Groundwater dynamics and streamflow generation in a mountainous headwater catchment: process understanding from field experiments and modeling studies/

Groundwater systems in mountainous headwater catchments significantly sustain downstream freshwater bodies and therefore play an important role in the regional water cycle. Complex interactions between atmospheric, subsurface and ecological variables occur that determine groundwater quantity and quality as well as streamflow-generation mechanisms at different spatiotemporal scales. An integrated understanding of the hydro(geo)logic processes in such areas is a necessary precursor to develop successful adaption methods in the face of climate change. For this, not only does our mechanistic understanding of groundwater flow in mountainous headwater catchments has to be improved, but also the complex land-atmosphere interactions with groundwater have to be understood. Although there exists a wide breadth of studies on hydrology in mountainous regions, research on groundwater dynamics in these settings still is comparably rare. <br> In order to close that research gap, an extensive field- and modeling study was carried out within this PhD project. Hydro-climatic data from a dense observation network in the Swiss pre-Alpine upper Rietholzbach Research Catchment (URHB, ~1km²) were used, where the major variables of the water cycle are continuously monitored at high temporal and spatial resolution. Scientifically significant results have been achieved in the four areas covered by this project, which refer to the first-order-controls of groundwater recharge (i.e., climatic forcing and landscape properties) and to the hydrologic responses driven by groundwater discharge (i.e., streamflow generation and solute transport). In the first project phase, six well-established groundwater recharge estimation techniques were evaluated systematically. From the inconsistencies among the applied GR estimation methods first-order controls of GR were identified that helped to better understand GR mechanisms. With the focus on groundwater discharge, a more detailed analysis of groundwater dynamics at the event-time scale was pursued in the second part of this thesis to identify dominant streamflow-generating mechanisms and threshold-responses. It was found that groundwater discharge from the shallow aquifer in the valley bottom of the URHB represents the dominant fraction of peak flow during most rainfall periods. The conceptual description of the hydro(geo)logic system in the URHB was evaluated in the third part of this thesis with an analytical model that consists of two linear reservoirs for event-flow generation and a baseflow storage with relatively constant discharge rates. Here, rainfall-driven event flow is generated in the riparian zones and the adjacent hillslopes, while baseflow was assumed to originate from the deep fractured-rock aquifer and to be rather constant. The model adequately reproduced the observed streamflow signal, however, the performance improved after implementation of the variable contributing area concept. Although the shrinking/expansion of the riparian zones was small compared to the total catchment area (up to 14 %), this process strongly controlled the streamflow hydrograph when wet antecedent moisture conditions coincide with high-intensity rainfall periods. <br> Overall, this PhD compiles various a practical approaches to analyze and characterize groundwater systems and streamflow-generation mechanisms in mountainous headwater catchments. By focusing on the two first-order controls on groundwater recharge, climate and subsurface properties, an important foundation for future research is provided that deals with potential negative effects of climate change and land use on water quality and quantity in mountainous headwater catchments.

Mots clés: décharge, nappe phréatique, propriétés de la terre, modélisation, versant amont, chute de pluie - ruisellement, recharge, génération de débits, seuils, domaines contribuant variable Thèse de doctorat : Université de Neuchâtel, 2015