Characterizing low and high flow spells and their temporal transitions using baseflow estimates

Abstract. Extreme hydrometeorological events such as floods and droughts cause severe socio-economic and environmental impacts. These impacts can be amplified if hazards occur successively before the system can recover. While the drivers of individual extremes are well understood, the spatial variability and timescales of transitions between high and low flow spells remain understudied, especially regarding their implications for operational management. We propose an analytical framework to detect and characterize these spells using daily streamflow data from 643 catchments in France (CAMELS-FR) over the 1970–2021 period. We use a mixed threshold approach combined with baseflow estimation as an indicator for catchment recovery to identify the spells and analyze their frequency, duration, and temporal transitions. The analysis is carried out at catchment scale and at the scale of French operational flood forecasting centers. We find that short duration high flow spells are predominant across France, while long duration high flow spells are concentrated in northern France. Regarding transitions, they are predominantly consecutive occurrences of the same spell type, with consecutive high flow spells being more common. Our analysis reveals that transitions occurring in less than a month from low to high flows show distinct spatial variability, with the shortest transition durations concentrated in the Rhone-Mediterranean and Rhine-Meuse basins. These short term transitions predominantly occur in autumn and early winter. On the other hand, transitions from high to low flows are typically slow, developing over more than 90 days. These findings highlight the importance of enhancing our knowledge on compound events to better adapt flood disaster and drought management to local contexts and their characteristics.