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

Experiences in using INCA-CH precipitation nowcasting for Urban Flood Nowcasting

Presentación realizada en la 3rd European Nowcasting Conference, celebrada en la sede central de AEMET en Madrid del 24 al 26 de abril de 2019

MULTI-BUNCH EFFECT OF RESISTIVEWALL IN THE CLIC BDS

Wake fields in the CLIC Beam Delivery System (BDS) can cause severe single or multi-bunch effects leading to luminosity loss. The main contributors in the BDS are geometric and resistive wall wake fields of the collimators and resistive wall wakes of the beam pipe. The present work focuses only on the multi-bunch effects from resistive wall. Using particle tracking with wake fields through the BDS, we have established the aperture radius, above which the effect of the wake fields becomes negligible. Our simulations were later extended to include a realistic aperture model along the BDS as well as the collimators. The two cases of 3 TeV and 500 GeV have been examined

Could electrical conductivity replace water level in rating curves for alpine streams?

Streamflow time series are important for inference and understanding of the hydrological processes in alpine watersheds. Because streamflow is expensive to continuously measure directly, it is usually derived from measured water levels, using a rating curve modeling the stage-discharge relationship. In alpine streams, this practice is complicated by the fact that the streambed constantly changes due to erosion and sedimentation by the turbulent mountain streams. This makes the stage-discharge relationship dynamic, requiring frequent discharge gaugings to have reliable streamflow estimates. During an ongoing field study in the Val Ferret watershed in the Swiss Alps, 93 streamflow values were measured in the period 2009–2011 using salt dilution gauging with the gulp injection method. The natural background electrical conductivity in the stream, which was measured as by-product of these gaugings, was shown to be a strong predictor for the streamflow, even marginally outperforming water level. Analysis of the residuals of both predictive relations revealed errors in the gauged streamflows. These could be corrected by filtering disinformation from erroneous calibration coefficients. In total, extracting information from the auxiliary data enabled to reduce the uncertainty in the rating curve, as measured by the root-mean-square error in log-transformed streamflow relative to that of the original stage-discharge relationship, by 43.7%

Controls on the diurnal streamflow cycles in two subbasins of an alpine headwater catchment

In high-altitude alpine catchments, diurnal streamflow cycles are typically dominated by snowmelt or ice melt. Evapotranspiration-induced diurnal streamflow cycles are less observed in these catchments but might happen simultaneously. During a field campaign in the summer 2012 in an alpine catchment in the Swiss Alps (Val Ferret catchment, 20.4 km2, glaciarized area: 2%), we observed a transition in the early season from a snowmelt to an evapotranspiration-induced diurnal streamflow cycle in one of two monitored subbasins. The two different cycles were of comparable amplitudes and the transition happened within a time span of several days. In the second monitored subbasin, we observed an ice melt-dominated diurnal cycle during the entire season due to the presence of a small glacier. Comparisons between ice melt and evapotranspiration cycles showed that the two processes were happening at the same times of day but with a different sign and a different shape. The amplitude of the ice melt cycle decreased exponentially during the season and was larger than the amplitude of the evapotranspiration cycle which was relatively constant during the season. Our study suggests that an evapotranspiration-dominated diurnal streamflow cycle could damp the ice melt-dominated diurnal streamflow cycle. The two types of diurnal streamflow cycles were separated using a method based on the identification of the active riparian area and measurement of evapotranspiration

Geomorphic signatures on Brutsaert base flow recession analysis

This paper addresses the signatures of catchment geomorphology on base flow recession curves. Its relevance relates to the implied predictability of base flow features, which are central to catchment-scale transport processes and to ecohydrological function. Moving from the classical recession curve analysis method, originally applied in the Finger Lakes Region of New York, a large set of recession curves has been analyzed from Swiss streamflow data. For these catchments, digital elevation models have been precisely analyzed and a method aimed at the geomorphic origins of recession curves has been applied to the Swiss data set. The method links river network morphology, epitomized by time-varying distribution of contributing channel sites, with the classic parameterization of recession events. This is done by assimilating two scaling exponents, β and bG, with |dQ/dt| â̂ Q β where Q is at-a-station gauged flow rate and N(l) â̂ N(l)â̂G(l)bG where l is the downstream distance from the channel heads receding in time, N(l) is the number of draining channel reaches located at distance l from their heads, and G(l) is the total drainage network length at a distance greater or equal to l, the active drainage network. We find that the method provides good results in catchments where drainage density can be regarded as spatially constant. A correction to the method is proposed which accounts for arbitrary local drainage densities affecting the local drainage inflow per unit channel length. Such corrections properly vanish when the drainage density become spatially constant. Overall, definite geomorphic signatures are recognizable for recession curves, with notable theoretical and practical implications. Key Points signatures of catchment geomorphology on base flow recession curves Analysis of streamflow data and DEM for 27 catchments in Switzerland New conceptual model accounting for uneven drainage densit

Spatial variability of near surface soil moisture in an alpine catchment: application of a wireless network of meteorological stations

Soil moisture is an essential control on hydrological and meteorological behavior and knowledge of its spatial variability is considered to be of high importance for the performance of distributed hydrological models. Few studies have tried to assess the spatial variability of soil moisture in mountainous catchment. Since 2008, an alpine watershed in the Swiss Alps has been intensely monitored with a network of wireless meteorological stations. We present some preliminary results from statistical and geostatistical analysis of the soil moisture dataset. Moreover, we investigate the information content that antecedent soil moisture measurements add to rainfall measurements in order to predict and understand both runoff events and recession flows

Hydrological monitoring and modeling of an alpine catchment

Since 2008 we have been monitoring a watershed in the alps to assess the impact of the spatial heterogeneity of meteorological variables over a complex terrain and identify the mechanisms generating streamflow. The study area is located in the Swiss Alps along the southernmost ridge that borders Italy. The watershed is heavily monitored with wireless meteorological stations, measuring precipitation, solar radiation, air temperature and humidity, skin temperature, wind speed and direction, soil temperature and humidity, and soil matrix potential. During the 2011 field campaign, the discharge of two sub-basins has been monitored in order to compare the results with a semi-distributed model. Moreover, water from different rivers of the basin has been sampled automatically, along with water samples from precipitation, in order to proceed to a two-component hydrograph separation with stable isotopes analysis. We discuss the relative importance of snowmelt, precipitation and local geomorphology, based on the results of different models in combination with the data collected in the past years

Controls on diurnal streamflow cycles in a high altitude catchment in the Swiss Alps

We study the diurnal streamflow cycles in a high altitude catchment where a large field campaign has been undertaken. In one monitored sub-basin, the cycle is evapotranspiration-dominated, in another, the cycle is icemelt-dominated. The two processes happen at the same time of the day but with opposite signs. Our results suggest that the amplitude of icemelt dominated streamflow is damped by evapotranspiration in the riparian area

Application of a meteorological wireless sensor network to a small alpine watershed

This study focuses on the spatial variability of meteorological parameters in an alpine watersehd in the Swiss Alps, where a wireless sensor network has been deployed. We present some results from the previous summer campaigns, sharing experiences and challenges on the use of a wireless network in a complex environment