Flow and Temperature Dynamics in the Hydrologic Response of Alpine Catchments: Travel Time Formulation and Geomorphologic Signatures

We present a travel time formulation of water and energy transport at sub-catchment scale. The derived equations are implemented in Alpine3D, a physically-based model of snow processes, which provides the necessary boundary conditions to perform hydro-thermal response simulations of Alpine catchments. The model set-up accounts for advective and non- advective energy fluxes to perform spatially distributed simulations of streamflow and temperature in river networks having an arbitrary degree of geomorphological complexity. The model gives reliable predictions of streamflow and tem- perature, as shown by comparing modeled and measured hydrographs and thermographs at the outlet of the Dischma catchment (45 km2) in the Swiss Alps. Our model setup is applied to investigate the role of hillslope aspects, representing the main control on radiation and snowmelt patterns, in the flow regime of the study catchment. The distributed simu- lation results show that snowmelt-induced discharge exhibits a visible geomorphologic signature of aspects at sub-catch- ment scale, but this progressively fades out going from headwater streams to the outlet. Accordingly, the geomorphologic signature is scale-dependent: it is significant at small scales where the high aspect correlation generates predominant orientations but is lost at larger scales where aspects are de-correlated and different orientations are averaged out. We further apply the model to investigate the geomorphologic signature of drainage density in the thermal regime of the study catchment. The results show that the contribution of the advective energy fluxes becomes progressively smaller when the drainage density increases, while the one of the non-advective energy fluxes becomes larger. Moreover, such variations balance out at the catchment outlet, where the temperature signal is not sensitive to the increasing drainage density. The relevance of the performed invetigations stems from the increasing scientific interest concerning the impacts of the warming climate on water resources management and temperature-influenced ecological processes.

Flow and Temperature Dynamics in the Hydrologic Response: Travel Time Formulation and Application to Alpine Catchments

Francesco Comola, Michael Lehning, Andrea Rinaldo, Bettina Schaefli

We present a travel time formulation of water and energy transport at sub-catchment scale, resulting in a closed form solution for water temperature evolution in the subsoil compartment. The derived equations are implemented in Alpine3D, a physically-based model of snow processes, which provides the necessary boundary conditions to perform hydro-thermal response simulations of Alpine catchments. The model set-up accounts for advective and non-advective energy fluxes to perform spatially distributed simulations of streamflow and temperature in river networks having an arbitrary degree of geomorphological complexity. The model performance is illustrated comparing modeled and measured hydrographs and thermographs at the outlet of the Dischma catchment (45 km2) in the Swiss Alps. The model gives reliable predictions of streamflow and temperature. Our results suggest that the advective energy fluxes provide a large contribution to the thermal regime of the study catchment. The non-advective energy fluxes are also influential, as they contribute to cool the water in the winter and to warm it in the summer. The analysis of the distributed model results shows a notable streamflow spatial variability during a snowmelt event, as opposed to a more homogeneous response during a rainfall event. The stream temperature, instead, progressively increases from headwater streams to the outlet both during the snowmelt and the rainfall events. The proposed model completes the surface processes simulation framework of Alpine3D. The relevance of the work stems from the increasing scientific interest concerning the impacts of the warming climate on water resources management and temperature-influenced ecological processes.

2014

Thermodynamics in the hydrologic response: Travel time formulation and application to Alpine catchments

Francesco Comola, Michael Lehning, Andrea Rinaldo, Bettina Schaefli

This paper presents a spatially-explicit model for hydro-thermal response simulations of Alpine catchments, accounting for advective and non-advective energy fluxes in stream networks characterized by arbitrary degrees of geomorphological complexity. The relevance of the work stems from the increasing scientific interest concerning the impacts of the warming climate on water resources management and temperature-controlled ecological processes. The description of the advective energy uxes is cast in a travel time formulation of water and energy transport, resulting in a closed form solution for water temperature evolution in the soil compartment. The application to Alpine catchments hinges on the boundary conditions provided by the fully-distributed and physically-based snow model Alpine3D. The performance of the simulations is illustrated by comparing modeled and measured hydrographs and thermographs at the outlet of the Dischma catchment (45 km2) in the Swiss Alps. The Monte Carlo calibration shows that the model is robust and that a simultaneous fitting of stream ow and stream temperature reduces the uncertainty in the hydrological parameters estimation. The calibrated model also provides a good fit to the measurements in the validation period, suggesting that it could be employed for predictive applications, both for hydrological and ecological purposes. The temperature of the subsurface flow, as described by the proposed travel time formulation, proves warmer than the stream temperature during winter and colder during summer. Finally, the spatially-explicit results of the model during snowmelt show a notable hydro-thermal spatial variability in the river network, owing to the small spatial correlation of infilltration and meteorological forcings in Alpine regions.

American Geophysical Union2015

Implications of observation-enhanced energy-balance snowmelt simulations for runoff modeling of Alpine catchments

Tobias Jonas

Snow is an important component of the water balance of many mountain watersheds worldwide. In a warming climate, snowmelt modeling and consequent soil water input, is often challenged by complex conditions such as rain-on-snow situations. This is why detailed physics-based snow models are increasingly being used. These models however have much higher input data requirements, where in many cases accurate forcing fields are very difficult to provide. This study investigates whether the latest advances in the development of snow model framework actually translate into improved discharge simulations. To this end we integrated a distributed multi-layer energy-balance snow model with two recently developed methods of updating snow model mass and energy fluxes using snow observations to improve snow accumulation and depletion predictions. Surface water input from these simulations was used as input for subsequent streamflow modeling of 25 catchments in the Swiss Alps over four hydrological years. Our analysis clearly demonstrates the benefits of accurate snow simulations for hydrological modeling in Alpine catchments. Simulations that included the flux updates improved streamflow predictions, and offered best performance at high elevation, where snow most prominently affected watershed hydrology. These results were consistently achieved when analyzing model performance over entire hydrological years, over the snowmelt season only, and for individual events.

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