共查询到20条相似文献，搜索用时 46 毫秒
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G.E. Wickens 《Journal of Arid Environments》1997,37(4):649-663
Desertification was reported in the Sahel during the higher rainfall regime of the 1950s and has become increasingly evident as human and livestock populations increase as a result of medical and veterinary care, which has created ever increasing population pressures for food, fodder, fuel, etc. The Great Drought of the mid 1960s and the ensuing lower rainfall has further aggravated the situation. With the help of NGO projects rural communities should be able to combat desertification and maintain a sustainable agriculture/livestock economy. The urban situation is undoubtedly extremely difficult if not beyond salvation. The future effects of climatic change is uncertain. 相似文献
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Little is known about the issues facing geographers working on fractional appointments in higher education institutions in the UK. This paper draws on a survey of UK geography departments and personal experiences to explore some of that territory for the first time. The purpose is both to set the agenda for institutional policy issues and to provide some immediate points of reflection for staff intending to take up fractional work. We consider the place of fractional working in the dominant discourses around paid work, problems of workload division and the impact on professional standing (including the RAE). We suggest that while personal 'life' reasons for fractional working are often positive, fractional workers in the academy risk professional marginalization. 相似文献
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《The Journal of geography》2012,111(3):150-156
Abstract The geography of Black America represents a legitimate and a distinctive sub-field of the discipline. While research time and attention is being focused increasingly on Black America much remains to be done to incorporate its study into introductory courses at both the secondary and college levels. Since many existing geography courses are inadequately structured to accommodate a meaningful analysis of Black America, it will be necessary to restructure them so that they become relevant not only to Black America but to emerging world-wide cultural patterns as well. The most logical and efficacious framework for such courses is one focused on the geography of cities. Courses dealing with urban topics can be devised so that major concepts of city structure can be presented as concomitants to topics dealing with the role and the contribution of the Negro in the city. 相似文献
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Leila De Floriani Paola Magillo 《International journal of geographical information science》2013,27(5):541-561
Abstract We present a model for describing the visibility of a polyhedral terrain from a fixed viewpoint, based on a collection of nested horizons. We briefly introduce the concepts of mathematical and digital terrain models, and some background notions for visibility problems on terrains. Then, we define horizons on a polyhedral terrain, and introduce a visibility model, that we call the horizon map. We present a construction algorithm and a data structure for encoding the horizon map, and show how it can be used for solving point visibility queries with respect to a fixed viewpoint. 相似文献
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The population structure, educational level and the livelihoods of 82 households of pastoral nomads, the organization of livestock husbandry and its impact on the grassland and forest ecosystems of the Dayan high valley (>2000 m a.s.l.) in the Mongolian Altai, western Mongolia, were surveyed using interviews and secondary information from official sources. Changes following the transition from centrally planned (before 1990) to market economy were analyzed. Two thirds of the monthly mean income of ca. 310 USD per nomad household is cash (ca. 55 USD) or non-cash (ca. 165 USD) income from livestock husbandry. Cashmere sale accounts for 70% of the cash income from livestock husbandry, which has led to a strong increase of goat numbers after 1990. Forests are used for livestock grazing, fuel wood collection, logging, and fruit collection. Livestock breeding and the seasonal migration of the nomad households are no longer organized by the government. To avoid transportation costs, two thirds of the families have reduced their seasonal migrations. This trend was favored by rising temperatures and earlier snowmelt during the last few decades, but resulted in a shortage of fodder and intensified forest use. Therefore, the use of grasslands and forests in the Mongolian Altai is no longer considered to be sustainable. 相似文献
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Summary. The ascent of a hot spherical body through a fluid with a strongly temperature-dependent viscosity has been studied using an axisymmetric finite element method. Numerical solutions range over Peclet numbers of 10−1 – 103 from constant viscosity up to viscosity variations of 105 . Both rigid and stress-free boundary conditions were applied at the surface of the sphere. The dependence of drag on viscosity variation was shown to have no dependence on the stress boundary condition except for a Stokes flow scaling factor. A Nusselt number parameterization based on the stress-free constant viscosity functional dependence on the Peclet number scaled by a parameter depending on the viscosity structure fits both stress-free and rigid boundary condition data above viscosity variations of 100. The temperature scale height was determined as a function of sphere radius. For the simple physical model studied in this paper pre-heating is required to reduce the ambient viscosity of the country rock to less than 1022 cm2 s−1 in order for a 10 km diapir to penetrate a distance of several radii. 相似文献
