首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5248篇
  免费   531篇
  国内免费   297篇
测绘学   1047篇
大气科学   283篇
地球物理   822篇
地质学   1473篇
海洋学   1105篇
天文学   12篇
综合类   475篇
自然地理   859篇
  2024年   8篇
  2023年   17篇
  2022年   71篇
  2021年   124篇
  2020年   157篇
  2019年   154篇
  2018年   128篇
  2017年   256篇
  2016年   200篇
  2015年   234篇
  2014年   297篇
  2013年   384篇
  2012年   306篇
  2011年   347篇
  2010年   308篇
  2009年   316篇
  2008年   362篇
  2007年   351篇
  2006年   340篇
  2005年   280篇
  2004年   239篇
  2003年   173篇
  2002年   190篇
  2001年   153篇
  2000年   118篇
  1999年   137篇
  1998年   83篇
  1997年   79篇
  1996年   40篇
  1995年   46篇
  1994年   34篇
  1993年   33篇
  1992年   17篇
  1991年   13篇
  1990年   15篇
  1989年   11篇
  1988年   16篇
  1987年   9篇
  1986年   4篇
  1985年   7篇
  1984年   9篇
  1983年   3篇
  1982年   3篇
  1981年   1篇
  1978年   1篇
  1977年   1篇
  1971年   1篇
排序方式: 共有6076条查询结果,搜索用时 15 毫秒
951.
In this commentary, we wish to draw from Kathryn's experience in Askole to complicate the argument we developed in favor of an autoethnographic sensibility in the earlier article in this issue (Butz and Besio 2004 ButzDavid KathrynBesio 2004 The value of autoethnography for field research in transcultural settings. The Professional Geographer 56 3 350 60 [Google Scholar]). Just as we used David's first-person singular voice in much of that article, we use Kathryn's voice here to reflect the central influence of her research circumstances on the points that we make. We will return to the first-person plural voice in the commentary's conclusion where we attempt to synthesize the lessons of our two sets of research experiences.  相似文献   
952.
The antiquated and undemocratic rules operating in the United Nations Security Council do not reflect and serve today’s integrated global society, nor does the institution adequately represent the diversity of member states. Despite General Assembly discussions that have lasted a quarter-century, no result has materialized on the expansion issue. The author concludes that regional rivalries, the veto, the onerous Charter amendment process, and the failure to devise a truly representative model are responsible for the lack of progress. A model with eighteen rotating seats, based on population, area, and regional position, and four elected seats deserves consideration as an alternative. Key Words: metageography, political geography, population and area ratings, United Nations Security Council.  相似文献   
953.
The northeastern Gulf of Mexico contains some of the most diverse and productive marine habitat in the United States. Much of this habitat, located on the shelf edge in depths of 50 to 120 m, supports spawning for many economically important species, including groupers. Here, we couple acoustic surveys with georeferenced videography to describe the primary spatial and geologic features of spawning aggregation sites for four economically important species: gag (Mycteroperca microlepis), scamp (M. phenax), red grouper (Epinephelus morio), and red snapper (Lutjanus campechanus), with notes on fish distribution and abundance and spawning activities. We provide information on movement patterns of reef fish determined using acoustic telemetry. Finally, we discuss the possible coupling of geomorphology with hydrographic features to influence the overall productivity of the region and the importance of spatial fishery management in sustaining that productivity.  相似文献   
954.
Abstract

Zionist geographers in Israel were highly involved in institutionalizing Israel's national identity. In search for interpretative schemes, they were highly committed both to Western paradigms and to Zionist narratives. Until the 1970s, organicist models of regional geography were popular despite contradictions between the use of tragic and comic narratives in Western models and the need to adopt romantic narratives for Zionist purposes. The emergence of historical geography in the 1950s helped to solve these contradictions by adopting a contextual mode of explanation in which the national story has been described in terms of a unique one and as an example for the power of Zionist civilization to transform bewildered nature. During the last two decades, new forms of criticism have emerged that changed the discourse from a particularistic story of nation building to a more general story of colonization and territorial control.  相似文献   
955.
Investment returns are produced by combining financial assets with human capital, the decision-making protocols of investment houses, and the electronic infrastructure that supports the flow of information about investment. At the center of the production process stand senior managers; their power and authority in the production process is fundamental to the performance of investment organizations. This article provides a model of the production of investment returns in financial centers and spatially extensive financial markets. We begin with Coase's theory of the firm but go beyond models of the firm that represent commodity-producing industries. Having substantiated the model of investment management, it is applied to institutions that seek investment returns in geographically extensive financial markets. Operating in financial markets at a distance from home jurisdictions is an increasingly important aspect of investment management. Given recent turmoil in global financial markets, financial houses have sought higher rates of return from markets in which they have little direct experience. At issue is how different types of financial organizations (large and small, growing and declining) produce investment returns in this new environment.  相似文献   
956.
