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   4865篇
  免费   1155篇
  国内免费   956篇
测绘学   359篇
大气科学   699篇
地球物理   1590篇
地质学   2153篇
海洋学   700篇
天文学   57篇
综合类   309篇
自然地理   1109篇
  2024年   10篇
  2023年   66篇
  2022年   154篇
  2021年   150篇
  2020年   166篇
  2019年   204篇
  2018年   217篇
  2017年   237篇
  2016年   218篇
  2015年   238篇
  2014年   325篇
  2013年   279篇
  2012年   292篇
  2011年   279篇
  2010年   229篇
  2009年   302篇
  2008年   291篇
  2007年   304篇
  2006年   319篇
  2005年   280篇
  2004年   268篇
  2003年   257篇
  2002年   243篇
  2001年   225篇
  2000年   194篇
  1999年   153篇
  1998年   160篇
  1997年   135篇
  1996年   133篇
  1995年   121篇
  1994年   108篇
  1993年   99篇
  1992年   83篇
  1991年   64篇
  1990年   47篇
  1989年   42篇
  1988年   36篇
  1987年   18篇
  1986年   8篇
  1985年   3篇
  1984年   4篇
  1983年   2篇
  1981年   1篇
  1979年   1篇
  1978年   1篇
  1954年   5篇
  1921年   1篇
  1908年   1篇
  1900年   1篇
  1872年   1篇
排序方式: 共有6976条查询结果,搜索用时 31 毫秒
631.
Starting from the assumption that the radiation source at the origin of the cosmic microwave background (CMB) could not have a luminosity larger than the maximum energy in ordinary matter divided by the minimum time allowed by causality, one arrives at an expression that gives the energy density of CMB as a function of the main cosmological parameters. Also, by defining a radiation charge as the hypothetical charge that opposes the congregation of a cloud of particles around a source of electromagnetic radiation, on arrives at another expression for the energy density of CMB that agrees exactly with the measured value for a value of the Hubble constant equal to 72.09 km s−1 Mpc−1. Both expressions are independent of the redshift.  相似文献   
632.
BOOK REVIEWS     
Books reviewed in this article: Aerial Photography and Remote Sensing Aerial photography and image interpretation for Resource Management . DAVID P. PAINE . Analytical Models and Techniques Spatial Processes Models and Applications A. D. CLIFF AND J. K. ORD Quantitative and Statistical Approaches to Geography: A Practical Manual JOHN A. MATTHEWS . Cartography and Maps Map Data Processing . HERBERT FREEMAN AND GOFFREDO G. PIERONI , eds. Computer-assisted Cartography: Principles and Prospects, 1982 . MARK S. MONMONIER . Cultural Navajo Architecture: Forms, History, Distributions . STEPHEN C. JETT AND VIRGINIA E . SPENCER . Rational Landscapes and Humanistic Geography . EDWARD RELPH . Economic The Structure and Control of a State Economy . W. L. L'ESPERANCE . Economic Geography . JAMES O. WHEELER AND PETER O. MULLER . The Myth of the Family Farm: Agribusiness Dominance of U.S. Agriculture . INGOLF VOCELER . Libya: The Experience of Oil . J. A. ALLEN . Geography of Public Finance, Welfare Under Fiscal Federalism and local Government Finance . ROBERT BENNETT . Environmental Management, Resources, and Systems Mountains and Man . LARRY W. PRICE . Environmental Geology . DONALD R. COATES . An Introduction to Environmental Systems . G. H. DURY . The Environment: Chinese and American Views . Edited by LAURENCE J. C. MA AND ALLEN G. NOBLE . Land Use in America . RICHARD H. JACKSON . Medical Conceptual and Methodological Issues in Medical Geography . MELINDA S. MEADE (ed.). Philosophy and Geographic Thought Birds in Egg/Eggs in Bird . GUNNAR OLSSON . PhysicaI Climatology: Selected Applications . J. E. OLIVER . Geomorphological Techniques . ANDREW GOUDIE (ed.). Political Politics, Geography and Behaviour . RICHARD MUIR AND RONAN PADDISON . The State of the World Atlas . MICHAEL KIDRON AND RONALD SECAL . Population The Population of the South . DUDLEY L. POSTON , JR . AND ROBERT H. WELLER , eds. Regional Latin America: Economic Development and Regional Differentiation . ARTHUR MORRIS . Developing the Amazon . EMILIO F. MORAN . Western Europe: A Systematic Human Geography . BRIAN W. ILBERY . Settlement Systems in Sparsely Settled Regions: The United States and Australia . RICHARD E. LONSDALEAND JOHN H. HOLMES , eds. African Perspectives: The Economic Geography of Nine African States . HARM DE BUJ AND ESMOND MARTIN , eds. Rural Nonmetropolitan America in Transition . AMOS H. HAWLEY AND SARA MILLS MAZIE , eds. Social Crime and Environment . R. NORMAN DAVIDSON . The Ghetto: Readings with Interpretations . JOE T. DARDEN , ed. Urban Urban Problems and Planning in the Developed World . MICHAEL PACIONE (ed.) Problems and Planning in Third World Cities . Edited by MICHAEL PACIONE . The City in West Europe . D. BURTENSHAW , M. BATEMAN , AND C. J. ASHWORTH .  相似文献   
633.
634.
