Generalized drought index: A novel multi-scale daily approach for drought assessment

Droughts are recognized as one of the most complex, widespread, and devastating natural extremes presenting a formidable challenge for human societies and the ecosystems due to water scarcity. The impact of droughts is far-reaching, affecting water management, agriculture, and health, and negatively affecting the economy. Amongst all natural hazards, droughts developed slowly over time, having the longest duration, and spreading further, where often their effects persist long after their occurrence. In the recent past however, the concept of flash drought is surging. These types of events are characterized by a short duration but a fast intensification, threatening the water supply and can significantly impact critical stages of plant development, depending on the time of year. Flash droughts can only be identified with daily drought indices, due to their sub-monthly nature. Nowadays, and allied with the widespread of model and observation based daily datasets, this study proposes a novel daily drought index, the Generalized Drought Index (GDI). The GDI has the capability of standardizing precipitation or the balance between precipitation and potential evapotranspiration, offering several advantages over existing indices such as the Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), or a simple Z-Score transformation. The GDI is evaluated using data from the Iberian Gridded dataset and from the Coordinated Regional Climate Downscaling Experiment for Europe (EURO-CORDEX), for the 1989-2009 period. Seven accumulation periods (7, 15, 30, 90, 180, 360, 720 days) and three drought levels (moderate, severe, extreme) are considered. An assessment of the projections of droughts in a climate change context is also performed. To this end, data from the historical and future simulations from the EURO-CORDEX are also considered for the periods of 1971-2000 as a reference and 2011-2040, 2041-2070, and 2071-2100. The evaluation includes direct comparisons against the SPI, SPEI, and Z-Score, added value assessments, and bias differences. Results reveal that the new index effectively characterizes flash droughts, while at the same time still demonstrating added value, especially for longer accumulation periods, outperforming the existing indices. Compared to the Z-Score, the new index exhibits slightly greater advantages, particularly for extreme drought events at shorter accumulation periods. On the other hand, the main differences between the GDI against the SPI or SPEI lies towards the extreme values of the index.  Overall, the findings suggest the new index's enhanced performance and added value, making it a valuable tool for daily drought assessment.   This work was supported by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDL, DHEFEUS (https://doi.org/10.54499/2022.09185.PTDC). JC, RMC, AR, and DCAL are supported by the Portuguese Foundation for Science and Technology (FCT) financed by national funds from the MCTES through grants SFRH/BD/139227/2018, https://doi.org/10.54499/2022.01167.CEECIND/CP1722/CT0006 and https://doi.org/10.54499/2022.03183.CEECIND/CP1715/CT0004, respectively. Pedro MM Soares would like 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).