Drought-to-flood transitions – When and where do they occur?

Drought-to-flood transitions are both a challenge and opportunity for water management. While the two extremes are often studied separately, their close succession can have severe impacts. The timespan between events can range from rapid transitions happening within a few days to long transitions taking many years. Still, the drivers and frequency of those transitions in specific river basins remain unknown. Therefore, we ask ‘when, where and how often do transitions from streamflow droughts to floods occur?’

To answer these questions, we analyse over 1000 catchments in the contiguous US from the GAGES-II database, identify streamflow droughts and floods, and calculate transition times between both types of extremes. Then, we relate the time and frequency of occurrence and the timespan between extremes to local climate and topographic characteristics. We distinguish between winter and summer transitions to identify hydro-meteorological processes important in different seasons and focus on particularly rapid transitions. 

We find that the duration and frequency of transitions show large spatial variability. Regionally, rapid transitions occur during a typical time of the year which is often related to the presence of snow and melt processes. Snow also dictates seasonal differences in rapid transition frequencies between summer and winter. Snow-free catchments have a lower frequency but higher variability of transitions which makes the phenomenon less predictable. Additionally, reservoirs reduce the occurrence of snow-affected rapid transitions. We conclude that management challenges related to drought-to-flood transitions are particularly pronounced in natural and rainfall-dominated catchments.