Vegetation and hydraulic-morphological interactions at the individual plant, patch and channel scale

Interactions between vegetation, flow and sediment are a key ingredient for the development of vegetated islands in highly dynamic, fluvial alpine ecosystems such as the Tagliamento River, north-east Italy. There has been substantial research on factors influencing the establishment of vegetation and feedback mechanisms between vegetation, hydraulic, and geomorphological processes in such environments. This has yielded the development of conceptual models identifying different trajectories of vegetation and landform development from bare gravel to established floodplain forest. Nevertheless, some of the finer-scale processes underpinning such interactions are not well understood and parameterisation concepts that augment our knowledge from process understanding to quantified data and prediction models are not available until now. This paper identifies mechanisms and parameters of vegetation-flow interaction at the individual scale that are reflected at a patch or even at the channel scale. These mechanisms are reviewed from a multi-disciplinary perspective and concepts and analogies are proposed that provide ideas to progress research towards the development of predictive vegetation-flow models. Such models must incorporate both hydraulic and ecological components and this is demonstrated for a simplified force-bending model of Salicaceae seedlings. The development of such models demands advances in the individual disciplines of hydraulics, morphology, plant ecology and biomechanics, which offers many possibilities for multidisciplinary research between these disciplines.     

Factors Controlling Total Concentration and Aqueous Speciation of Aluminium in an Acidic Headwater Stream of the Bavarian Forest National Park: a Modelling Approach

The purpose of the present paper is to analyse factors controlling total concentration and aqueous speciation of aluminium in the Große Ohe River, using a thermodynamic equilibrium model and a mixing approach. A model compound for humic substances is derived on the basis of the relation between anion deficit and the organic carbon content in the river as well as literature data. An equilibrium speciation model for aluminium is set up, considering this model compound and relevant inorganic solutes. Although the model cannot be verified directly, its results may be viewed as qualitatively correct. Applying the model to measured stream water samples highlights that aqueous speciation of aluminium is mainly controlled by the pH value and discharge and that free aluminium concentrations reach clearly toxic levels during acidic episodes. Comparing measured concentrations of sulfate and H⁺ and calculated concentrations of Al³⁺ with solubility curves of gibbsite like minerals and jurbanite clearly shows that total aluminium concentrations are not controlled by equilibria with these mineral phases alone. The observed relationship can be better explained from a mixture of two distinct waters, representing lowflow and highflow chemistry, and the resulting equilibrium concentrations. This indicates that total aluminium concentration, in particular during high discharge events, is mainly controlled by the mixture of waters with differing chemistry and flowpaths.     

What controls equatorial Atlantic winds in boreal spring?

The factors controlling equatorial Atlantic winds in boreal spring are examined using both observations and general circulation model (GCM) simulations from the coupled model intercomparison phase 5. The results show that the prevailing surface easterlies flow against the attendant pressure gradient and must therefore be maintained by other terms in the momentum budget. An important contribution comes from meridional advection of zonal momentum but the dominant contribution is the vertical transport of zonal momentum from the free troposphere to the surface. This implies that surface winds are strongly influenced by conditions in the free troposphere, chiefly pressure gradients and, to a lesser extent, meridional advection. Both factors are linked to the patterns of deep convection. Applying these findings to GCM errors indicates, that, consistent with the results of previous studies, the persistent westerly surface wind bias found in most GCMs is due mostly to precipitation errors, in particular excessive precipitation south of the equator over the ocean and deficient precipitation over equatorial South America. Free tropospheric influences also dominate the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that the free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Niños), the results imply that interannual variability in the region may rely far less on coupled air–sea feedbacks than is the case in the tropical Pacific.     

Zum mittleren Kluftabstand aus der Anzahl von Kluftanschnittlinien

ZusammenfassungZum mittleren Kluftabstand aus der Anzahl von Kluftschnittlinien Der mittlere Kluftabstand wird anhand von zwei Methoden bestimmt: In der ersten Methode werden Kluftgefüge typisiert und durch drei senkrecht aufeinanderstehende Kluftscharen approximiert. Von diesen drei Kluftscharen wie dem gesamten Kluftgefüge wird der mittlere Kluftabstand ermittelt. Je nach Gefügetyp stellt der mittlere Kluftkörper einen Quader, ein Prisma mit quadratischer Grundfläche oder einen Würfel dar. Gemessen wird die Anzahl von Kluftanschnittlinien bzw. Sehnen entlang von Meßgeraden, die parallel zu Gefügekoordinaten orientiert werden.Die zweite Methode ermöglicht, den Kluftabstand vonn Kluftscharen festzustellen. Meßgrößen sind wiederum die Anzahl von Kluftanschnittlinien und außerdem die Raumstellung der Klüfte. Die Meßgeraden können beliebig orientiert sein.Dazu werden die Gleichungen für den mittleren Kluftabstand hergeleitet. An einem Verwitterungsprofil eines oberdevonischen Siltsteines werden beide Methoden getestet.

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SummaryAbout the Mean Joint Spacing from the Number of Dissecting Lines Between Joints and Rock Faces The mean joint spacing is determined by two methods: The first method typifies the fabric and approximates the joints by three orthogonally sets of joints. The mean joint spacing is determined by these sets of joints as well as by the entire fabric of the joints. Depending upon the fabric type the mean rock block defined by joints is a rectangular parallelepiped, a prism with a quadratic basal surface or a cube. The number of dissecting lines between joints or chords are measured along secants, which are parallel to fabric coordinates. The second method allows to determine the spacing ofn shares of joints. Measuring units are once again the number of dissecting lines between joints and moreover the position in space of the joints. The orientation of the measuring secants may be at random.In addition to this equations are derived for the mean joint spacing. A weathering profile of Upper Devonian siltstone is used to test both methods.

