Gerfried Winkler

Analytical approximations of discharge recessions for steeply sloping aquifers in alpine catchments

The validity and applicability of various methods to infer hydraulic properties of sloping aquifers in alpine settings using the power law relationship between the discharge recession and its first time derivative is explored. For this purpose, a synthetic spring catchment implemented in the numerical groundwater flow model MODFLOW as well as the example of a relict rock glacier in an alpine setting is examined. The various approaches are found to differ particularly in the late time domain, whereas most of them agree fairly well in the early time domain and at the transition point between the two time domains. As the early recession may be affected by uncertainties from inappropriate initial conditions, it is proposed to use the transition point for estimating aquifer thickness and transmissivity. Using only prolonged winter recessions in the analysis of the field data from the relict rock glacier yields estimates of aquifer thickness and hydraulic conductivity consistent with results from a geophysical survey and tracer tests, respectively. In the other seasons, the recession is frequently interrupted by minor recharge events, and using the lower envelope of the entire data is found to yield estimates that are too high in the given case. It is thus recommended to focus on the winter recession in the analysis of hydrograph data from alpine settings.

Widespread occurrence of ephemeral funnel hoarfrost and related air ventilation in coarse‐grained sediments of a relict rock glacier in the seckauer tauern range, austria

Abstract This paper examines the occurrence of ephemeral hoarfrost crystals at funnel openings (funnel hoarfrost) detected between large blocks at the surface of the presumably relict chöneben ock lacier. Field mapping on 25 November 2011 identified 51 individual funnel openings with notable hoarfrost crystals distributed over the entire rock glacier. Hoarfrost was no longer detectable a few days after the initial mapping campaign. At least in the period 20–25 November 2011 temperature conditions at the rock glacier surface were favourable for hoarfrost formation and preservation as indicated by different types of measurements. A period of 24–48 h of hoarfrost‐suitable weather conditions would have been sufficient to form the observed hoarfrost if crystal growth rates of 2–4 mm h−1 are assumed. The void systems with funnel hoarfrost seem to be rather localised and limited in horizontal (10s of metres) and vertical (some metres) extent. Presumably the observed funnel hoarfrost was caused by the so‐called chimney effect, although no larger reversible air circulation systems with warm air exhalation were identified. Continuous ground temperature data from several sites at the rock glacier surface (period November 2011–December 2012) showed that hoarfrost sites are cooler and thermally buffered compared with non‐hoarfrost sites at similar elevations. This seems to be related to the decoupling of the air above the rock glacier and the pore air during periods of atmospheric warming. Only the combination of specific micro‐climatic (temperature/humidity), geometric (open void systems) and sedimentological (grain size/sediment structure) conditions allow the formation of the ephemeral funnel hoarfrost at this rock glacier.

Investigating groundwater flow components in an Alpine relict rock glacier (Austria) using a numerical model

