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   30552篇
  免费   648篇
  国内免费   408篇
测绘学   887篇
大气科学   2333篇
地球物理   6350篇
地质学   10977篇
海洋学   2493篇
天文学   6959篇
综合类   130篇
自然地理   1479篇
  2021年   274篇
  2020年   308篇
  2019年   343篇
  2018年   765篇
  2017年   740篇
  2016年   950篇
  2015年   571篇
  2014年   929篇
  2013年   1590篇
  2012年   1045篇
  2011年   1228篇
  2010年   1179篇
  2009年   1509篇
  2008年   1327篇
  2007年   1312篇
  2006年   1258篇
  2005年   977篇
  2004年   937篇
  2003年   846篇
  2002年   810篇
  2001年   705篇
  2000年   718篇
  1999年   648篇
  1998年   652篇
  1997年   592篇
  1996年   478篇
  1995年   472篇
  1994年   492篇
  1993年   371篇
  1992年   350篇
  1991年   313篇
  1990年   360篇
  1989年   325篇
  1988年   289篇
  1987年   335篇
  1986年   280篇
  1985年   359篇
  1984年   397篇
  1983年   396篇
  1982年   363篇
  1981年   301篇
  1980年   315篇
  1979年   268篇
  1978年   256篇
  1977年   276篇
  1976年   209篇
  1975年   246篇
  1974年   224篇
  1973年   221篇
  1972年   145篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
When the observation of small headwater catchments in the pre-Alpine Alptal valley (central Switzerland) started in the late 1960s, the researchers were mainly interested in questions related to floods and forest management. Investigations of geomorphological processes in the steep torrent channels followed in the 1980s, along with detailed observations of biogeochemical and ecohydrological processes in individual forest stands. More recently, research in the Alptal has addressed the impacts of climate change on water supply and runoff generation. In this article, we describe, for the first time, the evolution of catchment research at Alptal, and present new analyses of long-term trends and short-term hydrologic behaviour. Hydrometeorological time series from the past 50 years show substantial interannual variability, but only minimal long-term trends, except for the ~2°C increase in mean annual air temperature over the 50-year period, and a corresponding shift towards earlier snowmelt. Similar to previous studies in larger Alpine catchments, the decadal variations in mean annual runoff in Alptal's small research catchments reflect the long-term variability in annual precipitation. In the Alptal valley, the most evident hydrological trends were observed in late spring and are related to the substantial change in the duration of the snow cover. Streamflow and water quality are highly variable within and between hydrological events, suggesting rapid shifts in flow pathways and mixing, as well as changing connectivity of runoff-generating areas. This overview illustrates how catchment research in the Alptal has evolved in response to changing societal concerns and emerging scientific questions.  相似文献   
2.
3.
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   
4.
5.
6.
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   
7.
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   
8.
9.
A spectacular change in the lower corona on the south-west limb has been found in solar images taken by the Yohkoh soft X-ray telescope. The event is characterized by a large topological change in magnetic field and a large intensity decrease observed after the X1. 1/1B flare on 9 November, 1991. A coronal mass ejection (CME) was observed by the Mark III K-coronameter (MK3) at the HAO/Mauna Loa Observatory. Both the MK3 (white-light) and soft X-ray observations showed that one leg of this CME was located above the flare site. An interplanetary shock associated with this event was observed by Pioneer Venus Orbiter, and, possibly, by IMP-8.Also Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, U.S.A.  相似文献   
10.
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