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   1442篇
  免费   12篇
公路运输   453篇
综合类   60篇
水路运输   466篇
铁路运输   42篇
综合运输   433篇
  2023年   4篇
  2022年   20篇
  2021年   13篇
  2020年   12篇
  2019年   13篇
  2018年   35篇
  2017年   43篇
  2016年   71篇
  2015年   25篇
  2014年   51篇
  2013年   215篇
  2012年   77篇
  2011年   84篇
  2010年   66篇
  2009年   82篇
  2008年   65篇
  2007年   56篇
  2006年   34篇
  2005年   32篇
  2004年   22篇
  2003年   23篇
  2002年   25篇
  2001年   22篇
  2000年   29篇
  1999年   20篇
  1998年   23篇
  1997年   26篇
  1996年   25篇
  1995年   41篇
  1994年   7篇
  1993年   22篇
  1992年   15篇
  1991年   14篇
  1990年   9篇
  1989年   4篇
  1988年   6篇
  1987年   10篇
  1986年   9篇
  1985年   8篇
  1984年   10篇
  1983年   8篇
  1982年   5篇
  1981年   7篇
  1980年   12篇
  1979年   14篇
  1978年   10篇
  1977年   6篇
  1975年   9篇
  1974年   7篇
  1972年   4篇
排序方式: 共有1454条查询结果,搜索用时 15 毫秒
991.
During 2005–2008 species composition, abundance patterns, natural mortality rates and salinity effects on zooplankton were studied in the Bosphorus regions of the Black and Marmara Seas. The tendencies to diminish for abundance and biomass of the Black Sea originated zooplankton species and to increase for the proportion of their carcasses in the direction from the Black Sea toward the Marmara Sea were found. The mortality in the Black Sea species increased with depth in the Marmara Sea. The contribution of organic matter of carcasses of the Black Sea originated organisms to bacterial processes in the deep strata of the Marmara Sea was estimated. Different salinity and temperature regimes restrict mutual penetration of the species in these seas.  相似文献   
992.
Due to the importance of drayage operations, operators at marine container terminals are increasingly looking to reduce the time a truck spends at the terminal to complete a transaction. This study introduces an agent-based approach to model yard cranes for the analysis of truck turn time. The objective of the model is to solve the yard crane scheduling problem (i.e. determining the sequence of drayage trucks to serve to minimize their waiting time). It is accomplished by modeling the yard crane operators as agents that employ reinforcement learning; specifically, q-learning. The proposed agent-based, q-learning model is developed using Netlogo. Experimental results show that the q-learning model is very effective in assisting the yard crane operator to select the next best move. Thus, the proposed q-learning model could potentially be integrated into existing yard management systems to automate the truck selection process and thereby improve yard operations.  相似文献   
993.
In this paper we introduce, for the first time, a methodology from the most recent literature of finance to reveal the duration of shipping cycles and then show the benefit of the use of this information to make more successful shipping loans. This is so as banks are willing to finance, during boom periods, shipping loans for new buildings but by this way ‘create’ oversupply and thus depress the freight market by their own actions. The information about cycles, especially their forecasting, is mostly important as shipping loans are based on project financing/cash flow financing, which means that ship revenue is of utmost importance. The Rescaled Range Analysis is applied here to 379 monthly freight trips—made stationary—between 1971 and 2002 (July), due to Hurst 1 Hurst, HE. 1950. Long-term storage capacity of reservoirs. April1950. pp.770808. American Society of Civil Engineers. Paper No. 2447 [Google Scholar] and elaborated and popularized by Mandelbrot 2 Mandelbrot, BB. 1975. Stochastic models for the earth's relief, the shape and the fractal dimensions of the coastlines, and the number-area rule for islands. Proceedings of the National Academy of Sciences. 1975, USA 72. pp.38253828. [Crossref] [Google Scholar]. The most important effect, however, is that shipping freight series exhibit non-normality and long-run dependence rendering the use of random walk models such as GARCH (Generalized Autoregressive Conditional Heteroscedasticity) problematic. Thus an adequate literature review is carried out with criticism against the models used. The cycles have been calculated as equal to 4.5 years and 2.25 years. This is almost compatible with the most recent paper of Stopford 3 Stopford, M. 21 September 2001. “Forecasting the dry bulk, tanker and container markets”. In Maritime Cyprus 21 September,  [Google Scholar]. The Hurst exponent was found equal to 0.93, alternating over the periods examined (0.65, 0.73, 0.62, 0.59 and 0.55) and indicating long-term persistence but seriously away from normal/random walk domain. Most studies have said the same using the Jarque--Bera test for normality but provided no alternative.  相似文献   
994.
This paper forecast/predicted the one-year time charter weekly freight rates earned by a 65 000 dwt bulk carrier using 996 weeks of data from 1989 to 2008. First, the need and the importance, but also the futility, of forecasting is discussed in shipping. This is a volatile industry that can be easily likened to the roulette. The introduction is followed by a literature review that has examined the principal recent works in this area and presented a critique of earlier works. Most of the research studied dealt with the shipping industry per se. Since the methods used are considered as a departure from the classical Random Walk, a comprehensive section of the paper is devoted to the methodology of nonlinear, chaotic and deterministic methods. The relevant time series have been transformed into stationary ones, as this is the proper practice (using first logarithmic differences). The time series were tested for randomness (identically and independently distributed) and for long-term correlation using BDS statistic. The methods used were: Rescaled Range Analysis and the related Hurst Exponent; Power Spectrum Analysis; V-statistic and BDS Statistic (using software MATLAB 5.3 and NLTSA V.2.0/2000). The analysis of the data was presented in three separate sections. The relevant ‘attractor’ of the system has been graphically shown. System's dimension has been calculated, which was found to be non-integer, fractal and equal to 3.95. This finding permitted us to proceed to forecasting, as this is a case of a low dimensional chaos (3.95 < 10 dimensions). In order for the predictions to be robust, the prediction horizon allowed was found equal to 8.24 weeks, as indicated by the positive maximum Lyapunov exponent (0.12 rounded). Then NLTSA software was used to make prediction inside- and forecasting outside- the sample, using by selection nonlinear Principal Components and Kernel Density Estimation methods.  相似文献   
995.
