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   35149篇
  免费   787篇
  国内免费   472篇
测绘学   931篇
大气科学   3306篇
地球物理   7523篇
地质学   12686篇
海洋学   2545篇
天文学   7273篇
综合类   162篇
自然地理   1982篇
  2020年   242篇
  2019年   244篇
  2018年   659篇
  2017年   632篇
  2016年   914篇
  2015年   606篇
  2014年   907篇
  2013年   1695篇
  2012年   969篇
  2011年   1205篇
  2010年   1100篇
  2009年   1478篇
  2008年   1244篇
  2007年   1081篇
  2006年   1159篇
  2005年   1000篇
  2004年   938篇
  2003年   937篇
  2002年   935篇
  2001年   823篇
  2000年   845篇
  1999年   717篇
  1998年   704篇
  1997年   713篇
  1996年   639篇
  1995年   590篇
  1994年   542篇
  1993年   462篇
  1992年   443篇
  1991年   447篇
  1990年   452篇
  1989年   437篇
  1988年   404篇
  1987年   498篇
  1986年   468篇
  1985年   490篇
  1984年   591篇
  1983年   603篇
  1982年   536篇
  1981年   524篇
  1980年   481篇
  1979年   468篇
  1978年   477篇
  1977年   427篇
  1976年   382篇
  1975年   389篇
  1974年   431篇
  1973年   419篇
  1972年   257篇
  1971年   238篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
961.
962.
963.
964.
965.
966.
967.
968.
This paper presents a statistical approach to study the spatial relationship between landslides and their causative factors at the regional level. The approach is based on digital databases, and incorporates such methods as statistics, spatial pattern analysis, and interactive mapping. Firstly, the authors propose an object-oriented conceptual model for describing a landslide event, and a combined database of landslides and environmental factors is constructed by integrating the various databases within such a conceptual framework. The statistical histogram, spatial overlay, and dynamic mapping methods are linked together to interactively evaluate the spatial pattern of the relationship between landslides and their causative factors. A case study of an extreme event in 1993 on Lantau Island indicates that rainfall intensity and the migration of the center of the rainstorm greatly influence the occurrence of landslides on Lantau Island. A regional difference in the relationship between landslides and topography is identified. Most of the landslides in the middle and western parts of the island occurred on slopes with slope angles of 25–35°, while in the eastern part, the corresponding range is 30–35°. Overlaying landslide data with land cover reveals that a large number of landslides occurred in the bareland and shrub-covered area, and in the transition zones between different vegetation types. The proposed approach can be used not only to analyze the general characteristics of such a relationship, but also to depict its spatial distribution and variation, thereby providing a sound basis for regional landslide prediction.  相似文献   
969.
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   
970.
In areas of broadly distributed extensional strain, the back‐tilted edges of a wider than normal horst block may create a synclinal‐horst basin. Three Neogene synclinal‐horst basins are described from the southern Rio Grande rift and southern Transition Zone of southwestern New Mexico, USA. The late Miocene–Quaternary Uvas Valley basin developed between two fault blocks that dip 6–8° toward one another. Containing a maximum of 200 m of sediment, the Uvas Valley basin has a nearly symmetrical distribution of sediment thickness and appears to have been hydrologically closed throughout its history. The Miocene Gila Wilderness synclinal‐horst basin is bordered on three sides by gently tilted (10°, 15°, 20°) fault blocks. Despite evidence of an axial drainage that may have exited the northern edge of the basin, 200–300 m of sediment accumulated in the basin, probably as a result of high sediment yields from the large, high‐relief catchments. The Jornada del Muerto synclinal‐horst basin is positioned between the east‐tilted Caballo and west‐tilted San Andres fault blocks. Despite uplift and probable tilting of the adjacent fault blocks in the latest Oligocene and Miocene time, sediment was transported off the horst and deposited in an adjacent basin to the south. Sediment only began to accumulate in the Jornada del Muerto basin in Pliocene and Quaternary time, when an east‐dipping normal fault along the axis of the syncline created a small half graben. Overall, synclinal‐horst basins are rare, because horsts wide enough to develop broad synclines are uncommon in extensional terrains. Synclinal‐horst basins may be most common along the margins of extensional terrains, where thicker, colder crust results in wider fault spacing.  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