Physics and Chemistry of the Earth, Part B: Hydrology, Oceans and Atmosphere最新文献

Saltwater intrusion from saline formation waters in the shallow lowlands of the state of Brandenburg, Germany, is simulated using a 2D density-dependent (dd) flow and transport model. Based on the geological situation and the present-day chemical composition of the groundwaters in that region, migration scenarios with and without inclusion of density effects are modelled. We find that, due to the shallowness of the aquifer system, the surficial topography has a large effect on the flow and migration patterns and, especially, gives rises to upwelling flow underneath the discharge area of the Nuthe river. Comparing models, with and without density effects included, we then investigate possible saltwater upconing due this natural discharge flow pattern. A sensitivity study of the hydrodynamic dispersion and of the anisotropy of the aquifer is at the heart of the investigation. The results of the models show that density effects are diminishing for large values of the dispersivity and high anisotropy ratios. This means that for the management of saltwater intrusion, instead of using a complicated dd flow and transport model, it may be sufficient in most practical situation to use a passive transport model, which would have a much smaller computational time.

On the UK-scale, sulphur dioxide (SO₂) dry deposition has declined at a faster rate than SO₂ emissions, but S wet deposition has declined at a slower rate than emissions. Three hypotheses have been put forward to explain this non-linear response to changes in emissions: changes in the effective release height of emissions; oxidation of SO₂ may have been oxidant limited in previous years; the canopy resistance of SO₂ has changed in response to changing ammonia concentrations. Of these three hypotheses, the first has been tested here using a 10-layer Lagrangian acid deposition model. The model reproduced the rate of decline of observed total S deposition but not the magnitude. The mismatch in magnitude between modelled and observed UK S deposition is consistent with the model framework and its limitations. More importantly, the model did not reproduce the changes in the proportions of wet and dry deposition to the UK that have been observed using two different sets of UK emission data. The UK-derived emissions data show a small increase in the proportion of low-level SO₂ sources to the total and a small decrease in proportion of the high-level SO₂ sources over the period 1989–1994. These changes are not apparently consistent with observed changes in SO₂ concentrations nor changes in wet and dry S deposition. Thus, it is unsurprising that the modelling does not simulate the observed changes in fractions of wet and dry deposition. The current constraints of the modelling and the emissions data have the consequence that future scenarios of rates of change of total S deposition may be optimistic for locations that are dominated by wet deposition, remote from sources.

Agricultural land use series are investigated in a plain catchment area of the Tisza River, almost identically represented by six administrative counties. Each county, commonly covering 34,000 km², is characterised by high percentage (70–80 %) of managed vegetation. Effects of area coverage variations between the different plants are computed for the period 1951–1993 by applying results of a literature-based syntheses, specified for Hungary. The latter studies estimate surface-albedo values and proportion between real and potential evapotranspiration for the great majority of the plants grown in the region. Potential evapotranspiration and relative soil moisture content are estimated from the monthly meteorological series of temperature, precipitation and atmospheric water vapour pressure. Product of these plant-specific characteristics and the relative area coverage yield in monthly series of surface albedo and real evapotranspiration. Furthermore, these values are related to the energy balance of the surface-atmosphere system by using a radiative-convective model adjusted for the given location. Two questions are investigated:

i) Are there monotonous trends in the given terms of the energy and water budget?

ii) Are these changes comparable to the effects caused by other external forcings?

Our computations give positive answer to both questions.

The Amazon river has the biggest flow in the world, 0.2 Sv (1 Sv = 10⁶ m³/s), and is responsible for a large part of the low Sea Surface Salinity (SSS) in the west tropical Atlantic ocean. Very few Ocean General Circulation Model (OGCM) include runoff which brings a specific contribution to the Ocean physics. Comparison of simulations with constant or monthly runoffs shows that the spread of the Amazon fresh waters offshore of the north Brazilian coast is controlled by the ocean circulation and not by the Amazon flow. Therefore, in the model like in the observations, SSS minimum is observed in summer three months after the Amazon flood. In agreement with observations, a thick (more than 40 m) Barrier Layer (BL) is present every summer north of the Amazon mouth. Because of the strong and shallow salinity gradient associated with the Amazon freshwater, an important part of the solar radiation is trapped in the BL and creates an inversion of the vertical gradient of temperature. However with this forced model, BL does not seem to have a clear impact on SST and so on the air-sea interaction. Freshwater flux is also able to bend the sea surface. The geostrophic part of the North Brazilian Current (NBC) retroflection is then lightly weaker in presence of the Amazon runoff.

