全文获取类型
收费全文 | 1999篇 |
免费 | 302篇 |
国内免费 | 347篇 |
专业分类
测绘学 | 178篇 |
大气科学 | 264篇 |
地球物理 | 440篇 |
地质学 | 710篇 |
海洋学 | 488篇 |
天文学 | 48篇 |
综合类 | 151篇 |
自然地理 | 369篇 |
出版年
2024年 | 10篇 |
2023年 | 34篇 |
2022年 | 68篇 |
2021年 | 89篇 |
2020年 | 75篇 |
2019年 | 105篇 |
2018年 | 83篇 |
2017年 | 83篇 |
2016年 | 60篇 |
2015年 | 76篇 |
2014年 | 91篇 |
2013年 | 178篇 |
2012年 | 103篇 |
2011年 | 86篇 |
2010年 | 103篇 |
2009年 | 94篇 |
2008年 | 105篇 |
2007年 | 160篇 |
2006年 | 152篇 |
2005年 | 128篇 |
2004年 | 124篇 |
2003年 | 88篇 |
2002年 | 88篇 |
2001年 | 74篇 |
2000年 | 73篇 |
1999年 | 75篇 |
1998年 | 49篇 |
1997年 | 39篇 |
1996年 | 27篇 |
1995年 | 18篇 |
1994年 | 28篇 |
1993年 | 20篇 |
1992年 | 14篇 |
1991年 | 6篇 |
1990年 | 8篇 |
1989年 | 15篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有2648条查询结果,搜索用时 15 毫秒
61.
In order to understand the structure of fish assemblages in the modified Lima basin (Northern Portugal), two distinct datasets concerning the presence and abundance of fish species were subjected to multivariate analysis. On the River Lima two types of flow modification are present within kilometres of one another: (a) a reduced and constant flow due to hypolimnetic release; and (b) an intense and irregular flow. A comparison of their influence on fish assemblages revealed a gradient of assemblage types from tributaries to main river sites. The latter were characterised by a strong dominance of cyprinids, particularly Iberian barbel (Barbus bocagei). The former harboured two kinds of fish assemblages: those closer to the river mouth were dominated by the cyprinids Iberian chub (Squalius carolitertii) and Iberian nase (Chondrostoma polylepis), which were also frequently present in the main river; while in those further upstream the predominant species was the brown trout (Salmo trutta). Although explanatory variables such as distance from source, altitude, substrate coarseness and width were the primary correlates of fish assemblage composition, dam construction and flow regulation also had a significant effect upon assemblage structure, particularly by: i) reducing the importance of migratory species; ii) constraining the presence of trout in the regulated segments; and iii) simplifying the community, especially in the case of the constant and reduced flow regime. 相似文献
62.
63.
人工土层冻结法加固在盾构出洞施工中的应用 总被引:10,自引:0,他引:10
软土地区盾构出洞施工中洞口土体易失稳、渗水,上海明珠线二期工程浦东大道站至张扬路站区间,隧道在盾构出洞施工中,为确保地面建筑及地下管线的安全及正常使用,首次采用了人工土层冻结加固,取得了良好的效果;本文介绍了该工程出洞口土体加固的方案选择、关键技术处理及实际取得的效果,并探讨了人工土层冻结加固在含水松软土层的地下工程中的应用前景. 相似文献
64.
国外国土开发整治与规划的经验及启示 总被引:3,自引:1,他引:3
本文简要回顾了20世纪以来国外国土开发整治与规划的过程,总结了国外在国土规划方面取得的经验:即国土规划的权威性、系统性、针对性、可操作性、实效性和参与性等特点。在此基础上,结合我国国情,针对当前面临的形势,提出发展和完善我国国土规划的基本思路:重视国土规划;将国土规划纳入规范化、法制化的轨道;强化国土规划的系统调控和管理等。 相似文献
65.
66.
67.
James D Sidaway 《Area》2005,37(4):373-377
This paper examines a key aspect of how the global geometries of finance and trade articulate the euro–dollar relationship. Here, European integration extends into a global financial and trading system replete with contradictions. The paper examines these, drawing upon a literature from international political economy, foregrounding the spatialities that they signal. 相似文献
68.
The swing of the main channel of the Qiantang River is controlled by the high-water and low-water changes in the river, as well as the impact of large-scale reclamation of tidal flats. Its evolution in modern times is the result of the combined functions of natural and man-made factors. This paper analyzes the cause of the formation of the South Channel and Xisan Tidal Furrow and proposes the regulation principle of “To regulate the river and reclaim tidal flats by taking the advantage of local topography”. It is suggested to cut off the South Channel and Xisan Tidal Furrow completely to restrict the swing of the main channel and to increase the reclamation area of the tidal flat at the same time. 相似文献
69.
广西南丹地区尾矿分布于大厂、车河、芒场等矿区。尾砂来自以锡石为主的多金属矿床,尾砂中除Sn,Pb,Zn,Ag,Sb的质量分数达到工业指标外,Au亦达到综合利用指标。通过试验证明,对尾矿中的细微粒锡石可采用风力分级选矿、磁选和化学选矿等方法回收;尾砂中的非金属资源可作为填充材料、水泥原料利用,使尾砂型人工矿床充分的资源化。 相似文献
70.