Browsing by Author "Pulido-Calvo, I."
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- Regional frequency analysis of droughts: portuguese casePublication . Santos, João Filipe; Portela, Maria Manuela; Pulido-Calvo, I.[Poster introduction] A common problem in drought risk analysis relates to the assessment of the rarity of the events, such as long duration droughts or high magnitude droughts. Being a frequent phenomena in the Southern Europe and in others regions of the world drought constitute a primary natural hazard for human activities. For this reason, and for an improved drought risk management, the preparation of drought hazard maps is an important and urgent task. The drought definition based on deviations from normal conditions or from reference stages implies that droughts can occur in any hydroclimatological region and at any time of the year with the same probability. In order to due so, a large number of quantitative drought characteristics must been considered. Two common approaches to select extreme events from a drought index time series were analyzed: the annual maximum series (AMS) and the partial duration series (PDS) approaches.
- Regionalization of droughts in PortugalPublication . Santos, João Filipe; Portela, M.M.; Pulido-Calvo, I.Droughts are complex natural hazards that distress large worldwide areas every year with serious impacts on society, environment and economy. Despite their importance they are still among the least understood extreme weather events. This paper is focused on the identification of regional patterns of droughts in Mainland Portugal based on monthly precipitation data, from September 1910 to October 2004, in 144 rain gages distributed uniformly over the country. The drought events were described by means of the Standardized Precipitation Index (SPI) applied to different time scales. To assess the spatial and temporal patterns of droughts, a principal component analysis (PCA) and K-means clustering method (KMC) were applied to the SPI series. The study showed that, for the different times scales, both methods resulted in an equivalent areal zoning, with three regions with different behaviours: the north, the centre and the south of Portugal. These three regions are consistent with the precipitation spatial distribution in Portugal Mainland, which in general terms decrease from North to South, with the central mountainous region representing the transition between the wet north and the progressively dry south. As the mean annual precipitation decreases southwards the hydrological regime becomes more irregular and consequently more prone to droughts.
- Variabilidade temporal e espacial das secas em Portugal ContinentalPublication . Santos, João Filipe; Pulido-Calvo, I.; Portela, Maria ManuelaÉ apresentada uma análise de secas em Portugal Continental baseada em séries de precipitação mensal, de Setembro de 1910 a Outubro de 2004, em 144 postos udométricos uniformemente distribuídos pelo País. Os eventos de seca foram caracterizados pelo índice de precipitação padronizada, SPI (Standardized Precipitation Index), aplicado a diferentes escalas temporais, designadamente 1, 6 e 12 meses consecutivos, 6 meses de Abril a Setembro e 12 meses de Outubro a Setembro. Para o estudo dos padrões temporal e espacial das secas aplicou-se às séries de SPI a análise de componentes principais (PCA) e a análise de clusters não-hierárquica, algoritmo de k-médias (KMC). Desta forma obtiveram-se três regiões diferentes e espacialmente bem definidas com diferentes padrões temporais de seca: norte, centro e sul de Portugal. Os padrões dos SPI obtidos com base na análise de componentes principais e na análise de clusters foram testados recorrendo a análise espectral, utilizando o algoritmo da transformada rápida de Fourier, tendo-se obtido um ciclo de 3.6 anos no padrão representativo do sul de Portugal e ciclos de 2.4 e 13.4 anos, no do norte do Pais. Conclui-se, assim, que a análise dos períodos de seca suporta a ocorrência de ciclos mais frequentes de seca no sul (secas moderadas a extremas aproximadamente cada 3.6 anos) do que no norte (secas de severas a extremas aproximadamente cada 13.4 anos). Contudo, é necessário prosseguir com a investigação de modo a avaliar a origem desses ciclos.