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   52638篇
  免费   718篇
  国内免费   1261篇
测绘学   2108篇
大气科学   4021篇
地球物理   10123篇
地质学   21235篇
海洋学   3607篇
天文学   8380篇
综合类   2211篇
自然地理   2932篇
  2021年   236篇
  2020年   284篇
  2019年   351篇
  2018年   5277篇
  2017年   4559篇
  2016年   3320篇
  2015年   728篇
  2014年   847篇
  2013年   1505篇
  2012年   1846篇
  2011年   3806篇
  2010年   3039篇
  2009年   3643篇
  2008年   3086篇
  2007年   3533篇
  2006年   1310篇
  2005年   1133篇
  2004年   1320篇
  2003年   1240篇
  2002年   1066篇
  2001年   743篇
  2000年   748篇
  1999年   640篇
  1998年   635篇
  1997年   572篇
  1996年   468篇
  1995年   448篇
  1994年   460篇
  1993年   361篇
  1992年   360篇
  1991年   309篇
  1990年   364篇
  1989年   318篇
  1988年   311篇
  1987年   329篇
  1986年   282篇
  1985年   392篇
  1984年   434篇
  1983年   414篇
  1982年   394篇
  1981年   323篇
  1980年   348篇
  1979年   284篇
  1978年   270篇
  1977年   270篇
  1976年   229篇
  1975年   232篇
  1974年   211篇
  1973年   206篇
  1972年   144篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
To date, passive flux meters have predominantly been applied in temperate environments for tracking the movement of contaminants in groundwater. This study applies these instruments to reduce uncertainty in (typically instantaneous) flux measurements made in a low-gradient, wetland dominated, discontinuous permafrost environment. This method supports improved estimation of unsaturated and over-winter subsurface flows which are very difficult to quantify using hydraulic gradient-based approaches. Improved subsurface flow estimates can play a key role in understanding the water budget of this landscape.  相似文献   
2.
3.
4.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk.  相似文献   
5.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin.  相似文献   
6.
7.
8.
This paper presents an introductory overview of recently developed stochastic theories for tackling spatial variability problems in predicting groundwater flow and solute transport. Advantages and limitations of the theories are discussed. Lastly, strategies based on the stochastic approaches to predict solute transport in aquifers are recommended.  相似文献   
9.
Nowadays, the research works on landscape at fine scales using high-resolution images are uncommon.This research is based on the analysis of the combination of remote sensing data (1KONOS imagery acquired in 2002 and historical aerial photo taken in 1942). In the paper, the ecotopes in Qiujiadou and Xishao villages in Yixing City of Jiangsu Province in 1942 and 2002 were compared and landscape changes as well as the causes of the considerable changes were analyzed. It was found that the ecotope changes were at greater level in some aspects such as water surface and perennial vegetation coverage etc. This study at fine scale is globally significant for the rural areas, especially for the subsistence agricultural land, which occupies larger percentage in the earth. And it analyzes the structure of landscape based on a new landscape classification system--stratifications method.  相似文献   
10.
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM.  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