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   1153篇
  免费   188篇
  国内免费   126篇
测绘学   98篇
大气科学   313篇
地球物理   302篇
地质学   477篇
海洋学   107篇
天文学   1篇
综合类   58篇
自然地理   111篇
  2024年   7篇
  2023年   12篇
  2022年   25篇
  2021年   28篇
  2020年   29篇
  2019年   31篇
  2018年   15篇
  2017年   40篇
  2016年   42篇
  2015年   50篇
  2014年   79篇
  2013年   58篇
  2012年   65篇
  2011年   64篇
  2010年   63篇
  2009年   76篇
  2008年   89篇
  2007年   63篇
  2006年   85篇
  2005年   83篇
  2004年   71篇
  2003年   51篇
  2002年   66篇
  2001年   42篇
  2000年   31篇
  1999年   41篇
  1998年   34篇
  1997年   33篇
  1996年   18篇
  1995年   13篇
  1994年   17篇
  1993年   15篇
  1992年   12篇
  1991年   7篇
  1990年   2篇
  1989年   4篇
  1988年   1篇
  1987年   1篇
  1984年   1篇
  1983年   2篇
  1982年   1篇
排序方式: 共有1467条查询结果,搜索用时 31 毫秒
971.
Previous studies of neustonic debris have been limited to surface sampling. Here we conducted two trawl surveys, one before and one shortly after a rain event, in which debris and zooplankton density were measured at three depths in Santa Monica Bay, California. Surface samples were collected with a manta trawl, mid-depth samples with a bongo net and bottom samples with an epibenthic sled, all having 333 micron nets. Density of debris was greatest near the bottom, least in midwater. Debris density increased after the storm, particularly at the sampling site closest to shore, reflecting inputs from land-based runoff and resuspended matter. The mass of plastic collected exceeded that of zooplankton, though when the comparison was limited to plastic debris similar to the size of most zooplankton, zooplankton mass was three times that of debris.  相似文献   
972.
Different levels of eutrophication and pollution in the Baltic coastal waters in general, and in estuaries particularly define variations in the buffering capacity of ecosystems of these important filter zones between rivers and the adjacent Baltic Proper. Phyto- and zooplankton in the estuaries form important components of the pelagic food webs that participate in producing and structuring the matter, energy, and information fluxes in the ecosystems. Due to salinity gradients, estuaries provide a large variety of aquatic habitats for native populations of marine, brackish and fresh water plankton species. Non-indigenous planktonic species that expand their bio-geographical range and invade the Baltic coastal ecosystems also find favourable environments, establish permanent populations there, and impact aboriginal ecosystems. The dynamics of abundance, species diversity, population structure, productivity of planktonic communities, and peculiarities of trophic interactions in different Baltic estuaries are influenced by a number of common environmental factors. Being measured on the unified basis and monitored, changes (or stability) of structural and functional parameters of plankton communities can serve as indicators of alterations to the ecosystems. Comparative analyses and quantitative estimates of long-term changes in phyto- and zooplankton can contribute to evaluation of functional response of Baltic coastal ecosystems to anthropogenic stress. Both theoretical (modeling) and empirical studies of interactions within plankton communities aimed at the assessment of the role of planktonic organisms in water quality regulation, and applicability of these data for the evaluation of the status of ecosystems and their management can be considered as major goals for the future investigations in Baltic estuaries.  相似文献   
973.
Various human activities threaten the groundwater quality and resource under urban areas, and yet residents increasingly depend on it for their livelihood. The anticipated expansion of the worlds urban population from 3 to 6 billion in the coming 50 years does not only pose a large water management threat but also provides an opportunity to conserve groundwater in a better way than up to now. The authors argue for a new way to manage urban activities in order to conserve the precious groundwater resource. The focus is on the quality of the discharged water after use in households. Restrictions on what is added to water while using it, e.g. detergents, excreta, paint residues, oils, and pharmaceuticals, are important to simplify the treatment and reuse of used water. Avoiding mixing different wastewater flows has the same positive effect. If increased volumes of wastewater can be treated and reused, the demand on the groundwater resource is reduced, as also occurs with demand management measures. Reduced discharge of polluted water to the environment from households and utilities also conserves the quality of groundwater and reduces sophisticated treatment costs.
Resumen La urbanización lleva a una demanda elevada y concentrada de agua de calidad adecuada, acompañada por el vertido de volúmenes análogamente mayores de aguas residuales. Los alimentos se importan a ciudades donde los microorganismos y nutrientes de los excrementos humanos son descargados a ríos, lagos y, también, aguas subterráneas. Más aún, gran número de los bienes de consumo son eliminados vía las tuberías de las cloacas. Las deudas medioambientales, es decir, el empobrecimiento de las condiciones medioambientales, que requerirán de aportaciones humanas y económicas para rehabilitarlas, son comunes a todas las ciudades, sobretodo en el Hemisferio Sur, donde, según la Valoración Global de Suministro de Agua y Saneamiento (OMS y UNICEF), la mayor parte de las aguas residuales urbanas no son tratadas (65% en Asia, 86% en Latinoamérica, 100% en África). La tarea pendiente consiste en proteger los recursos de aguas subterráneas para que estén disponibles de forma rápida para los habitantes urbanos también el futuro. En los próximos 50 años, se espera que la población urbana aumente de 3,000 a 6,000 millones de personas, según estimaciones de las Naciones Unidas, por lo que es imperativo dejar de contaminar las aguas subterráneas existentes bajo las nuevas áreas urbanas en construcción. En este artículo, se escudriña la calidad de las aguas subterráneas y su protección a la luz de los cambios recientemente introducidos en las políticas del agua, esto es, desde la gestión de los abastecimientos de agua a la gestión de la demanda y, eventualmente, hastaa la gestión de la reutilitzación.

