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   30935篇
  免费   575篇
  国内免费   250篇
测绘学   774篇
大气科学   2420篇
地球物理   6089篇
地质学   11358篇
海洋学   2668篇
天文学   6477篇
综合类   68篇
自然地理   1906篇
  2021年   296篇
  2020年   329篇
  2019年   317篇
  2018年   774篇
  2017年   764篇
  2016年   907篇
  2015年   508篇
  2014年   867篇
  2013年   1664篇
  2012年   978篇
  2011年   1310篇
  2010年   1130篇
  2009年   1447篇
  2008年   1268篇
  2007年   1261篇
  2006年   1250篇
  2005年   898篇
  2004年   899篇
  2003年   812篇
  2002年   814篇
  2001年   749篇
  2000年   729篇
  1999年   585篇
  1998年   540篇
  1997年   617篇
  1996年   486篇
  1995年   485篇
  1994年   482篇
  1993年   396篇
  1992年   394篇
  1991年   374篇
  1990年   381篇
  1989年   340篇
  1988年   338篇
  1987年   370篇
  1986年   332篇
  1985年   423篇
  1984年   412篇
  1983年   454篇
  1982年   421篇
  1981年   372篇
  1980年   409篇
  1979年   326篇
  1978年   305篇
  1977年   299篇
  1976年   266篇
  1975年   263篇
  1974年   274篇
  1973年   259篇
  1971年   174篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
33.
34.
Measurements of 18O concentrations in precipitation, soil solution, spring and runoff are used to determine water transit time in the small granitic Strengbach catchment (0·8 km2; 883–1146 m above sea level) located in the Vosges Mountains of northeastern France. Water transit times were calculated by applying the exponential, exponential piston and dispersion models of the FlowPC program to isotopic input (rainfall) and output (spring and stream water) data sets during the period 1989–95. The input function of the model was modified compared with the former version of the model and estimated by a deterministic approach based on a simplified hydrological balance. The fit between observed and calculated output data showed marked improvements compared with results obtained using the initial version of the model. An exponential piston version of the model applied to spring water indicates a 38·5 month mean transit time, which suggests that the volume in the aquifer, expressed in water depth, is 2·4 m. A considerable thickness (>45 m) of fractured bedrock may be involved for such a volume of water to be stored in the aquifer. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
35.
36.
37.
Photographic spectra of SN1987A in the LMC have been obtained from 1987 February 25 to 1988 June 30. Microdensitometer tracings of these have been reduced to intensity and corrections for instrumental response have been applied to the spectra. This paper presents these data in an atlas format, discusses the reduction procedures in detail, and presents radial velocity measurements of selected lines in the spectra  相似文献   
38.
39.
A radiative seasonal model which incorporates a multilayer radiative transfer treatment at wave-lengths longward of 7 μm is presented and applied to Saturn's stratosphere. Opacities due to H2-He, CH4, C2H2, and C2H6 are included. Season-dependent insolation is shown to produce a strong hemispheric asymmetry decreasing with depth at the Voyager encounter times, and seasonal amplitudes of 30°K at the poles are predicted in the high stratosphere. The ring-modulated dependence of the insolation and the orbital eccentricity are shown to have a significant effect. Calculations agree closely with the Voyager 1 and 2 radio occultation ingress profiles recorded at 76°S and 36.5°S for CH4/H2 = 3.5 + 1.4/? 1.0 × 10?3;the estimated errors include modeling systematic errors and uncertainties in the occultations profiles. The possible role of aerosols in the stratospheric heating is analyzed. The Voyager 2 egress profile recorded at 31°S cannot be reproduced by calculations. Some constraints on the C2H2 and C2H6 abundances are derived. The upper portion of the occultation profiles (p < 3mbar) can be matched for C2H2/H2 = 1.0 + 1.3/?0.6 × 10?7, C2H6/H2 = 1.5 + 1.8/?0.9 × 10?6 at 76°S and C2H2/H2 = 4 + 6/?4 × 10?8, C2H6/H2 = 6 + 9/?6 × 10?7 at 36.5°N. At the northern occultation latitude, the discrepancy with the concentrations derived from analysis of IRIS spectra by R. Courtin, D. Gautier, A. Marten, B. Bézard, and R. Hanel (1984, Astrophys. J.287) can be explained by a sharp variation of the mixing ratios of these gases with altitude in the upper stratosphere. Other interpretations are discussed.  相似文献   
40.
The metasediments in the Chamba region experienced three phases of deformation: DF1, DF2 and DF3.Folded quartz veins are co-folded with the F2 crenulation folds. Their geometric and tectonic significance is studied by microstructures and shortening adjacent to the discrete crenulation cleavage, S2. Microstructures of folded vein segments, their geometric changes and truncation to cleavage (S2) are mainly due to pressure-solution phenomena and the estimated volume loss from reconstructed vein segments range from 16 to 25.5%,which is closely related to volume decrease (26%) calculated from the polydeformed slates of North Wales areas.  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