Axial convergence in a well-mixed estuary     

R.A. Nunes  J.H. Simpson 《Estuarine, Coastal and Shelf Science》1985,20(5):637-649

Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (～0.1 ms^?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators.  相似文献   

Qualitative study of motion under the potentials     

J. Casasayas  A. Jorba  A. Nunes 《Celestial Mechanics and Dynamical Astronomy》1987,42(1-4):129-139

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Quaternary geology of the Amazonian Lowland     

Georg Irion  Jens Müller  Jose Nunes de Mello  Wolfgang J. Junk 《Geo-Marine Letters》1995,15(3-4):172-178

The Quaternary history of the Amazon lowlands is characterized by deposition of sediments of Andean provenance and by the influences of changing sea levels. Areas well above the present water tables were not reached by Pleistocene high-water stages. These areas have been intensively weathered since the Tertiary, forming hard lateritic weathering horizons. These weathering horizons are best explained by the relatively constant, humid tropical climate throughout the Quaternary. In the western Amazonian Lowland, flood plains corresponding to the different Pleistocene sea-level heights were formed. During low sea level, erosion in the drainage areas increased and the water levels of the central Amazon River system were lowered. Valleys drowned and lakes formed in the lower reaches of rivers and creeks during high sea-level stages. These lakes (ria lakes) remained in the valleys with rivers having a low sediment load. Seismic profiling (3.5 kHz) in some of these lakes clearly showed deposits of the three last periods of Quaternary high sea-level stages.  相似文献   

Genericity of the non-periodic solutions of the central force problem     

Ana Nunes  Josefina Casasayas 《Astrophysics and Space Science》1990,165(1):95-99

We study the classical problem of two-dimensional motion of a particle in the field of a central force proportional to a real power of the distancer. for negative energy and (0, 2), each energy levelI _h is foliated by the invariant toriI _hc of constant angular momentumc and, by Liouville-Arnold's theorem, the flow on eachI _hc is conjugated to a linear flow of rotation number _h (c).A well-known result asserts that if we require _h (c) to be rational for every value ofh andc, the, must be equal to one (Kepler's problem). In this paper we prove that for almost every (0, 2) _h (c) is a non-constant continuous function ofc, for everyh<0. In particular, we deduce that motion under central potentials is generically non-periodic.Partially supported by CIRIT under grant No. EE88/2.  相似文献   

Limiting the soil degradational impacts of wildfire in pine and eucalyptus forests in Portugal: A comparison of alternative post-fire management practices     

Richard A Shakesby  David J Boakes  Celeste de OA Coelho  AJ Bento Gonçalves  Rory PD Walsh 《Applied geography (Sevenoaks, England)》1996,16(4):337-355

In newly burnt and unburnt pine and eucalyptus forest in Portugal, overland flow and soil losses were monitored to assess the impacts of the following post-fire treatments: application of different quantities of logging litter; rip-ploughing compared with minimum tillage prior to planting eucalyptus seedlings; and clearance of pine needles and vegetation. Eucalyptus logging litter reduced soil losses by up to 95 per cent. The impact of pine logging litter was equivocal, but removal of pine needles increased soil losses elevenfold. Implications for soil longevity, soil quality and land management strategy are discussed.  相似文献   

Some Morphological Aspects and Hydrological Characterization of the Tagus Floods in the Santarém Region, Portugal   总被引：1，自引：0，他引：1  

T. M. Azevêdo  E. Nunes  C. Ramos 《Natural Hazards》2004,31(3):587-601

In Portugal, few studies have been made of the historical floods of the RiverTagus (the longest in the Iberian Peninsula). This fact led to the study of theLower Tagus (Santarém region) using written historical documents,cartographic documents and hydrological data, consisting mainly of waterlevel records. With the support of the historical documents and the analysisof all the maps, it was possible to verify that the human intervention has beenfundamental in the morphological changes of the Tagus' channel. It becameaware that the river changed from a braided to a single channel with alternatebars. From the hydrological data, return periods for the floods were determined,using the Pearson Type 3 distribution. The selection of the most important floods,from 1855 to 1998, enabled to build a ``flood hazard scale' for this region.  相似文献   

Assessing the effects of land cover and future climate conditions on the provision of hydrological services in a medium‐sized watershed of Portugal          下载免费PDF全文

Claudia Carvalho‐Santos  João Pedro Nunes  António T. Monteiro  Lars Hein  João Pradinho Honrado 《水文研究》2016,30(5):720-738

