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

Abstract Southern British Columbia, Canada experienced successive summer streamflow droughts in 2023 and 2024, with flows ~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. Relative to the counterfactual climate, summer flows in the factual climate are ~8%–31% lower and drought severity has intensified. Future projections suggest increasing 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 in the region.