Dirk Radny

Implications of hydrologic connectivity between hillslopes and riparian zones on streamflow composition

Hydrological responses in mountainous headwater catchments are often highly non-linear with a distinct threshold-related behavior, which is associated to steep hillslopes, shallow soils and strong climatic variability. A holistic understanding of the dominant physical processes that control streamflow generation and non-linearity is required in order to assess potential negative effects of agricultural land use and water management in those areas. Therefore, streamflow generation in a small pre-Alpine headwater catchment (Upper Rietholzbach (URHB), ~1km(2)) was analyzed over a 2-year period by means of rainfall-response analysis and water quality data under explicit consideration of the joint behaviors of climate forcing and shallow groundwater dynamics. The runoff coefficients indicate that only a small fraction of the total catchment area (1-26%) generates streamflow during rainfall events. Hereby, the valley bottom areas (riparian zones) were the most important event-water source whereas only the lower parts of the hillslopes became hydrologically connected to the river network with higher antecedent moisture conditions. However, a distinct threshold-like behavior could not be observed, suggesting a more continuous shift from a riparian-zone to a more hillslope-dominated streamflow hydrograph. Regular manure application on the hillslopes in combinations with lateral hillslope groundwater flux and long groundwater residence times in the riparian zones resulted in a higher mineralization (e.g., total phosphorous) and significant denitrification in the valley bottom area. Despite the important role of the riparian zones for event-flow generation in the URHB, their nutrient buffer capacity is expected to be small due to the low permeability of the local subsurface material. The findings of this integrated analysis are summarized in a conceptual framework describing the hydrological functioning of hillslopes and riparian zones in the URHB.

Characterization of a managed aquifer recharge system using multiple tracers

Knowledge about the residence times of artificially infiltrated water into an aquifer and the resulting flow paths is essential to developing groundwater-management schemes. To obtain this knowledge, a variety of tracers can be used to study residence times and gain information about subsurface processes. Although a variety of tracers exists, their interpretation can differ considerably due to subsurface heterogeneity, underlying assumptions, and sampling and analysis limitations. The current study systematically assesses information gained from seven different tracers during a pumping experiment at a site where drinking water is extracted from an aquifer close to contaminated areas and where groundwater is artificially recharged by infiltrating surface water. We demonstrate that the groundwater residence times estimated using dye and heat tracers are comparable when the thermal retardation for the heat tracer is considered. Furthermore, major ions, acesulfame, and stable isotopes (δ2H and δ18O) show that mixing of infiltrated water and groundwater coming from the regional flow path occurred and a vertical stratification of the flow system exist. Based on the concentration patterns of dissolved gases (He, Ar, Kr, N2, and O2) and chlorinated solvents (e.g., tetrachloroethene), three temporal phases are observed in the ratio between infiltrated water and regional groundwater during the pumping experiment. Variability in this ratio is significantly related to changes in the pumping and infiltration rates. During constant pumping rates, more infiltrated water was extracted, which led to a higher dilution of the regional groundwater. An infiltration interruption caused however, the ratio to change and more regional groundwater is extracted, which led to an increase in all concentrations. The obtained results are discussed for each tracer considered and its strengths and limitations are illustrated. Overall, it is demonstrated that aquifer heterogeneity and various subsurface processes necessitate application of multiple tracers to quantify uncertainty when identifying flow processes.

Estimating the spatial distribution of artificial groundwater recharge using multiple tracers

ABSTRACT Stable isotopes of water, organic micropollutants and hydrochemistry data are powerful tools for identifying different water types in areas where knowledge of the spatial distribution of different groundwater is critical for water resource management. An important question is how the assessments change if only one or a subset of these tracers is used. In this study, we estimate spatial artificial infiltration along an infiltration system with stage–discharge relationships and classify different water types based on the mentioned hydrochemistry data for a drinking water production area in Switzerland. Managed aquifer recharge via surface water that feeds into the aquifer creates a hydraulic barrier between contaminated groundwater and drinking water wells. We systematically compare the information from the aforementioned tracers and illustrate differences in distribution and mixing ratios. Despite uncertainties in the mixing ratios, we found that the overall spatial distribution of artificial infiltration is very similar for all the tracers. The highest infiltration occurred in the eastern part of the infiltration system, whereas infiltration in the western part was the lowest. More balanced infiltration within the infiltration system could cause the elevated groundwater mound to be distributed more evenly, preventing the natural inflow of contaminated groundwater. Dedicated to Professor Peter Fritz on the occasion of his 80th birthday

Patterns of streamflow regimes along the river network: The case of the Thur river