The separated and combined effects of land‐cover scenarios and future climate on the provision of hydrological services were evaluated in Vez watershed, northern Portugal. Soil and Water Assessment Tool was calibrated against daily discharge, sediments and nitrates, with good agreements between model predictions and field observations. Four hypothetical land‐cover scenarios were applied under current climate conditions (eucalyptus/pine, oak, agriculture/vine and low vegetation). A statistical downscaling of four General Circulation Models, bias‐corrected with ground observations, was carried out for 2021–2040 and 2041–2060, using representative concentration pathway 4.5 scenario. Also, the combined effects of future climate conditions were evaluated under eucalyptus/pine and agriculture/vine scenario. Results for land cover revealed that eucalyptus/pine scenario reduced by 7% the annual water quantity and up to 17% in the summer period. Although climate change has only a modest effect on the reduction of the total annual discharge (?7%), the effect on the water levels during summer was more pronounced, between ?15% and ?38%. This study shows that climate change can affect the provision of hydrological services by reducing dry season flows and by increasing flood risks during the wet months. Regarding the combined effects, future climate may reduce the low flows, which can be aggravated with eucalyptus/pine scenario. In turn, peak flows and soil erosion can be offset. Future climate may increase soil erosion and nitrate concentration, which can be aggravated with agriculture scenario. Results moreover emphasize the need to consider both climate and land‐cover impacts in adaptation and land management options at the watershed scale. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Influence of prey abundance and abiotic factors on the long-term home-range and movement dynamics of spotted grunter Pomadasys commersonnii in an intermittently open estuary     

BA Maree  PD Cowley  TF Næsje  A-R Childs  AI Terörde  EB Thorstad 《African Journal of Marine Science》2016,38(2):171-180

Long-term home-range and movement dynamics of spotted grunter Pomadasys commersonnii and the distribution of their dominant prey, the sandprawn Callichirus kraussi, were investigated in the intermittently open East Kleinemonde Estuary, South Africa. In addition, the effects of fish length, diel cycle, season and other abiotic factors on home range and area use were examined. Nine adult spotted grunter (326–489?mm total length) were tagged with acoustic transmitters, and their movements were studied by manual tracking on six consecutive days and nights during each of five tracking sessions from March to November 2004. In addition, movement patterns were recorded by stationary data-logging receivers. Kernel home ranges (95% utilised distribution) varied in size (mean: 76 797?m²?, range: 26 296–165 321?m²), but were all located in the lower one-third of the 3.6 km-long estuary. There was no effect of season, the diel cycle or fish length on the home-range estimates. However, the distribution of prey corresponded with the home-range distribution of tagged fish. Although sandprawn distribution was highly variable within and among transects and depth classes, sandprawns were most abundant in the lower region of the estuary, which corresponded to the home-range distribution of spotted grunter. Spatial distribution of prey appears to be a dominant factor influencing home-range parameters of this species within an intermittently open estuary.  相似文献   

Seismic hazard analysis in zones of time and space interdependence: An application to São Miguel Island,Azores     

C. S. Oliveira  A. C. Costa  V. H. Forjaz  J. C. Nunes 《Natural Hazards》1990,3(1):15-29

The Azores Islands are located in the mid-Atlantic region near the triple junction where the Euro-Asiatic-African-American plates join together. Seismic activity in the area is very high, as can be observed either from historical events since the fifteenth century, from present day microseismicity, and from direct and indirect measurements of recent tectonic deformation. Volcanic activity is also present throughout the region. All available information, even data exhibiting low quality, was used to develop hazard models of São Miguel Island. Source zones were established based on both the global tectonic behaviour of the region and on the local active fault structures. Recurrence laws for São Miguel Island, for which historical information seemed quite incomplete, were obtained from the large events in the entire archipelago and from their remarkable pattern of time and space dependence, and complimented by information on long-term fault deformation (for the longer recurrence periods) and on high precision instrumental network (for the very short recurrence periods). Attenuation laws were derived from data on events felt and/or recorded in the Island. Hazard maps were obtained through a modified version of McGuire's algorithm for several geometries of source areas and results compared with the maximum observed intensity of historical events.  相似文献   

The early twentieth century Arctic high — fact or fiction?     

PD Jones 《Climate Dynamics》1987,1(2):63-75

Reconstructions of monthly mean sea-level grid-point pressure from station pressure data over the Arctic region using an objective technique have revealed two serious errors in the gridded data. First, all gridded data (from the U.S. Historical Weather Map series) for the Arctic Ocean area away from the North Atlantic sector appears to be between 4 and 6 mb too high before about 1931. The semi-permanent Arctic high produced in the early historical map series was the result of a lack of basic station data, and the belief amongst many North American meteorologists of the 1920s and 1930s of the existence of a polar or glacial anticyclone. The second error affects only north-western North America for the years 1899–1909. Here pressure is again too high compared with reconstructed data. The cause of this error is not immediately apparent, but is probably the result of an erroneous correction to sea level for stations in the region. Both errors seriously affect the homogeneity of the gridded Arctic series. The objective scheme used to reveal the errors, can also be used to derive correction factors for the gridded data prior to 1931.  相似文献