In this article, we argue that sectoral adaptation efforts to climate change, e.g. of the ports sector, are also struggles to reshape economic space according to sectoral needs. Addressing globalisation, the effects on economic spatial hierarchies among regions and the active promotion of regionalisation are seen as important. Applied to the port industry, this allows approaching the fierce competition among European north range ports from an action- and power-oriented perspective. Climate adaptation of ports is predominantly referred to as technical responses to extreme events (e.g. coastal protection). A differentiated conceptualisation (based on still ongoing research), however, also addresses impacts on specific elements of the transport chain, and effects on the spatial function of a port (e.g. the changing competitiveness within the European port system). Hence, the ability of a specific port to adapt will also encompass the management of regional target conflicts, and of multi-level relations. Thus, climate adaptation becomes part of positional struggles in spatial hierarchies among regions and of conflicts about political priorities within them. At the same time, the limits of exclusively regional approaches in addressing sustainability issues without higher level support become evident. The article gives an overview of the literature on climate adaptation and its application to ports and provides a preliminary typology of forms of sectoral adaptation to climate change.  相似文献   
996.
The coastal zone has critical natural, commercial, recreational, ecological, industrial, and esthetic values for current and future generations. Thus, there are increasing pressures from population growth and coastal land development. Local coastal land use planning plays an important role in implementing the U.S. Coastal Zone Management Act (CZMA) by establishing goals and performance policies for addressing critical coastal issues. This study extends the CZMA Performance Measurement System from the national level to the local land use level by measuring coastal zone land use plan quality and political context in fifty-three Pacific coastal counties. Plan quality is measured using an evaluation protocol defined by five components and sixty-eight indicators. The results indicate a reasonable correspondence between national goals and local coastal zone land use planning goals, but a slight gap might exist between the national/state versus local levels in the overall effectiveness of coastal zone management (CZM) efforts. The results show many U.S. Pacific coastal counties lack strong coastal zone land use plans because the average plan quality score was only 22.7 out of 50 points. Although these plans set relatively clear goals and objectives, they are somewhat weaker in their factual basis, identify a limited range of the available planning tools and techniques, and establish few coordination and implementation mechanisms. The regression analysis results indicate that CZM plan quality was not significantly related to any of the jurisdictional characteristics.  相似文献   
997.
Oysters have been harvested on the east coast of Australia for many thousands of years. Coastal Aboriginal communities used the extensive estuarine oyster resource and may have farmed oysters by establishing shell cultch beds in shallow areas of estuaries. The British colonization of Australia commenced in 1788 and oysters were initially used for food and production of lime. Concerns about unsustainable exploitation led to introduction of legislation that directed the oyster industry to aquaculture in 1884. Translocation of oyster stock for fattening, from New Zealand to Australian east coast estuaries, was encouraged. Here evidence is presented that this activity resulted in “mudworm disease” appearing in oyster farming estuaries on the Australian east coast between 1880 and 1900. The pandemic permanently destroyed natural sub-tidal oyster reefs and forced the oyster industry to adopt avoidance farming techniques including intertidal farming to cope with mudworm.  相似文献   
998.
The Gulf of California hosts astounding biodiversity that supports numerous economic activities in the region. These activities, and emerging threats, are placing pressure on the region's ecosystems. Government and civil society are working to address threats through several conservation and management mechanisms. Nevertheless, the use and incorporation of scientific information—a key component for creating effective and durable management—is still deficient. This article presents the concept of science integration and discusses the findings of a study that assesses the regional landscape, existing institutional arrangements, and capacity for using science to inform policy and management decisions. It also explores the current use of science within fisheries policy and management and the capacity of the National Network of Information and Research of Fisheries and Aquaculture (RENIIPA) and the State Fisheries and Aquaculture Councils, two mechanisms in the region. Finally, it shares lessons learned and offers recommendations on how the region can strengthen science-based decision-making. Results indicate that while there are some actors in the Gulf of California producing relevant science, there is varying capacity of intermediary groups connecting producers with users of science, or mechanisms in place to ensure that science is being utilized in decision-making processes. Moreover, despite having a well-developed landscape of producers and intermediaries and mechanisms in place for fisheries management in the region, effective science integration is not occurring.  相似文献   
999.
Abstract

An analysis of ecological systems that both sustain and are sustained by coastal waters provides the key to a biophysical procedure for delineating inland coastal management boundaries. This analysis entails two basic tasks: (1) mapping the ecosystems that compose coastal waters and adjacent areas, and (2) charting sustaining flows among these systems. The resulting boundary encompasses all environments of coastal waters (subaqueous areas containing a measurable quantity of seawater) and all shore‐lands (either emergent or submergent environments that interchange sustaining materials, energy, or biota with coastal waters). As this biophysical procedure depends on the precise location of, and functional transfers among, coastal ecosystems, it provides a means both for assessing the consequences of human actions and for establishing a landward boundary for a management program. Alternative boundaries not based on locations and operations of coastal ecosystems would probably be either arbitrary or of undue extent, nor would such “alternative”; boundaries be an integral part of an ongoing management process.  相似文献   
1000.
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