# How do flood-rich and drought-rich periods arise and change?

Floods and droughts are not spread evenly through time: they cluster into decades with many events, separated by quieter ones. Why does this happen, is the clustering changing, and what does that mean for planning that assumes the past record is representative?

## Question

How do flood-rich and drought-rich periods, runs of years or decades with unusually many or severe floods or droughts, arise? Are they changing, and if so, why? This covers detection (are such periods statistically distinct?), attribution (climate variability and teleconnections, climate change, land use, river engineering) and their characteristics.

## Background

The question merges three top-ranked ("gold") questions from the community process behind the twenty-three unsolved problems in hydrology, on the detection, attribution and characteristics of such periods. It is related to the Hurst phenomenon, studied from the 1970s for reservoir design mainly with statistical methods; climate change brought a more process-based view that singles out teleconnections and goes beyond trend analysis [1].

## What is known

- A database of more than 100 documentary flood series shows that the past three decades were among the most flood-rich periods in Europe in 500 years. Of nine flood-rich periods (including 1560–1580, 1760–1800, 1840–1870 and 1990–2016), the recent one is unusually warm and more seasonal: 55% of central European floods now occur in summer, against 41–42% before [2].
- Five decades of observed European flood discharges show regional trends from an increase of about 11% per decade to a decrease of 23% per decade, consistent with changes in autumn and winter rainfall, evaporation and snowmelt [3].
- In southwestern North America, 2000–2021 was the driest 22-year period since at least 800 CE; about 19% of the exceptional 2021 drought severity is attributable to human-caused climate trends [4].

## What would count as progress

No single result settles a question like this; contributions like these would move it forward:

- Long, open flood and drought chronologies for more regions, combining instrumental, documentary and natural archives, with a reproducible definition of a "rich" period.
- Attribution studies that test whether specific rich periods are explained by circulation patterns or teleconnections, checked on independent periods or regions.
- Stochastic or process-based models that reproduce the observed clustering out of sample, and show how design floods and drought plans should change.

## References

1. Blöschl, G. et al. (2019). [Twenty-three unsolved problems in hydrology (UPH) – a community perspective](https://doi.org/10.1080/02626667.2019.1620507). *Hydrological Sciences Journal* 64(10), 1141–1158.
2. Blöschl, G. et al. (2020). [Current European flood-rich period exceptional compared with past 500 years](https://doi.org/10.1038/s41586-020-2478-3). *Nature* 583(7817), 560–566.
3. Blöschl, G. et al. (2019). [Changing climate both increases and decreases European river floods](https://doi.org/10.1038/s41586-019-1495-6). *Nature* 573(7772), 108–111.
4. Williams, A. P., Cook, B. I. & Smerdon, J. E. (2022). [Rapid intensification of the emerging southwestern North American megadrought in 2020–2021](https://doi.org/10.1038/s41558-022-01290-z). *Nature Climate Change* 12(3), 232–234.

## Source

Imported from [Twenty-three unsolved problems in hydrology, question 9](https://doi.org/10.1080/02626667.2019.1620507), curated by Blöschl et al. Listed as open in the source (2019) and still an active research question as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.