Storylines of summer streamflow droughts in western Canadian watersheds: historical attribution and future projections

Southern British Columbia, Canada experienced successive summer streamflow droughts in 2023 and 2024, with flows approximately 23%–43% below the 1955–2024 means and substantial impacts on water and energy supplies. Here, we characterize these events in a storyline framework by driving a large-scale hydrological model with meteorological forcings from factual, counterfactual and future climates. The results showed that the 2023 drought was primarily driven by anomalously high May–June temperatures, whereas the 2024 drought was primarily caused by an exceptionally low snowpack. Long-term climate change has reduced summer flows by approximately 8%–31% and intensified the severity of both droughts. Future projections suggest increased frequency and severity of summer streamflow droughts, with events exceeding the historically extreme 2023 drought becoming more common, and the compounding effects of meteorological and snow droughts becoming more prevalent. Overall, these results underscore the need to enhance resiliency to summer streamflow droughts.