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1. Definition
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Name
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SOIL
SURFACE STABILITY
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Brief
definition
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An
indicator of soil aggregate stability
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Unit
of measure
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The
stability of the soil surface horizon is measured
either in terms of resistance to break-up (torvane
method), and in this case the unit of measure is in
Kg/cm², or by means of a test of dispersion
of particles in water, and in this case the unit of
measurement is represented by two different qualitative
indices estimated visually.
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2.
Position within the logical framework DPSIR
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Type
of Indicator
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State.
A soil property determining vulnerability to the splash
of rain drops and to surface runoff.
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3.
Target and political pertinence
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Objective
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Contribution
to the identification of Environmentally Sensitive
Areas by assessing how the soil fulfils the function
of water storage and conservation, and of resistance
to erosion.
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Importance
with respect to desertification
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This
indicator is part of a set of tools to identify and
mitigate land degradation, developed in the MEDALUS
target area of the Alentejo. Together with the indicators
of soil infiltration capacity and data relative to
the occurrence of extreme rain events, it contributes
to assessing land vulnerability and consequently to
the elaboration of development strategies compatible
with the resources available in a given area.
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International
Conventions and agreements
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The
UNCCD emphasizes the fact that combating desertification
must be tackled within the general framework of actions
to promote sustainable development.
Within
Agenda 21 soil surface stability is relevant to Chapter
12 - management of fragile ecosystems: combating desertification
and drought.
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Secondary
objectives of the indicator
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4.
Methodological description and basic definitions
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Definitions
and basic concepts
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The
stability of the soil surface layer is defined by
means of resistance to the break-up of the soil itself
and on the basis of the degree of soil particle breakdown
in water. Resistance to the breaking force exercised
by gravity, by the movement of fluids and by mechanical
means on soil. Together with infiltration capacity
and aggregate stability, it influences erodibility
and the soil's capacity to store water. The degree
of breakdown in water of particles of an undisturbed
soil sample is an indicator of resistance to break-up
and to the impact of raindrops and is also a factor
determining soil erodibility.
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Benchmarks
Indication of the values/ranges of value
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Methods
of measurement
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Two
methods are used to determine the stability of the
soil surface horizon: i) the torvane method (Imeson
et al., 1999) and ii) the dispersion test.
The
torvane method is **
The
dispersion test is a simple laboratory test.
In order to describe the results of the test, the
author proposes two indices. The first is the Breakdown
index (B), which distinguishes the degree to which
aggregates breakdown in water in: I) no breakdown
(there is no breakdown and the original lump remains
unmodified); ii) slight breakdown (the clear-cut edges
of the lump breakdown); iii) moderate breakdown (the
edges breakdown and the lump divides into smaller
lumps); iv) strong breakdown (some small lumps can
still be distinguished amongst the dispersed particles);
v) complete breakdown (all the lumps are divided into
single particles). The second index, on the other
hand, the Suspension index (S), based on the milkiness
of the sample, and distinguishes samples: I) not milky
(the water is perfectly clear); ii) slightly milky
(water is slightly muddy with a milky suspension at
the bottom) iii) moderately milky ( water is muddy,
but it is still possible to see through the beaker);
iv) very milky (only the lower part of the beaker
has a degree of turbidity making it impossible to
see the bottom)v) extremely milky (a muddy suspension
fills the whole beaker). Both indices, B and S are
estimated visually.
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Limits
of the indicator
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These
are mainly limits of an operational nature. Especially
the dispersion test, which is based on visual observation
(and thus highly subjective from the outset ) requires
further adjustments to be considered valid, which
will necessarily lead to an increase in experimental
errors.
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Linkages
with other indicators
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Soil
surface stability along with Infiltration capacity, is part of a
system designed to identify, characterise in detail and then
to classify, a series of source areas, (areas that after the
advent of rainfall of varying intensity become sources of
sediment and surface flow).
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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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Data
has to be collected by means of specific field surveys
and laboratory analyses.
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Data
sources
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Availability
of data from national and international sources
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6.
Institutions that have participated in developing the indicator
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Main
institutions responsible
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University
of Amsterdam, University of Lisbon.
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Other
contributing organizations
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7.
Additional information
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Bibliography
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Imeson
A.C., Suryana N., Bergkamp G., Bolwidt L., Haring
R., van Leuzen P., Seijmonsbergen H., Hoogteiling
D., (1999). Developing and applying indicators of
desertification derived from soil-water-vegetation
relationships. Mediterranean Desertification and Land
Use - Final report phase III (1996-1999). Contract
ENV4-CT95-0119. Thatcham, UK, pp. 47-85.
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Other
references
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Contacts Name and address
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Prof. A. C. Imeson
Dept. Physical Geography and Soil Science - University
of Amsterdam, Amsterdam, The Netherlands.
Tel: (31) 20 525 7457
Fax: (31) 20 525 7431
Email: A.C.Imeson@frw.uva.nl
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