Abstract A modeling framework for point-wise prediction of the probability density function and flow duration curve of streamflows along complex river networks is presented. The predictions are based on catchment-scale climatic and morphological features, without calibration on observed discharge time-series. The framework was applied to a test basin in north-eastern Switzerland, and relevant flow statistics were validated at six sub-catchment outlets with satisfactory results. Spatial patterns of flow regime exhibit a strong climatic signature, mostly driven by reduced rainfall depths and increasing effective rainfall frequency in the downstream areas. The increasing non-linearity of the catchment response with contributing area is reflected by the observed increase in the recession parameters along the main river channel. This framework offers a novel approach for assessing the spatial patterns of streamflows based on limited information, which is important for evaluation of human and ecological functions in riverine systems.

Improved water resource management for a highly complex environment using three-dimensional groundwater modelling

A three-dimensional groundwater model was used to improve water resource management for a study area in north-west Switzerland, where drinking-water production is close to former landfills and industrial areas. To avoid drinking-water contamination, artificial groundwater recharge with surface water is used to create a hydraulic barrier between the contaminated sites and drinking-water extraction wells. The model was used for simulating existing and proposed water management strategies as a tool to ensure the utmost security for drinking water. A systematic evaluation of the flow direction between existing observation points using a developed three-point estimation method for a large number of scenarios was carried out. It is demonstrated that systematically applying the developed methodology helps to identify vulnerable locations which are sensitive to changing boundary conditions such as those arising from changes to artificial groundwater recharge rates. At these locations, additional investigations and protection are required. The presented integrated approach, using the groundwater flow direction between observation points, can be easily transferred to a variety of hydrological settings to systematically evaluate groundwater modelling scenarios.RésuméUn modèle tridimensionnel d’eau souterraine a été utilisé pour améliorer la gestion des ressources en eau pour une zone d’étude dans le Nord-Ouest de la Suisse, où la production d’eau potable est à proximité d’anciens sites d’enfouissement et des zones industrielles. Pour éviter la contamination de l’eau potable, la recharge artificielle des eaux souterraines avec de l’eau de surface est utilisée pour créer une barrière hydraulique entre les sites contaminés et les puits d’exploitation d’eau potable. Le modèle a été utilisé pour simuler les stratégies existantes et proposées de gestion de la ressource en eau comme outil pour assurer la plus grande sécurité pour l’eau potable. Une évaluation systémique de la direction des écoulements entre les piézomètres existants en utilisant une méthode d’estimation à trois points développée pour un grand nombre de scénarios a été réalisée. Il est démontré que l’application systématique de la méthodologie développée aide à identifier les endroits vulnérables qui sont sensibles aux changements des conditions aux limites, telles que celles résultant des changements de taux de recharge artificielle des eaux souterraines. Dans ces endroits, des études et protection complémentaires sont requises. L’approche intégrée présentée, en utilisant la direction de l’écoulement des eaux souterraines entre les piézomètres, peut être facilement transférée à une variété de contextes hydrogéologiques pour évaluer de manière systématique des scénarios de modélisation des eaux souterraines.ResumenSe utilizó un modelo tridimensional de agua subterránea para mejorar la gestión de los recursos hídricos de un área de estudio en el noroeste de Suiza, donde la producción de agua potable está cerca de los antiguos vertederos de residuos y zonas industriales. Para evitar la contaminación del agua potable, la recarga artificial de agua subterránea con agua superficial se utiliza para crear una barrera hidráulica entre los sitios contaminados y los pozos de extracción de agua potable. El modelo se utilizó para simular las estrategias existentes y propuestas de gestión del agua como una herramienta para garantizar la máxima seguridad para el agua potable. Se llevó a cabo una evaluación sistemática de la dirección del flujo entre puntos de observación existentes utilizando un método de estimación de tres puntos desarrollado para un gran número de escenarios. Se demuestra que la aplicación sistemática de la metodología desarrollada ayuda a identificar lugares vulnerables que son sensibles a las condiciones cambiantes de los límites, como los que surgen de los cambios en las tasas de recarga artificial de agua subterránea. En estos lugares, se requieren investigaciones adicionales y de protección. El enfoque integrado que se presenta, que utiliza la dirección del flujo de agua subterránea entre los puntos de observación, puede transferirse fácilmente a una variedad de ambientes hidrológicos para evaluar sistemáticamente los escenarios a partir del modelado del agua subterránea.摘要在瑞士西北部一个研究区,饮用水生产靠近过去的填埋场及工业区,在这里利用三维地下水模型提高水资源的管理水平。为了避免饮用水遭到污染,采用地表水人工补给地下水,在污染的场地和饮用水抽水井之间构建水力屏障。模型用作进行模拟现有的和建议的水管理策略的工具,以确保饮用水的绝对安全。利用开发出的针对各种不同方案的三点估算法对现有观测点之间的水流方向进行了系统评估。证明系统应用开发的方法有助于确定易受物探的位置,这些位置对变化的边界条件诸如人工地下水补给量导致的边界变化非常敏感。在这些位置,需要进行额外的调查和保护。所展示的利用观测点之间水流方向的综合方法可容易地应用到各种水文地质背景下,系统地评估地下水模拟方案。ResumoUm modelo tridimensional de águas subterrâneas foi utilizado para aprimorar o gerenciamento de recursos hídricos para uma área de estudo no noroeste da Suíça, onde a produção de água potável está próxima de antigos aterros e áreas industriais. Para evitar a contaminação da água potável, a recarga artificial de águas subterrâneas com água superficial é utilizada para criar uma barreira hidráulica entre os locais contaminados e os poços de extração de água potável. O modelo foi utilizado para simular estratégias existentes e propostas de gerenciamento da água, como uma ferramenta para garantir a máxima segurança para a água potável. Foi realizada uma avaliação sistemática da direção do fluxo entre os pontos de observação existentes utilizando um método de estimativa de três pontos desenvolvido para um grande número de cenários. Demonstra-se que a aplicação sistemática da metodologia desenvolvida ajuda a identificar locais vulneráveis que são sensíveis a modificações nas condições de contorno, tais como os decorrentes de mudanças nas taxas de recarga artificial de água subterrânea. Nesses locais, são necessárias investigações adicionais e proteção. A abordagem integrada apresentada, utilizando a direção do fluxo das águas subterrâneas entre os pontos de observação, pode ser facilmente transferida a uma variedade de configurações hidrológicas para avaliar sistematicamente cenários de modelagem de águas subterrâneas.

