Regional Frequency Analysis of Droughts in Portugal |
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Authors: | João Filipe Santos Maria Manuela Portela Inmaculada Pulido-Calvo |
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Affiliation: | 1.Dpto. Engenharia, ESTIG,Instituto Politécnico de Beja,Beja,Portugal;2.Dpto. Engenharia Civil, SHRH,Instituto Superior Técnico (Lisboa),Lisboa,Portugal;3.Dpto. Ciencias Agroforestales, Escuela Técnica Superior de Ingeniería, Campus La Rábida,Universidad de Huelva,Palos de la Frontera,Spain |
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Abstract: | This study investigated the frequency of droughts for the period September 1910 to October 2004 in mainland Portugal, based
on monthly precipitation data from 144 rain gauges distributed across the country. The drought events were characterized using
the standardized precipitation index (SPI) applied to time scales of 1, 3, 6 and 12 consecutive months. Based on the SPI time
scale series a regional frequency analysis of drought magnitudes was undertaken using two approaches: annual maximum series
(AMS) and partial duration series (PDS). Three spatially defined regions (north, central and south) were identified by cluster
analysis and analyzed for homogeneity. Maps of drought magnitude were developed using a kriging technique for several return
periods. Similar uniform spatial patterns were found throughout the country using the AMS and PDS approaches. For several
SPI time scales a comparison of the observed and estimated maximum magnitude (269-year empirical return period) showed that
the AMS combined with the selected probability distribution models (Pearson type III, general Pareto and Kappa) provided better
results than the PDS approach combined with the same models. A general and simplified characterization of drought duration
revealed a relatively uniform pattern of droughts events across the country. |
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