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11.
E. I. Ducheyne R. R. De Wulf B. De Baets 《International journal of geographical information science》2013,27(8):917-928
Forest‐management decision‐support systems are largely monolithic structures. Spatial details are left out during the optimization process and are elaborated during the operational planning. This might produce misleading results and plans that are impossible to implement. In this paper, a forest‐management spatial decision‐support systems is presented, in which spatial formulation needed for wildlife models is included during the optimization process. To this end, a multiple‐objective genetic algorithm is combined with a geographical information system. An online spatial evaluation of the objective functions is made possible. This is illustrated by a pilot study in Kirkhill forest, Aberdeen. 相似文献
12.
Hyperspectral airborne remote sensing images and in-situ data are combined to assess the spatial and temporal sediment dynamics in the tidal Scheldt river. A log-linear empirical relationship has been developed between a near-infrared reflectance difference and total suspended matter. The relationship was shown to be relatively insensitive to the varying cirrus cloud cover occurring during data acquisition. The produced sediment maps show good agreement with known variations of turbidity over the tidal cycle: maximum turbidity around high water, gradual settling of the sediment in the succeeding slack water and resuspension at the onset of the ebb flow stage. 相似文献
13.
Effects of slope angle and aspect on plant cover and species richness in a humid Mediterranean badland 总被引：1，自引：0，他引：1 
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下载免费PDF全文 Estela Nadal‐Romero Kristien Petrlic Els Verachtert Esther Bochet Jean Poesen 《地球表面变化过程与地形》2014,39(13):1705-1716
Soil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands occupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi‐arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south‐facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant colonization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions. The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south‐facing slopes, which contrasts with the results obtained in semi‐arid badlands. North‐facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope‐aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
14.
Retrieval of Suspended Sediment from Advanced Hyperspectral Sensor Data in the Scheldt Estuary at Different Stages in the Tidal Cycle 总被引：1，自引：0，他引：1
Hyperspectral airborne remote sensing images and in-situ data are combined to assess the spatial and temporal sediment dynamics in the tidal Scheldt river. A log-linear empirical relationship has been developed between a near-infrared reflectance difference and total suspended matter. The relationship was shown to be relatively insensitive to the varying cirrus cloud cover occurring during data acquisition. The produced sediment maps show good agreement with known variations of turbidity over the tidal cycle: maximum turbidity around high water, gradual settling of the sediment in the succeeding slack water and resuspension at the onset of the ebb flow stage. 相似文献
15.
Different estimates were used to assess the diversity of the total macrofauna and its major taxonomic groups separately from a broad bathymetric range at a site in the NE Atlantic. In the Goban Spur region, a transect was sampled from the shelf to the abyssal plain over a depth range from 200 to 4500 m and in the Porcupine Sea Bight two stations were sampled (at 3670 m and 4115 m). Species diversity (the number of species per number of individuals) increased with increasing water depth, both when expressed as Hurlbert's E(Sn) and as Shannon's H′log e. The expected number of species in a 100-individual sample E(S100) of total macrofauna increased from 30 on the shelf to 68 on the abyssal plain. Evenness (the proportional abundance of species), estimated with Shannon's J′, also increased with water depth from 0.66 to 0.91, whereas dominance (Simpson's D) decreased from 0.09 to 0.01. Species richness (the number of species per unit of area), however, showed a parabolic pattern with a peak at the upper slope. The largest number of species was found at the slope station at 1425 m (232 species within 0.66 m2). It is argued that species richness is not a synonym of species diversity, but that species richness depends both on species density (which decreases with increasing water depth) and on species diversity. Across the whole bathymetric range (200 to 4500 m) a total of 696 species within 8327 specimens in a total sampled area of 4.12 m2 were counted, yielding mean values of 12 individuals per species and 169 species per m2. Different communities were found to exist on the shelf, slope and abyss. It is suggested that this could have been caused by different selection processes. Differences in life-history strategies and organic-matter supply could (at least partly) explain the different community structures and diversity patterns found along the depth gradient. 相似文献
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17.