To examine the influence of buffelgrass land conversion and pasture management on native species diversity and regeneration patterns, we describe community attributes and population structure in four different active pastures in thornscrub vegetation from eastern Sonora, Mexico. We compare a relatively undisturbed thornscrub community with a contiguous five years old active pasture, to identify species able to regenerate under current management practices. Buffelgrass conversion has a significant influence on species diversity and other community attributes. Active pastures have lower species diversity, crown cover and basal area of native species than the studied native thornscrub. An active pasture subjected to heavy grazing and without management of the thorny legume, Acacia cochliacantha, showed the lower species diversity and an almost monospecific stand of this legume. It is likely that the reduction in species diversity is caused by the conversion process, inadequate management and regeneration barriers that limit seedling establishment in active pastures. Our data recorded eight native species (13%) that were able to regenerate in active pastures. However, the great majority of native species were unable to regenerate under pastures. This study shows that under current management, active buffelgrass pastures maintain only a small fraction of native species with regenerating populations in the thornscrub.  相似文献   
957.
Rates of soil disruption from hikers and vehicle traffic are poorly known, particularly for arid landscapes. We conducted an experiment in Organ Pipe Cactus National Monument (ORPI) in western Arizona, USA, on an air-dry very fine sandy loam that is considered to be vulnerable to disruption. We created variable-pass tracks using hikers, an all-terrain vehicle (ATV), and a four-wheel drive vehicle (4WD) and measured changes in cross-track topography, penetration depth, and bulk density. Hikers (one pass = 5 hikers) increased bulk density and altered penetration depth but caused minimal surface disruption up to 100 passes; a minimum of 10 passes were required to overcome surface strength of this dry soil. Both ATV and 4WD traffic significantly disrupted the soil with one pass, creating deep ruts with increasing passes that rendered the 4WD trail impassable after 20 passes. Despite considerable soil loosening (dilation), bulk density increased in the vehicle trails, and lateral displacement created berms of loosened soil. This soil type, when dry, can sustain up to 10 passes of hikers but only one vehicle pass before significant soil disruption occurs; greater disruption is expected when soils are wet. Bulk density increased logarithmically with applied pressure from hikers, ATV, and 4WD.  相似文献   
958.
This paper traces three key phases in Newfoundland's resource economy: the collapse of cod, the rise of shellfish and recent efforts to re-establish cod. I argue that these changes in the province's fish economy may be productively understood through the ‘new resource geography’. The paper explores three themes: the relationship between knowledge practices and resource management; the way in which fish resources are defined in both material and discursive forms; and the different sites and institutions involved in the regulation of the cod resource.  相似文献   
959.
As managers and researchers of protected natural areas continue to seek balance in promoting visitor use but limiting negative experiential and natural resource impacts, the integration of social and physical spatial data may play a critical role in understanding how visitors and the community interface with the landscape within these protected areas. These spatial considerations are important for inventorying, monitoring, and managing the conditions of natural resources within parks. Specifically related to the visitor impacts of natural resources in parks are the use and condition of multiple-use trails. Technology such as GPS and GIS may allow for a unique assessment of the relationship between factors that influence this resource. This paper focuses on how visitor use distribution (measured through GPS tracking), activity type, and trail design influence the impacts to trail conditions. This paper also addresses statistical concerns related to spatial dependency. Results suggest that failure to account for spatial dependency can lead to erroneous Type I findings. Additionally, activity type (specifically horseback riders) and trail design were found to best predict trail impacts when controlling for spatial dependency.  相似文献   
960.
In the Rhine-Meuse delta in the south-western part of the Netherlands,the morphology of the river branches is highly dependent on the erodibility of the subsoil.Erosion processes that were initiated after closure of the Haringvliet estuary branch by a dam(in 1970),caused a strong incision of several connecting branches.Due to the geological evolution of this area the lithology of the subsoil shows large variations in highly erodible sand and poorly erodible peat and clay layers.This study shows how the geological information can be used to create 3D maps of the erodibility of the sub-soil, and how this information can be used to schematize the sub-soil in computational models for morphological simulations.Local incisement of sand patches between areas with poorly erodible bed causes deep scour holes,hence increasing the risk on river-bank instability(flow slides) and damage to constructions such as groynes,quays,tunnels, and pipelines.Various types of mathematical models,ranging from 1D(SOBEK) to quasi-3D(Delft3D) have been applied to study the future development of the river bed and possible management options.The results of these approaches demonstrate that models require inclusion of a layer-bookkeeping approach for sub-soil schematization, non-uniform sediment fractions(sand-mud),tidal and river-discharge boundary conditions,and capacity-reduction transport modeling.For risk-reducing river management it has been shown how the development of the river bed can be addressed on a large scale and small scale.For instance,the use of sediment feeding and fixation of bed can be proposed for large-scale management,while monitoring and interventions at initiation of erosion can be proposed as response to small-scale developments that exceed predefined intervention levels.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号