A unified streamflow drought index for both perennial and intermittent rivers at global scale

The modelling of streamflow drought is usually performed with different approaches for perennial and intermittent rivers, as no-flow periods require a dedicated statistical treatment. Here, a unified drought index is proposed by extending the threshold method to all flow regimes. The method generalizes the approach commonly adopted for low-flow droughts by accounting for the probability of occurrence in a yearly reference period. Application to a global dataset for 1991-2020, derived from ERA5-forced LISFLOOD model simulations, demonstrates how consistent outcomes can be obtained in all major perennial and intermittent streams. A simplified version of the method is also discussed, detailing how reliable drought detection can be achieved in many parts of the world (e.g. central Europe, the eastern US for low-flow, and northern Russia and the Middle East for no-flow) even when the frequency distribution of the spells in time is replaced by the simpler average number of events.