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1. Definition
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Name
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ARIDITY INDEX
(2)
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Brief definition
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An index of the
average water available in the soil, defined as the
ratio between mean annual precipitation (P) and mean
annual evapotranspiration- (ETP) calculated with the
Penman formula.
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Numeric value
of P/ETP ratio
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Spatial scale
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Regional
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Temporal scale
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Single measure
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2. Position within
the logical framework DPSIR
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Type of Indicator
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State. Represents
the typical climatic condition of the area under study.
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3. Target and political
pertinence
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Objective
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The primary objective
is the identification of desertification prone areas,
with particular reference to degradation of soil and
water resources.
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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, used by many of the Annex IV Focal
Points. Along with the soil loss index and the drought
index, it contributes to producing a scale of the
state of health of soil and water resources and consequently
to the elaboration of development strategies compatible
with the resources available in a given area.
According to
the UNCCD desertification cannot exist without a certain
level of aridity (between 0.05 and 0.65).
Within Agenda
21 the aridity index (2) is relevant to Chapter 12
- Management of fragile ecosystems: combating desertification
and drought. Aridity is one of the main causes of
soil vulnerability as regards the degradation factors
involved in desertification. Aridity causes open vegetation
covers and a high soil salinization risk.
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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.
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Secondary objectives
of the indicator
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To elucidate
the desertification rate and its geographical distribution,
as a starting point for prioritising mitigation actions.
To generate aridity maps at sub-watershed levels.
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4. Methodological description
and basic definitions
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Definitions and
basic concepts
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The Aridity index (2)
measures the average water available in the soil of a region
and reflects the moisture content of the soil and the stress
derived from water scarcity.
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Benchmarks Indication
of the values/ranges of value
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There are three
classes for the P/ETP ratio
Each class corresponds
to climatic zones (humid/dry sub-humid; semi-arid;
and arid respectively) and a different score is assigned
to each.
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Methods of measurement
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This index is
the one proposed by the UNCCD to identify arid, semi-arid,
dry sub-humid areas, potentially at risk of desertification.
It provides a measure of the average water available
for plants and the relation between mean annual precipitations
and mean annual evapotranspiration calculated with
the Penman formula.
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Limits of the
indicator
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These are mainly
linked to the difficulty of calculating the ETP with
the Penman method, which in itself is rather complicated
and requires a lot of data that it is not always easy
to obtain.
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Linkages with
other indicators
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Climate index, Drought index, Rainfall, Rainfall
seasonality, Vegetation cover, Vegetation
cover rs, Soil salinity index, 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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For the definition
of the P parameter, the data required are annual rainfall
statistics obtainable from the various stations located
in the area under study. As for the indirect calculation
of the ETP, using the Penman formula, data requirements
for the period under study consist of temperature,
ambient humidity, wind speed and radiation means.
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Data sources
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Data can be obtained
from the different regional, national or international
institutions dealing with the collection and elaboration
of such data, or, if not available, they must be collected
by the user with the help of meteorological stations
located within the area under study. A record of at
least 30 years is necessary.
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Availability
of data from national and international sources
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Data on rainfall
are generally readily available and are reasonably
cost effective, while those relative to the ETP calculation
are more difficult to gather.
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6. Institutions that
have participated in developing the indicator
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Main institutions
responsible
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DGF - Direcçao-Geral
das Florestas. Ministèrio da Agricultura, do Desenvolvimento
Rural e das Pescas.
Dirección General
de Conservación de la Naturaleza (DGCN), Secretaría
de Medio Ambiente, Ministerio de Medio Ambiente, Spain.
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Other contributing
organizations
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7. Additional information
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Bibliography
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Programa de Acción Nacional
Contra la Desertificación (DGCN, Ministerio de Medio Ambiente,
Spain).
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Other references
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http: //www.dgf.min-agricultura.pt.
Desertification
Indicators for the European Mediterranean Region (Enne,
G. and Zucca,C. 2000)Mapa de Aridez (Servicio Meteorológico
Nacional, Spain).
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Contacts Name
and address
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Direcçao-Geral
das Florestas
Av.a Joao Crisotomo, n. 28-5.
1069-040 Lisboa- Portugal
Tel: +351.1.312 49 58
FAX: +351.1.312 49 89
Email dgf.web@mail.telepac.pt
Leopoldo Rojo
Serrano <LRojo@mma.es>
Dirección General de Conservación de la Naturaleza
Ministerio de Medio Ambiente
Gran Vía de San Francisco 4
28005 Madrid (Spain)
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