Relict rock glaciers are complex hydrogeological systems that might act as relevant groundwater storages; therefore, the discharge behavior of these alpine landforms needs to be better understood. Hydrogeological and geophysical investigations at a relict rock glacier in the Niedere Tauern Range (Austria) reveal a slow and fast flow component that appear to be related to the heterogeneous structure of the aquifer. A numerical groundwater flow model was used to indicate the influence of important internal structures such as layering, preferential flow paths and aquifer-base topography. Discharge dynamics can be reproduced reasonably by both introducing layers of strongly different hydraulic conductivities or by a network of highly conductive channels within a low-conductivity zone. Moreover, the topography of the aquifer base influences the discharge dynamics, which can be observed particularly in simply structured aquifers. Hydraulic conductivity differences of three orders of magnitude are required to account for the observed discharge behavior: a highly conductive layer and/or channel network controlling the fast and flashy spring responses to recharge events, as opposed to less conductive sediment accumulations sustaining the long-term base flow. The results show that the hydraulic behavior of this relict rock glacier and likely that of others can be adequately represented by two aquifer components. However, the attempt to characterize the two components by inverse modeling results in ambiguity of internal structures when solely discharge data are available.RésuméDes glaciers de reliquats rocheux sont des systèmes hydrogéologiques complexes qui pourraient agir comme des zones pertinentes de stockage d’eaux souterraines. Par conséquent, le comportement des débits de ces reliefs alpins doit être mieux compris. Des investigations hydrogéologiques et géophysiques sur un glacier de reliquats rocheux dans la chaîne de montagnes du Tauern Niedere (Autriche) révèlent une composante d’écoulement lent et une composante d’écoulement rapide qui semblent être liées à la structure hétérogène de l’aquifère. Un modèle numérique d’écoulement d’eaux souterraines a été utilisé pour démontrer l’influence des structures internes importantes telles que la superposition des couches, les cheminements d’écoulement préférentiel et la topographie de la base de l’aquifère. Les dynamiques des débits peuvent être reproduites de manière raisonnable aussi bien en introduisant des couches avec des contrastes importants de conductivités hydrauliques qu’un réseau de chenaux très conducteurs au sein d’une zone à faible conductivité. En outre, la topographie de la base de l’aquifère influence les dynamiques du débit, qui peuvent être observées particulièrement dans des aquifères de structure simple. Des différences de conductivité hydraulique de trois ordres de grandeur sont nécessaires pour prendre en considération le comportement observé des débits : une couche fortement conductrice et/ou un réseau de chenaux contrôlant les réponses rapides et éclairs de la source aux événements de recharge, par opposition à des accumulations sédimentaires moins conductrices qui maintiennent le débit de base sur du long terme. Les résultats montrent que le comportement hydraulique du glacier de reliquat rocheux et probable, peut être représenté de manière adéquate par les deux composantes de l’aquifère. Cependant, la tentative de caractérisation des deux composantes par modélisation inverse donne des résultats ambigus vis-à-vis des structures internes lorsque seules des données de débits sont disponibles.ResumenLos glaciares rocosos relictos son sistemas hidrogeológicos complejos que podrían actuar como almacenamientos relevantes de agua subterránea. Por lo tanto, el comportamiento de la descarga de esta morfología alpina necesita ser mejor entendida. Las investigaciones hidrogeológicas y geofísicas en el glaciar rocoso relicto de Niedere Tauern (Austria) revelan una componente de flujo lento y rápido que parece estar relacionada con la estructura heterogénea del acuífero. Se utiliza un modelo numérico de flujo de agua subterránea para indicar la influencia de importantes estructuras internas tales como la estratificación, las trayectorias de flujo preferencial y la topografía de la base del acuífero. La dinámica de la descarga puede ser reproducida razonablemente tanto introduciendo capas de conductividades hidráulicas con fuertes diferencias como por una red de canales de alta conductividad dentro de una zona de baja conductividad. Por otra parte, la topografía de la base del acuífero influye en la dinámica de la descarga, que puede ser particularmente observada en acuíferos de estructura sencilla. Se requieren diferencias conductividad hidráulica de tres órdenes de magnitud para explicar el comportamiento observado en la descarga: una capa de alta conductividad hidráulica y / o una red de canales que controlan las rápidas y repentinas respuestas de los manantiales en eventos de recarga, en oposición la acumulación de sedimentos menos conductivos que sostienen el caudal de base a largo plazo. Los resultados muestran que el comportamiento hidráulico de este glaciar rocoso relicto y probablemente en otros pueden ser adecuadamente representados por dos componentes del acuífero. Sin embargo, el intento de caracterizar estas dos componentes la modelización inversa da como resultado una ambigüedad de las estructuras internas cuando únicamente los datos de descarga están disponibles.摘要残余岩石冰川是复杂的水文地质系统,可以充当相关的地下水储存设施。因此,需要更好地了解阿尔卑斯山脉这些地形的排泄特性。(奥地利)Niedere Tauern山脉残余岩石冰川的水文地质和地球物理调查结果揭示,有一个缓慢和快速的水流成分,似乎和含水层的非均质结构相关。利用数值地下水流模型显示了重要内部结构诸如分层的、优先水流通道及含水层基底地形的影响。通过采用完全不同的水力导水系数层或者通过传导性很弱地带内传导性很强的渠道网合理地再现了流量动态。此外,含水层基底的地形影响着流量动态,流量动态尤其是在简单结构的含水层中可以观察到。需要三个量级的水力传导系数差来说明观测到的排泄特征:控制泉对排泄事件快速及瞬间反应的传导性很强的层及/或渠道网络,与维持长期基流的传导性很低的泥沙堆积截然相反。结果显示,残余岩石冰川的水力特征及其他类似冰川的水力特征可通过两个含水层成分充分地展示出来。然而,在仅有排泄资料的情况下,通过反演模拟描述两个成分将导致内部结构的含糊不清。ResumoGeleiras de rochas relictas são sistemas hidrogeológicos complexos que podem atuar como importantes reservatórios de águas subterrâneas. Desse modo, o comportamento da descarga desses terrenos alpinos precisa de um melhor entendimento. Investigações hidrogeológicas e geofísicas em uma geleira de rochas relictas na cordilheira de Niedere Tauern (Áustria) revelam um componente de fluxo lento e outro rápido que parecem estar relacionados à estrutura heterogênea do aquífero. Um modelo numérico do fluxo das águas subterrâneas foi utilizado para indicar a influência de estruturas internas importantes como estratificação, direção de escoamento preferencial e topografia aquífero-base. A dinâmica da descarga pode ser razoavelmente reproduzida pela inserção de camadas com condutividades hidráulicas muito diferentes, ou por uma rede de canais com alta condutividade dentro de uma zona de baixa condutividade. Ademais, a topografia da base do aquífero influencia a dinâmica da descarga, o que pode ser observado particularmente, em aquíferos estruturados de maneira simples. Diferenças de condutividade hidráulica em três ordens de magnitude são necessárias para explicar o comportamento de descarga observado: uma camada com alta condutividade e/ou redes de canais controlando a rápida e expressiva resposta da nascente aos eventos de recarga, em oposição a menos condutiva acumulação de sedimentos sustentando o fluxo basal de longo prazo. Os resultados demonstram que o comportamento hidráulico dessa geleira de rocha relicta e provavelmente a de outros, pode ser representado adequadamente por dois componentes aquíferos. Entretanto, a tentativa de caracterizar os dois componentes por modelagem inversa resulta em ambiguidade das estruturas internas quando somente são disponíveis dados de descarga.