Hydrological models are often used to evaluate future changes in streamflow. Despite the strong aware- ness of non-stationarity in hydrological system characteristics, model parameters are often assumed stationary and obtained through calibration on past conditions. The representation of system change in hydrological models is challenging, as a lot of uncertainty abounds on changes in future climate and ecosystems. However, it is shown that ecosystems co-evolve with the prevailing climate conditions. There is increasing evidence that vegetation adapts its root zone storage capacity - considered as a key parameter in any hydrological model - corresponding to moisture deficits in the root zone. This is the main assumption underlying the water balance method. In combination with long-term water budget estimates from the Budyko framework, this method has the potential to meaningfully describe future climate-vegetation interactions within the context of process-based hydrological models. Accordingly, this study provides an exploratory analysis for six catchments in the Austrian Alps to in- vestigate future changes in root zone storage capacity and their impact on modelled streamflow in the past and under two emission scenarios in the future. Our findings show that, although parameter ranges of the root zone storage capacity significantly narrow-down for climate-based estimates, modelling performance on past streamflow is similar when using the calibrated and climate-based parameter sets. Following climate projections from 14 cli- mate models, adaptive climate-based parameter estimates are predicted to increase by 10-100% in all catchments in the future. However, little to no dissimilarity in modelled future streamflow is found when adaptation in root zone storage capacity is included in the hydrological model. Modelled differences in annual mean, maximum, and minimum flows remain within 5%, with slight increases for monthly streamflow and runoff coefficients. Thereby, little to no evidence is found that time-dynamic representation of root zone storage capacity significantly alters modelled future streamflow and suggests limited necessity for its inclusion in hydrological models to obtain rea- sonable descriptions of future streamflow in the investigated Alpine catchments.

Global hydrological models (GHMs) have become an increasingly valuable tool in a range of global impact studies related to water resources. However, glacier parameterization is often overly simplistic or non-existent in GHMs. The representation of glacier dynamics and evolution, including related products such as glacier runoff, can be improved by relying on dedicated global glacier models (GGMs). In this study we test the hypothesis that coupling a GGM to a GHM can lead to increased GHM predictive skills and decreased GHM uncertainty through better glacier parameterization. To this end, the GGM GloGEM is coupled with the GHM PCR-GLOBWB 2 within the eWaterCycle II framework. For the years 2001-2012, the coupled model is evaluated against the uncoupled benchmark in 25 large (>50.000 km2) glacierized basins. Across all basins, the coupled model produces higher runoff throughout the melt season. In July and August, it ranges between 100.07% and 352% of the mean monthly benchmark runoff in lowly and highly glaciated basins respectively. The difference can primarily be explained by the inability of PCR-GLOBWB 2 to simulate snow redistribution and glacier retreat, causing an underestimation of glacier runoff. The coupled model better reproduces basin runoff observations primarily in highly glaciated basins, i.e. where the coupling has the most impact. This study underlines the importance of glacier representation in GHMs and demonstrates the potential of coupling a GHM with a GGM for better glacier representation and runoff predictions in glaciated basins.

The melting of mountain glaciers worldwide contributes significantly to global sea level rise, and is considered an indicator of climate change. The accurate modelling of glacier heights is crucial for determining their past, current and future state. The new lidar altimeters ICESAT-2 and GEDI were launched in 2018, which could provide valuable new data in this regard. ICESAT-2 uses a novel photon counting technique that delivers elevation data at an unprecedented 0.7 m along-track resolution. This also poses questions about how such data should be processed. GEDI uses a more conventional full waveform sensor, which creates a vertical profile of surface returns every 60 m representing a footprint with a 30 m diameter. In this thesis the potential of ICESAT-2 and GEDI for detecting glacier changes in Austria is examined. The vertical errors on the Austrian glaciers of the ATL03 geolocated photons and ATL06 Land ice elevation data products of ICESAT-2 were found to be 1.94 and 1.72 meters respectively, which outperforms most alternative sensors. The GEDI L2A product provides an error of 5.80 m for the same area. For both sensors, the horizontal geolocation error was found to be relatively large, which results in a strong relation between the surface gradient and the vertical error. The high resolution and accuracy of ICESAT-2 can provide valuable detailed information about glacier height differences worldwide. The inclusion of GEDI data can help provide observations in sparsely sampled areas, but considering its relatively poor resolution and accuracy better alternatives may be available. The satellite observations were confined to glacier inventory outlines and compared to a high quality Digital Elevation Model of Austria. The results show an average glacier height loss of 5.85 m for ICESAT-2 and 4.86 m for GEDI, with the majority of observations representing the period 2010-2019. The height differences were compared with their local elevation, slope, orientation and their distance to the glacier edge, in order to find what features could assist in extrapolating the observations. All features show some relation with the glacier height differences, but the elevation and distance to glacier edge were found to contribute the most information.
Spaceborne lidar data is confined to narrow profiles of the surface, so they require extrapolation to make conclusions about the total state of the glaciers. The total mass difference of Austria was estimated first by simply applying the average height difference to the total glacier area, and secondly by ordinary cokriging with the elevation. Both techniques agree on a total mass loss of 1.59 gigatons, while the cokriging method additionally results in a more detailed image of mass loss for individual glaciers. For the Ötztal region a mass loss of 77.07 megatons annually was estimated for the period 2010-2019.

Hydrological regimes of alpine catchments are expected to be strongly influenced by climate change due to their dependence on snow dynamics. While seasonal changes have been studied extensively, studies on changes in the timing and magnitude of annual extremes remain rare. This study investigates the effects of climate change on runoff patterns in six alpine catchments in Austria by using a topography-driven semi-distributed hydrological model and 14 climate projections for both RCP 4.5 and RCP 8.5. The study catchments represent a range of alpine catchments, from pluvial-nival to nivo-glacial, as the study focuses on exploring the effects of climate change on catchments of different altitudes.
Simulations of 1981-2010 were compared to projections of 2071-2100 and changes in timing and magnitude of annual maximum and minimum flows as well as monthly discharges and melt were examined. Our results indicate a substantial shift to earlier occurrences in annual maximum flows by 9 to 31 days and an extension of the potential flood season by 1 to 3 months for high elevation catchments. For lower elevation catchments, changes in timing of annual maximum flows are less pronounced. Magnitudes of annual maximum flows are likely to increase, with four catchments exhibiting larger increases under RCP 4.5 than RCP 8.5. The timing of minimum annual discharges shifts to earlier in the winter months for high elevation catchments, whereas for lower elevation catchments a shift from winter to autumn is observed. While all catchments show an increase in mean magnitude of minimum flows under RCP 4.5, this is only the case for four catchments under RCP 8.5. Our results suggest a relationship between the altitude of catchments and changes in timing of annual maximum and minimum flows and magnitude of low flows, whereas no relationship between altitude and magnitude of annual maximum flows could be distinguished. The degree of future change in timing and monthly discharges is larger under RCP 8.5, a change of up to twice as large in monthly discharges is found for RCP 8.5 compared to RCP 4.5.