Power spectral characteristics of drought indices in the Ebro river basin at different temporal scales
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ORIGINAL PAPER
Power spectral characteristics of drought indices in the Ebro river basin at different temporal scales L. Telesca • S. M. Vicente-Serrano J. I. Lo´pez-Moreno
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Published online: 7 October 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract A deep spectral investigation of the monthly time series of Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) in 45 meteorological stations in the Ebro basin (Spain) from 1950 to 2006 for timescales ranging from 1 to 48 months was performed. In order to summarize the results for the whole basin, the spectral analysis was also carried out on the four principal components of SPI and SPEI. Results confirm that SPI and SPEI presents very similar spectral characteristics. At the shorter time scales, the signal of SPI and SPEI is characterized by purely random temporal fluctuations. The longer time scales tend to feature the signal as a smoothly varying time series or persistent, mostly due to the aggregated nature of the indices calculation. The comparative analysis of the spectral properties of the drought indices for all the 45 sites in the Ebro basin lead to the identification of global or regional effects discriminated by local effects. It was found that some periodical signals are common to almost all the sites, while others where only identified in specific meteorological stations. Keywords Standardized Precipitation Index Standardized Precipitation Evapotranspiration Index Power spectrum Principal component analysis
L. Telesca (&) National Research Council, Institute of Methodologies for Environmental Analysis, C.da S.Loja, 85050 Tito, PZ, Italy e-mail: [email protected] S. M. Vicente-Serrano J. I. Lo´pez-Moreno Instituto Pirenaico de Ecologı´a, CSIC, Campus de Aula Dei. Avda. Montan˜ana 1005, 50.059 Zaragoza, Spain
1 Introduction Droughts are one of the main natural hazards that affect the Mediterranean region. In this area droughts have severe consequences for agriculture and natural vegetation (La´zaro et al. 2001; Reichstein et al. 2002; Iglesias et al. 2003), increasing the frequency of fires (Colombaroli et al. 2007; Pausas 2004) and significantly reducing water availability for urban and tourist consumption (Morales et al. 2000). The identification of drought events is a difficult task since we identify a drought by its effects at different levels, but there is not a physical variable we can measure to quantify droughts. In addition it is very difficult to clearly identify their beginning, the duration and quantify its magnitude in both, time and space (Wilhite 1993). The impact of a given drought on different natural or socioeconomic frames will depend largely on the temporal persistence of the drought conditions, as the different subsystems of the water cycle (i.e. soil moisture, streamflows, snowpack, aquifers or water stored in lakes or artificial reservoirs) have a different time of response to antecedent climatic conditions (VicenteSerrano et al. 2011). For this reason, drou
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