全文获取类型
收费全文 | 48879篇 |
免费 | 8196篇 |
国内免费 | 11489篇 |
专业分类
测绘学 | 4222篇 |
大气科学 | 5494篇 |
地球物理 | 9840篇 |
地质学 | 30208篇 |
海洋学 | 5392篇 |
天文学 | 2183篇 |
综合类 | 3359篇 |
自然地理 | 7866篇 |
出版年
2024年 | 254篇 |
2023年 | 705篇 |
2022年 | 1393篇 |
2021年 | 1567篇 |
2020年 | 1668篇 |
2019年 | 1882篇 |
2018年 | 1559篇 |
2017年 | 1810篇 |
2016年 | 2001篇 |
2015年 | 2138篇 |
2014年 | 2564篇 |
2013年 | 2607篇 |
2012年 | 2889篇 |
2011年 | 3054篇 |
2010年 | 2645篇 |
2009年 | 3141篇 |
2008年 | 3104篇 |
2007年 | 3448篇 |
2006年 | 3317篇 |
2005年 | 2937篇 |
2004年 | 2718篇 |
2003年 | 2619篇 |
2002年 | 2375篇 |
2001年 | 2043篇 |
2000年 | 1902篇 |
1999年 | 1765篇 |
1998年 | 1528篇 |
1997年 | 1395篇 |
1996年 | 1264篇 |
1995年 | 1080篇 |
1994年 | 1130篇 |
1993年 | 948篇 |
1992年 | 749篇 |
1991年 | 540篇 |
1990年 | 485篇 |
1989年 | 379篇 |
1988年 | 291篇 |
1987年 | 195篇 |
1986年 | 121篇 |
1985年 | 87篇 |
1984年 | 36篇 |
1983年 | 31篇 |
1982年 | 32篇 |
1981年 | 22篇 |
1980年 | 26篇 |
1979年 | 28篇 |
1978年 | 35篇 |
1977年 | 24篇 |
1975年 | 4篇 |
1954年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
331.
本文介绍了国家标准《潜水员水下用电安全技术规范》的研究、编制过程,以及编者对该标准性质、适用范围、总体结构、电气安全数值取舍等的基本认识,并就该标准制定实施后的经济、社会效益进行了初步预测。 相似文献
332.
IwrincrIONOn the evolution of palcoenvironment, salt-water encroachmnt and the distributionof subsurface brine in the LaizhOu Bay area, many researches had been carried out, and muchknowledge abeut the gcolOgical features in this area since Late QUaternary was aCquired' ) (H8nand Wu, l992; Han and Meng, l994; Wen et al. l989). However there was IittIe studyon the sediment geOChemical characteristics since Quaternary. In this Paper, the element strati-graphic characteristics and evolut… 相似文献
333.
334.
Pyruvate kinase (PK) activity measurements are used to assess the role of carbohydrates in global feeding of mesozooplankton communities inhabiting an estuary plume. As a consequence of a remarkably low freshwater discharge rate, the sea surface layers of the area under estuarine influence showed a very moderate salinity fall and a nearly total depletion in nitrates, whereas higher levels of these nutrients were found in deeper, more saline, layers. Small-scale PK activity variations in mesozooplankton appear to be closely correlated to nitrate integration values within the water column. The results were analysed in comparison with literature reports. The study produced a coherent overall interpretation, which strongly supports the reliability of this new biochemical tool in detecting assimilation of trace carbohydrates in the diet of mesozooplankton. 相似文献
335.
336.
As a fundamental study to evaluate the contribution of the Kuroshio to primary production in the East China Sea (ECS), we
investigated the seasonal pattern of the intrusion from the Kuroshio onto the continental shelf of the ECS and the behavior
of the intruded Kuroshio water, using the RIAM Ocean Model (RIAMOM). The total intruded volume transport across the 200m isobath
line was evaluated as 2.74 Sv in winter and 2.47 Sv in summer, while the intruded transport below 80m was estimated to be
1.32 Sv in winter and 1.64 Sv in summer. Passive tracer experiments revealed that the main intrusion from the Kuroshio to
the shelf area of the ECS, shallower than 80m, takes place through the lower layer northeast of Taiwan in summer, with a volume
transport of 0.19 Sv. Comparative studies show several components affecting the intrusion of the Kuroshio across the 200 m
isobath line. The Kuroshio water intruded less onto the shelf compared with a case without consideration of tide-induced bottom
friction, especially northeast of Taiwan. The variations of the transport from the Taiwan Strait and the east of Taiwan have
considerable effects on the intrusion of the Kuroshio onto the shelf. 相似文献
337.
ABSTRACT Understanding temporal patterns in restored environments is important for identifying potential barriers to recovery and improved management of degraded habitats. In this paper, we use temporal beta diversity analyses to compare invertebrate community recovery trajectories in three restored agricultural stream sites under different integrated catchment plans, a native forest reference site, and two unmodified pasture control stream sites over 24 years. The restored sites diverged from their initial community composition over time and became more similar to the reference site community, which was relatively stable over time. Variation partitioning showed that prior to restoration beta diversity was primarily associated with environmental and spatial drivers, whereas post-restoration beta diversity was more influenced by temporal and environmental drivers, including changes in substrate size, fine sediments, water clarity, and nutrients, as well as temperature and flow regime. Species’ contributions to beta diversity varied between sites and years, with sensitive EPT taxa contributing more in reference and control sites. However, contributions of some EPT species, particularly mayflies, increased in restored sites post-ICM. In summary, after nearly two decades of ICM, restored stream sites show recovery towards reference conditions, yet differences persist, indicating that rehabilitation may take longer, depending on the restoration goals. 相似文献
338.
In recent years many fuller ship hull forms have been designed and constructed in various shipbuilding countries, but the data available for the development of the fuller forms are inadequate from the point of view of preliminary ship design. In this paper the authors describe how they have systematically tested vessel forms of block coefficients ranging from 0.80 to 0.90. The analysis and presentation of the test results have been made in such a manner that designers can produce hull forms like those of tankers and other bulk carriers quickly and reliably. 相似文献
339.
应用多年的实测资料 ,分析了曹娥江河口高水位成因 ,结果表明 ,曹娥江河口洪水位具有山溪性和可冲性的特点 ,人类活动对高水位有较大影响。应用统计分析法和成因分析法推求 1 %设计高水位 ,讨论了成因分析法中上、下边界条件的选取 ,其选取原则对其他潮汐河口确定设计高水位具有借鉴意义 相似文献
340.
根据海南岛榆林验潮站1954~1992年连续39a潮位观测资料,经过统一基准面校正后,运用多种统计分析方法研究,得出近40a来海南岛南岸的相对海平面变化呈上升趋势,相应的平均上升速率为0.64mm/a,这一数值较近百年来1~2mm/a的全球海平面上升速率明显偏小,反映出同期海南岛南岸的地面是微弱抬升的。 相似文献