A new ensemble-based SPI and SPEI index to depict droughts projections for the Iberia Peninsula with the EURO-CORDEX

Droughts are one of the major natural hazards, affecting the flora and fauna, but also human activities and health. Such events impact water management and agriculture, potentially causing increased mortality and economical losses. Drought analysis is a complex and challenging task, as it is quite difficult to accurately determine the spatial and temporal dimensions of drought events. Synthetic tools, like drought indices mostly based on the climate information, are often used to tackle this problem. Both the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) were widely used to characterize droughts. These indices usually consider a monthly aggregation of either precipitation or a water balance (precipitation minus evapotranspiration), adjusting the data to a theoretical Probability Density Function (PDF), in order to get a standardized time-series. However, the input of such a small amount of data into the PDFs could potentially lead to uncertainties and it is not the best for some types of applications which respond on a smaller timescale. Nowadays, observational data are more readily available at a daily time-step. Thus, a new daily index is here proposed, relying on an empirical PDF built from the data. This new daily-SPI and daily-SPEI indices were applied to the World Meteorological Organization - Coordinated Regional Climate Downscaling Experiment for the European domain (EURO-CORDEX). In total 13 Regional Climate models were considered for the 1971-2100 period, following the Intergovernmental Panel on Climate Change – Representative Concentration Pathways (RCP) 2.6, 4.5 and 8.5 from 2006 onwards. The versatility of these new indices allows a building of an ensemble PDF, featuring all model data. A timescale of accumulation with 7-, 15-, 90-, 180 and 360- days were considered. The EURO-CORDEX is then used to assess drought projections throughout the 21^st century in terms of intensity, frequency, and mean duration of events for moderate, severe, and extreme droughts. It is projected an increase of intensity along the century, more pronounced for the RCP 8.5. While for the RCP 2.6, the intensity peak occurs for the mid century (2041-2070). As for the frequency of drought events, the timescale of 15 days reveals a noticeable increase, being constantly above other timescales, particularly for the daily SPEI. Moreover, the mean duration of events reveals a higher increase at the longer accumulation periods, while small to no changes occur for the 7 and 15 days.

Acknowledgements

The authors wish to acknowledge the financial support of FCT through project UIDB/50019/2020 – IDL and EEA-Financial Mechanism 2014-2021 and the Portuguese Environment Agency through Pre-defined Project-2 National Roadmap for Adaptation XXI (PDP-2). J. Careto is supported by the Portuguese Foundation for Science and Technology (FCT) with the Doctoral Grant SFRH/BD/139227/2018 financed by national funds from the MCTES, within the Faculty of Sciences, University of Lisbon.