全文获取类型
收费全文 | 682篇 |
免费 | 8篇 |
专业分类
公路运输 | 165篇 |
综合类 | 31篇 |
水路运输 | 228篇 |
铁路运输 | 24篇 |
综合运输 | 242篇 |
出版年
2023年 | 3篇 |
2022年 | 15篇 |
2021年 | 4篇 |
2020年 | 6篇 |
2019年 | 8篇 |
2018年 | 21篇 |
2017年 | 18篇 |
2016年 | 30篇 |
2015年 | 8篇 |
2014年 | 9篇 |
2013年 | 107篇 |
2012年 | 28篇 |
2011年 | 31篇 |
2010年 | 34篇 |
2009年 | 28篇 |
2008年 | 27篇 |
2007年 | 30篇 |
2006年 | 18篇 |
2005年 | 19篇 |
2004年 | 9篇 |
2003年 | 10篇 |
2002年 | 10篇 |
2001年 | 11篇 |
2000年 | 21篇 |
1999年 | 12篇 |
1998年 | 14篇 |
1997年 | 12篇 |
1996年 | 10篇 |
1995年 | 19篇 |
1994年 | 5篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1973年 | 3篇 |
1972年 | 4篇 |
排序方式: 共有690条查询结果,搜索用时 265 毫秒
211.
212.
Irwin P. Levin 《Transportation》1982,11(1):71-85
In each of two experiments evaluations were obtained of alternative carpooling situations varying in driving arrangement, size of carpool, distance traveled, and amount of time to pick up and deliver passengers. College students in Experiment I rated each situation in terms of comfort, economy, convenience and overall desirability. Shared driving emerged as the most desirable driving arrangement, and results suggested that this was due to a tradeoff of the perceived economic advantages of being the driver and the perceived greater comfort and convenience of being a rider. Driving all the time was the least preferred arrangement for females, and riding all the time was the least preferred arrangement for males. Desirability ratings decreased as carpool size increased, and this was interpreted to indicate that perceived economic advantages of large pools were outweighed by large perceived decreases in comfort and convenience. The major results of Experiment I were replicated in Experiment II, using university employees who were either currently in carpools or seeking to find carpools. Results are discussed in terms of the practical implications of the findings concerning driving arrangement and carpool size preferences, and in terms of the usefulness of controlled experimental designs for understanding the processes-underlying ride-sharing attitudes and behavior. 相似文献
213.
214.
Rapid urbanization has taken place in China since the Open Policy in 1978. In face of the growing demand for mobility in large cities, new metro systems were developed in large cities like Guangzhou and Shanghai. At present, there are seven cities with 10 metro lines under construction. What are the policy issues and major challenges of developing metro systems in China? This paper systematically reviews the current situation and highlights three major gaps that Chinese cities have to overcome in planning metro systems. They are the technology gap, the financing gap and the affordability gap. A discussion of these gaps in the Chinese context leads us to the conclusion that early planning and careful studies are important in the development of metro systems in China. Moreover, the existing official criteria (population and economic power) for approving the building of metro systems are insufficient and should be supplemented by more vigorous evaluation criteria. 相似文献
215.
Long-term variability of the biogeochemical properties during the formation of central waters in the Eastern North Atlantic were analyzed between 42–47°N and 10–20°W from the dataset gathered during the Galicia VII (GVII) and C. Darwin 58/59 (CD58/59) cruises. These cruises that showed important changes in the thermohaline properties and the nutrient abundance of the upper layers were carried out under contrasting conditions of the North Atlantic Oscillation (NAO) index. The different climate forcing led a meridional shift of the transition zone between the formation regions of subpolar and subtropical Eastern North Atlantic Central Water (ENACWp and ENACWt, respectively). This displacement conditioned the presence of each ENACW in the study region and so the thermohaline and biogeochemical properties. The effect of the observed variability at decadal scale on the air–sea CO2 gradient (ΔfCO2) and exchange (FCO2) was analyzed using 1D model approach throughout 11 weekly-steps that simulated the development of a spring bloom during the shoaling of the mixed layer. The outputs of the model showed an intensification of the ocean CO2 uptake due to higher biological CO2 drawdown, during positive NAO conditions and its weakening under negative NAO influence. 相似文献
216.
A recent metaanalysis identified certain management attributes that are associated with successful management of threshold-based systems. However, high variance among case studies indicates that these attributes do not guarantee good conservation outcomes, suggesting that additional factors may be at play. To better understand these additional factors, we compiled a list of effective governance attributes from the literature, and developed guidance for systematically evaluating their presence, absence, and the extent to which each attribute is actually manifested in a given case study. We also examine the distribution of rights and responsibilities within a system, and the resulting impacts on stewardship incentives. Here we present the results of this analysis as applied to Kāneohe Bay, Hawai‘i. Our results confirm that absent or incomplete effective governance attributes can negatively impact conservation outcomes. In Kāneohe Bay, a public-private partnership temporarily compensated for gaps and weaknesses in the governance system, thereby creating conditions conducive to successfully reducing populations of invasive algae. However, this partnership has since dissolved and current capacity to address this and other issues in this system is again lacking. Failure to fix governance weaknesses may compromise the continued health and functioning of the Kāne'ohe Bay system. 相似文献
217.
