Technical note: What does the Standardized Streamflow Index actually reflect? Insights and implications for hydrological drought analysis

Additionally, we analyzed the impacts of adopting commonly used timescales on propagation analyses of specific drought events – from meteorological to soil moisture and hydrological drought – with focus on their duration and intensity. The results reveal that the choice of timescale for the SSI has larger effects on correlations with explanatory variables in rainfall-dominated regimes compared to snowmelt-driven basins, especially when simulated fluxes and storages are aggregated to timescales longer than 9 months.

In all the basins analyzed, the strongest relationships (Spearman rank correlation values over 0.7) were obtained when using 6-month timescales to compute the SSI and 9–12 months to compute the explanatory variables, excepting aquifer storage in snowmelt-driven basins. Finally, the results show that the trajectories of drought propagation obtained with the Standardized Precipitation Index (SPI), the Standardized Soil Moisture Index (SSMI), and the SSI may change drastically with the selection of timescale.

Overall, this study highlights the need for caution when selecting standardized drought indices and associated timescales, since their choice impacts event characterizations, monitoring, and propagation analyses.