全文获取类型
收费全文 | 15978篇 |
免费 | 3434篇 |
国内免费 | 3205篇 |
专业分类
测绘学 | 234篇 |
大气科学 | 988篇 |
地球物理 | 2758篇 |
地质学 | 11484篇 |
海洋学 | 2295篇 |
天文学 | 20篇 |
综合类 | 710篇 |
自然地理 | 4128篇 |
出版年
2024年 | 80篇 |
2023年 | 304篇 |
2022年 | 701篇 |
2021年 | 807篇 |
2020年 | 734篇 |
2019年 | 917篇 |
2018年 | 761篇 |
2017年 | 919篇 |
2016年 | 951篇 |
2015年 | 903篇 |
2014年 | 1124篇 |
2013年 | 1217篇 |
2012年 | 1075篇 |
2011年 | 1093篇 |
2010年 | 926篇 |
2009年 | 1115篇 |
2008年 | 1084篇 |
2007年 | 1170篇 |
2006年 | 1009篇 |
2005年 | 846篇 |
2004年 | 750篇 |
2003年 | 683篇 |
2002年 | 491篇 |
2001年 | 424篇 |
2000年 | 421篇 |
1999年 | 382篇 |
1998年 | 260篇 |
1997年 | 265篇 |
1996年 | 221篇 |
1995年 | 189篇 |
1994年 | 191篇 |
1993年 | 132篇 |
1992年 | 143篇 |
1991年 | 101篇 |
1990年 | 66篇 |
1989年 | 48篇 |
1988年 | 36篇 |
1987年 | 8篇 |
1986年 | 15篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
321.
Arantxa Luzón Angel González Arsenio Muñoz Belén Sánchez-Valverde 《Journal of Paleolimnology》2002,28(4):441-456
The lacustrine facies from two sections (Candasnos and Fraga) ofthe Oligocene-Miocene Torrente de Cinca lithostratigraphic Unit in thecentral part of the Ebro Basin (Spain) have been analysed to determine theinfluence of orbital parameters in lacustrine sedimentation. The unit ispredominantly composed of limestones and marls, and represents a shallowlacustrine freshwater system. The sedimentological features of the faciesstudied demonstrate that the lower part of the Candasnos section representsoffshore lacustrine subenvironments whereas the upper part, and the whole ofthe Fraga section, characterise marginal lacustrine areas. Series of stratalthickness variations of limestone, marl, and limestone/marl couplets fromboth sections have been analysed using spectral analysis. This shows thatinformation corresponding to periodic cycles only appears in the offshorefacies, that is to say, in the lower part of Candasnos section, and disappearsin the marginal facies where non-periodic cycles exist. Furthermore, thespectral analysis of the offshore facies highlights the existence of a peak inthe power spectrum with a period of around 7 (6.8 to 7.8) that can berecognised in the field as shallowing-upward lacustrine sequences.Magnetostratigraphic data from the Candasnos section allow us to establish atime span of 2,808 years for the limestone/marl couplet from the lower partof this section, and between 19,000 and 22,000 years for the periodic cycleidentified, thus representing the climatic precession cycle. Shallowingsequences from marginal areas do not correspond with any periodiccycle. 相似文献
322.
323.
324.
Jorge Dinis Jacques Rey Pierre-Charles de Graciansky 《Comptes Rendus Geoscience》2002,334(10):757-764
. Three transgressive–regressive 2nd-order cycles were identified in the Upper Aptian–Albian fluvial and marine deposits of the Lusitanian Basin. Its widespread nature, probably including eustatic origin, allows correlation between the southern package, with precise stratigraphic positioning, and the northern series with a poorly constrained age. The main unconformities can be related to the onset of an oceanic crust, in the western margin of Galicia during the Late Aptian, in the bay of Biscay during the Early Albian, and, to the northwest of the Galicia triple point, during the Middle to Late Albian transition, and, at the Albian–Cenomanian boundary, to a probable compressive event with Africa due to the rotation of Iberia. To cite this article: J. Dinis et al., C. R. Geoscience 334 (2002) 757–764. 相似文献
325.
Hussein A. Amery 《The Geographical journal》2002,168(4):313-323
This paper explains how hydropolitical dynamics and spatial variables almost triggered a water war between Israel and Lebanon because the latter was building a pump on the Wazzani Spring, a tributary of the Jordan River. The convergence of a regional drought, history of violent confrontations between the two riparians, distrust, varying development needs and territorial disputes almost culminated in a war between these east Mediterranean neighbours. While most international water disputes in the Middle East will be resolved peacefully, some are likely to trigger violent confrontations threatening political stability in the Middle East in the next few decades. 相似文献
326.
The hydraulic and sedimentary characteristics of the spawning habitat of Atlantic salmon (Salmo salar) in tributary and mainstem locations in a river system in north-east Scotland are described. Salmon used spawning sites with a relatively wide range in sediment characteristics, although measures of central tendency were all in the gravel (2–64 mm) size-class. The dominant factor differentiating the sediment characteristics of study sites was the level of fine sediment, which accounted for significant differences between tributary and mainstem samples. The ranges of depth and velocity in areas used for spawning by salmonids were found to be similar in all tributary study sites. However, due to the interdependence of depth and velocity, major differences were observed between tributary and mainstem study sites in that spawning in larger streams tended to be associated with deeper, faster flowing water. Spawning locations were shown to have similar Froude number, despite different sized streams and species of salmonid. Due to its dimensionless nature and significance in characterising flow hydraulics, the Froude number is proposed as a potentially useful variable for describing the habitat of aquatic organisms. 相似文献
327.
Martin Roberge 《The Professional geographer》2002,54(2):175-189
In‐stream gravel mining, massive bridge piers, and channelization have all contributed to the geomorphic instability of the Lower Salt River channel in Arizona. Dam closure, changing dam operating rules, and the frequent modification of the channel bed have decreased our ability to predict the Salt River hydrology. Engineering practice has adapted to this situation and to a public that is increasingly intolerant of service disruptions by constructing larger bridges and extending levees. Building these larger structures may be counterproductive; future construction should not constrict the channel and should re‐establish a braided river to decrease the energy available to the system. 相似文献
328.
近、现代黄河三角洲地貌形态反演 总被引:5,自引:0,他引:5
1855年黄河改夺大清河河道后于山东利津县东北入海。至今黄河尾癌河道历经10次大的改道变迁,塑造了近、现代黄河三角洲,形成了7个主要的亚三洲堆积体。该文利用黄河尾癌河道改道资料,近、现代河口流路变迁图,近、现代黄河三角洲体系图以及1985-1986年的水文工程地质钻孔资料,在研究黄河三角洲沉积模式的基础上,对工程地质钻孔数据进行沉积层序分析,将今论古,反推近、现代黄河三角洲沉积地貌,研究结果近似展示了近、现代黄河三角洲二维平面的发育演变和其时其地的地貌形态。 相似文献
329.
汾河流域水环境地理信息系统设计 总被引:1,自引:0,他引:1
该文结合汾河流域水环境地理信息系统设计的具体实践,论述了系统所采用的三层结构体系、设计的技术策略,重点对系统的功能特别是数据库设计和应用系统功能进行了详细的描述,是地理信息系统在水环境管理领域应用的有益探索。 相似文献
330.