Are rainfall warning levels ready for climate change? A case study from Catalonia, Spain

Extreme precipitation events, characterised by significant rainfall amounts over short periods, are projected to intensify and occur more frequently under the influence of climate change. These projected changes, combined with rapid urbanisation, will likely lead to more frequent and extreme pluvial flood events (urban and flash floods) due to the precipitation intensity rapidly and easily exceeding the current capacity of natural and artificial drainage systems. Assessing the impact of future climate scenarios on extreme precipitation is therefore critical for identifying and designing sustainable adaptation and mitigation actions for at-risk communities and their citizens.

As part of the EU Horizon 2020 project CLIMAAX, an extreme precipitation workflow has been developed to provide step-by-step guidelines for communities and regions to identify and assess how their critical rainfall thresholds could shift in both magnitude and frequency under climate projections. In this work, a critical rainfall threshold is defined as the precipitation intensity necessary to trigger unsustainable or unacceptable impacts in a specific location or area. These thresholds are commonly used in designing drainage systems and flood protection infrastructure and serve as decision support values for triggering rainfall warnings or advisory information during emergencies. By employing the workflow to assess how these critical rainfall thresholds are projected to change, communities can make informed decisions about the most appropriate long-term adaptation measures to enhance their overall climate resilience. Moreover, the flexible workflow structure facilitates the integration of diverse hazard, exposure and vulnerability datasets at multiple scales (e.g., CORDEX, WorldPoP), making it adaptable to specific regional needs.

The extreme precipitation workflow has been applied in the Catalonia Region, Spain, to evaluate how the current rainfall thresholds used for triggering rainfall warnings for Dangerous Meteorological Situations will vary due to the influence of climate change. Model combinations of EURO-CORDEX climate projections at a 12km spatial resolution for the different Representative Concentration Pathways (RCPs) were employed for assessing future rainfall projections. Considering the increased number of extreme precipitation events in the region over the past years, the impacts associated with these and the number of triggered warnings per year, the results are expected to provide authorities with valuable insights into the frequency and magnitude shifts of these extreme events in the region.