全文获取类型
收费全文 | 16440篇 |
免费 | 2802篇 |
国内免费 | 2668篇 |
专业分类
测绘学 | 342篇 |
大气科学 | 1547篇 |
地球物理 | 4413篇 |
地质学 | 7617篇 |
海洋学 | 2678篇 |
天文学 | 58篇 |
综合类 | 760篇 |
自然地理 | 4495篇 |
出版年
2024年 | 103篇 |
2023年 | 328篇 |
2022年 | 663篇 |
2021年 | 739篇 |
2020年 | 705篇 |
2019年 | 730篇 |
2018年 | 663篇 |
2017年 | 743篇 |
2016年 | 723篇 |
2015年 | 772篇 |
2014年 | 1021篇 |
2013年 | 1206篇 |
2012年 | 938篇 |
2011年 | 1026篇 |
2010年 | 883篇 |
2009年 | 1011篇 |
2008年 | 1114篇 |
2007年 | 1067篇 |
2006年 | 1055篇 |
2005年 | 801篇 |
2004年 | 750篇 |
2003年 | 705篇 |
2002年 | 543篇 |
2001年 | 489篇 |
2000年 | 478篇 |
1999年 | 429篇 |
1998年 | 335篇 |
1997年 | 303篇 |
1996年 | 275篇 |
1995年 | 234篇 |
1994年 | 207篇 |
1993年 | 185篇 |
1992年 | 164篇 |
1991年 | 129篇 |
1990年 | 114篇 |
1989年 | 73篇 |
1988年 | 61篇 |
1987年 | 24篇 |
1986年 | 22篇 |
1985年 | 19篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 14篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 16篇 |
1977年 | 6篇 |
1976年 | 2篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
1985年5、8月中法联合调查黄河口海域的营养盐,其样品采用常规的分光光度法测定。营养盐的分布总趋势是河口附近浓度高,外海浓度低,河口与外海浓度之比达数百倍。在河口及其邻近海域营养盐的分布梯度与盐度的分布一致。在河口控制营养盐含量的主要因素是河水和海水的混合过程。在119°30′~50′E,37°25′~38°5′N,PO_4—P,SiO_3—Si出现低值区。在此范围附近恰恰浮游植物的总量比较高。对8月份的02站进行了25h的连续观测,其结果是底层营养盐与盐度有良好的负相关性,而表层的相关性较差 黄河具有很高的氮磷比值,这很可能是农业上用氮肥量增多引起的。 相似文献
72.
73.
74.
75.
The mixing characteristics of particles such as dredged sediment of variable size discharged into cross flow are studied by a 3D numerical model, which is developed to model the particle-fluid two-phase flow. The Eulerian method with the modified k-ε parameterization of turbulence for the fluid phase is used to solve fluid phase, while a Lagrangian method for the solid phase (particles), both the processes are coupled through the momentum sources. In the model the wake turbulence induced by particles has been included as additional source term in the k-ε model; and the variable drift velocities of the particles are treated efficiently by the Lagrangian method in which the particles are tracked explicitly and the diffusion process is approximated by a random walk model. The hydrodynamic behavior of dumping a cloud of particles is governed by the total buoyancy of the cloud, the drag force on each particle and the velocity of cross-flow. The computed results show a roughly linear relationship between the displacement of the frontal position and the longitudinal width of the particle cloud. The particle size in the cloud and the velocity of cross flow dominate the flow behavior. The computed results are compared with the results of laboratory experiments and satisfactory agreement is obtained. 相似文献
76.
Wave induced excess flow of momentum(WIEFM)is the averaged flow of momentum over a wave period due to wave presence,which may also be called 3-D radiation stress.In this paper,the 3-D current equations with WIEFM are derived from the averaged Navier-Stokes equations over a wave period,in which the velocity is separated into the large-scale background velocity,the wave particle velocity and the turbulent fluctuation velocity.A concept of wave fluctuating layer(WFL)is put forward,which is the vertical column from the wave trough to wave ridge.The mathematical expressions of WIEFM in WFL and below WFL are given separately.The parameterized expressions of WIEFM are set up according to the linear wave theory.The integration of WIEFM in the vertical direction equals the traditional radiation stress(namely 2-D radiation stress)given by Longuet-Higgins and Stewart. 相似文献
77.
Two distinct series of slumps deform the upper part of the sedimentary sequence along the continental margin of the Levant.
One series is found along the base of the continental slope, where it overlies the disrupted eastern edge of the Messinian
evaporites. The second series of slumps transects the continental margin from the shelf break to the Levant Basin. It seemed
that the two series were triggered by two unrelated, though contemporaneous, processes. The shore-parallel slumps were initiated
by basinwards flow of the Messinian salt, that carried along the overlying Plio-Quaternary sediments. Seawater that percolated
along the detachment faults dissolved the underlying salt to form distinctly disrupted structures. The slope-normal slumps
are located on top of large canyons that cut into the pre-Messinian sedimentary rocks. A layer of salt is found in the canyons,
and the Plio-Quaternary sediments were deposited on that layer. The slumps are bounded by large, NW-trending faults where
post-Messinian faulted offset was measured. We presume that the flow of the salt in the canyons also drives the slope-normal
slumps. Thus thin-skinned halokynetic processes generated the composite post-Tortonian structural patterns of the Levant margin.
The Phoenician Structures are a prime example of the collapse of a distal continental margin due to the dissolution of a massive
salt layer. 相似文献
78.
The abandoned Yellow River Delta coast is a typical erodible silty and muddy coast in China. The paper analyses the marine dynamic characteristics and the mechanism of beach erosion of this area. Analysis and calculation show that in this sea area wave and tidal current action should be considered. Based on the above analysis, an equilibrium beach profile calculation model is developed, in which the wave-current interaction is considered while sediment supply and sediment re-deposition are neglected. The model consists of four parts: (1) calculation of wave parameters, (2) calculation of velocity due to wave-current interaction at different water depth, (3) calculation of friction velocity and shear stress at different water depths, and (4) calculation of the amount of sediment erosion, erosion intensity and variation of beach profile. Calculated results are in good agreement with observed data. Finally, the evolution tendency is discussed and the equilibrium beach profile of this coast is calculated. B 相似文献
79.
黄河三角洲地貌环境体系 总被引:4,自引:0,他引:4
黄河三角洲是由三角洲上部冲积平原一下部冲积海积平原(超潮滩)─周边潮滩三个区带的地貌系列构成.它们之间形成三条地貌环境岸线,是一个复杂而统一的陆上─潮滩堆积地貌体系.该体系研究对三角洲发育.环境及其开发利用有一定理论和实践意义. 相似文献
80.
Stable carbon isotope ratios have been used to study the sources of particulate organic carbon (POC) and total dissolved inorganic carbon in the Orinoco Basin. The isotopic composition of total dissolved inorganic carbon shows a range of from -8·1 to -23·0 ppt, an indication of dominance of biological processes. The isotopic composition of POC exhibits a range of from -24·1 to -34·6 ppt with little seasonal variation. The isotopic evidence indicates that the POC is predominantly of terrestrial origin rather than a result of in situ planktonic production. The similarity of isotopic composition of POC and coastal sediments suggests that riverine organic detritus has been transported 30-50 km offshore in a direction parallel to the Orinoco river channel. 相似文献