The transit time of a pulsar signal is influenced by the density fluctuations of the interstellar plasma. This effect is calculated in the one-dimensional and spherically symmetrical case, using a WKB-type approximation for the transverse electric field. The influence is found to be small compared to Lerche's result, and the difference is explained by the fact that Lerche's formula refers to a mean signal.This work has been undertaken as part of the joint research programme of the Institut für Plasmaphysik and Euratom. 相似文献
18.
B. G. Els 《Sedimentary Geology》1998,120(1-4):205-224
Studies of the auriferous Witwatersrand placers and associated rocks have revealed that certain palaeo-environments, especially braided fluvial, are particularly well represented in this part of the rock record. However, there is a paucity of lithofacies indicating certain other palaeo-environments. Possible reasons for their scarceness or absence are suggested in this paper. It is generally assumed that pre-vegetational fluvial systems would have been characteristically braided, because of the absence of land plants necessary to stabilise river banks. A question arising is what kind of downstream changes such pre-vegetational braided streams underwent. A recent study of a braided stream placer revealed that the depositing system retained its braided character right down to the palaeo-shoreline. However, gravel did not reach the palaeo-beach. As with many modern examples, beach conglomerates are rare in the Witwatersrand rocks. The paucity of conglomeratic beach placers is ascribed to the low probability of gravel being transported across coastal plains, because of the relatively low slopes or depths of rivers here. The Witwatersrand fluvial channels are generally considered to have had high width-to-depth ratios, because of the absence of land vegetation to stabilise channel banks. However, two examples of deep, relatively narrow scour features, with predominantly fine-grained fills, occur near the base of the Central Rand Group. The low width-to-depth ratios of these scour features, which probably represent palaeo-valleys, are ascribed to severe incision during a rapid sea-level fall. The auriferous fluvial systems of the Witwatersrand are generally considered to have been entirely braided, due to the lack of bank stability. However, the fluvial B placer of the Welkom goldfield is confined to discrete channels. Their banks are thought to have been stable, due to the cohesive nature of the lutite into which the gravelly streams incised. In addition, braiding sensu stricto was probably inhibited by initial incision and a low sediment supply. No deposits of specifically deltaic sub-environments have been found in the Central Rand Group. Their absence is attributed to the following factors: (a) the paucity (absence?) of well-defined palaeo-river mouths; (b) low concentrations of suspended sediment; (c) intermittent sediment supply to the palaeo-coastline; and (d) reworking by tidal and longshore currents. Alluvial-fan deposits are also apparently absent in the Witwatersrand rocks. The absence of fan deposits is attributed to the poor potential for development and preservation of fan deposits in the compressive tectonic setting proposed for the Witwatersrand Basin. Although ventifacts have been found in the Witwatersrand rocks, no aeolianites have been reported. Their apparent absence is probably due to (a) reworking in a predominantly humid climate, and (b) transport of sand by dominant winds to areas unfavourable for the preservation of aeolianites. 相似文献
19.
