IMPROVING HYDROLOGIC MODEL REALISM USING STABLE WATER ISOTOPES IN THE SWISS ALPS

Climate change is modifying global precipitation patterns and bringing about unprecedented changes in the different facets of the water cycle. In order to be better prepared for the potentially adverse impacts of climate change on water resources, we need to improve our understanding of the water cycle. Environmental tracers such as stable water isotopes provide a useful medium to help untangle the complex web of Earth System processes. Stable water isotopes are naturally present in rainfall and snowfall, making them an ideal environmental tracer to track the journey of a water particle along its entire hydrologic life cycle. In this thesis, I use stable water isotopes to improve the representation of hydrological processes occurring within mountainous landscapes in rainfall-runoff models. In the first chapter, I undertake a comprehensive review of ways in which stable water isotopes have been used in snow hydrology, with a special focus on mountainous environments. This review explains the different transformations that a water particle undergoes once it enters the landscape through rainfall or snowfall. In the second chapter, I build a novel Bayesian mixing model that derives valuable information from stable water isotope data, while taking into account the numerous limitations of field hydrology. In the third chapter, I propose a new hydrologic modeling framework that uses information derived from stable water isotopes, as illustrated in Chapter 2, to build more reliable rainfall-runoff models by constraining both the celerity and velocity behavior of catchments. This modeling framework is comprehensively evaluated in a Swiss Alpine catchment called Vallon de Nant. Finally, in the fourth chapter, I use stable water isotopes, streamflow recession analysis, and a conceptual groundwater model to show how climate change may increase groundwater recharge in the Swiss Alps. This thesis therefore improves our understanding of the dominant hydrologic processes occurring in mountainous environments, and provides a novel approach to parameterize these processes within rainfall-runoff models. The key findings are summarized in the final chapter, where I also highlight practical challenges in isotope hydrology, and propose future research directions

Benefits from high-density rain gauge observations for hydrological response analysis in a small alpine catchment

Spatial rainfall patterns exert a key control on the catchment-scale hydrologic response. Despite recent advances in radar-based rainfall sensing, rainfall observation remains a challenge, particularly in mountain environments. This paper analyzes the importance of high-density rainfall observations for a 13.4 km2 catchment located in the Swiss Alps, where rainfall events were monitored during 3 summer months using a network of 12 low-cost, drop-counting rain gauges. We developed a data-based analysis framework to assess the importance of high-density rainfall observations to help predict the hydrological response. The framework involves the definition of spatial rainfall distribution metrics based on hydrological and geomorphological considerations and a regression analysis of how these metrics explain the hydrologic response in terms of runoff coefficient and lag time. The gained insights on dominant predictors are then used to investigate the optimal rain gauge network density for predicting the streamflow response metrics, including an extensive test of the effect of down-sampled rain gauge networks and an event-based rainfall–runoff model to evaluate the resulting optimal rain gauge network configuration. The analysis unravels that, besides rainfall amount and intensity, the rainfall distance from the outlet along the stream network is a key spatial rainfall metric. This result calls for more detailed observations of stream network expansions and the parameterization of along-stream processes in rainfall–runoff models. In addition, despite the small spatial scale of this case study, the results show that an accurate representation of the rainfall field (with at least three rain gauges) is of prime importance for capturing the key characteristics of the hydrologic response in terms of generated runoff volumes and delay for the studied catchment (0.22 rain gauges per square kilometer). The potential of the developed rainfall monitoring and analysis framework for rainfall–runoff analysis in small catchments remains to be fully unraveled in future studies, potentially also including urban catchments.</p

Studying the dynamic of a high alpine catchment based on multiple natural tracers

Hydrological processes in high elevation catchments are largely influenced by snow accumulation and melt, as well as summer rainfall input. The use of the stable isotopes of water as a natural tracer has become popular over recent years to characterize water flow paths and storage in such environments, in conjunction with electric conductivity (EC) and water temperature measurements. In this work, we analyzed in detail the potential of year round samples of these natural tracers to characterize hydrological processes in a snow-dominated Alpine catchment. Our results underline that water temperature measurements in springs, groundwater and in-stream are promising to trace flow path depth and relative flow rates. The stable isotopes of water are shown here to be particularly valuable to get insights into the interplay of subsurface flow and direct snowmelt input to the stream during winter and early snow melt periods. Our results underline the critical role of subsurface flow during all melt periods and the presence of snowmelt even during winter base flow. We furthermore discuss why reliably detecting the role of subsurface flow requires year-round water sampling in such environments. A key conclusion of our work is the added value of soil and water temperature measurements to interpret EC and isotope analyses, by giving additional information on snow-free periods and on flow path depths

Short Description of the Vallon de Nant Research Site

This document gives a short overview of the Vallon de Nant research catchment, including a map, currently installed meteo stations and an overview of recent and ongoing work, with numerous references where to find further details. This document is meant to very shortly introduce the experimental research catchment ot other researchers and to help navigate through available publication

Young Hydrologic Society (YHS): vision, mission, and strategy

The Young Hydrologic Society (YHS) is a grassroots initiative to stimulate the interaction and active participation of early career hydrologists within the hydrological sciences community and beyond. Our vision is to reform hydrologic community towards a more active and inclusive involvement of early career hydrologists. YHS mission is to connect and inform. With YHS, we want to (1) connect early career scientists (ECS) by regularly organizing scientific and professional development sessions/workshops, as well as social events at geoscience conferences e.g. EGU, AGU, and IAHS; (2) inform YHS members about successes/issues of the hydrologic community and the current and future research topics of the field. Social media, such as Twitter, blogs or Facebook have been instrumental for YHS to reach its goals. But we noticed that, even in the era of online communication, face-to-face contact remains to be the most effective way to foster connections globally and raise awareness about our network. Actively participating in workshops, seminars, and roundtable discussions at international conferences requires more resources, but the memories are longer lasting and can be even more meaningful/inspiring than what online posts provides. YHS takes pride in its strategy to diversify and empower National Initiatives (e.g. Canadian YHS) as well as actively collaborate with other ECS networks such as Student Subcommittee of AGU Hydrology Section (H3S), Young Earth System Scientists (YESS) community, and IAHS Early Career Committee. We recommend that diversifying ECS networks, engaging ECS in convening scientific sessions, in addition to expanding ECS-focused workshops/events should be an inalienable component of geoscience conferences