2.2 Regional degradation index to identify desertification risk at sub-national
to Mediterranean-wide scales (UNIVLEEDS and EC-JRC-EI)
Feeding into WP3.2, the development
of a desertification indicator system for Mediterranean Europe, this work
package will enable desertification risk to be identified for areas larger
than the sub-national, and national areas at which the ESAIS operates.
It also uses data from different sources, medium and high resolution remote
sensing data, digital elevation models, and global circulation models.
As such its use by a different part of the stake-holder community, UNCCD
National Committees, Mediterranean-wide regional managers (rather than
local land users) is foreseen.
The Regional Degradation Index (RDI)
is a methodology to forecast the risk of soil erosion by water. The underlying
physical basis is a one-dimensional hydrological model, which is used
to estimate potential vegetation cover, if required and storm runoff based
on climatic and vegetation data. The forecast runoff, accumulated across
the frequency distribution of storms, is used to give a climatic erosion
potential, which is then appropriately combined with measures of topography
and soil erodibility to estimate the expected rate of soil erosion at
a resolution of 250m - 1 km. The methodology is based on work funded by
a previous FP IV project (MEDALUS III), a tender under FP IV (MODEM),
a current FP V project (PESERA) and prior unfunded research.
Water erosion is known to be directly
controlled by a number of factors, including climate, vegetation, soil
properties and topography. Each factor is itself complex, and the various
factors interact with one another. In creating an estimate of regional
erosion risk, it is important to use a clear scientific rationale to combine
relevant measures of these factors into composite indicators. This analysis
is targeted on such a synthesis, working from a physical basis. The work
package contains five components.
The RDI model contains a clear basis
for downscaling over space down to single hillslope elements, and over
time down to the chosen time-step of the individual day. Downscaling over
space is achievable because the basis of the model is a sediment transport
equation, which can be evaluated at any point within a landscape. However,
the estimates of local basal slope from local relief in a DEM, and of
suitably weighted mean slope length inevitably mean that the parameters
(especially erodibility) are far from scale free, and that effective parameters
at the scale of the entire slope will need scale-dependent correction
for applying at local intra-catena resolutions.
Over time, the mean erosion is explicitly
integrated over the distribution, so that downscaling to the time step
of a day should be relatively straightforward. However, for the finer
resolutions which are certainly needed to comprehend the variability inherent
in possible distributions of a given rainfall within a given storm total,
there is a major intellectual challenge in temporal downscaling as well.
Theoretical investigations of the
best way to implement these two aspects of downscaling, the spatial and
the temporal, provide the largest area of scientific innovation within
the work package. However, it is clear that decisions on how to proceed
must be made at an early stage of the project to ensure that progress
is made with other aspects of the work plan, and its role within the project.
The particular aspect of downscaling
which is most important for DESERTLINKS is the explicit inclusion of indicators
which are being developed for ESAs within target areas (WP 2.1). Our objective
is to develop a basis for comparison between these scales which is scientifically
sound and can be demonstrated to end users.
Part of this downscaling work and
validation of the physical model is being explicitly addressed by UNIVLEEDS
and other partners within the PESERA project. These studies will be available
to DESERTLINKS and are not therefore charged to this project.
of salinity sub-model
Salinity is the second most important
form of desertification in Europe. Increased irrigation is leading to
secondary salinisation in many areas, including the Guadalentín.
Moving further east or into north Africa, the problems become still more
Preliminary work on salinity has concentrated
on models for the 1-dimensional movement of solutes within the soil profile.
For inclusion at a scale compatible with the RDI, the results of these
investigations must be parameterised in terms of monthly water balances
at each point, combined with a linearised model for solute uptake and,
where appropriate, re-deposition within the soil profile. These climatic
data must be combined with
With these data, a salinity model
can provide data on both current and potential risks, which can be compared
with data on saline soils to indicate areas of potentially increased risk
under various land use, irrigation and climate scenarios. This phase of
model development should follow the downscaling work, but should also
be completed in good time to allow map products to be prepared from the
of channel delivery model
Channel delivery is important for
the off-site impacts of erosion, which often have the highest short term
economic impact, causing damage to settlements and loss of life. There
are questions about how to manage flood plains, in relation to check dams,
urban land use and groundwater recharge.
For flow within the channel network,
sediment delivered to the slope base should be routed through the network.
