Transformation of phosphorus in the Elbe estuary     

J. E. E. van Beusekom  U. H. Brockmann 《Estuaries and Coasts》1998,21(4):518-526

The distribution of dissolved and particle-bound phosphorus (P) was investigated in the Elbe estuary during March 1995. The forms of particulate P were studied with a sequential extraction technique. Organic P dominated particle-bound P in the outer reaches of the estuary (52%), decreased to a minimum of 21% in the turbidity zone, and increased to 33% further upstream. Fe-bound P was the second most important P species in the outer reaches (27%) and dominated in the turbidity zone (up to 57%) and upstream of the turbidity zone (up to 48%). The P:Fe ratio increased with decreasing salinity, from 0.11 in the outer reaches to about 0.22 at zero salinity. Dissolved inorganic P release from reverine suspended matter was about two to three times larger than release, from marine suspended matter and was dominated by release of Fe-bound P. Dissolved inorganic P release from marine and from riverine organic matter were of equal importance. Because marine suspended matter dominates in the estuary, this suggests riverine organic matter is remineralized much faster than marine organic matter. This is in line with the refractory nature of marine organic matter (no phytoplankton bloom) and the easily degradable character of the riverine suspended matter (phytoplankton bloom) in the Elbe estuary during March 1995.  相似文献   

Role of Helicity in the Formation of Intermediate Filaments     

Mackay  D. H.  Priest  E. R.  Gaizauskas  V.  van Ballegooijen  A. A. 《Solar physics》1998,180(1-2):299-312

In the last few years new observations have shown that solar filaments and filament channels have a surprising hemispheric pattern. To explain this pattern, a new theory for filament channel and filament formation is put forward. The theory describes the formation of a specific type of filament, namely the intermediate filament which forms either between active regions or at the boundary of an active region. It describes the formation in terms of the emergence of a sheared activity complex. The complex then interacts with remnant flux and, after convergence and flux cancellation, the filament forms in the channel. A key feature of the model is the net magnetic helicity of the complex. With the correct sign a filament channel can form, but with the opposite sign no filament channel forms after convergence. It is shown how the hemispheric pattern of helicity in emerging flux regions produces the observed hemispheric pattern for filaments.  相似文献   

The influence of a stratified rheology on the flexural response of the lithosphere to (un)loading by extensional faulting   总被引：2，自引：0，他引：2  

M. ter Voorde  R. T. van Balen  G. Bertotti  & S. A. P. L. Cloetingh 《Geophysical Journal International》1998,134(3):721-735

We present a two-layered finite difference model for the flexural response of the lithosphere to extensional faulting. The model allows for three modes of flexure: (1) fully coupled, with the upper crust and mantle welded together by the lower crust; (2) fully decoupled, with the upper crust and mantle behaving as independent layers; and (3) partly decoupled, signifying that the response of the upper crust to small-wavelength loads is superimposed on the response of the entire lithosphere to long-wavelength loads. Which of these modes of flexure is to be expected depends on the rheology and especially the thermal state of the lithosphere. Coupled behaviour is related to a cold and strong lithosphere. The Baikal Rift Zone provides a typical example for this mode of flexure. A fully decoupled lithosphere is an exceptional case, related to anomalous high temperatures in the lower crust, and is observed in the Basin and Range province. The most common case is a partly decoupled lithosphere, with the degree of decoupling depending on the thickness and viscosity of the lower crust. This is inferred, for example, for the Bay of Biscay margin.  相似文献   

Modelling Spatial Patterns of Overbank Sedimentation on Embanked Floodplains   总被引：2，自引：0，他引：2  

Hans Middelkoop  & Marcel van der Perk 《Geografiska Annaler: Series A, Physical Geography》1998,80(2):95-109

This paper presents a GIS-based mathematical model for the simulation of floodplain sedimentation. The model comprises two components: (1) the existing hydrodynamic WAQUA model that calculates two-dimensional water flow patterns; and (2) the SEDIFLUX model that calculates deposition of sediment based on a simple mass balance concept with a limited number of model parameters. The models were applied to simulate floodplain sediment deposition over river reaches of several kilometres in length. The SEDIFLUX model has been calibrated and validated using interpolated raster maps of sediment deposition observed after the large magnitude December 1993 flood on the embanked floodplain of the lower river Rhine in the Netherlands. The model appeared to be an adequate tool to predict patterns of sediment deposition as the product of the complex interaction among river discharge and sediment concentration, floodplain topography, and the resulting water flow patterns during various discharge levels. In the investigated areas, the resulting annual average sedimentation rates varied between 0.5 mm/year and 4.0 mm/year. The role of the most important mechanisms governing the spatial patterns of overbank deposition, i.e. inundation frequency, sediment load, floodplain topography and its influence on the flow patterns over the floodplain, are discussed.  相似文献   

