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   11796篇
  免费   106篇
  国内免费   70篇
测绘学   240篇
大气科学   744篇
地球物理   2208篇
地质学   4605篇
海洋学   1181篇
天文学   2489篇
综合类   49篇
自然地理   456篇
  2022年   144篇
  2021年   206篇
  2020年   190篇
  2019年   227篇
  2018年   526篇
  2017年   431篇
  2016年   477篇
  2015年   188篇
  2014年   413篇
  2013年   606篇
  2012年   462篇
  2011年   562篇
  2010年   556篇
  2009年   643篇
  2008年   575篇
  2007年   573篇
  2006年   493篇
  2005年   327篇
  2004年   311篇
  2003年   281篇
  2002年   274篇
  2001年   245篇
  2000年   254篇
  1999年   169篇
  1998年   184篇
  1997年   171篇
  1996年   115篇
  1995年   131篇
  1994年   115篇
  1993年   97篇
  1992年   71篇
  1991年   98篇
  1990年   97篇
  1989年   68篇
  1988年   79篇
  1987年   99篇
  1986年   80篇
  1985年   107篇
  1984年   95篇
  1983年   104篇
  1982年   86篇
  1981年   81篇
  1980年   97篇
  1979年   88篇
  1978年   87篇
  1977年   77篇
  1976年   59篇
  1975年   63篇
  1974年   71篇
  1973年   64篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Analysis of the location of the fastest growing private companies in the United States suggests that a new corporate landscape is emerging. During the 1980s this corporate landscape has increasingly been dominated by companies in the service sector rather than in manufacturing. Sunbelt metropolises, especially in California, are more favored locations for the fast-growth firms than for the traditional Fortune 500 industrial and 500 service corporations. The fast-growth firms are also more likely to locate in suburban areas than the traditional corporations. These service, Sunbelt, and suburban characteristics of the new corporate landscape are interpreted as a consequence of the restructuring of the US economy.  相似文献   
142.
This article is an English translation of the work cited in 46: 3092. The authors review the conclusions of Pivovarov (see 44: Title 3118) concerning the nature of the urbanization process and optimal city size in the USSR. The relationship between urbanization and the characteristics of a given socioeconomic system is examined, and the possibility of determining optimal size for cities of different functional types is evaluated  相似文献   
143.
144.
145.
146.
This paper attempts to explore the migration patterns during the transition period in Estonia. A structuration approach was used to analyze data from the Estonian Statistical Office collected in 1997. Findings show that for migration between urban and rural areas, work-related reasons have been the most important motivating factor in urban growth during the transition period. Also considered are the family and education. In relation to sociodemographic structure of the population, men cite work, while women count family-related reasons, as the main motive for migrating. As to nonregistration, the most significant reason relates to issues of ownership. Because migrants are living in rented housing, it is not possible for them to register even if they desire to do so. Other reasons include "temporary", associated with study and work; "juridical", bureaucratic matters; and "multiple places of residence". This analysis, however, is incomplete because the attitudes and patterns of behavior have only partially or perfunctorily been related to the dramatic changes that have occurred in Estonian society. Proper statistical data are needed to help examine trends at a more disaggregated spatial level.  相似文献   
147.
The objectives of this research were to (a) study seasonal variation of the seed bank of grasses in two areas with different grazing history, (b) analyse the effect of the presence of shrubs on the seed bank and (c) study seed viability and dormancy in buried seeds of Piptochaetium napostaense and Stipa tenuis. The seed density of grasses was in general low and showed seasonal variation. Most of the grasses showed maximum seed density in December when seed dispersal occurs. Shrub cover did not show a marked influence in seed accumulation. The relatively high number of damaged seeds suggests that predation may be an important factor in seed bank dynamics in the Caldenal. Seeds of dominant perennial forage grasses showed dormancy that could explain, at least in part, persistence of these species in spite of periodical disturbance.  相似文献   
148.
An assessment was made of the mean levels of major and trace elements and of biogenic and organic matter in glacial and river waters within the Aktru river basin (Gornyi Altai, Russia). The analysis showed a general tendency for an increase in mineralization along the direction from the region of alimentation of glaciers to the mouth of the river. A relatively abrupt increase was revealed in Al, Zn, Cu, Pb, Si, NO2 ? and NH4 + concentrations in the source of the Aktru river and its glacial tributaries, with their subsequent decrease downstream. The mechanism for such changes is governed by the conditions of interaction of rocks with meltwaters and river waters.  相似文献   
149.
Eighty-nine juvenile Atlantic sturgeon,Acipenser oxyrhynchus oxyrhynchus, including 10 recaptures, were collected, tagged and released in the upper tidal Delaware River between July 1981 and December 1984. All were captured in the river channel between Roebling and Trenton, New Jersey, using bottom-set experimental gill nets during daylight hours. The species was present from July through December and collected in increased numbers in the past two years. The fork lengths ranged from 284 to 862 mm (mean±SD = 516 mm±106 mm) and the weights from 140 to 4,250 g (mean±SD = 1,369 g±815 g). Ten juvenile Atlantic sturgeon were recaptured between September 1983 and December 1984 and were at large from one to 418 d. Recapture data suggest that these sturgeon utilize this area annually from July through December, possibly as a nursery. Juvenile Atlantic sturgeon appear to utilize the upper tidal portion of the Delaware River for a much longer period of time and at lower temperatures than in other river systems.  相似文献   
150.
The long-term data (1950–2017) on the chemical composition of the water from the source of the Angara River, which is the only run-off from Lake Baikal, a large and clean fresh water reservoir, are presented. The absence of water contamination despite the anthropogenic influence is shown. The similarity and constancy of the current composition of the waters in the lake and the Angara River are recorded. It is recommended to continue geochemical studies of this unique world heritage site.  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