We study the spatial patterns formed by interacting populations or reacting chemicals under the influence of chaotic flows. In particular, we have considered a three-component model of plankton dynamics advected by a meandering jet. We report general results, stressing the existence of a smooth-filamental transition in the concentration patterns depending on the relative strength of the stirring by the chaotic flow and the relaxation properties of planktonic dynamical system. Patterns obtained in open and closed flows are compared.

A three dimensional quasi-analytical model is introduced to determine the flow field and the altimetric response of movable-bed channels subject to periodic width variations. The basic assumptions underlying the analysis are those of small amplitude of width variations and wide channel, so that non linear effects and side wall effects are neglected. The aim of the work is to determine the conditions under which the channel is planimetrically stable or unstable, i.e. it tends to damp (or enhance) a given initial (infinitesimal) perturbation of the channel width due to bank erosion. A simple bank erosion model is adopted whereby the rate of bank retreat is related to the excess shear stress at the banks. Theoretical results suggest that the equilibrium bottom profile is mainly constituted by two components. The first component represents a purely longitudinal bottom deformation, which induces deposition at the widest section and scour at the constraint, where the cross sectionally averaged velocity attains its maximum value. The second component is mainly originated by three dimensional effects and induces a transverse deformation of the bed in the form of a central bar. Its relative position with respect to the former component changes with the length of width variations: under suitable conditions the flow divergence induced by the central bar leads to a maximum velocity at the banks in wide sections, which implies that width variations tend to amplify.

The recent growth in neural network hydrological modelling has focused on the provision of river flow estimates of one kind or another. Little or no scientific research has been undertaken to assess the potential benefits for modelling sediment transfer. Some initial pathfinder experiments were therefore conducted to assess the competence of a backpropagation network to produce a combined model of sediment transfer occurring under different types of agriculture and land management conservation regimes. The results of this investigation demonstrate that a neural network solution is able to exceed the limitations of traditional multiple linear regression. The potential to create multiple solutions at different levels of generalisation and robust solutions that can be transferred to unknown catchment types is illustrated.

Northeastern Brazil, an example of vulnerable semi-arid regions on the earth, is characterised by water scarcity and vulnerability of natural resources, pronounced climatic variability and social stress situations. Integrated studies involving hydrology, ecology, meteorology, climatology, pedology, agronomy, and social and economic sciences are required not only for analysing consequences of the adverse natural conditions but also to assess possible strategies to make semi-arid regions less vulnerable to the present and changing climate.

Integrated modelling of the relationships between water availability, quality of life and migration, including aspects of global change processes, in the Semi-arid Integrated Model (SIM) serves to understand the dynamics of the system. An overview of the concept, structure and application results of the integrated model are given.

A sensitivity study on climate change illustrates potentially significant adverse effects on the region. Moreover, it shows that a regional integrated model is a suitable tool for complex and interdisciplinary studies. The present version of SIM still is in a development phase, especially in its representation of socio-economic processes.

A two-dimensional vertically integrated hydrodynamic model coupled to a particle tracking model is applied to study the dispersion processes and residence time in Ria de Aveiro lagoon (Portugal).

The only dispersion process that is considered in this study is the advection, according to the main characteristics of the local hydrodynamic. The particle tracking model computes the particles position at each time step, using a fourth-order Runge-Kutta integration method. The dispersion of passive particles released along the lagoon and in critical areas are studied in this work. The residence time is also determined for the entire lagoon.

The results show that the mixture between particles coming from different channels of the lagoon is negligible in a time scale higher than 2 tidal cycles. The residence time for the lagoon central area is about 2 days, revealing a strong marine influence in this area. At the upper reaches of the channels were found values higher than 2 weeks.

A numerical model that treats density-dependent variably saturated flow and miscible salt transport is used to investigate the occurrence of seawater intrusion in the Korba coastal plain of northeastern Tunisia. We examine the effects of and interplay between pumping, artificial recharge, soil/aquifer properties, and the unsaturated zone. The data processing steps undertaken in this study are briefly described, and a critical assessment is given of the data availability and of the requirements for successful monitoring and modeling of seawater intrusion risks in heavily exploited coastal aquifers such as those found in the semi-arid regions of the Mediterranean basin. An idea of the extent of over-exploitation of the Korba aquifer is obtained by examining the pumping and rainfall/infiltration data, and the simulation results support groundwater pumping as the mechanism for and seawater intrusion as the origin of the salt contamination observed in the soils and subsurface waters of the Korba plain.