New insights from fully-integrated surface-subsurface hydrological modeling in high-elevation glaciated environments

Recent modeling efforts go towards distributed physics-based modeling for improved hydrological process understanding. The sparse hydro-climatological observations and complex topography in high-elevation mountainous environments have, however, hampered the application of such data-intensive and sophisticated techniques for detailed process exploration. In particular, characterizing surface-subsurface water exchange processes in these terrains remains understudied. Here we implement a fully-integrated and fully-distributed surface-subsurface hydrological model, WaSiM, to quantify the observed streamflow variations and their interactions with groundwater in the high-elevation glaciated environment (Martell Valley) in the central European Alps since the 2000s. Extensive field observations (meteorology, vegetation, glacier mass balance, soil properties, river discharge), especially in-situ groundwater levels, were collected to investigate hydrological processes and constrain model parameters in the surface and subsurface. We observe that in contrast to the delayed responses typically observed in aquifers, shallow alpine groundwater responds nearly as quickly as streamflow to peak snowmelt and extreme rainfall events, highlighting the need for improved subsurface parametrization in hydrological modeling. Surprisingly, we are able to obtain satisfactory model performance when subsurface lateral flow is constrained to zero, indicating its limited influence in river discharge generation at the site and providing new insights into hydrological processes in such an environment. Moreover, to overcome the challenges of integrating point-scale groundwater observations into a fully-distributed hydrological model in high-elevation catchments, we suggest pre-calculating the Topographic Wetness Index to guide piezometer placement. Despite the inherent uncertainties associated with the necessary assumptions of modeling, this study sheds new light on surface-subsurface hydrological processes in high-alpine landscapes.