INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 30: 1526–1537 (2010) Published online 18 August 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1999 Spatial–temporal dynamics of precipitation extremes in southern Portugal: a geostatistical assessment study R. M. Dur˜ ao, a * M. J. Pereira, a A. C. Costa, b J. Delgado, c G. del Barrio d and A. Soares, a a Instituto Superior T´ ecnico, CERENA, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Universidade Nova de Lisboa, ISEGI; Campus de Campolide, 1070-312 Lisboa, Portugal c Dpto. Ingenier´ ıa Cartogr´ afica, Geod´ esica y Fotogrametr´ ıa, Universidade de Ja´ en, Spain d CSIC-Consejo Superior de Investigaciones Cient´ ıficas, EEZA – Estaci´ on Experimental de Zonas Aridas; General Segura, 1 – 04001 Almer´ ıa, Spain ABSTRACT: Most of the recent studies and projections of precipitation patterns, based on records observed in the past and climate change scenarios for the Mediterranean basin, suggest a relatively slow decrease in rainfall amounts over the years but an increase in the frequency of extreme precipitation events. These are key factors in desertification processes and these will cause social and environmental impacts in the short term, mainly because changes in heavy rainfall events may have severe implications and impacts on soil erosion, resulting in increased risk of soil degradation. The main objective of the present work is to evaluate the spatial–temporal dynamics of extreme precipitation events in southern Portugal, using a direct sequential simulation algorithm (DSS models) in order to assess the relationships between spatial and temporal extreme rainfall patterns. Local probability density functions (pdfs) and spatial uncertainty are evaluated by a set of equiprobable simulated images of the chosen extreme precipitation indices. The used dataset in this work comprises a set of 105 station records of observed daily precipitation within the period 1961–2000. Two indices of extreme precipitation were selected: one representing the frequency of extremely heavy precipitation events (R30) and another characterizing the occurrence of dry events (RL10), both obtained from observed daily precipitation series. Results show that the spatial continuity of extreme precipitation events has increased in the last 40 years in southern Portugal. It also demonstrates a decrease in spatial variability, implying that extreme precipitation events tend to be more spatially homogeneous, which may have a severe impact on water resources, agriculture and soil erosion, particularly when associated with desertification risks. Copyright  2009 Royal Meteorological Society KEY WORDS precipitation extremes; space–time patterns; stochastic simulation; spatial uncertainty Received 2 September 2008; Revised 16 July 2009; Accepted 16 July 2009 1. Introduction Although there is a general agreement on the trend decline of precipitation over the years, the most impor- tant factors on desertification and other social and envi- ronmental short-term impacts in Mediterranean regions are extreme precipitation events – severe droughts and floods. In Mediterranean climate regions, precipitation patterns are highly variable as far as the time, space, amount and duration of the events is concerned. Most of the studies and projections of future precipitation pat- terns, based on past observed records and climate change scenarios for the Mediterranean Basin, indicate a decrease in rainfall amounts over the years and an increase in the frequency of heavy/intense rainfall events in autumn and winter seasons, particularly in winter (Brunetti et al., 2001; Kostopoulou and Jones, 2005). In some regions, the * Correspondence to: R. M. Dur˜ ao, Instituto Superior T´ ecnico - CERENA, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: rmdurao@ist.utl.pt increasing number of heavy precipitation events increases the flood risk (Hidalgo et al., 2003). Issues such as drought and erosive rainfall have been raising concern about the risks of land degradation and desertification (L´ azaro et al., 2001). These changes in extreme precipitation events may also have severe implications and impacts on soil erosion, resulting in increased soil degradation risks. A common tool used to understand and assess the precipitation pattern over a region is the extreme precipi- tation indices based on daily precipitation series, as indi- cators of climate change (Jones et al., 1999; Karl et al., 1999; Brunetti et al., 2001). Generally, these indices can be split into two main categories: one involves arbitrary fixed thresholds, such as the number of days per year with daily precipitation exceeding a specific amount or threshold (in millimetres) (e.g. Klein Tank and K¨ onnen, 2003; Kostopoulou and Jones, 2005) and the other cate- gory is based on statistical quantities such as percentiles, which are more appropriate for regions that contain a broad range of climates (Haylock and Nicholls, 2000; Copyright  2009 Royal Meteorological Society