the focal point of
the research requiring measurements at the appropriate scale.
A field observation, for example of soil erosion
measurement, has in itself little value taken alone.
The problem is that the value measured will reflect
the unique space–time position within the nested
hierarchical scales. This is why relationships of soil
erosion rates to runoff or precipitation are so hard to
model, especially in areas of frequent disturbance.
By transferring the emphasis from a measurement of
a flux Žfor example soil loss. to a process–pattern
feature resulting from an erosion process, a way of
avoiding this problem is presented. The concepts of
hierarchy theory can be applied for this purpose.
Process–pattern phenomena are indicative of the
inertia and resilience of a system subject to erosion.
Projection of the impact of climate on erosion over a
period of 30–50 yr requires understanding how the
resilience of the eco-geomorphological system is affected by climate. The interaction of processes at
different hierarchical levels is important because different conclusions will be reached according to the
scale of study. Process–pattern phenomena may provide a useful means of up-scaling erosional phenomena because the patterns and structures found in the
landscape at different scales can be used as indicators. The interpretation of patterns and structures as
indicators requires an understanding of their evolution in time. It is suggested that this should be one of
the future research objectives of transect studies.
Acknowledgements
We gratefully appreciate the contributions to this
paper from the many people with whom we have
discussed this work. In particular, we would like to
thank Ger Bergkamp, Erik Cammeraat, F. PerezTrejo, Adolfo Calvo-Cases and all of those who have
helped to collect the field data upon which this paper
depended. We gratefully acknowledge the financial
support for the research from The German-Israeli
Programme of Scientific Cooperation ŽDISUM-029.
and from the European Community Environmental
Programme ŽEV5V-0023..
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