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   7278篇
  免费   279篇
  国内免费   79篇
测绘学   271篇
大气科学   584篇
地球物理   1684篇
地质学   2402篇
海洋学   580篇
天文学   1365篇
综合类   29篇
自然地理   721篇
  2022年   38篇
  2021年   85篇
  2020年   114篇
  2019年   129篇
  2018年   183篇
  2017年   183篇
  2016年   246篇
  2015年   192篇
  2014年   211篇
  2013年   428篇
  2012年   270篇
  2011年   376篇
  2010年   311篇
  2009年   420篇
  2008年   372篇
  2007年   316篇
  2006年   296篇
  2005年   273篇
  2004年   276篇
  2003年   217篇
  2002年   246篇
  2001年   133篇
  2000年   163篇
  1999年   117篇
  1998年   123篇
  1997年   98篇
  1996年   94篇
  1995年   97篇
  1994年   97篇
  1993年   79篇
  1992年   95篇
  1991年   70篇
  1990年   61篇
  1989年   58篇
  1988年   69篇
  1987年   59篇
  1986年   66篇
  1985年   79篇
  1984年   81篇
  1983年   83篇
  1982年   80篇
  1981年   75篇
  1980年   69篇
  1979年   86篇
  1978年   67篇
  1977年   48篇
  1976年   37篇
  1975年   41篇
  1974年   40篇
  1973年   39篇
排序方式: 共有7636条查询结果,搜索用时 0 毫秒
11.
12.
Central elements of the challenges of the future are the population, supply and disposal trap, the question of how the increasing world population will be supplied with long‐lasting and effective sources of energy, raw materials and food? How do we deal with the problems of greenhouse effect, ozone hole, forest dieback, ground erosion, desertification and other environmental threats? Driven by the dynamics of the technical progress the world is in the midst of experiencing the transition from an industrial to an information society. The process of globalization is intrinsically tied to the information society and plays a vital role in the path to a new, digital development of the world. Therefore, new challenges have been added to the traditional ecological challenges at the beginning of the 21st century, which the world community must react swiftly to.  相似文献   
13.
Expressions for both the rectilinear and rotational inertial and damping coefficients for a circular monolithic tower of uniform radius are derived. The analysis matches the fluid velocity, derived from potential theory, with the structural velocity in sway. That is, the motions of the tower are assumed to be in a vertical plane. The analysis is then applied to a tower composed of (lumped-mass) elements, where the expressions for the added-mass and damping coefficients are shown to be functions of wave number. The added-mass is shown to be a product of two wave systems: a travelling wave system, which is responsible for the radiation damping, and a standing wave system, called the evanescent system, which is attached to the structure. The added-mass of the evanescent system is negative for small wave numbers, while that of the travelling waves is positive. The negative sign simply means that the inertial force of the evanescent waves is 180° out of phase with that of the travelling system. Furthermore, it is shown that the contributions of the two wave systems to the total added-mass of the structure counteract each other, resulting in a total added-mass which varies gradually with the wave number. Finally, the analysis is applied to an experiment, and results of the analysis and the experiment are found to agree rather well.  相似文献   
14.
15.
Coring through glaciotectonically stacked Quaternary sediments situated below sea level on the island of Møn, Denmark, recovered a succession of interstadial sediments of Middle Weichselian age. Plant and animal remains including insects found in laminated sand and mud indicate deposition in a lake surrounded by dwarf shrubs, herbs, mosses and rare trees. The insect fauna indicates a mean July temperature of 8–12 °C, suggesting an arctic to sub‐arctic environment, while winter temperatures around ?8 to ?22 °C suggest periglacial conditions with permafrost. Luminescence dating of sediment samples gave ages from 48–29 ka, and radiocarbon dating indicates deposition of plant fragments between 45 and 36 ka BP. The fossil assemblage from Møn shows close resemblance to those from other sites with similar ages found in the vicinity of the western Baltic Basin.  相似文献   
16.
17.
Arsenic in Ground Water of the Western United States   总被引:12,自引:0,他引:12  
  相似文献   
18.
A popular and contemporary use of numerical groundwater models is to estimate the discrete relation between groundwater extraction and surface-water/groundwater exchange. Previously, the concept of a “capture map” has been put forward as a means to effectively summarize this relation for decision-making consumption. While capture maps have enjoyed success in the environmental simulation industry, they are deterministic, ignoring uncertainty in the underlying model. Furthermore, capture maps are not typically calculated in a manner that facilitates analysis of varying combinations of extraction locations and/or reaches. That is, they are typically constructed with focus on a single reach or group of reaches. The former of these limitations is important for conveying risk to decision makers and stakeholders, while the latter is important for decision-making support related to surface-water management, where future foci may include reaches that were not the focus of the original capture analysis. Herein, we use the concept of a response matrix to generalize the theory of the capture-map approach to estimate spatially discrete streamflow depletion potential. We also use first-order, second-moment uncertainty estimation techniques with the concept of “risk shifting” to place capture maps and streamflow depletion potential in a stochastic, risk-based framework. Our approach is demonstrated for an integrated groundwater/surface-water model of the lower San Antonio River, Texas, USA.  相似文献   
19.
20.
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