全文获取类型
收费全文 | 16855篇 |
免费 | 3518篇 |
国内免费 | 4442篇 |
专业分类
测绘学 | 1448篇 |
大气科学 | 3934篇 |
地球物理 | 3782篇 |
地质学 | 8964篇 |
海洋学 | 1808篇 |
天文学 | 423篇 |
综合类 | 1670篇 |
自然地理 | 2786篇 |
出版年
2024年 | 109篇 |
2023年 | 256篇 |
2022年 | 487篇 |
2021年 | 655篇 |
2020年 | 646篇 |
2019年 | 787篇 |
2018年 | 616篇 |
2017年 | 782篇 |
2016年 | 809篇 |
2015年 | 894篇 |
2014年 | 1064篇 |
2013年 | 1252篇 |
2012年 | 1158篇 |
2011年 | 1149篇 |
2010年 | 1024篇 |
2009年 | 1117篇 |
2008年 | 1153篇 |
2007年 | 1265篇 |
2006年 | 1228篇 |
2005年 | 1058篇 |
2004年 | 960篇 |
2003年 | 871篇 |
2002年 | 749篇 |
2001年 | 701篇 |
2000年 | 639篇 |
1999年 | 580篇 |
1998年 | 500篇 |
1997年 | 446篇 |
1996年 | 351篇 |
1995年 | 301篇 |
1994年 | 264篇 |
1993年 | 232篇 |
1992年 | 169篇 |
1991年 | 116篇 |
1990年 | 109篇 |
1989年 | 69篇 |
1988年 | 71篇 |
1987年 | 35篇 |
1986年 | 26篇 |
1985年 | 33篇 |
1984年 | 15篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1979年 | 5篇 |
1978年 | 11篇 |
1977年 | 13篇 |
1976年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
41.
Complexity theory has received considerable attention over the past decade from a wide variety of disciplines. Some who write on this topic suggest that complexity theory will lead to a unifying understanding of complex phenomena; others dismiss it as a passing and disruptive fad. We suggest that for the analysis of coupled natural/human systems, the truth emerges from the middle ground. As an approach focused as much on the connections among system elements as the elements themselves, we argue that complexity theory provides a useful conceptual framework for the study of coupled natural/human systems. It is, if nothing else, a framework that leads us to ask interesting questions about, for example, sustainability, resilience, threshold events, and predictability.In this paper we attempt to demystify the ongoing discussions on complexity theory by linking its evocative and overloaded terminology to real-world processes. We illustrate how a shift in focus from system elements to connections among elements can lead to meaningful insight into human-environment interactions that might otherwise be overlooked. We ground our discussion in ongoing interdisciplinary research surrounding Yellowstone National Park’s northern elk winter range; a tightly coupled natural/human system that has been the center of debate, conflict, and compromise for more than 135 years. 相似文献
42.
Klauea historical summit lavas have a wide range in matrix 18OVSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, 18O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine 18O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma 18O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine 18O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination 相似文献
43.
This article describes a unique flood hazard, produced by the dramatic expansion of wetlands in Nelson County, located within
the North American Prairie Pothole Region of North Dakota, USA. There has been an unprecedented increase in the number, average
size, and permanence of prairie wetlands, and a significant increase in the size of a closed lake (Stump Lake) due to a decade-long
wet spell that began in 1993 following a prolonged drying trend. Base-line land cover information from the 1992 USGS National
Land Cover Characterization dataset, and a Landsat TM scene acquired 9 July 2001 are used to assess the growth of the closed
lake and wetland pond surface areas, and to analyze the type and area of various land cover classes inundated between 1992
and 2001. The open water profile in Nelson County changed from one marked by relatively comparable coverage of closed lake
and wetland pond areas in 1992, to one in which wetland open water accounted for the vast majority of total open water in
2001. The bulk of the wetland pond area expansion occurred by displacing existing wetland vegetation and agricultural cropland.
Producers responded to the flood hazard by filing Federal Crop Insurance Corporation (FCIC) claims and enrolling cropland
in the Conservation Reserve Program (CRP), a federal land retirement program. Land taken out of agricultural production has
had an enormous impact upon the agricultural sector that forms the economic base of the rural economy. In 2001 the land taken
out of production due to CRP enrollment and preventive planting claims represented nearly 42% of Nelson County’s 205.2 K ha
base agricultural land. The patterns obtained from this detailed study of Nelson County are likely to be the representative
of the more publicized flood disaster occurring within the Devils Lake Basin of North Dakota. 相似文献
44.
从混凝土管材的选定、工作坑的设计及施工、顶管顶进中若干关键问题入手,介绍了手掘式顶管施工技术在成都市中心城水环境综合整治工程G标段管道排水工程中的应用。 相似文献
45.
46.
47.
48.
应用车贝雪夫多项式分析过渡季节(6—7月)澳大利亚冷高压与赤道辐合带的关系,得出了对赤道辐合带北移的主要形势以及过程演变特征,最后用合成方法得到它的演变概念模式,为赤道辐合带的北移影响西太平洋及南海地区提供了中期预报的依据。 相似文献
49.
刘连吉 《中国海洋大学学报(自然科学版)》1997,(3)
指出ISD单片语言器件的独特之处,是采用直接模拟存储技术,语音信号以其原本的模拟形式直接存入模拟量存储器中并长远保存。省去了传统的A/D和D/A数模转换机制,能高保真的将语音内容再生。这是一种高新录/放技术。另外,给出的ISD基本录/放电路十分简单易用。可方便地为微机系统、测控仪表电器和广播等设备所采用,使设备具有高效功能 相似文献
50.
GUI Maochang * WU Lingjuan . Institute of Oceanology Chinese Academy of Sciences Qingdao China. Laboratory of Numerical Modeling for Atmospheric Sciences Geophysical Fluid Dynamics Institute of Atmospheric Physics Chine-se Academy of Sciences Beijing China. Graduate School Chinese Academy of Sciences Beijing China 《海洋学报(英文版)》2005,(4)
1 IntroductionIt is well known that interaction between the trop-ical ocean and atmosphere produces the largest inter-annual climate signal, El Nino-Southern Oscillation(ENSO). In past decades many efforts have been madein understanding and predicting ENSO: such as the hy-pothesis of Bjerknes (1969) that ENSO arises as aself-sustained cycle in which SSTA in the Pacific O-cean causes the trade winds to strengthen or slackenand that this in turn drives the ocean circulation changesthat … 相似文献