Streamflow droughts in Sweden: Spatiotemporal patterns emerging from six decades of observations

Sweden Study focus: Although Sweden has historically been a country abundant with water, observed changes in temperature and precipitation patterns during the past century have perturbed regional hydrologic regimes, including the severity, frequency and duration of streamflow droughts. This study utilizes the standardized streamflow index (SSI) and the threshold-level method to provide an unprecedented overview of spatiotemporal patterns of streamflow droughts in 50 Swedish catchments over the past six decades. The study catchments were categorized into five clusters, of which each was analyzed for changes in various drought characteristics over the period 1961–2020. Multiple severe streamflow drought events were detected over the past 60 years. Remarkably, droughts in 1976 and 1996 were identified as the most extreme and wide-spread events, also compared to the latest 2018 drought. Southern catchments were generally more often and more severely affected than northern catchments. Our results suggest a wetting tendency over the past six decades across the entire country. This occurs in conjunction with less severe, shorter and less frequent droughts, especially during colder winter months. Only in the southernmost regions, a slight drying trend in spring and summer was found. Thus, we argue that a better understanding and regional management of streamflow droughts is essential to secure the needs of the environment, society and economy now and in the future.