全文获取类型
收费全文 | 8489篇 |
免费 | 1831篇 |
国内免费 | 2661篇 |
专业分类
测绘学 | 959篇 |
大气科学 | 1570篇 |
地球物理 | 1911篇 |
地质学 | 5115篇 |
海洋学 | 1466篇 |
天文学 | 226篇 |
综合类 | 872篇 |
自然地理 | 862篇 |
出版年
2024年 | 48篇 |
2023年 | 156篇 |
2022年 | 383篇 |
2021年 | 521篇 |
2020年 | 418篇 |
2019年 | 523篇 |
2018年 | 492篇 |
2017年 | 472篇 |
2016年 | 488篇 |
2015年 | 512篇 |
2014年 | 574篇 |
2013年 | 578篇 |
2012年 | 674篇 |
2011年 | 678篇 |
2010年 | 649篇 |
2009年 | 633篇 |
2008年 | 637篇 |
2007年 | 572篇 |
2006年 | 529篇 |
2005年 | 421篇 |
2004年 | 324篇 |
2003年 | 272篇 |
2002年 | 215篇 |
2001年 | 224篇 |
2000年 | 230篇 |
1999年 | 241篇 |
1998年 | 208篇 |
1997年 | 214篇 |
1996年 | 178篇 |
1995年 | 147篇 |
1994年 | 144篇 |
1993年 | 130篇 |
1992年 | 92篇 |
1991年 | 70篇 |
1990年 | 64篇 |
1989年 | 54篇 |
1988年 | 52篇 |
1987年 | 33篇 |
1986年 | 25篇 |
1985年 | 20篇 |
1984年 | 16篇 |
1983年 | 16篇 |
1982年 | 14篇 |
1981年 | 9篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1973年 | 2篇 |
1958年 | 2篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
南极冰藻是南大洋碳流和能量流的重要组成部分,在该区域的生态系统中发挥着极其重要的作用.低温、低光照和高盐度对类囊体膜光系统的伤害必将影响冰藻对光能的吸收、传递和转换,从而影响冰藻的正常生长繁殖.事实上,冰藻为了能够在这寒冷骤变的环境中生存、繁衍,类囊体膜光系统在生理、代谢和遗传上进行了复杂的适应性改变,冰藻在低光照条件下保持很高的光合效率.因此研究南极冰藻类囊体膜光系统具有重要的理论意义.综述了南极冰藻类囊体膜光系统的研究进展,以期为进一步了解南极冰藻光系统作用机理提供依据. 相似文献
92.
An embedded cylinder is a large-diameter cylinder embedded in a soil foundation. The state of failure of such an embedded cylindrical structure shows large deflection instead of slide and overturn of the traditional gravity type of structure placed on a rubble base or foundation base. The critical value of deflection of the embedded cylindrical structure, which is the maximum allowable deflection for stability calculation of the cylinder, is a vital control value. Through investigation on deflection and soil pressures on an embedded cylinder by model experiments, the variations of the angle of rotation θ of a cylinder with effective anti-overturning ratio η and moment MH of thrust are discussed. On the basis of experimental study, the critical value of deflection of the cylindrical structure is proposed in the paper. Meanwhile, the formulas for calculating deflection of cylinders are derived. 相似文献
93.
94.
Determining the age of water and long-term transport timescale of the Chesapeake Bay 总被引:5,自引:0,他引:5
The concept of age of water (AW) is applied to the Chesapeake Bay to investigate the long-term transport properties for dissolved substances. A real-time calibrated hydrodynamic Chesapeake Bay model in 3 Dimensions (CH3D), employing a boundary-fitted curvilinear grid, is used for the study. The long-term transport properties, represented by AW, are investigated under the conditions of low river inflow of 1995 and high river inflow of 1996, as well as for constant mean inflows. The influences of freshwater, density-induced circulation, and wind-induced transport on age distribution have been investigated. Model results show that river inflows, wind stress, and density-induced circulation play important roles in controlling the long-term transport in the Bay. The model results shows that it requires 120–300 days for a marked change in the characteristics of the pollutant source discharged into the Bay from the Susquehanna River to affect significantly the conditions near the mouth under different hydrodynamic conditions. An increase of river discharge results in increases of downstream residual current and gravitational circulation, and thus reduces AW. The density-induced circulation contributes to the transport substantially. The dissolved substances discharged into the Bay are transported out of the Bay more rapidly when the estuary becomes more stratified. Southeasterly and southwesterly winds have strong impacts on the transport compared to the northeasterly and northwesterly winds. The former increases lateral and vertical mixing significantly. Consequently, the gravitational circulation is reduced and the transport time is increased by 50%. The model results provide useful information for understanding the long-term transport processes in the Bay. 相似文献
95.
Estimation of Land Subsidence Based on Groundwater Flow Model 总被引:8,自引:0,他引:8
This article presents an approach for estimating land subsidence due to withdrawal of groundwater. The proposed method calculates the groundwater seepage in 3-D-condition and calculates the land subsidence one-dimensionally. The governing equation on groundwater seepage is based on the three-dimensional mass conservation law and the principle of effective stress. The land subsidence calculation method is derived based on the following assumptions: (1) displacements occur only in the vertical direction, and (2) in vertical direction the total stresses do not change. The governing equation is solved by numerical method, i.e., finite element method (FEM) in spatial discretization and finite difference method (FDM) in time series discretization. In FEM Galerkin method is adopted and in FDM, lumped matrix method is employed. The proposed method is calibrated via analyzing 1-D consolidation problem and the results are compared with those from Terzaghi's one-dimensional consolidation theory and oedometer test. The proposed method is employed to analyze the consolidation of a soft layer due to withdrawal of groundwater from an aquifer under it. Moreover, this method is also applied to a field case of land subsidence due to groundwater pumping in a gas production field in Japan. The analytical results are compared with the field observed data. The results show that this approach simulates the field case well. 相似文献
96.
胶州湾鱼类复殖吸虫的研究 总被引:1,自引:0,他引:1
胶州湾水域的鱼类复殖吸虫,早在30年代秦素美(1933)曾报道过7种;作者自1958年起陆续在该水域进行吸虫标本采集,先后剖检鱼类49种,共529尾,找到吸虫21种,其中4种为新种。本文为新种记述,并附胶州湾吸虫及宿主鱼类名录。
文内测量单位一律为mm,卵子为μm。模式标本存放在中国科学院海洋研究所。 相似文献
97.
98.
胶州湾营养盐的现状和变化 总被引:28,自引:3,他引:28
报道了1991年5月至1994年2月12个航次胶州湾NO3-N,NO2-N,NH4-N,PO4-P和SiO3-Si的时空分布基本特征,60年代于至90年代的变化表明,胶州湾营养盐的浓度和分布已发生显变化。尤其从60年代至80年代,30年来,胶州湾中东部水城PO4-P,NO3-N和NH4-N浓度分别增加2.2,7.3和7.1倍,TON浓度也增加了3.5倍,TIN/PO4-P从10增加至24.2;T 相似文献
100.