Hydrological Processes in a High Alpine Watershed

Alpine hydrology is particularly challenging due to the complexity of mountainous terrain and the spatial and temporal variability of meteorological parameters such as precipitation, temperature and evaporation. Yet improving our understanding of hydrological processes in alpine regions is critical for freshwater management and for protection against natural hazards. Since 2009, the upper Val Ferret watershed in the Swiss Alps is monitored with a large variety of instruments to measure hydrological, meteorological and pedological parameters at high temporal and spatial resolution. In this dissertation, the data collected during three consecutive summer field campaigns from 2011 to 2013 has been utilized. We deployed a wireless network of meteorological stations, continuously measured the stream discharge at three locations and made use of a one meter resolution Digital Elevation Model of the watershed. In particular, we focused on the influence of the geomorphology on several streamflow generation processes. The topographical and geomorphological complexity of the Val Ferret watershed is illustrated by the structure of the channel network, which was carefully mapped in the field. The spatial distribution of the channel network was particularly heterogeneous, with certain areas characterized by a high density of groundwater channel heads and perennial streams. This highly uneven drainage density had a significant impact on modeling storm hydrographs. Other channel networks, extracted from the Digital Elevation Model using classical methods, were not able to capture this spatial variability. More advanced extraction techniques relying on curvature and openness showed superior efficacy. Recession events are defined as periods without precipitation during which water stored in the watershed is released and the streamflow is in decline. Although they have been widely studied previously, we have analyzed their link to the basin geomorphology in detail. Based on a recent study, we propose a simple conceptual model in which the temporal variation of the river network is computed and linked to a classic power law parametrization of recession curves. Furthermore, we analyzed daily streamflow fluctuations that occurred during recession events in the upper Val Ferret watershed. In one of the monitored sub basins, we observed that evaporation from the perennial riparian area was inducing a diurnal streamflow cycle. In another sub basin, we observed a diurnal streamflow cycle induced by ice melt from a small glacier. Both cycles occurred at similar times of the day but with perturbations in opposite directions. However, the evaporation induced streamflow cycle was never observed at the outlet of the watershed as the amplitude of the glacier-fed ice cycle was larger than the one due to evaporation. The geomorphology of the riparian area contributing to the evaporation cycley was analyzed cautiously and linked to the ice melt diurnal cycle, given that it is damped by evaporation. In order to better understand streamflow generation during precipitation events, we analyzed threshold relations between antecedent soil water potential and runoff coefficients above which the latter ones were increasing and the response time of the catchment shortened. Unlike many recent studies, we did not observe threshold behaviors between antecedent soil moisture and runoff coefficients, pointing out the importance of hysteresis effects at the hillslope scale.

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