Carlier, Claire

2020-3-1, Carlier, Claire, Wirth, Stefanie, Cochand, Fabien, Hunkeler, Daniel, Brunner, Philip

Knowing how stream discharge in an ungauged catchment reacts to dry spells is a major challenge for managing water resources. The role of geology on these dynamics is poorly understood. For the Swiss Molasse basin, we therefore explored how the geology influences the groundwater contribution to stream flow during low-flow conditions. Using existing data from geological reports and maps as well as from deep boreholes, we constructed a basin-wide overview of the hydrogeological quality of the bedrock and investigated five catchments in 3D. We found that catchments with the most permeable sedimentary bedrock are least sensitive to low flows (marine sandstone, K = 10−4 to 10−5 m/s, Peff = 5–10%). In contrast, if bedrock K is low (K < 10−6 m/s), the presence of a productive Quaternary volume becomes decisive for groundwater contribution to stream flow. Limitations exist due to a restricted database for K and Peff values of the Molasse and limited information on continuation of lithologies with depth. This emphasizes the need for more hydrogeologically relevant data for the future management of water resources. Our results highlighting what lithotypes favor groundwater contribution to stream flow are valid also in other regions for the assessment of a catchment’s sensitivity to low flows

2019-1, Carlier, Claire, Wirth, Stefanie, Cochand, Fabien, Hunkeler, Daniel, Brunner, Philip

This study investigates how catchment properties influence low-flow dynamics. With 496 synthetic models composed of a bedrock and an alluvial aquifer, we systematically assess the impact of the hydraulic conductivity of both lithologies, of the hillslope and of the river slope on catchment dynamics. The physically based hydrogeological simulator HydroGeoSphere is employed, which allows obtaining a range of low-flow indicators. The hydraulic conductivity of the bedrock K bedrock , a proxy for transmissivity, is the only catchment property exerting an overall control on low flows and explains 60% of the variance of Q95/Q50. The difference in dynamics of catchments with same K bedrock depends on hillslope gradients and the alluvial aquifer properties. The buffering capacity of the bedrock is mainly related to K bedrock and the hillslope gradient. We thus propose the dimensionless bedrock productivity index (BPI) that combines these characteristics with the mean net precipitation. For bedrock only models, the BPI explains 82% of the variance of the ratio of Q95 to mean net precipitation. The alluvial aquifer can significantly influence low flows when the bedrock productivity is limited. Although our synthetic catchment setup is simple, it is far more complex than the available analytical approaches or conceptual hydrological models. The direct application of the results to existing catchments requires nevertheless careful consideration of the local geological topographic and climatic conditions. This study provides quantitative insight into the complex interrelations between geology, topography and low-flow dynamics and challenges previous studies which neglect or oversimplify geological characteristics in the assessment of low flows. © 2018, National Ground Water Association.

2018-8, Carlier, Claire, Wirth, Stefanie, Cochand, Fabien, Hunkeler, Daniel, Brunner, Philip

Relating stream dynamics to catchment properties is essential to anticipate the influence of changing environ-mental conditions and to predict flows of ungauged rivers. Although the importance of subsurface processes incatchment hydrology is widely acknowledged, geological characteristics are rarely explicitly included in studiesassessing physiographic controls on catchment dynamics. In this investigation of 22 catchments of the SwissPlateau and Prealpes, we use a simple linear regression approach to analyze the relationship between streamflowindicators and various geological and hydrogeological properties of the bedrock and quaternary deposits, alongwith meteorological, soil, land use and topographical characteristics. We use long-term discharge percentiles, aswell as dimensionless flow duration curves (FDC, standardized by long-term mean discharge) that allow toevaluate the catchment response to climate forcing. While climate conditions dominate the high to mediumdischarge percentiles (Q5–Q50), the capacity of the catchments to buffer the meteorological forcing can only beattributed to geological characteristics. The sandstone proportion in the catchments explains 54% of the varianceof both extremities of the dimensionless FDC (Q5/Qmean and Q95/Qmean) and productive quaternary depositsare responsible of 55% resp. 58% of the variance of the two ratios. Examining the hydrogeological characteristicsof both bedrock and quaternary lithologies considerably improves the understanding of catchment dynamics.