全文获取类型
收费全文 | 1900篇 |
免费 | 39篇 |
专业分类
公路运输 | 730篇 |
综合类 | 273篇 |
水路运输 | 441篇 |
铁路运输 | 174篇 |
综合运输 | 321篇 |
出版年
2023年 | 20篇 |
2022年 | 58篇 |
2021年 | 44篇 |
2020年 | 28篇 |
2019年 | 24篇 |
2018年 | 38篇 |
2017年 | 36篇 |
2016年 | 71篇 |
2015年 | 36篇 |
2014年 | 106篇 |
2013年 | 177篇 |
2012年 | 121篇 |
2011年 | 131篇 |
2010年 | 115篇 |
2009年 | 133篇 |
2008年 | 108篇 |
2007年 | 82篇 |
2006年 | 80篇 |
2005年 | 67篇 |
2004年 | 47篇 |
2003年 | 41篇 |
2002年 | 35篇 |
2001年 | 27篇 |
2000年 | 28篇 |
1999年 | 23篇 |
1998年 | 17篇 |
1997年 | 16篇 |
1996年 | 22篇 |
1995年 | 23篇 |
1994年 | 12篇 |
1993年 | 14篇 |
1992年 | 6篇 |
1991年 | 10篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 6篇 |
1981年 | 11篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 15篇 |
1974年 | 5篇 |
1972年 | 3篇 |
排序方式: 共有1939条查询结果,搜索用时 621 毫秒
61.
Livelihood diversity factors such as flexibility within fisheries, geographical mobility, reallocation of fishing effort into the broader economy, and the non-material benefits that fisheries provide are important areas of research in marine policy. We use two small-scale fisheries related socio-economic surveys of communities in the Tigak Islands of Papua New Guinea. The first conducted 5 years before a ban on the harvesting of sea cucumbers was imposed and the second from the present day, 5 years after the initiation of the ban—with the objectives of exploring changes in household fishing strategies (types and numbers of species targeted) and to identify any important socioeconomic factors that help explain those changes. Fishing's contribution to total household income has increased significantly (p = 0.019) up from 61% in 2004 to 73% in 2014 with the percentage of female residents living in a household now positively and significantly (p = 0.018) associated with fishing income. The average number of species categories targeted per household increased insignificantly while households with more women are significantly (p = 0.018) less likely to target more species than households with more men. Moreover, customary management practices contribute to this difference. Together, these results show that households are not worse off financially 5 years after the ban on sea cucumber harvesting and that gendered seascape use has implications for the role of livelihood diversity as a marine policy tool. 相似文献
62.
Michiel C. J. Bliemer Mark P. H. Raadsen Luuk J. N. Brederode Michael G. H. Bell Luc J. J. Wismans Mike J. Smith 《运输评论》2017,37(1):56-78
This paper presents a review and classification of traffic assignment models for strategic transport planning purposes by using concepts analogous to genetics in biology. Traffic assignment models share the same theoretical framework (DNA), but differ in capability (genes). We argue that all traffic assignment models can be described by three genes. The first gene determines the spatial capability (unrestricted, capacity restrained, capacity constrained, and capacity and storage constrained) described by four spatial assumptions (shape of the fundamental diagram, capacity constraints, storage constraints, and turn flow restrictions). The second gene determines the temporal capability (static, semi-dynamic, and dynamic) described by three temporal assumptions (wave speeds, vehicle propagation speeds, and residual traffic transfer). The third gene determines the behavioural capability (all-or-nothing, one shot, and equilibrium) described by two behavioural assumptions (decision-making and travel time consideration). This classification provides a deeper understanding of the often implicit assumptions made in traffic assignment models described in the literature. It further allows for comparing different models in terms of functionality, and paves the way for developing novel traffic assignment models. 相似文献
63.
文章首先分析了公路建设过程中对环境造成的影响,而后阐述了公路工程在路线设计时应注意的环境问题,最后列举了提升公路设计线路与环境环保的举措,希望降低工程在建设过程中对环保造成的不利影响。 相似文献
64.
65.
Dynamics of inorganic nutrient species in the Bohai seawaters 总被引:3,自引:0,他引:3
J. Zhang Z. G. Yu T. Raabe S. M. Liu A. Starke L. Zou H. W. Gao U. Brockmann 《Journal of Marine Systems》2004,44(3-4):189
Within the frame of a Sino-German Joint Research Program, two cruises of “R/V Dong Fang Hong 2” were carried out in September–October 1998 and April–May 1999, respectively, to understand the dynamics of nutrients in the Bohai. Nutrient species (NO3−, NO2−, NH4+, PO43− and SiO32−) are determined colorimetrically on board for five anchor and 30 grid stations. In situ incubation experiments are performed to determine planktonic nutrient uptake and benthic exchange flux. Nutrient concentrations display short-term variability and seasonal change in the Bohai, with higher levels in shallow coastal waters than in the Central Bohai. The influence of riverine discharge on nutrient levels can be seen from salinity isopleths, nutrient distribution and species ratios. Near-bottom (nb) waters have similar nutrient concentrations as to the surface waters in the Central Bohai, whereas stratification takes place in the Bohai Strait and North Yellow Sea. In situ incubation experiments provide evidence that the uptake ratio (i.e. N, P) by phytoplankton is proportional to the ratios among nutrient species in ambient waters. Based on the data of this study and previously publications, a preliminary estimate of nutrient budgets via riverine input and atmospheric deposition is established. The results indicate that atmospheric deposition gains importance over rivers in delivering nutrients into the Bohai and sustain the new production, following recent decrease in riverine inflow caused by drought periods in North China and damming practices. A historical review of nutrient data indicates that concentrations of nitrogen increase and phosphorus and silica decrease in the Central Bohai over last 40 years. This potentially has an important influence on the health of ecosystem in Bohai (e.g. food web and community structure), though further study is needed to examine the scenario in more detail. 相似文献
66.
67.
船舶在雾中航行,由于能见度低,瞭望、定位困难,极易发生海上交通事故,造成人员、船舶的经济损失及海洋环境的污染。据统计,在雾航中发生的船舶碰撞事故占整个海上碰撞事故总数的70%以上,因此,对雾航安全的研究一直是航运界关注的重点。琼州海峡位于海南岛与雷州半岛之间,是我国南海沟通北部湾和华南沿海的一个主要海峡通道,同时也 相似文献
68.
多智能体(Agent)系统能有效解决分布式问题,电力系统属于分布式结构。文章以智能变电站为研究对象,将Agent技术与IEC 61850标准相结合,分析了Agent技术解决IEC 61850通信时可能的通信对象冲突问题。将IED设备作为Agent,在相同局域网内通过实现多Agent自治算法协调系统,设置消息过滤和创建轮询Agent,来避免通信的混乱;在不同局域网间依靠TCP/IP协议传输消息,解决了变电站复杂的分布式通信问题。 相似文献
69.
70.