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RésuméDistance de failles moyenne établie par le nombre des lignes de failles coupées La distance de failles moyenne se détermine à l'aide de deux méthodes: La première méthode sert à typiser les textures de failles et à rapprocher ces textures par trois séries de cassures orthogonales. Pour chacune de ces trois séries de cassures aussi bien que pour la texture de failles intégrale on établit la distance de failles moyenne. Selon le type de texture, le polyèdre moyen représente un parallélépipède orthogonal, un prisme à base quadratique ou un cube. On mesure le nombre des lignes de failles coupées respectivement des cordes le long des lignes de mesures qui se trouvent en relation avec les coordonnées de texture. La seconde méthode permet à déterminer la distance de failles den séries de cassures. Les quantités de mesure sont de nouveau le nombre des lignes de failles coupées et plus l'orientation des failles. Les lignes de mesure peuvent être orientées de manière quelconque.En outre on déduit les équations pour la distance de failles moyenne. On met à l'épreuve ces deux méthodes à l'exemple d'un profil de désagrégation dans le schiste argileux du Dévonien supérieur.

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Mit 12 Abbildungen     

The influence of cosmic-ray production on extinct nuclide systems

Variations in the atomic abundances of ⁵³Cr, ⁹²Zr, ⁹⁸Ru, ⁹⁹Ru, and ¹⁸²W in meteorites and lunar samples relative to terrestrial values may imply the early decay of radioactive ⁵³Mn, ⁹²Nb, ⁹⁸Tc, ⁹⁹Tc and ¹⁸²Hf, respectively. From this one can deduce nucleosynthetic sites and early solar system timescales. Because these effects are very small, production and consumption of the respective isotopes by cosmic-ray interactions is a concern. It has recently been demonstrated that ¹⁸²W production by neutron capture reactions on ¹⁸¹Ta is crucial for most lunar samples (Leya et al., 2000a). In this study the neutron fluence of each sample was estimated from its nominal cosmic-ray exposure age as deduced from noble gas data. This approach overestimates the true cosmogenic isotopic shift for samples that might have been irradiated very close to the regolith surface. Here we therefore combine our model calculations with the neutron dose proxies ¹⁵⁷Gd/¹⁵⁸Gd and ¹⁴⁹Sm/¹⁵⁰Sm. This allows us to accurately correct the measured W isotopic data for cosmic-ray induced shifts without the explicit knowledge of the exposure age or the shielding depth of the sample simply by measuring ¹⁵⁷Gd/¹⁵⁸Gd and/or ¹⁴⁹Sm/¹⁵⁰Sm in an aliquot. In addition we present new model results for the GCR-induced effects on ⁵³Mn-⁵³Cr, ⁹²Nb-⁹²Zr and ⁹⁸Tc-⁹⁹Tc-⁹⁸Ru-⁹⁹Ru. For each of these systems, except Tc-Ru, a proper cosmic-ray dose proxy is given, permitting the accurate correction of measured isotopic ratios for cosmogenic contributions.     

Phytoplankton und Primärproduktion einiger ostschweizerischer Seen und des Bodensees

Ohne Zusammenfassung     

Using global node-based velocity in random walk particle tracking in variably saturated porous media: application to contaminant leaching from road constructions

Precise and efficient numerical simulation of transport processes in subsurface systems is a prerequisite for many site investigation or remediation studies. Random walk particle tracking (RWPT) methods have been introduced in the past to overcome numerical difficulties when simulating propagation processes in porous media such as advection-dominated mass transport. Crucial for the precision of RWPT methods is the accuracy of the numerically calculated ground water velocity field. In this paper, a global node-based method for velocity calculation is used, which was originally proposed by Yeh (Water Resour Res 7:1216–1225, 1981). This method is improved in three ways: (1) extension to unstructured grids, (2) significant enhancement of computational efficiency, and (3) extension to saturated (groundwater) as well as unsaturated systems (soil water). The novel RWPT method is tested with numerical benchmark examples from the literature and used in two field scale applications of contaminant transport in saturated and unsaturated ground water. To evaluate advective transport of the model, the accuracy of the velocity field is demonstrated by comparing several published results of particle pathlines or streamlines. Given the chosen test problem, the global node-based velocity estimation is found to be as accurate as the CK method (Cordes and Kinzelbach in Water Resour Res 28(11):2903–2911, 1992) but less accurate than the mixed or mixed-hybrid finite element methods for flow in highly heterogeneous media. To evaluate advective–diffusive transport, a transport problem studied by Hassan and Mohamed (J Hydrol 275(3–4):242–260, 2003) is investigated here and evaluated using different numbers of particles. The results indicate that the number of particles required for the given problem is decreased using the proposed method by about two orders of magnitude without losing accuracy of the concentration contours as compared to the published numbers.     

A comparison of methods used to estimate the height of sand dunes on Mars

The collection of morphometric data on small-scale landforms from other planetary bodies is difficult. We assess four methods that can be used to estimate the height of aeolian dunes on Mars. These are (1) stereography, (2) slip face length, (3) profiling photoclinometry, and (4) Mars Orbiter Laser Altimeter (MOLA). Results show that there is good agreement among the methods when conditions are ideal. However, limitations inherent to each method inhibited their accurate application to all sites. Collectively, these techniques provide data on a range of morphometric parameters, some of which were not previously available for dunes on Mars. They include dune height, width, length, surface area, volume, and longitudinal and transverse profiles. The utilization of these methods will facilitate a more accurate analysis of aeolian dunes on Mars and enable comparison with dunes on other planetary surfaces.