Generalised Drought Index: a novel Multi-Scale Daily Drought Index

Droughts are among the most impactful and complex climatic phenomena, widely recognised as pervasive natural hazards posing serious challenges to ecosystems and societies. Assessing droughts is challenging due to difficulties in accurately determining their spatial and temporal dimensions. This study introduces a novel daily drought index, the Generalized Drought Index (GDI), calculated using the Iberian Gridded Dataset (1971–2015) with precipitation and precipitation minus potential evapotranspiration data. The daily resolution enables the identification of flash droughts, which proves highly valuable for future research efforts.

A comparative analysis was conducted against the daily Standardised Precipitation Index (SPI), the Standardised Precipitation Evapotranspiration Index (SPEI), and a Z-Score standardisation. Seven accumulation periods (7, 15, 30, 90, 180, 360, 720 days) were evaluated, with focus on direct comparison amongst indices in their ability to conform to the standard normal distribution. Results showed that GDI, SPI, and SPEI follow the normal standard distribution, while Z-Score depends on the data's original distribution. Using the Distribution Added Value (DAV) technique, GDI demonstrated gains over other indices, with DAV up to 35% compared to SPI and SPEI. The spatial extent of the 2004–2005 drought was also analysed, with GDI, SPI, and SPEI providing similar results, while Z-Score exhibited limitations at shorter accumulation periods.

GDI was also applied to the Coordinated Regional Climate Downscaling Experiment for Europe (EURO-CORDEX), using data from 13 regional climate models (1971–2100) for RCP2.6, RCP4.5, and RCP8.5. An ensemble-based index approach was considered using all models and the same accumulation periods. Findings show Iberia's high vulnerability to climate change, with increases in drought severity, frequency, and duration, especially by mid-century. Under RCP2.6, changes were observed early in the century. RCP4.5 projects similar impacts in mid and end century, with up to 30 additional events at shorter timescales and 50 extra moderate drought days for longer periods. RCP8.5 projects dramatic increases, with over 50 severe events at shorter scales and durations exceeding 100 days for longer periods by the century's end. These results highlight the urgent need for mitigation policies and targeted adaptation strategies to address water challenges and minimize drought impacts in Iberia.

 

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UID/50019/2025 and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).