Résumé Lurbanisation conduit à une demande élevée et concentrée deau de qualité adéquate, accompagnée du rejet dimportants volumes correspondants deaux usées. La nourriture est importée dans les villes tandis que les micro-organismes et les nutriments provenant des excrétas humains sont rejetés dans les rivières, les lacs et aussi les eaux souterraines. De plus, une large gamme de biens de consommation est évacuée par les égouts. Les créances environnementales, cest-à-dire lappauvrissement des conditions environnementales qui demandera des apports humains et économiques pour la réhabilitation, sont habituelles dans toutes les villes, et pas seulement dans lhémisphère sud, comme cela est indiqué dans le rapport sur lalimentation en eau et la santé publique du monde (publié par lOMS et UNICEF), établissant que la plupart des eaux résiduaires des zones urbaines reste non traitée (65% en Asie, 86% en Amérique latine et 100% en Afrique). La tâche à réaliser pour lhomme est de protéger les ressources en eau souterraine en sorte quelle reste disponible pour les habitants des villes dans le futur. Dans les prochaines 50 années, il faut sattendre à ce que la population urbaine saccroisse de 3 à 6 milliards de personnes selon les estimations des Nations Unies; aussi il est impératif de ne pas continuer à polluer les eaux souterraines sous les nouvelles zones urbaines en cours de construction. Dans cet article, nous analyserons la qualité des eaux souterraines et leur protection à la lumière des récents changements de politique de leau, cest-à-dire de la gestion de lalimentation en eau jusquà une gestion de la demande et en abordant éventuellement la période de gestion du recyclage.
  相似文献   
974.
李洋 《世界地理研究》2006,15(4):6-10,22
在当今全球化的文明社会里,还生存着数以亿计现代奴隶,其绝对数远远超过了200年前的黑奴,呈现出多元化、国际化等特征,加大了打击的难度,要铲除现代社会的奴隶现象,应采取建立国际经济新秩序,消除国家间贫富差距;加强国际合作,加大国际人口走私的打击力度;健全相关法规政策,加大执法力度等对策。  相似文献   
975.
The Pb contents and 206Pb/207Pb and 208Pb/206Pb isotopic ratios were studied in the soils and wines (2004 harvest) of three vineyard areas of the Czech Republic. The areas differ in their geological basements and anthropogenic loading. The isotopic compositions of wine in areas with intensive industry (Most, North Bohemia 206Pb/207Pbwine = 1.178 ± 0.004) and the agricultural areas of Central Bohemia (Roudnice nad Labem 206Pb/207Pbwine = 1.176 ± 0.007) are similar to the Pb isotopic composition of airborne particulate material typical of polluted and industrial environments (206Pb/207Pb = 1.17–1.19). The isotopic composition of wine from Prague (206Pb/207Pbwine = 1.174 ± 0.003) is different from that of the soil, which was severely contaminated in the past by vehicular Pb (206Pb/207Pbsoil = 1.147–1.168). This fact shows that interception of airborne Pb by plants is greater than its uptake by the root system.  相似文献   
976.
遥感图像判读训练与应用系统的设计与实现   总被引:1,自引:0,他引:1  
遥感图像判读与应用技术对深刻认知环境和辅助决策具有重要的意义.文中介绍了遥感图像判读训练与应用系统的设计思想和主要功能,对各部分涉及的关键技术进行了分析与研究.系统由影像管理、判读训练、地形快速修测和实时态势标绘四部分组成,实现了影像数据库管理,遥感图像与地图对照以及屏幕立体观察,地图与图像坐标系匹配的地形快速修测和实时态势标绘的操作等功能.  相似文献   
977.
商丘市的大雾多形成在大陆高压型、冷锋前暖区型、均压场型、(低压)倒槽型天气形势中。空气中的污染物不仅增加了水汽凝结核,那些可溶性气溶胶还降低了饱和水汽压,使空气中的水汽更容易饱和,因此被污染的空气更容易形成雾。而大雾天气抑制了近地面层污染物的垂直扩散,加剧了城市污染。  相似文献   
978.
The impact that volcanic eruptions may have upon environments far from the volcanic source is conventionally assumed to depend on climatic modification by emitted gases. However, recent research has suggested that the damage caused by the direct impact of volcanic gases, mainly H2SO4, may be profound. This paper highlights the severity of this mechanism by reference to human sickness and death in France and contiguous with the eruption of the Laki fissure in Iceland in 1783. This work demonstrates the gains which may be made by interdisciplinary teams of researchers and illustrates the valuable knowledge that remains to be revealed by further research in the French historical record. To cite this article: J. Grattan et al., C. R. Geoscience 337 (2005).  相似文献   
979.
John Grattan   《Lithos》2005,79(3-4):343-353
This paper is based on the premise that research into the environmental impact of continental flood basalt (CFB) volcanism has paid insufficient attention to the potential ecosystem damage that would result from the direct deposition of hundreds of megatons (Tg) of sulphur and other volatiles. The environmental impacts of the 1783 Laki Fissure eruption are reviewed in outline. It is shown that in a relatively brief period of volcanic activity, volatiles emitted by the eruption damaged and destroyed vegetation from the Arctic Ocean to the Mediterranean. Air pollution was so intense that human health was affected and the national death rate increased dramatically in both England and France.

