Abstract
The paper refers to a study on droughts in a small Portuguese Atlantic island, namely Madeira. The study aimed at addressing the problem of dependent drought events and at developing a copula-based bivariate cumulative distribution function for coupling drought duration and magnitude. The droughts were identified based on the Standardized Precipitation Index (SPI) computed at three and six-month timescales at 41 rain gauges distributed over the island and with rainfall data from January 1937 to December 2016. To remove the spurious and short duration-dependent droughts a moving average filter (MA) was used. The run theory was applied to the smoothed SPI series to extract the drought duration, magnitude, and interarrival time for each drought category. The smoothed series were also used to identify homogeneous regions based on principal components analysis (PCA). The study showed that MA is necessary for an improved probabilistic interpretation of drought analysis in Madeira. It also showed that despite the small area of the island, three distinct regions with different drought temporal patterns can be identified. The copulas approach proved that the return period of droughts events can differ significantly depending on the way the relationship between drought duration and magnitude is accounted for.
Highlights
Droughts, perceived as prolonged and regionally extensive occurrences of below average natural water availability, are among the most destructive hazards and can arise virtually everywhere on the planet [1]
This work presents a systematic analysis of the effect of the moving average filter on drought assessment based on the Standardized Precipitation Index (SPI) series (SPI3 and SPI6) from 1936 to 2016, and of the jointly modeling of drought characteristics with bivariate copulas for Madeira
Planning and management of water resources systems under drought conditions often require the estimation of return periods of the exceptional drought events [59,85]
Summary
Droughts, perceived as prolonged and regionally extensive occurrences of below average natural water availability, are among the most destructive hazards and can arise virtually everywhere on the planet [1]. As stated by the Intergovernmental Panel on Climate Change, IPCC, in its periodical assessment reports [5,6,7], compared to continental areas, islands are more vulnerable to natural hazards due to their lower adaptive capacity, and are more often affected by extreme hydrological. Water 2019, 11, 2489 events (e.g., floods and droughts) and climate change, especially the so-called small islands with areas between 100 km and 5000 km2 [8]. The pronounced hydrological temporal and spatial variability in some of the small islands makes drought complex to analyze and simultaneously a poorly understood extreme hydrological events (e.g., compared to floods) [1]. Examples are Madeira with a very pronounced wet season and with notable differences in rainfall between northern and southern slopes; the nearby Porto Santo
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