Channel sediment transport is generally considered to be strongly buffered
by the effects of valley floor sedimentation or erosion, so that periods
of high hillslope erosion lead to valley floor sedimentation, and periods
of lowered hillslope input lead to channel incision. It is also widely
observed that reservoir construction, by impounding sediment from upstream,
leads to erosion below dams. In models for long term landscape development,
the concept of the effective bedload fraction (ebf) has been developed
as a simplifying approach. The ebf is the ratio of actual to capacity
sediment transport, reflecting the transition from transport limited by
capacity for coarse debris to transport limited by the supply of fines.
This therefore depends explicitly on the grainsize distribution of source
material and the availability of each grainsize fraction within the channel
environment. Thus, as high erosion rates lead to valley floor sedimentation,
the ebf rises towards its upper limit of 100%, giving a less than proportional
increase in channel sediment transport. Similarly as sediment input falls,
the ebf also falls, leading to scour of previously deposited fines and
ultimately valley floor incision.
This approach to hillslope-channel
sediment coupling offers an innovative re-working of existing data which
is consistent with observed patterns of valley sedimentation, both over
time and between reaches where there are downstream variation in hillslope
sediment delivery. This aspect of the modelling should also follow the
initial work on downscaling, and has considerable potential for application
to regional problems of offsite sedimentation and reservoir siltation.
of updated AVHRR land cover for Mediterranean Europe, and ofvegetation
cover for target areas (EC-JRC-EI)
MODEM showed the potential and limitations
of direct use of RS-based indicators on their own. We can use vegetation
indicators effectively and they can give observable trends in relation
to changes in grazing intensity and conversion to/from arable land or
tree crops, but this is only one part of the erosion story.
Spectral unmixing techniques have
proven their potential to provide semi-quantitative measures of vegetation
cover density, which can be more directly compared at different spatial
scales than conventional vegetation indices. Vegetation abundance, even
if derived from data with quite different spectral and spatial characteristics,
appears to be reasonably coherent through scales and thus has a strong
potential to be further used to derive land cover and bio-physical vegetation
characteristics from the target areas to the region. In this context alternative
methods of regional scale (Mediterranean wide) land cover classification
and vegetation cover attribute mapping (density, fragmentation, bio-mass
etc) will be investigated and applied, based upon spectral mixture analysis
applied to multi-temporal signatures of vegetation.
Change detection techniques such as
regression and change vector analyses will be applied to identify areas
of major changes. The assessment and interpretation of these observed
changes will refer to and incorporate land use and climate change scenarios.
Using these methods, it will become possible both to extend the coverage
for Mediterranean Europe over time, and to add detail for the target areas.
This is a crucial resource for both RDI maps and for linkage with ESAIS
for target areas.
of European and regional scale maps for erosion and salinity risk
With the developments in modelling,
one of the key deliverables is the production of maps for Europe. This
requires land cover data, 1-km DEM and the European Soils Data Base.
It is proposed to provide monthly
maps of average erosional loss, summed over the frequency distribution
of storm events. The methodology also allows production of maps which
express the erosion expected in events with an average recurrence interval
of 10 or 100 years, which provide a basis for planning the responses to
individual extreme events. In addition, the same maps will be generated
for a range of likely land use and climate scenarios, to highlight areas
where there are likely to deteriorating conditions. The primary basis
for land use scenarios at a European scale and for target areas will be
from research within the ongoing MEDACTION project. These will be integrated
within an interactive web interface to a Synoptic Prediction System which
integrates predictions of the climate, physical and socio-economic environment
to create scenario-based forecasts of agricultural land-use and land degradation
of the Mediterranean regional scale.
Although maps provide an important
deliverable from the work package, they lack the flexibility to forecast
the impact of all alternative scenarios or policy options. In Work Package
3.2 it is therefore proposed to provide the software to the Annex IV National
Committees and other policy makers directly. This will then provide a
dynamic tool for evaluating the impact on desertification of policy alternatives.
This software will clearly be delivered at a late stage in the project,
when it has been validated and debugged as fully as possible.
The output from this work package is intended to be directly useable by
policy makers, as end users at both regional and European scales. Maps
are provided of erosion risk, vegetation and land use history (Deliverables
2.2a and 2.2b). The results of this work package will also underpin research
in WP 3.2 and 3.4, and contribute to deliverables 3.2a-d.