1. Climate and desertification: indicators for an assessment methodology
M. Sciortino, E. Caiaffa, G. Fattoruso
ENEA
Italian National Agency for New Technologies, Energy and the Environment
Introduction Aridity and drought are the main climatic phenomena that can affect the complex processes causing the desertification. The United Nation Convention to Combat Desertification (UNCCD) agreed to use the Aridity Index[1] (AI) to identify the areas sensitive to desertification. The AI maps at global, national or sub-national scale are used to classify arid, semi arid, dry-sub humid and humid areas[2] by modeling climatic parameters to obtain desertification relevant indicators. Since the adoption of UNCCD, AI maps have been published at global (Middleton and Thomas 1997, Millennium Ecosystem Assessment, 2005), continental (EEA 2005) and national scales for different time periods.
The UNCCD definition, attributing to climatic variations and human activities, the reason of desertification, introduces the time evolution concept for natural pressures on land resources. As consequence of the insufficient availability of data, scarce attention has been devoted, until now, to indicators addressing the time dependent dimension. The time evolving climatic forcing on land resources has been mostly addressed in terms of temperature and precipitation trends. The precipitation decrease over the Sicilian region, from 1921 to 2003, is 81,4 mm (Alecci S., Rossi G., 2007) and the temperature increase over Italian territory, from 1961 to 2004, is 0,96 °C (Desiato F., 2007 ) or 1°C from 1881 to 2003 (Brunetti M. et al, 2006). The regional hydro-climatic monitoring network in Sicily has a number of temperature and precipitation records allowing the assess of the AI time evolution.
[1] The Aridity Index is defined as the ratio of annual precipitation to potential evapotranspiration.
[2] Areas with Aridity Index values falling in range.
Data and methods
The hydro climatic service of Sicily[3] collects and publishes since 1921 regional hydrological annals that have been recently made available on internet. The requirement of contemporary availability of temperature and precipitation limited the number of records to 47 stations that have been used for the calculation of the AI. In this study the FAO-UNEP methodology adopted by UNCCD has been used to estimate the AI as described in World Atlas of Desertification (Middleton and Thomas, 1997).
The AI is computed yearly, for each monitoring station, as the ratio of precipitation to potential evapotranspiration (AI = ∑ i=1,12 Pi / ETPi ) on monthly basis. The yearly values have been averaged on thirty years’ time periods (AI , AI , AI , AI , AI ) following the
same methodology applied to obtain the global aridity map published in the World Atlas of Desertification.
The potential evapotranspiration is computed using the Thornthwaite empirical relationship (Thornthwaite 1948). Although the Thornthwaite method is known to underestimate ETP for dry conditions and over estimate values for moister condition, this limitation does not affect the evaluation of the changes of dryland areas that is the main purpose of this work. The Aridity Index maps (figure 1) have been carried out using geostatistical analysis techniques provided by the kriging module of the ArcMap (ESRI-ArcGIS 9.x). AI maps (figure 1) show the extension of semi arid, dry sub humid, humid areas from 1931 to The maps show a remarkable increase of aridity.
[3] Agenzia Regionale per i Rifiuti e per le Acque,
Results and discussion
In order to have a further interpretation of the results shown in figure 1, it seemed interesting to perform an overlay of the AI map on the AI map. The result of this overlay is shown in figure 2.
Through the ArcGIS facilities it has been possible to calculate how much and where the humid has become dry sub humid , how much of dry sub humid has become semi arid , how much of the semi arid has remained semi arid, how much of dry sub humid has remained dry sub humid, and how much of the humid has remained humid.
From this type of elaboration of AI maps, over the regional territory of Sicily, it is possible to identify the extension of the areas most sensitive to desertification and their time evolution.
The results (Figure 2) indicate that, considering the time period from to , semiarid zones are increased of km2 (17%), from 5,2% to 22,8% of the total Sicilian territory. In the same period dry sub humid
Figure 2. Map of the semi arid and dry sub humid areas change from the , to
zones are increased of km2 (+8%), from 38,5% to 46,9%, and the humid zones, consequently are decreased of km2 (-26%) from 56% to 30%.
The climate variations (precipitation decrease and temperature increase) in the time period considered, extended Sicilian dry lands of the 38%. In the period , the 69% ( km2) of the Sicilian territory is below the AI threshold of 0,65 and can therefore be considered sensitive to desertification (figure 1).
Figure 1. Extension of areas characterized by the
Aridity Index in the Sicily region.
The progressive drying process affected the whole regional territory. The AI values decreased mostly in the internal territories (Figure 3) where the transition from the humid conditions (in period) to dry sub humid (in the period) has been more significant. The changes of the AI in the coastal areas have been less significant although aridity classes transitions occurred as shown in Figure 2.
Figure 3. AI % change = 100 * (AI AI ) / AI
Conclusions
The time evolution of the territories’ surface in different aridity classes provides desertification sensitivity indicators that should be utilized to identify where to monitor desertification processes. The areas identified by this work are the most sensitive to losses of land biological and economical productivity and therefore more suitable for future desertification studies.
Bibliography
Middleton, N. and Thomas D., 1997: World Atlas of Desertification, Arnold, London
Millennium Ecosystem Assessment, Ecosystems and Human Well-being: Desertification Synthesis. World Resources Institute, Washington, DC.
European Environment Agency, The European environment - State and outlook 2005, Technical Report 1/2005, EEA, Copenhagen.
Alecci S., Rossi G., Siccità, Analisi Monitoraggio e Mitigazione, Applicazioni in Sicilia, Nuova Editoriale BIOS, ISBN pp79-149
Desiato F., Toreti, Temperature trend over Italy from 1961 to 2004 , Theor. Appl. Climatol.
Brunetti M, Maugeri M, Nanni T., Temperature and precipitation variability in Italy in the last two centuries from homogenized instrumental time series. Int J.  Climatol 26: 345–381
INTERNATIONAL CONFERENCE ON DESERTIFICATION IN MEMORY OF PROFESSOR JOHN B. THORNES, (Murcia, Spain, september 2009)