Abstract. The cockle Cerastoderma edule is one of the large and important species of the tidal flats of the Wadden Sea. C. edule disturbs the upper sediment layer due to its crawling and “shaking” behaviour. About 7% of a cockle population move within one week. The mean distance of movement is -4 cm per week, which, in the case of a 2-year-old cockle with a length of ?3 cm, means that ?20 cm2 of sediment is disturbed to a depth of ?3 cm. Besides these measurable movements, cockles also regularly “shake” themselves, thereby disturbing the sediment around them to at least ?0.5 cm. At a density of 500 cockles per m2 of a mean length of 3 cm, ?16% of the sediment surface is occupied by cockles and another ?29% is disturbed by cockles within one week. To study the effects of cockles on recruitment of other benthic species, densities of 0, 125, 250, 375 and 500 cockles (mean length 3 or 3.5 cm) were added to 1-m2 plots within large depopulated squares in April. Subsequently, sediment samples were sieved once a month (from June to September) and the densities of all macrozoobenthic species were estimated. The presence of cockles significantly reduced the densities of juveniles of the bivalve species C. edule, Macoma balthica, Mya arenaria, Tellina (Angulus) tenuis, and Ensis directus and of the worm species Pygospio elegans, Lanice conchilega, Eteone longa, Anaitides spec., Nephtys hombergii, Heteromastus filiformis, Scoloplos armiger, and Tharyx marioni and of the amphipods Corophium volutator and C. arenarium. Non-significant effects were found only in Capitella capitata and Nereis diversicolor. For all species mentioned above a negative effect was already observed at the lower cockle densities of 125 - 250 per m2. Reductions of about 50% or more were found at densities of 500 per m2, which was more than could be expected based on the area occupied by the cockles (?16%), but close to the area occupied plus disturbed (?45%). It is concluded that (dense) assemblages of adult cockles can have a strong influence on the macrozoobenthic community. 相似文献
20.
A sedimentological study of the early Proterozoic Black Reef Quartzite Formation in the south-western parts of the Transvaal province of South Africa was undertaken with the primary aim of examining the sedimentological controls of gold mineralization in the Black Reef placer, which occurs at the base of this Formation. A second aim of the study was to investigate the early history of the basin in which the Transvaal Sequence of South Africa was deposited. The thin, siliciclastic Black Reef Quartzite Formation, which is informally subdivided into a lower Conglomerate Unit and an upper Quartzite Unit, is underlain by Archaean rocks belonging to the basement complex and the Witwatersrand and Ventersdorp Supergroups, and is overlain by a thick succession of carbonate rocks of the Malmani Subgroup. The pre-Transvaal palaeosurface is characterised by elongated northeast to southwest trending grabens and partly-eroded horst blocks. The Black Reef Quartzite Formation, which has a maximum thickness of about 30 m in the study area, typically comprises a succession of interbedded arenites and mudstones, with a sporadically-developed basal Conglomerate Unit. Thickness trends are similar to the dominant structural trend of the pre-Transvaal palaeosurface. At localities where the Conglomerate Unit is absent, the Formation invariably overlies quartzites of the Witwatersrand Supergroup directly. The palaeocurrent distribution of the Conglomerate Unit is unimodal, with modes towards the southwest in the southern parts of the study area and towards the north in the northern regions. Most of the palaeocurrent distributions of the Quartzite Unit are unimodal, too, but bimodal distributions were found at three localities. Pebble size of the Black Reef placer is largest in the northeastern parts of the study area, but no orderly lateral size variation was found. Pebble roundness, too, varies greatly and apparently randomly. The composition of the pebble assemblage is not constant, but no systematic lateral change could be detected. A petrographic study of the arenites of the Formation reveals a remarkable textural and mineralogical maturity, especially for the upper beds. It is concluded that the pre-Transvaal palaeosurface had a palaeorelief of up to 30 m and that the topography of the palaeo-landscape was the dominant factor controlling early sedimentation in the basin. The palaeo-grabens probably constituted the valleys of shallow braided stream systems that drained south-westwards and northwards from a palaeo-drainage divide in the northern parts of the study area. Sediment, including detrital gold, was derived from erosion of Witwatersrand rocks and fed to the graben valleys via several alluvial fans. During a subsequent transgression, the fluvial systems became drowned and transgressive estuarine conditions ensued. During the final stages of siliciclastic sedimentation, the upper quartzite beds of the Formation were probably reworked by shallow marine processes before carbonate precipitation commenced. The cause of the marine transgression is not known beyond doubt. It is suggested, however, that lithospheric rifting, which initiated the extrusion of the underlying Ventersdorp lavas, resumed during early Transvaal times, resulting in complete severing of the continental crust and the creation of a linear sea. 相似文献