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.

The geomorphic origin of recession curves

Nevena Tomasic

An important study domain in hydrology is dedicated to understand how the spatial organization of river networks is reflected in the hydrologic response to rainfall inputs (Biswal and Marani, 2010). Base flow (or recession flow) is the discharge rate in a river that results from the natural release of the water stored in the upstream river channels and adjoining riparian aquifers in the absence of precipitation, snowmelt or other inputs (Brutsaert W. , 2005). Biswal and Marani (2010) sustain that the recession flow is dominated by aquifer drainage and they find a link between recession flows and network morphology. Since geomorphologic characteristics can be readily obtained from maps and air photos, a relationship between the groundwater outflow rate and the geomorphologic parameters of the basin would permit to extract the characteristics of recession hydrographs solely from Digital Terrain Models (DTMs). The practical importance of base flow prediction stems mainly from the fact that it is the rate of flow that a given catchment can sustain in the absence of precipitation and in the absence of artificial storage works (Brutsaert & Nieber, 1977). In the first chapter is presented the theoretical background dedicated to the study of the processes controlling recession flows with a particular attention to the aquifer drainage process. A method to describe the behaviour of the base flow rate at the outlet of the basin is presented, introducing the concept of recession slope analysis. The same method is used to define aquifer hydraulic parameters using Boussinesq equations, which are shortly described. Finally the relationship with geomorphic parameters found by Biswal and Marani (2010) is presented. In the second chapter the data and methods used to analyse the recession flows of the Chamberonne river are explained. In the last part of the report the results of the analysis are showed, followed by some propositions for the future work.

2011

Hydrologic response of an alpine watershed: Application of a meteorological wireless sensor network to understand streamflow generation

Guillermo Barrenetxea Kobas, Marc Diebold, François Ingelrest, Daniel Nadeau, Simone Padoan, Marc Parlange, Silvia Simoni, Martin Vetterli

A field measurement campaign was conducted from June to October 2009 in a 20 km2 catchment of the Swiss Alps with a wireless network of 12 weather stations and river discharge monitoring. The objective was to investigate the spatial variability of meteorological forcing and to assess its impact on streamflow generation. The analysis of the runoff dynamics highlighted the important contribution of snowmelt from spring to early summer. During the entire experimental period, the streamflow discharge was dominated by base flow contributions with temporal variations due to occasional rainfall-runoff events and a regular contribution from glacier melt. Given the importance of snow and ice melt runoff in this catchment, patterns of near-surface air temperatures were studied in detail. Statistical data analyses revealed that meteorological variables inside the watershed exhibit spatial variability. Air temperatures were influenced by topographic effects such as slope, aspect, and elevation. Rainfall was found to be spatially variable inside the catchment. The impact of this variability on streamflow generation was assessed using a lumped degree-day model. Despite the variability within the watershed, the streamflow discharge could be described using the lumped model. The novelty of this work mainly consists in quantifying spatial variability for a small watershed and showing to which extent this is important. When the focus is on aggregated outputs, such as streamflow discharge, average values of meteorological forcing can be adequately used. On the contrary, when the focus is on distributed fields such as evaporation or soil moisture, their estimate can benefit from distributed measurements.

American Geophysical Union2011

BIOPHYSICAL CONTROLS ON EVAPOTRANSPIRATION: NUMERICAL MODELS AND EXPERIMENTAL OBSERVATIONS

Federico Croci

Water and food are basics of human life, unfortunately climate changes and water pollution have reduced the amount of water available for human uses. In this context the study of the hydrologic cycle is becoming of growing importance in order to optimize the anthropogenic exploitation of water resource. Evapotranspiration is the term used to describe the part of the water cycle which removes liquid water from an area with vegetation into the atmosphere by the processes of both transpiration and evaporation. Evapotranspiration is an important part of the hydrologic cycle because it represents a considerable percentage of water losses in watersheds. This process is also one of the main consumers of solar energy at the Earth's surface. The estimation of evapotranspiration it is of great importance for agriculture and water resources management, for example, by knowing the rate of water loss from a region, farmers will be better placed to efficiently manage the irrigation strategies. The rate of evapotranspiration at any location on the Earth's surface is controlled by several factors such as energy availability, humidity gradient, wind speed immediately above the surface, water availability, type of vegetation, stomatal resistance and soil characteristics. The problem of estimating evapotranspiration has been widely studied. During the past several decades, different approaches have been proposed and many formulas are available in the literature to fulfill this task. The complexity of the phenomena and its great variability all over the world make difficult to find a universal and robust method for the evapotranspiration prediction and estimation. Evapotranspiration can be obtained by many estimation methods. Some of these methods need many weather parameters as inputs while others need fewer. Factors such as data availability, the intended use, and the time scale required by the problem must be considered when choosing the evapotranspiration calculation technique. The Penman-Monteith equation was derived from the classic Penman equation adding to the terms already considered in the Penman formulation (aerodynamic resistance and energy balance) the canopy resistance. This approach requires many meteorological data, such as maximum and minimum air temperatures, relative humidity, solar radiation, and wind speed. If some of these data are not available, alternative techniques have been developed to overcome this drawback. However, this estimations of meteorological data affect the quality of the result, alternatively, in such cases other simpler methods may be used. The Penman-Monteitu equation is assumed to be the most reliable because is based on physical principles and because it considers all the climatic factors, which affect evapotranspiration.

2013