Forest expansion and irrigated agriculture reinforce low river flows in southern Europe during dry years

• Southern Europe shows a strong decrease in streamflow not directly linked to climate dynamics. • Non-climate factors, such as forest growth and irrigated agriculture, drive negative streamflow trends in southern Europe. • During dry years, green water consumption surpasses blue water generation, exacerbating streamflow decline. • Nature-based solutions and expanding irrigated agriculture may reduce water resources and increase low flow periods. This study analyses the evolution of annual streamflow across Europe between 1962 and 2017, focusing on the connection of streamflow trends with climate dynamics and physiographic and land cover characteristics and changes. The spatial pattern of trends in streamflow shows strong agreement with the spatial patterns of climate trends, suggesting a climate control of these trends. However, analysing temporal evolution at the basin scale shows that the strong decrease in streamflow in southern Europe cannot be directly associated with climate dynamic. In fact, a negative trend related to non-climate factors clearly emerges. Rather, we show that forest growth and irrigated agriculture are the leading drivers of negative streamflow trends in southern Europe, particularly during dry years due to the greater proportion of green water consumption compared to blue water generation. These findings have significant implications, particularly in the context of widely embraced nature-based solutions for mitigating climate change, including carbon sequestration through forests and the planned expansion of irrigated agricultural lands in central and northern European countries as a response to rising crop water demands. These developments could potentially diminish water resources availability, leading to an increased occurrence and severity of low flow periods.