It is proposed that the events of 1783 may be used as a paradigm for the environmental impacts of a CFB lava flow, and the emissions of 1783 are scaled up to illustrate this point. Thus, if a Laki style event were to erupt for a year it would approach the physical scale of a single episode of the Roza flow in the Columbia River CFB and potentially yield 576 Tg of sulphur gases which could have been oxidised into approximately 945 Tg of aerosol. This could generate a tropospheric aerosol mass of approximately 708 Tg H2SO4.

The ecosystem impact of the deposition of acids on this scale would be profound and, as with the actual Laki event, be continental in scale. All parts of the plant life cycle would be disrupted, including photosynthesis and fruiting. Inevitably, with the disruption of food webs animals would also be affected. Poorly buffered inland waters would be acidified, as would Boreal soils, reducing their biodiversity.

In our already polluted and interdependent world, any future event on this scale would have serious consequences for human health and trade.  相似文献   

980.
To determine selected water pollution parameters of the Anyang River (one of the biggest contributory branches of the Han River in Korea) and its main tributaries, the geological and topographical and rainfall features in its basin were investigated, and the resulting data were tabulated. Samples were collected at the upper, mid and down parts of the Anyang River and its branches and were analyzed based on biochemical and chemical methods, Korean biotic index (KBI) and Saprobien systems. Selected parameters of concern include BOD, heavy metals, nonpoint pollution and sewage discharge. The Anyang River basin has a torrential heavy rainfall; however, the rate of rainfall significantly varies from season to season. Water pollution levels in the dry season increase dramatically. The mainstream of the Anyang River is classified as fifth grade polysaprobic water according to Saprobien system. In addition, the biotic index is over 2.5 in overall. General pollution at the junction of the Anyang River and each branch stream varies. Possible countermeasures to improve the water quality of the river include intercept the non-treated waste water and sewage at the Anyang River junction and each branch stream, enforcement of water management during the rainy season, and continuous investment on environmental restoration.  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