Nitrogen loss by anaerobic ammonium oxidation in unconfined aquifer soils

Anaerobic ammonium oxidation (anammox) is recognized as an important process for nitrogen cycling, yet little is known about its role in the subsurface biosphere. In this study, we investigated the presence, abundance, and role of anammox bacteria in upland soil cores from Tianjin, China (20 m depth) and Basel, Switzerland (10 m depth), using isotope-tracing techniques, (q)PCR assays, and 16 S rRNA & hzsB gene clone libraries, along with nutrient profiles of soil core samples. Anammox in the phreatic (water-saturated) zone contributed to 37.5–67.6% of the N-loss (up to 0.675 gN m−2 d−1), with anammox activities of 0.005–0.74 nmolN g−1 soil h−1, which were even higher than the denitrification rates. By contrast, no significant anammox was measured in the vadose zone. Higher anammox bacterial cell densities were observed (0.75–1.4 × 107 copies g−1 soil) in the phreatic zone, where ammonia-oxidizing bacteria (AOB) maybe the major source of nitrite for anammox bacteria. The anammox bacterial cells in soils of the vadose zone were all <103 copies g−1 soil. We suggest that the subsurface provides a favorable niche for anammox bacteria whose contribution to N cycling and groundwater nitrate removal seems considerably larger than previously known.

What Do They Have in Common? Drivers of Streamflow Spatial Correlation and Prediction of Flow Regimes in Ungauged Locations

The spatial correlation of daily streamflows represents a statistical index encapsulating the similarity between hydrographs at two arbitrary catchment outlets. In this work, a process-based analytical framework is utilized to investigate the hydrological drivers of streamflow spatial correlation through an extensive application to 78 pairs of stream gauges belonging to 13 unregulated catchments in the eastern United States. The analysis provides insight on how the observed heterogeneity of the physical processes that control flow dynamics ultimately affect streamflow correlation and spatial patterns of flow regimes. Despite the variability of recession properties across the study catchments, the impact of heterogeneous drainage rates on the streamflow spatial correlation is overwhelmed by the spatial variability of frequency and intensity of effective rainfall events. Overall, model performances are satisfactory, with root mean square errors between modeled and observed streamflow spatial correlation below 10% in most cases. We also propose a method for estimating streamflow correlation in the absence of discharge data, which proves useful to predict streamflow regimes in ungauged areas. The method consists in setting a minimum threshold on the modeled flow correlation to individuate hydrologically similar sites. Catchment outlets that are most correlated (q > 0:9) are found to be characterized by analogous streamflow distributions across a broad range of flow regimes.