Deep pore water profiles reflect enhanced microbial activity towards tidal flat margins

Microbial activity in permeable tidal flat margin sediments is enhanced by two main processes. First, organic matter is supplied by rapid sedimentation at prograding tidal flat margins. Second, surface and deep pore water advection lead to a replenishment of the dissolved organic matter and sulfate pools. Increasing microbial activity towards the low water line is reflected in sulfate and methane profiles as well as in total cell numbers, sulfate reduction rates, and remineralization products. The impact of high sedimentation rates on pore water biogeochemistry is confirmed by inverse modeling reproducing the depth profiles obtained by measurements. In central parts of the tidal flats, low sedimentation rates and pore water flow velocities limit microbial activity despite the high availability of electron acceptors for microbial respiration such as sulfate. Therefore, tidal flat margins with high microbial activity are of special importance for budgeting biogeochemical cycling in tidal flat areas.     

Numerical study of mean flow patterns in the South China Sea and the Luzon Strait

Previous investigations have suggested that wind stress curl, the balance of influx- and outflux-induced upwelling, as well as a positive vorticity source fed from the left flank of the Kuroshio are all possible mechanisms that contribute to a persistent cyclonic gyre in the South China Sea (SCS). Studies have also suggested that the loop current that forms from the Kuroshio intrusion in the Luzon Strait, similar to the Loop Current in the Gulf of Mexico (GOM), has rarely been observed in the northern SCS. In this research, an idealized numerical model driven by annual mean wind stress was adopted to investigate the relative importance of dynamic processes that control the mean flow pattern of Kuroshio in the Luzon Strait and regulate circulation in the SCS. An analysis of results drawn from numerical experiments suggests that the three mechanisms are of approximately equal importance in the formation of the persistent cyclonic gyre in the northern SCS. Unlike the Gulf Stream which enters the Gulf of Mexico through the Yucatan Channel, the two topographic ridges that align nearly meridionally in the Luzon Strait keep the Kuroshio flowing roughly northward without distinct intrusion into the SCS. Unsurprisingly, an anticyclonic loop current similar to the Gulf Stream pathway in the GOM was barely observed in the northern SCS.     

Groundwater-surface water interactions: New methods and models to improve understanding of processes and dynamics

Interest in groundwater (GW)-surface water (SW) interactions has grown steadily over the last two decades. New regulations such as the EU Water Framework Directive (WFD) now call for a sustainable management of coupled ground- and surface water resources and linked ecosystems. Embracing this mandate requires new interdisciplinary research on GW-SW systems that addresses the linkages between hydrology, biogeochemistry and ecology at nested scales and specifically accounts for small-scale spatial and temporal patterns of GW-SW exchange. Methods to assess these patterns such as the use of natural tracers (e.g. heat) and integrated surface-subsurface numerical models have been refined and enhanced significantly in recent years and have improved our understanding of processes and dynamics. Numerical models are increasingly used to explore hypotheses and to develop new conceptual models of GW-SW interactions. New technologies like distributed temperature sensing (DTS) allow an assessment of process dynamics at unprecedented spatial and temporal resolution. These developments are reflected in the contributions to this Special Issue on GW-SW interactions. However, challenges remain in transferring process understanding across scales.     

Sediment-charged flash floods on Montserrat: The influence of synchronous tephra fall and varying extent of vegetation damage

On 20th May 2006 the Soufrière Hills Volcano on the Caribbean island of Montserrat experienced a large dome collapse and intense rainfall generated flash floods. The floods had very high loads of volcanic debris derived both from this and previous eruptions and can thus be classified as lahars. The floods reached unusually high water levels and caused substantial geomorphic change in the Belham Valley. Detailed rainfall and geomorphological data, coupled with the precise timing of events and yewitness accounts have facilitated an assessment of the relative importance of rainfall volume and intensity, older volcanic debris, pre- and syn-flood tephra fall and the extent of pre-flood vegetation damage for the behavior of this and subsequent sediment-laden floods in this setting. The change in runoff behavior was controlled by preexisting vegetation damage and synchronous tephra fall and this was critically important in controlling the impact of these flash floods. Although rainfall intensity and volume have some control on flood occurrence they are not the critical control on flash flood impact on the geomorphology in the Belham Valley. A significant conclusion of this study is that the extreme nature of the flash floods was not caused by extreme rainfall (as is commonly believed to be the primary cause of flash floods) but rather it was the result of changed runoff behaviour caused by the widespread syn-flood tephra deposition and importantly the widespread vegetation damage by volcanic-associated acid rain in the preceding weeks.     

Propagation velocity and radiation properties of induced tensile cracks

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A3 ionospheric absorption measurements on 1539 kHz at Panská Ves

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A semi-implicit scheme for the shallow water equations for application to shelf sea modelling

A semi-implicit scheme for the numerical solution of the shallow water equations is proposed. The scheme is suitable for the simulation of shelf sea dynamics as is demonstrated by some examples of successful application covering a range of grid sizes typical for shelf sea models. The basic outlines of the method are presented. Some practical aspects of computation are discussed which illustrate that an explicit model can be modified easily to the semi-implicit version proposed here. Compared to explicit schemes the semi-implicit approach has two major advantages: (1) its economy (a saving of at least 50% in computing time can be achieved); (2) a closer match is obtained between the time-stepping procedure and the time scales of processes, the spatial scales of which are close to the lower limit of the resolution of the model grid.