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1. Definition
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Name
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VEGETATION
COVER (remote sensing)
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Brief
definition
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The
percentage of soil, which is covered by green vegetation
as seen in a vertical view from top. Typically it
is % fractional vegetation cover which is derived
from remote sensing data.
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Unit
of measure
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%
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Spatial
scale
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Vegetation
cover is an important variable for assessing the state
of degradation of landscape units from the slope to
watershed, but also at scales allowing national and
Mediterranean wide assessments. Depending on the purpose,
vegetation cover may be determined at all spatial
resolutions air- and space-borne remote sensing systems
are offering, ranging from high resolutions between1x1
m² and 30x30 m² up to medium
and coarse resolutions from 250x250 m²
to 1x1 km² and beyond.
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Temporal
scale
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Monthly
to annual
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2.
Position within the logical framework DPSIR
3.
Target and political pertinence
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Objective
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Assessment
of the state and trends of land degradation, assessment
of desertification risk, identification and monitoring
of desertification hot spots, contribution to definition
and mapping of ESAs as well as to RDI modelling of
Mediterranean wide soil erosion risk.
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Importance
with respect to desertification
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Vegetation
cover is considered one of the most important factors
of land degradation. Long term trends of decrease
in vegetation cover and changes in annual cycles are
among the most important indicators proposed for regional
scale long term early warning and response systems
(LTEWRS). Vegetation cover and functioning play a
key role in protecting the soil from degradation processes
finally resulting in loss of productivity and severe
erosion. Hence vegetation cover is a crucial input
in the RDI soil erosion modelling and ESA definition.
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International
Conventions and agreements
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The
UNCCD emphasizes that combating desertification must
be tackled within the general framework of actions
to promote sustainable development. Furthermore the
UNCCD requires the creation of a working method to
lay the foundation and provide the cognitive elements
required to implement the convention at a regional
level (ICCD/COP(4)/3/Add.3 (B). Besides the Convention
on Desertification, precise information on vegetation
cover is highly relevant to the Convention against
global climate change and the implementation of the
Kyoto protocol as well as to the Convention for Biodiversity.
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Secondary
objectives of the indicator
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Vegetation
cover is closely related to climatic indicators but
also to assessments of soil organic matter status,
soil depth, fire risk, land cover and land use change.
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4.
Methodological description and basic definitions
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Definitions
and basic concepts
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In
principal, remote sensing derived fractional vegetation
cover corresponds to a single layered projection of
green foliage cover, known also under the term projected
foliage cover (PVC). Closely related is the leaf area
index (LAI), which is defined as cumulative leaf area
of a canopy projected to a plane and gives the area
of leaves of a canopy in m² per one m²
of the ground.
Annual
average vegetation cover is closely related to the
production of above-ground biomass and as such an
indicator for the productivity of an ecosystem. Many
authors demonstrated that in a wide range of environments,
both run-off and sediment loss decrease exponentially
as the percentage of vegetation cover increases. A
value of 30 to 40% vegetative cover is considered
critical below which accelerated erosion dominates
in a sloping landscape. This threshold may be modified
for different types of vegetation, rain intensity
and land attributes. In the case of poor vegetation
cover during periods with high rainfall intensities,
the erosional processes may be very active and the
regeneration of natural vegetation may be irreversible.
Therefore the seasonal variability of vegetation cover
is also highly relevant
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Benchmarks
Indication of the values/ranges of value
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<10%;
10%-30%;
30%-50%;
>50%
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Methods
of measurement
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Fractional
vegetation cover can be estimated from simple spectral
indices such has the NDVI, SAVI or GEMI, which however
have some disadvantages e.g. in terms of site specific
calibration to minimise the influence of different
soil backgrounds. Another problem is the comparability
of index values derived from remote sensing systems
with different spectral characteristics.
The
approach to derive fractional vegetation cover at
different levels of spatial and spectral resolution
upon the basis of spectral mixture analysis (SMA)
has shown that vegetation cover derived in this way
is more coherent across spatial resolutions and has
a fractal dimension which allows its comparison in
different scales.
Coarse
resolution/high revisit rate (1 to 3 days) remote
sensing systems such as NOAA-AVHRR (1 to 8 km resolution),
SPOT VEGETATION (1 km), TERRA-MODIS (0.25 to 1 km)
or MERIS (0.3 to 1 km) allow monthly observation over
tens of years and over large areas (e.g. Mediterranean
wide), but at the cost of spatial detail. High resolution
systems instead, e.g. Landsat-TM or SPOT HR, may allow
only seasonal observation maintaining spatial detail,
however, as required for mitigation planning and control
at ESA level.
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Limits
of the indicator
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The
detection of important characteristics of vegetation
cover such as structure and composition is limited.
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Linkages
with other indicators
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Close
relationships with numerous other indicators such as Soil
Organic Matter content, Soil depth,
Erosion risk (RDI), Soil
erosion, Erosion protection, Ecosystem resilience, Land use type, Fire
risk.
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5.
Evaluation of data needs and availability
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Data
required to calculate the indicator
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Primarily
optical remote sensing data, preferably corrected
for ground surface reflectance.
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Data
sources
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High
resolution data from commercial providers, national
and regional mapping or environmental agencies. Very
high resolution data (1x1 m²) is still
fairly expensive from commercial providers. Coarse
resolution data easily available at low to moderate
cost.
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Availability
of data from national and international sources
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Availability
of calibrated data of fractional vegetation cover
is limited. Time series of reflectance channels and
vegetation indices are available from agencies such
as ESA (ENVISAT), USGS (NOAA AVHRR pathfinder, TERRA-MODIS),
EC JRC (SPOT VEGETATION).
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6.
Institutions that have participated in developing the indicator
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Main
institutions responsible
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Institute
for Environment and Sustainability, DG JRC, European
Commission, Ispra, Italy
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Other
contributing organizations
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Universities
of Basilicata, Valencia, Trier
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7.
Additional information
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Bibliography
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Hill
J., Sommer S., Mehl W. & Megier J., 1996: A Conceptual
Framework for Mapping and Monitoring the Degradation
of Mediterranean Ecosystems with remote Sensing. In:
The use of remote sensing for land degradation and
desertification monitoring in the Mediterranean basin,
Eds. J. Hill & D. Peters, Report EUR 16732 EN,
pp.23-43.
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Other
references
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Lacaze
B., Caselles C., Coll C., Hill J., Hoff S., de Jong
S., Mehl W., Negendank J., Riezebos H., Rubio E.,
Sommer S., Teixeira Filho J. & Valor E., 1996:
DeMon Integrated Approaches to Desertification Mapping
and Monitoring in the Mediterranean Basin. Ed. J.
Hill, Final Report of the DeMon-1 project. EUR 16448
EN, JRC Ispra, 176pp.
Kosmas,
C., Kirkby, M. and Geeson, N. 1999. Manual on: Key
indicators of desertification and mapping environmentally
sensitive areas to desertification. European Commission,
Energy, Environment and Sustainable Development, EUR
18882, 87 p
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Contacts
Name and address
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EC,
DG Joint Research Centre
Institute for Environment and Sustainability
Stefan Sommer <stefan.sommer@jrc.it>
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