Understanding Changes in Iceland’s Streamflow Dynamics in Response to Climate Change

Abstract. The hydrological cycle in high-latitude regions is undergoing significant changes due to climate change. Iceland, with its extensive data from undisturbed catchments, provides a unique opportunity to study these changes in snow- and glacier-melt dominated regions. The country's heavy reliance on hydropower, without any connections to other electricity markets, makes understanding these changes crucial. Recent decades have seen warming outpace global warming trends in Iceland, along with increased precipitation, reduced glacier mass, rising soil temperatures and expanded vegetation cover. The impacts of these environmental shifts on streamflow remain largely unexplored. Our study uses the LamaH-Ice dataset, which includes streamflow observations, atmospheric forcings from climate reanalyses, and catchment characteristics, to investigate changes in Iceland's streamflow dynamics over recent decades. We first examine the long-term variability in streamflow and its primary drivers, correlating it with major climate indices. We then analyze trends during the last 30 and 50 years in annual, seasonal, and daily streamflow volumes, the timing of the spring freshet, and extreme flow conditions, linking these changes to environmental conditions and catchment attributes. Results show high inter-annual variability, decadal fluctuations, and strong correlations with the Arctic Oscillation, as reported in earlier studies. Streamflow trends vary by location and river type, with increased precipitation driving higher annual average flows in most rivers, while summer flows have decreased in most rivers, which is linked to lower summer precipitation and increased evapotranspiration. This study is the first to report coherent regional and seasonal trends in Icelandic streamflow. Annual low flows have increased in most rivers. Glacial rivers show positive streamflow trends during the last 50 years, but negative trends during the last 30 years. The findings offer crucial insights into Iceland's hydrological changes amid rapid climatic shifts, with broader implications for reservoir operations and water resources management. This study enhances our understanding of Icelandic hydrology and contributes to global knowledge on climate-induced hydrological changes.