LAUSR

Abstract. Stream temperature is a key hydrological variable for ecosystem and water resources management and is particularly sensitive to climate warming. Despite the wealth of meteorological and hydrological data, few studies have quantified observed stream temperature trends in the Alps. This study presents a detailed analysis of stream temperatures in 52 catchments in Switzerland, a country covering a wide range of alpine and lowland hydrological regimes. The influence of discharge, precipitation, air temperature and upstream lakes on stream temperatures and their temporal trends is analysed from multi-decade to seasonal time scales. Stream temperature has significantly increased over the past 5 decades, with positive trends for all four seasons. The mean trends for the last 20 years are +0.37 °C per decade for water temperature, resulting from joint effects of trends in air temperature (+0.39 °C per decade) in discharge (−10.1 % per decade) and in precipitation (−9.3 % per decade). For a longer time period (1979–2018), the trends are +0.33 °C per decade for water temperature, +0.46 °C per decade for air temperature, −3.0 % per decade for discharge and −1.3 % per decade for precipitation. We furthermore show that in alpine streams, snow and glacier melt compensates air temperature warming trends in a transient way. Lakes, on the contrary have a strengthening effect on downstream water temperature trends at all elevations. The identified stream temperature trends are furthermore shown to have critical impacts on ecological temperature thresholds, especially in lowland rivers, suggesting that these are becoming more vulnerable to the increasing air temperature forcing. Resilient alpine rivers are expected to become more vulnerable to warming in the near future due to the expected reductions in snow- and glacier melt inputs.