Alpine river temperature drivers and riverine heatwaves across Europe

Water temperature is one of the main drivers of water quality in rivers. Due to climate change and anthropogenic land use changes, mean river water temperature has risen during the past 30 years, and extreme water temperatures over several consecutive days, so called riverine heatwaves, occur more often. Existing research on water temperature extremes has focussed on single rivers with few water temperature measurement stations, and the occurrence and spatio-temporal variability of riverine heatwaves across Europe has not been studied yet. Therefore, we aim to (1) improve the understanding of small-scale water temperature variability and its hydro-climatic drivers by conducting an extensive 3-year field campaign in an alpine catchment and to (2) study the large-scale variability of riverine heatwaves by analysing water temperature data over Europe.

To understand small-scale water temperature variability, we measure water temperature, discharge, air temperature, and relative humidity at 15 locations within the alpine Dischmá catchment (Switzerland) along a strong elevational gradient. With this data, we describe the variability of water temperature at different time scales, assess the impact of lakes, glacier ice, snowmelt, refreezing, shading, and groundwater on water temperature, and quantify the relative importance of hydrological, atmospheric and cryospheric drivers for the development of seasonal water temperature anomalies. Preliminary results show a dampening influence of groundwater influx on the diurnal water temperature amplitude, and raise questions as to whether diurnal valley winds may cool the river.

To study large-scale water temperature variability, we compile a dataset of water temperature and discharge data across Europe. Using this dataset, we analyse changes in the occurrence of riverine heatwaves over the past 30 years. Further, we track riverine heatwaves in space, observing longitudinal propagation within one river system and the spread of riverine heatwaves across different catchments.

An improved understanding of both small- and large-scale river water temperature variability will support efforts to counteract the negative ecological and economic impacts of warming rivers.