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   64896篇
  免费   632篇
  国内免费   1231篇
测绘学   2334篇
大气科学   4764篇
地球物理   12005篇
地质学   27132篇
海洋学   4648篇
天文学   10836篇
综合类   2214篇
自然地理   2826篇
  2021年   203篇
  2020年   255篇
  2019年   310篇
  2018年   7078篇
  2017年   6356篇
  2016年   4259篇
  2015年   704篇
  2014年   811篇
  2013年   1352篇
  2012年   2487篇
  2011年   5437篇
  2010年   4780篇
  2009年   5292篇
  2008年   4324篇
  2007年   5300篇
  2006年   1235篇
  2005年   1496篇
  2004年   1545篇
  2003年   1511篇
  2002年   1157篇
  2001年   686篇
  2000年   730篇
  1999年   585篇
  1998年   578篇
  1997年   534篇
  1996年   404篇
  1995年   401篇
  1994年   417篇
  1993年   317篇
  1992年   312篇
  1991年   266篇
  1990年   315篇
  1989年   280篇
  1988年   266篇
  1987年   285篇
  1986年   247篇
  1985年   328篇
  1984年   349篇
  1983年   343篇
  1982年   318篇
  1981年   277篇
  1980年   296篇
  1979年   223篇
  1978年   209篇
  1977年   220篇
  1976年   194篇
  1975年   196篇
  1974年   184篇
  1973年   174篇
  1972年   115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.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars.  相似文献   
5.
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   
6.
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   
7.
8.
9.
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   
10.
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   
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