Observations and origin of Rayleigh-wave amplitude anomalies   总被引：1，自引：0，他引：1  

S. van der Lee 《Geophysical Journal International》1998,135(2):691-699

This is a report of observations of amplitude anomalies of fundamental-mode Rayleigh waves ( R 1) between periods of 17 and 100  s. The anomalies are with respect to amplitudes predicted by Rayleigh-wave excitation for a reference earth model and catalogued centroid earthquake source parameters, such as are used in large-scale waveform inversions. The observations indicate that the amplitude anomalies are consistent for nearby recordings of the same event, while there is no obvious relation between the observed anomalies and the paths travelled by the waves. This is in contrast to Rayleigh-wave phase anomalies, which are consistent for similar propagation paths, and hence form the input in many inversions for along-path structure. The observations in this paper show that a similar inversion of intermediate-period amplitude anomalies for along- and near-path structure is not warranted without eliminating source effects, since the amplitude anomalies are dominated by scattering off near-source earth structure and by possible uncertainties in the source parameters. Sensitivity kernels that take the coupling between the moment tensor and displacement field into account demonstrate that Rayleigh-wave amplitude sensitivity is largest near the source. This report argues that the interaction between source-radiated Rayleigh waves and near-source earth structure may not be ignored in amplitude inversion procedures.  相似文献   

Prediction of the occurrence of slope instability phenomenal through GIS-based hazard zonation   总被引：1，自引：0，他引：1  

C. J. van Westen  N. Rengers  M. T. J. Terlien  R. Soeters 《International Journal of Earth Sciences》1997,86(2):404-414

Slope instability hazard assessment is based on the analysis of the terrain conditions at sites where slope failures occurred in the past. For the analysis of the causative factors the application of geographic information systems (GIS) is an essential tool in the data analysis and subsequent hazard assessment. Three scale levels of hazard mapping are defined. A direct experience-driven mapping at reconnaissance level, a statistical approach to determine the causative factors in a quantitative susceptibility mapping and a methodology at large-scale making using of deterministic models.  相似文献   

Macroscopic representation of structural geometry for simulating water and solute movement in dual-porosity media     

Horst H. Gerke  Martinus Th. van Genuchten 《Advances in water resources》1996,19(6):343-357

The structure of macroporous or aggregated soils and fractured rocks is generally so complex that it is impractical to measure the geometry at the microscale (i.e., the size and the shape of soil aggregates or rock matrix blocks, and the myriad of fissures or fractures), and use such data in geometry-dependent macroscale flow and transport models. This paper analyzes a first-order type dual-porosity model which contains a geometry-dependent coefficient, β, in the mass transfer term to macroscopically represent the size and shape of soil or rock matrix blocks. As a reference, one- and two-dimensional geometry-based diffusion models were used to simulate mass transport into and out of porous blocks of defined shapes. Estimates for β were obtained analytically for four different matrix block geometries. Values for β were also calculated by directly matching analytical solutions of the diffusion models for a number of selected matrix block geometries to results obtained with the first-order model assuming standard boundary conditions. Direct matching improved previous results for cylindrical macropore geometries, especially when relatively small ratios between the outer soil mantle and the radius of the inner cylinder were used. Results of our analysis show that β is closely related to the ratio of the effective surface area available for mass transfer, and the soil matrix volume normalized by the effective characteristic length of the matrix system. Using values of β obtained by direct matching, an empirical function is derived to estimate macroscopic geometry coefficients from medium properties which in principle are measurable. The method permits independent estimates of β, thus allowing the dual-porosity approach eventually to be applied to media with complex and mixed types of structural geometry.  相似文献   

Analytical Modeling of Effects of Application Frequency on Pesticide Concentrations in Wells   总被引：1，自引：0，他引：1  

W.H.J. Beltman  J.J.T.I. Boesten  S.E.A.T.M. van der  Zee J. J. Quist 《Ground water》1996,34(3):470-479

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Time series analysis of monthly mean data of temperature,salinity, nutrients,suspended matter,phyto- and zooplankton at eight locations on the northwest european shelf     

M. Visser  S. Batten  G. Becker  P. Bot  F. Colijn  P. Damm  D. Danielssen  D. Van den Eynde  L. FØyn  A. Frohse  G. Groeneveld  R. Laane  W. van Raaphorst  G. Radach  H. Schultz  J. Sündermann 《Ocean Dynamics》1996,48(3-4):299-323

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Comparison of changes in the annual variability of the seasonal cycles of chlorophyll,nutrients and zooplankton at eight locations on the northwest european continental shelf (1960–1994)     

P. V. M. Bot  W. van Raaphorst  S. Batten  R. W. P. M. Laane  K. Philippart  G. Radach  A. Frohse  H. Schultz  D. van den Eynde  F. Colijn 《Ocean Dynamics》1996,48(3-4):349-364

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