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   383篇
  免费   17篇
  国内免费   6篇
测绘学   7篇
大气科学   22篇
地球物理   112篇
地质学   146篇
海洋学   46篇
天文学   31篇
自然地理   42篇
  2021年   6篇
  2020年   4篇
  2019年   5篇
  2018年   12篇
  2017年   9篇
  2016年   6篇
  2015年   10篇
  2014年   8篇
  2013年   19篇
  2012年   13篇
  2011年   27篇
  2010年   14篇
  2009年   22篇
  2008年   15篇
  2007年   13篇
  2006年   16篇
  2005年   12篇
  2004年   16篇
  2003年   15篇
  2002年   17篇
  2001年   13篇
  2000年   4篇
  1999年   7篇
  1998年   9篇
  1997年   4篇
  1996年   6篇
  1995年   5篇
  1994年   7篇
  1993年   5篇
  1992年   3篇
  1991年   4篇
  1990年   8篇
  1989年   5篇
  1988年   4篇
  1987年   3篇
  1986年   6篇
  1985年   6篇
  1984年   6篇
  1983年   2篇
  1982年   3篇
  1981年   2篇
  1980年   5篇
  1978年   3篇
  1977年   3篇
  1976年   2篇
  1974年   4篇
  1973年   7篇
  1972年   2篇
  1970年   2篇
  1862年   2篇
排序方式: 共有406条查询结果,搜索用时 15 毫秒
91.
92.
93.
94.
95.
Twice monthly sampling over two 16 month periods at a shallow site on Newfoundland's east coast showed the fish assemblage to be dominated by four taxa (Gasterosteus aculeatus, G. wheatlandi, Osmerus mordax, Gadus spp.) that accounted for 96% of the individuals collected. Of the 16 479 fish measured, 65% were adults based on the estimated size of first spawning. The fish assemblage was dominated (86%) by species with demersal eggs, several of which spawn at the same shallow sites used by juveniles as nursery sites. Coastal spawning and demersal eggs maintain offspring in coastal nursery areas where survival is thought to be increased. Number of species and number of fish were both correlated with water temperature being highest from mid-summer to early autumn and lowest in winter. Temperature, time of spawning, and movements of juveniles and adults facilitated grouping species into five assemblages based on seasonal abundance: seasonal periodic species (summer and winter), regular species, regular species collected in all seasons except winter, and occasional (rare) species. At the diel scale, two consistent species groupings were observed: species that showed no significant difference between day and night and species caught primarily at night. Number of night species exceeded day species by a factor of two. No seine-caught species in shallow water exhibited significantly higher catches during the day. Observations by SCUBA divers indicated some species were more abundant during day time at slightly deeper depths. This observation in conjunction with day and night seining in shallower water, suggests these species aggregate in deeper water during day and move to shallow waters at night.  相似文献   
96.
97.
Woolly rhinoceros bones, from a number of sites in Britain, have been AMS radiocarbon dated following ultrafiltration pre-treatment. These determinations give a coherent set of ages between >50 and c. 35 cal ka BP. The youngest (35,864–34,765 cal BP) come from the area around Bishopbriggs in western central Scotland and are derived from glaciofluvial sand and gravel overlain by till, both deposited during the Last Glacial Maximum (LGM) glaciation. A previous radiocarbon date from the site suggested that woolly rhinoceros lived c. 27 14C ka BP and the region was ice-free at the time. This date has had significant influence on the timing of extinction of woolly rhinoceros and the onset of glaciation over Britain during the LGM. The new dates revise this earlier determination and confirm that woolly rhinoceros became extinct in Britain after c. 35 cal ka BP, that central Scotland was ice-free at this time, and glaciation extended across this region sometime after 35 cal ka BP.  相似文献   
98.
99.
100.
Rose S 《Ground water》2007,45(3):309-317
An iterative algorithm is presented that allows the user to model the subsurface residence time of shallow ground water comprising stream base flow based on decadal scale variation of tritium concentrations. The algorithm accounts for the effects of radioactive decay, the shallow subsurface mixing of ground water with precipitation, and ground water flux. The inverse of the best-fitting modeled flux through the saturated zone is equivalent to the residence time. The data required for this model include at least two measurements of tritium in base flow for a given stream location made at least a decade apart and the long-term tritium input in precipitation for the region of interest. The model is sensitive to relatively small changes in tritium concentrations and is limited by analytic uncertainties to an accuracy of approximately +/-5 years. The algorithm was applied to stream base flow for several basins in the Piedmont Province of Georgia in which tritium concentrations were measured during the early 1990s and again in the 2000s. The model results produced highly concordant residence times for three hydrogeologically similar basins in the Upper Ocmulgee Basin in North Central Georgia. The best estimate of the average residence time for ground water comprising base flow in this Piedmont basin using this new method is between approximately 14 and 18 years. These results are generally consistent with calculations made in previous studies, and these relatively long residence times can be attributed to the storage of water in the clay soils that dominate Piedmont Province watersheds.  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