Drought-to-flood transitions: exploring new indicators using large-sample datasets

The study of the occurrence of abrupt hydrological extremes changes, and in particular the shift from extended dry periods to flood events is of particular interest for the stakeholders dealing with the operational management of environmental hazards. In fact, when floods occur during or soon after drought periods, early warning systems can become inaccurate and water management practices may fall short in reducing the risk of both extremes (e.g. Fowler et al., 2020; Ward et al., 2020).

In this study, we take advantage of the hydrometeorological time-series from more CAMELS-type datasets around the globe to propose and test indicators to characterise the drought-to-flood transition, understood as the basin's propensity to generate flood volumes during or immediately after droughts.

In a first phase, high flow events are identified using a fixed streamflow threshold, while drought events are characterized by means of the Standardised Precipitation Evapotranspiration Index (SPEI). Then, a set of signatures based on the co-occurrence and severity of compound drought and high flow events are calculated for all the study catchments, and their spatial pattern across the different areas of the globe is analysed. In particular, such indicators consider both the magnitude and the seasonality of high flow volumes occurring during or after drought periods.

The proposed metrics could support the hydrologic community in understanding the dynamics guiding compound events across the continents. Moreover, they could be useful in climate change impact studies to assess the evolution of combined drought and flood periods with important implications for practical risk management.

References

Fowler, K., Knoben, W., Peel, M., Peterson, T., Ryu, D., Saft, M., Seo, K., & Western, A. (2020). Many Commonly Used Rainfall-Runoff Models Lack Long, Slow Dynamics: Implications for Runoff Projections. Water Resources Research, 56(5), e2019WR025286. https://doi.org/10.1029/2019WR025286

Ward, P. J., De Ruiter, M. C., Mård, J., Schröter, K., Van Loon, A., Veldkamp, T., Von Uexkull, N., Wanders, N., AghaKouchak, A., Arnbjerg-Nielsen, K., Capewell, L., Llasat, M. C., Day, R., Dewals, B., Di Baldassarre, G., Huning, L. S., Kreibich, H., Mazzoleni, M., Savelli, E., … Wens, M. (2020). The need to integrate flood and drought disaster risk reduction strategies. Water Security, 11, 100070. https://doi.org/10.1016/j.wasec.2020.100070