全文获取类型
收费全文 | 490篇 |
免费 | 35篇 |
国内免费 | 28篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 53篇 |
地球物理 | 218篇 |
地质学 | 136篇 |
海洋学 | 86篇 |
综合类 | 10篇 |
自然地理 | 44篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 11篇 |
2021年 | 9篇 |
2020年 | 5篇 |
2019年 | 9篇 |
2018年 | 7篇 |
2017年 | 12篇 |
2016年 | 12篇 |
2015年 | 9篇 |
2014年 | 20篇 |
2013年 | 37篇 |
2012年 | 6篇 |
2011年 | 25篇 |
2010年 | 23篇 |
2009年 | 40篇 |
2008年 | 33篇 |
2007年 | 22篇 |
2006年 | 33篇 |
2005年 | 19篇 |
2004年 | 25篇 |
2003年 | 24篇 |
2002年 | 21篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 13篇 |
1998年 | 15篇 |
1997年 | 12篇 |
1996年 | 6篇 |
1995年 | 8篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有553条查询结果,搜索用时 15 毫秒
51.
The mechanisms which control the formation and maintenance of pool–riffles are fundamental aspects of channel form and process. Most of the previous investigations on pool–riffle sequences have focused on alluvial rivers, and relatively few exist on the maintenance of these bedforms in boulder-bed channels. Here, we use a high-resolution two-dimensional flow model to investigate the interactions among large roughness elements, channel hydraulics, and the maintenance of a forced pool–riffle sequence in a boulder-bed stream. Model output indicates that at low discharge, a peak zone of shear stress and velocity occurs over the riffle. At or near bankfull discharge, the peak in velocity and shear stress is found at the pool head because of strong flow convergence created by large roughness elements. The strength of flow convergence is enhanced during model simulations of bankfull flow, resulting in a narrow, high velocity core that is translated through the pool head and pool center. The jet is strengthened by a backwater effect upstream of the constriction and the development of an eddy zone on the lee side of the boulder. The extent of flow convergence and divergence is quantified by identifying the effective width, defined here as the width which conveys 90% of the highest modeled velocities. At low flow, the ratio of effective width between the pool and riffle is roughly 1:1, indicating little flow convergence or divergence. At bankfull discharge, the ratio of effective width is approximately 1:3 between the pool and downstream riffle, illustrating the strong flow convergence at the pool head. The effective width tends to equalize again with a ratio of 1:1 between the pool and riffle during a modeled discharge of a five-year flood, as the large roughness elements above the pool become drowned out. Results suggest that forced pool–riffle sequences in boulder-bed streams are maintained by flows at or near bankfull discharge because of stage-dependent variability in depth-averaged velocity and tractive force. 相似文献
52.
The flow pattern around a cylinder, installed in a scoured channel bed, was experimentally investigated. Detailed measurements of the instantaneous 3D velocities were performed by using an Acoustic Doppler Velocity Profiler (ADVP), from which the profiles of the time-averaged velocities and turbulence stresses were obtained. It is shown that the influence of the cylinder and of the scour hole alters the approach flow; this is essentially confined to the vicinity of the cylinder and to the inside of the scour hole. The horseshoe vortex is measured as a flow reversal inside the scour hole, formed by the downward flow along the cylinder face and the reversed flow at the scour bed. 相似文献
53.
ASIMPLIFIEDAPPROACHTOMODELING3DSEDIMENT-LADENTURBULENTFLOWSDonghuoZHOUandSamS.Y.WANGAbstract:A3-dnumericalmodelforsimulatings... 相似文献
54.
计算机辅助绘制流域等流时线 总被引:4,自引:0,他引:4
采用地理信息系统方法,对流域的地形地貌特征进行分析,模拟水质点在流域上的运动轨迹,并根据水质点汇流时间与汇流路径,坡度,坡长的关系,建立了流域汇流等流时线模型,经实例验证,效果较好。 相似文献
55.
WINDOW FACTOR ANALYSIS:THEORETICAL DERIVATION AND APPLICATION TO FLOW INJECTION ANALYSIS DATA 总被引:1,自引:0,他引:1
EDMUND R.MALINOWSKI 《地理学报(英文版)》1992,(1)
Window factor analysis(WFA)is a self-modeling method for extracting the concentration profiles ofindividual components from evolutionary processes such as flow injection,chromatography,titrationsand reaction kinetics.The method takes advantage of the fact that each component lies in a specificregion along the evolutionary axis,called the‘window’.Theoretical equations are derived.The methodis used to extract the concentration profiles and spectra of seven bismuth species from data obtained byGemperline and Hamilton,who injected bismuth perchlorate into a flowing stream of hydrochloric acid. 相似文献
56.
57.
Influence of rainfall on the deformation and stability of a slope in overconsolidated clays: a case study 总被引:2,自引:0,他引:2
The behaviour of an instrumented unstable slope in a profile of weathered overconsolidated clay has been analysed. Available
field investigation data and laboratory tests were integrated in a coupled hydromechanical model of the slope. Particular
attention was given to the unsaturated soil conditions above the water table and to the influence of the rainfall record.
Recorded pore-water pressures helped to identify the hydrogeological conditions of the slope. The coupled model was used to
compute slope deformations and the variation of safety with time. Actual rainfall records were also integrated into the analysis.
Comparison of measurements and calculations illustrate the nature of the slope instability and the complex relationships between
mechanical and hydraulic factors.
Electronic Publication 相似文献
58.
The 1995 Hyogoken–Nambu earthquake caused severe liquefaction over wide areas of reclaimed land. Furthermore, the liquefaction induced large ground displacement in horizontal directions, which caused serious damage to foundations of structures. However, few analyses of steel pipe piles based on field investigation have so far been conducted to identify the causes and process of such damage. The authors conducted a soil–pile-structure interaction analysis by applying a multi-lumped-mass-spring model to a steel pipe pile foundation structure to evaluate the causes and process of its damage. The damage process analyzed in the time domain corresponded well with the results of detailed field investigation. It was found that a large bending moment beyond the ultimate plastic moment of the pile foundation structure was induced mainly by the large ground displacement caused by liquefaction before lateral spreading of the ground and that the displacement appeared during the accumulating process of the excess pore water pressure. 相似文献
59.
60.
地质条件约束下储层流动单元定量研究 总被引:2,自引:0,他引:2
由于流动单元定量研究中常用的数学方法不能反映储层的地质分布规律,采用了地质条件约束的研究思路,对塔中油田塔中11油藏志留系潮坪相储层进行了流动单元研究.在应用高分辨率层序地层学进行小层对比的基础上,首先确定储层内部的渗流屏障及连通体的分布规律,然后在连通体内部,综合采用基于孔渗的宏观参数FZI和微观参数R35建立流动单元的聚类样本和判别函数,对所有取心井和非取心井划分出流动单元类型.最后根据单井划分结果,在储层结构模型的约束下,优选随机建模方法对流动单元的分布进行横向预测,建立了流动单元分布的三维模型.实践证明,渗流屏障和连通体等地质条件控制下流动单元定量研究方法在潮坪相地层中有很好的应用效果. 相似文献