Event-based analysis of hydrological drought propagation across contrasting river basins

Hydrological droughts are complex spatio-temporal phenomena whose impacts propagate through river networks, affecting water availability, ecosystems, and water management systems (Van Loon, 2015). Understanding how drought events develop and propagate across space remains challenging, particularly when comparing river basins with contrasting hydroclimatic conditions, sizes, and buffering capacities (Wu et al., 2022). Despite the increasing availability of long discharge records, comparative and event-based analyses of hydrological drought propagation are still limited by the lack of scalable and reproducible methodological approaches (Van Huijgevoort et al., 2012).

In this study, we present an event-based framework for the detection, characterization, and propagation analysis of hydrological droughts, designed to enable consistent inter-basin comparisons. Hydrological drought events are identified using threshold-based methods applied to daily discharge series, from which key metrics describing event duration, severity, deficit and lag are derived. Drought propagation is analysed through a network-aware approach that quantifies temporal relationships between upstream and downstream gauging stations along the river network.

The framework has been tested on daily discharge records from the Ebro and Segura River Basins (Spain), representing markedly different basin sizes and hydroclimatic regimes. Results reveal clear contrasts in drought behaviour between both systems. In the Ebro Basin, drought events are associated with large absolute water deficits due to the high-flow regime of the basin, but display comparatively lower relative severity and a weak coupling between event duration and deficit when normalized by local flow conditions, consistent with strong system buffering and spatial heterogeneity. In contrast, the Segura Basin exhibits more recurrent drought events per station, smaller absolute deficits but substantially higher relative severity, longer persistence, stronger coupling between duration and deficit, and a high spatial coherence across the river network, reflecting limited buffering capacity under semi-arid conditions.

These results demonstrate the ability of the proposed framework to capture basin-specific drought regimes and propagation dynamics in a consistent manner. The event-based analysis provides new insights into the controls of hydrological drought persistence and severity and offers a robust basis for comparative drought studies, large-scale impact assessments, and future integration into climate impact analyses and basin-scale hydrological modelling frameworks.

Keywords: Comparative basin analysis; Drought propagation; Hydroinformatics; Hydrological drought; River network connectivity; Spatio-temporal analysis

References
[1] Van Huijgevoort, M. H. J., Hazenberg, P., Van Lanen, H. A. J., & Uijlenhoet, R. (2012). A generic method for hydrological drought identification across different climate regions. Hydrology and Earth System Sciences, 16(8), 2437-2451. https://doi.org/10.5194/hess-16-2437-2012
[2] Van Loon, A. F. (2015). Hydrological drought explained. WIREs Water, 2(4), 359-392. https://doi.org/10.1002/wat2.1085
[3] Wu, J., Yao, H., Chen, X., Wang, G., Bai, X., & Zhang, D. (2022). A framework for assessing compound drought events from a drought propagation perspective. Journal of Hydrology, 604, 127228. https://doi.org/10.1016/j.jhydrol.2021.127228