A box model has been implemented to understand the large-scale biogeochemical cycles of nitrogen, phosphorus, and silicon in the Gulf of Riga. The large data sets collected within the international Gulf of Riga Project in 1993/1995 were used to validate the model. The comparison to data was useful in scaling up to the gulf-wide level and scrutinizing the conclusions based on short-term field surveys and experimental studies. The simulations indicate that the limiting role was passing from silicon to phosphorus to nitrogen over the seasons of organic production. However, on an annual scale, nutrient limitation was close to the “Redfield equilibrium”. Mass balance considerations, based on modeled coupled fluxes, disagree with the conclusions on low sediment denitrification and high phosphorus retention in the pelagic system, which were derived from isolated measurements.Nutrient budgets constructed with the model revealed the high buffer capacity of the Gulf of Riga. The nutrient residence times span a range from 6 years for N to 70 years for Si. The buffering arises from intensive internal recycling in the water body and by the bottom sediments. The budgets indicate that the Gulf retains about two-thirds of external nitrogen and silicon inputs, while phosphorus retention is only 10%.A slow response to external perturbations is demonstrated with numerical experiments run for 15 years under 50% reductions of terrestrial nutrient inputs. These experiments imply that the most effective is the N+P reduction scenario, which resulted in a 20% decrease of primary production after 12 years. A reduction of P resulted in only a 6% decrease of primary production; however, it yielded an 80% drop in the amount of nitrogen fixation. 相似文献
218.
P. J. Minnett F. Bignami E. Bhm G. Budus P. S. Galbraith P. Gudmandsen T. S. Hopkins R. G. Ingram M. A. Johnson H. J. Niebauer R. O. Ramseier W. Schneider 《Journal of Marine Systems》1997,10(1-4)
A summary of the seasonal development of the Northeast Water Polynya ice cover characteristics is presented. This is based primarily on satellite remote sensing observations, with some in situ measurements, including both new and published data. 相似文献
219.
Francisco Pérez José A. Suárez Juan A. Clemente Antonio Souto 《Journal of Marine Science and Technology》2007,12(2):83-94
Nowadays, bulbous bow forms have become a common design feature in most conventional ship designs. The design of a bulbous
bow is usually attempted with the use of certain form parameters that are imposed using the designer's experience or regression
values that provide the optimum parameters based on experimental tests or computational fluid dynamics (CFD) calculations.
In this article, the geometric modelling of a bulbous bow form that complies with a series of parameters is presented. First,
a wire model was constructed with cubic B-spline curves that hold certain form parameters that the designer wants to impose.
Second, a B-spline surface that fits these splines was mathematically constructed. A review of the influence of the design
parameters used on the bulbous bow properties was made. This method could be used prior to numerical optimization of a bulbous
bow because different variations of the design can be easily generated. These variations can be tested numerically using CFD
software. A practical application example of the method is shown. 相似文献
220.
A study of the response of chlorophyll-a biomass to a winter upwelling event off Western Iberia using SeaWiFS and in situ data 总被引:2,自引:0,他引:2
Observations of a winter upwelling event off Western Iberia shelf/slope in the area of influence of the Western Iberia Buoyant Plume (WIBP) were conducted in February 2000. Spatial patterns and time evolution of the chlorophyll-a (chl-a) biomass are analysed, based on in situ and satellite data. SeaWiFS-derived chl-a concentration L2 products were used to track the chlorophyll front and estimate its westward migration velocity (maximum up to 29 km day−1), as well as to characterize the frontal system and its evolution. A method associating the type of spectral signature of a pixel to the fraction of chlorophyll probed by SeaWiFS enabled the estimation of the chl-a biomass within error intervals. High chlorophyll concentrations (for wintertime) were observed over the shelf and slope, up to large distances to the coast. Due to the WIBP, a shallow Ekman layer developed, being nearly coincident with the stratified upper meters. The transport comprised westward advection and stretching of the plume, with little entrainment with the offshore deep mixed layer waters. The relative enlargement of the total area of the Inside-Front Zone (IFZ) during the upwelling event was roughly accompanied by the maintenance of the average biomass per unit of area, considering the water column up to depths of interest. This suggests that there was a net increase of chl-a biomass inside the water column associated with the IFZ, roughly proportional to the increase in the IFZ area. Retention of phytoplankton in the shallow stratified nutrient-rich waters of the WIBP was a key factor for this increase in chl-a biomass. 相似文献