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The temperature distributions of different parts of a subgrade were analyzed based on the results of three years of monitoring data from the Harbin-Qiqihaer Passenger Dedicated Line, a high-speed railway, including the slope toes, shoulders, and natural ground. The temperature variation with time and the maximum frozen depths showed that an obvious sunny-shady effect exists in the railway subgrade, which spans a seasonal frozen region. Development of frost heave is affected by the asymmetric temperature distribution. The temperature field and the maximum frozen depths 50 years after the subgrade was built were simulated with a mathematical model of the unsteady phase transition of the geothermal field. 相似文献
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Borders are often portrayed in stark terms, perhaps as national‐scale threats, or as sites of suffering, or conversely as hosts to socio‐cultural symbiosis. Yet borders are many things all at once. In this paper, we use the comparative context of the US–Mexico border and the Mexico‐Guatemala border to critique what we call the ‘border as hegemony’, a borderscape constructed through obstructions, punitive policing and reinforcing the limits of state control. Instead, we propose a model of the ‘border as discord’. In our heuristic framework, diverse mobilities are embraced, interests of borderlanders are acknowledged and prioritized, and borders are interpreted not as a security threat but as a resource for change. 相似文献
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Karst landscapes are important assets that provide a source of water, host rare and endangered biota, and serve as sites of significant human historical artifacts. Previous studies have attempted to ascertain the extent of anthropogenic disturbance within karst areas using various standardized methods. A common limitation of these earlier approaches included lack of assessment of spatial variability. This paper recognizes the importance of spatial heterogeneity by using a spatially-explicit calculation of the Karst Disturbance Index (KDI) in a Geographic Information System (GIS) and applying it to a sub-catchment of the Biosfera de la Reserva Selve el Ocote in Chiapas, Mexico. With a raster-based GIS approach, this particular application is the highest resolution spatial representation of the KDI ever undertaken. As the study area is in a developing country, much of the data required for applying the KDI were drawn from remotely sensed data. The final disturbance scores found low to moderate levels of disturbance indicative of lightly populated area. However, this particular study can serve as a benchmark for future applications of the KDI in the study area, as well as in other karst watersheds in developed or developing countries. With the potential of future development looming, land managers can use the zones of pristine-to low-levels of disturbance to begin monitoring programs and justify mitigation and land management decisions. 相似文献
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Juan J. Ruiz F. Javier Ariza Elidia B. Blázquez 《International journal of geographical information science》2013,27(9):1439-1466
This article is centred on analysing the state of the art of the conflation processes applied to geospatial databases (GDBs) from heterogeneous sources. The term conflation is used to describe the procedure for the integration of these different data, and conflation methods play an important role in systems for updating GDBs, derivation of new cartographic products, densification of digital elevation models, automatic features extraction and so on. In this article we define extensively each conflation process, its evaluation measures and its main application problems and present a classification of all conflation processes. Finally, we introduce a bibliography which the reader may find useful to further explore the field. It tries to serve as a starting point and direct the reader to characteristic research in this area. 相似文献
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To provide a safe transportation system in an extremely cold region,evaluation needs to be conducted of the thickness and the volumetric water content of the active layer,as they significantly affect frost heave.The objective of this study was to evaluate the dielectric constant(κ)of the active layer using ground-penetrating radar(GPR)and a dynamic cone penetrometer(DCP);this evaluation was then used to estimate the thickness and the volumetric water content of the active layer.A field located in midwest Alaska was selected as the study site.A GPR survey and two DCP tests were conducted on the surface of the ground,and the ground temperature was measured.From the GPR survey,travel times of the electromagnetic wave in the active layer were obtained.In addition,the thickness of the active layer was determined by using the dynamic cone penetration index(DCPI)and ground temperature.By using the travel time and travel distance of the electromagnetic wave in the active layer,dielectric constants were calculated as 26.3 and 26.4 for two DCP points.From the mean dielectric constant,the volumetric water content was estimated to be 40%~43%,and the thickness of the active layer was evaluated along the GPR survey line.The spatial-scaled GPR image showed that the thickness of the active layer varied from 520 mm to 700 mm due to the presence of a puddle,which accelerated the heat exchange.The results show that evaluation of the dielectric constant using the GPR survey and the DCP test can be effectively used to estimate the thickness and the volumetric water content of the active layer. 相似文献
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N.N.Myagkov 《Geophysical Journal International》1998,133(1):209-211
A hydrodynamic model of a strong volcanic blast associated with large (caldera-forming) ash–gas-flow eruptions is presented. We show that the mass erupted from a vent influences considerably the damping of the shock wave. If the ejected mass flux is constant, the shock amplitude as a function of distance will decrease slower (in accordance with the inverse 4/3 power law) than that of an atomic explosion (the inverse cube power law). From the modelling, we extract a relation connecting the effective energy of a blast with the mass ejected during the blast phase. The mass released during the largest explosion of the 1883 eruption of Krakatau, and the lateral blast energy at Mount St. Helens on 1980 May 18 are estimated on the basis of the equation obtained. 相似文献