Changing river temperatures in northern Germany: trends and drivers of change

Climate change is one of the main drivers of river warming worldwide. However, the response of river temperature to climate change differs with the hydrology and landscape properties, making it difficult to generalize the strength and the direction, of river temperature trends across large spatial scales and various river types. Additionally, there is a lack of long-term and large-scale trend studies in Europe as well as globally. In this study, we investigated the long-term (25years; 132 sites) and the short-term (10years; 475 sites) river temperature trends, patterns and underlying drivers within the period 1985-2010 in seven river basins of Germany. The majority of the sites underwent significant river warming during 1985-2010 (mean warming trend: 0.03 degrees Cyear(-1), SE=0.003), with a faster warming observed during individual decades (1985-1995 and 2000-2010) within this period. Seasonal analyses showed that, while rivers warmed in all seasons, the fastest warming had occurred during summer. Among all the considered hydro-climatological variables, air temperature change, which is a response to climate forcing, was the main driver of river temperature change because it had the strongest correlation with river temperature, irrespective of the period. Hydrological variables, such as average flow and baseflow, had a considerable influence on river temperature variability rather than on the overall trend direction. However, decreasing flow probably assisted in a faster river temperature increase in summer and in rivers in NE basins (such as the Elbe basin). The North Atlantic Oscillation Index had a greater significant influence on the winter river temperature variability than on the overall variability. Landscape and basin variables, such as altitude, ecoregion and catchment area, induced spatially variable river temperature trends via affecting the thermal sensitivity of rivers, with the rivers in large catchments and in lowland areas being most sensitive. Copyright (c) 2016 John Wiley & Sons, Ltd.