Patterns and Processes of Diel Streamflow Cycles Along the Longitudinal Profile of an Alpine Headwater Stream

In high-alpine catchments, diel streamflow cycles are typically controlled by snow and ice melt during days without precipitation. Potentially co-occurring evapotranspiration (ET) controlled streamflow cycles are of opposite phase and can often not be identified in such environments. This study investigates the spatial and temporal differences in diel streamflow cycles along the longitudinal river profile of the Fundusbach catchment (13 km²; 1620–3095 m a.s.l.) in the Ötztal Alps, Austria. We investigated variations in the shape characteristics of diel hydrographs from June to October 2022 using hourly streamflow from four stations along the river. We identified the main processes controlling the diel streamflow cycles, and tried to quantify the volumes of water added or removed from streamflow due to melt processes or ET. Results show that the amplitude of diel streamflow cycles decreases with increasing catchment area and over the second half of the year, indicating a damping effect of the catchment and decreasing importance of meltwater processes along the river. Despite the dominance of meltwater-driven diel cycles, we show that ET occurs simultaneously and its influence becomes more pronounced in the lower reaches of the river. By synchronising the diel cycles and deducting the meltwater influence from upstream stations, we visualised the ET-controlled streamflow cycles at downstream stations. However, the quantification of ET volumes from daily hydrographs is subject to large uncertainty. Our study highlights the importance of considering both meltwater and ET processes to understand diel streamflow cycles in high alpine catchments, especially given the likely increasing relevance of ET in these areas in a warmer climate.