根据Marshall空间飞行中心(MSFC)太阳天文台的矢量磁场测量和云南天文台的黑子细节照相资料,作者们详细研究了1986年2月初太阳大活动区(AR4711)的形态和演化。主要结论如下: i)几乎在活动区中每处地方,相距五小时观测到横向磁场排列方向和黑子半影纤维形态之间存在良好的相似性。 ii)利用文[4]的方法,推断了本活动区强的垂直电流源和强的水平电流渠道。 iii)与1972年8月初著名的太阳活动区(McMath 11976)相类似,沿老活动区的中性线的新浮磁通管的两足点(偶极黑子)的分离运动导致了一个密集四极磁结构的形成。 iv)新浮磁通管似乎是本活动区最强的电流系统。 上述结论将为进一步研究本区电流/磁场环系的演化及其与耀斑活动的关系提供一个基础数据。  相似文献   
635.
The processes of stormflow generation were studied in a hill pasture catchment near Hamilton, New Zealand. Although rainfall was relatively evenly distributed throughout the year, stormflow was highly seasonal and over 65 per cent occurred during the winter. Three main processes contributing to stormflow were identified which could be related to soil type and physiographic position. On gleyed soils derived from rhyolitic colluvium, saturation overland flow was the dominant process. Hydrographs from ‘Whipkey’ throughflow troughs also indicated that there was a subsurface response (saturated wedge) from this soil type. On steeper convex slopes, more permeable soils were derived from weathered greywacke. The presence of ephemeral springs on the hillslopes and direct observation during storm events indicated that storm runoff was generated as return flow from this soil. It was noted that nitrate concentrations from subsurface sources were 5–10 times higher than surface runoff. This difference in concentration was utilized in a chemical mixing equation which partitioned stormflow sources. This was compared with the stormflow predicted from rain falling on to saturated areas. There was good agreement between the two models for winter-spring events with respect to the volumes of surface runoff predicted, however the saturated areas model underestimated total stormflow. The results of the study are briefly discussed in terms of the potential for water quality management.  相似文献   
636.
本文对1949年以来出现的10次ENSO事件进行了分析,得出如下结果:开始於东太平洋增暖和中大平洋增暖的两类ENSO现象分别对应四川地区粮食产量的减产和增产。此结果对四川地区粮食生产政策的制定及产量的预测研究均有较重要的参考价值。  相似文献   
637.
长江三峡阶地的成因机制   总被引:15,自引:0,他引:15  
根据对长江三峡阶地堆积物进行的野外调查与室内分析发现三峡阶地的成因具有以下特点:构成阶地上部的河漫滩相堆积、中部的冲积砾石层与作为阶地基座的基岩平台是在不同时期形成的;阶地上部河漫滩相堆积是在中全新世气候温暖、长江三峡高水位条件下形成的.因此,长江三峡阶地是在构造上升的基础上由于气候及长江三峡流量及水位变化而形成的,并非一定是间歇性构造上升的标志.  相似文献   
638.
639.
结合部门实际。对江西气象部门如何领会和贯彻落实2005年全省领导干部会议精神,提出了具体的指导意见。讲话指出,全省各级气象部门干部职工,要把思想和行动进一步统一到全省领导干部会议精神上来,围绕为全民创业提供优质气象服务,找准自己的位置,为不同创业主体提供精细化、个性化的气象服务产品。拓宽为全民创业服务的气象信息传递渠道,强化安全气象和资源气象服务,开展全民创业技能培训和创业资讯服务,营造优化有利于全民创业的气象政务环境。努力满足全民创业对气象服务的迫切需求。讲话还强调,全省气象干部职工要强化创业理念,大力培育气象创业文化;加强气象能力建设,加快提升气象现代化水平和效益;适应中国气象事业发展战略,加快全省新一轮气象业务技术体制改革;坚持改革创新,做大做强做优气象科技服务;加强队伍建设,为事业发展提供智力支持和组织保证,努力在全民创业的热潮中,实现江西气象事业在新的起点上更快更好的发展。另外。讲话在总结分析上半年工作的基础上,结合全省领导干部会议精神。对2005年下半年的工作进行了全面的部署。认为2005年下半年,全省气象部门要狠抓目标管理,积极做好气象服务工作,下大力气促科技服务上新台阶;要加强与有关部门的共建共享工作。及时完成“十一五”规划的编制。大力推进业务技术体制改革和现代化建设;要加强党建和气象文化建设,并根据《江西省气象局关于加强基层台站建设的实施意见》,以及《全省县(市、区)气象局“五大工程”建设考评办法》。抓好基层台站建设。  相似文献   
640.
Wu JY 《Marine pollution bulletin》2005,50(12):1661-1667
The present study investigated the genotoxicity of the surface water samples from the Yangtze Estuary with the Ames test in three seasons. Several important chemical parameters, such as COD inorganic nitrogen, active phosphate and heavy metals, were also analyzed at the same time. According the results, surface water samples from the south branch and a few of samples at the seaward end of the Yangtze Estuary show positive genotoxicity in some seasons. Chemical analysis revealed that the Yangtze Estuary was seriously polluted by inorganic nitrogen and by active phosphate. However, chemical parameters could not demonstrate the spatial variation of water quality of the estuary, and they could not assess adverse effects of chemicals in mixtures as well. Therefore, it is recommended that genotoxicity data, mutation rate of the Salmonella typhimurium strain TA98 at the dose of 1 l per plate, be taken as a new parameter assessing surface water quality of the Yangtze Estuary. Our studies also suggested that genetic toxicology assays, such as the Ames test, could be applied as a routine measure to monitoring marine environment in China. The paper proposes a development of the National Seawater Quality Standard of China.  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