Between scales and complexity: Integrated process functioning of evapotranspiration in alpine catchments

Evapotranspiration (ET) is an important process in the water-balance of alpine catchments but usually superimposed by snow and glacial processes. Due to complex topography, vegetation segmentation and scarcity of observation data,  large uncertainty exists in the description and modelling of ET processes in alpine terrain. This is especially the case when looking at detailed process functioning on the distributed sub-kilometer scale. To address this knowledge gap and better quantify the spatial distribution and temporal dynamics of ET rates in alpine areas, we are combining detailed field monitoring and distributed, physically based modelling in the 12 km² sized Fundusbach catchment (Tyrol, Austria).  The monitoring network includes four water level and water temperature loggers along the longitudinal river profile (since 2022), two meteorological stations (Temperature and Precipitation since 2022), three Bowen-Ratio Stations coupled with soil moisture sensors and placed at locations with different elevation and vegetation cover (since 2024), as well as one CNRS-probe (since 2025) to quantify distributed soil moisture. All sensors operate at hourly or 15-minute intervals. To bridge the gap between the point- and catchment scale, we use a high-resolution water balance model (WaSiM, 25 m, 1 h). Based on the temporal resolution of the observations and the model, we are able to include both seasonal as well as diel cycles of the water balance.

In a two-step approach we first use the measurements and model to investigate integrated process functioning within the last decade. While changes in the diel streamflow cycles along the longitudinal river profile helped disentangling the contradictory signals of melt and ET, they were not sufficient to investigate spatial patterns of process behaviour. Bowen-Ratio measurements add some spatial information, but observations are still sparse with reference to the complex micrometeorological conditions and  topography. However, all observations proved crucial for the calibration and validation of the model, particularly given the goal of capturing process interactions and quantifying ET volumes.
In a second step, we apply an ensemble of six EURO-CORDEX climate projections to investigate how process dominance might change until the end of the century. Furthermore, we investigate the key role of vegetation distribution on spatial process behaviour. In this regard, we combine the climate projections with different land use scenarios. We hypothesize that especially the distribution of shrubs will have major influence on the partitioning of water within the catchment, potentially limiting water availability for lower elevation forested areas.