Recent years have shown that droughts can have serious socio-economical impacts. While the underlying cause of a drought (a lack of water compared with normal conditions) appears trivial, the exact definition is not. Especially since droughts can occur in different components of the hydrological cycle, each with unique spatial and temporal characteristics.
Despite that droughts are considered to be relatively rare, the spatial variability and complexity across different components of the hydrological can cause drought occurrence in some parts of the system to become relatively common. A conceptual example of this is shown in the figure below. Here, droughts are defined using the same metric for each variable (20th percentile threshold). This figure shows that droughts in at least one variable occur more frequently than is expected based on the drought frequency.
This drought frequency paradox is also observed in the Dutch province of Gelderland (see also link to infographic below). The figure below shows the drought occurrence and frequencies for each of the five considered components of the hydrological cycle. Based on the drought definition, it is expected that droughts occur for 20% of the time. However, all variables are more often in drought conditions than the definition. When considering how often simultaneous droughts occur, it appears that the province has at least one variable in drought conditions for 73% of the time. The actual drought frequency matches the definition when between 3 and 4 variables are simultaneously in drought conditions.
This recent study by Buitink et al. (2021), which was published in Frontiers in Water in a special issue on Challenges of Hydrological Drought Monitoring and Prediction, explains why drought can be rare by definition, but yet can be a near-continuous problem when different sectors are considered. For Gelderland, this is illustrated by news coverage on drought by Omroep Gelderland, which until recently was still reporting on the impacts and recovery of the 2018/2019 droughts, for instance on drought in brooks or groundwater near Wageningen. The drought frequency paradox explains why a rare event can cause near-continuous impacts, and possibly provides a new perspective on how we should look at, and define, drought from the impact on different parts of the hydrological system and different socio-economic sectors.
Further reading
Buitink, J.; T. C. van Hateren & A. J. Teuling (2021), Hydrological systems complexity induces a drought frequency paradox. Frontiers in Water, 3, doi:10.3389/frwa.2021.640976.
Kumar, R.; J. Musuuza; A. F. van Loon; A. J. Teuling; R. Barthel; J. ten Broek; J. Mai; L. Samaniego & S. Attinger (2016), Multiscale evaluation of the Standardized Precipitation Index as a groundwater drought indicator. Hydrology and Earth System Sciences, 20, 1117–1131, doi:10.5194/hess-20-1117-2016.