全文获取类型
收费全文 | 6221篇 |
免费 | 734篇 |
国内免费 | 1025篇 |
专业分类
测绘学 | 484篇 |
大气科学 | 331篇 |
地球物理 | 1328篇 |
地质学 | 2401篇 |
海洋学 | 494篇 |
天文学 | 1556篇 |
综合类 | 377篇 |
自然地理 | 1009篇 |
出版年
2024年 | 7篇 |
2023年 | 47篇 |
2022年 | 135篇 |
2021年 | 114篇 |
2020年 | 154篇 |
2019年 | 190篇 |
2018年 | 152篇 |
2017年 | 166篇 |
2016年 | 204篇 |
2015年 | 215篇 |
2014年 | 255篇 |
2013年 | 322篇 |
2012年 | 276篇 |
2011年 | 263篇 |
2010年 | 256篇 |
2009年 | 418篇 |
2008年 | 437篇 |
2007年 | 491篇 |
2006年 | 473篇 |
2005年 | 400篇 |
2004年 | 353篇 |
2003年 | 333篇 |
2002年 | 303篇 |
2001年 | 275篇 |
2000年 | 249篇 |
1999年 | 258篇 |
1998年 | 241篇 |
1997年 | 150篇 |
1996年 | 126篇 |
1995年 | 110篇 |
1994年 | 111篇 |
1993年 | 109篇 |
1992年 | 83篇 |
1991年 | 61篇 |
1990年 | 56篇 |
1989年 | 49篇 |
1988年 | 32篇 |
1987年 | 34篇 |
1986年 | 21篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 6篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1972年 | 1篇 |
1954年 | 6篇 |
排序方式: 共有7980条查询结果,搜索用时 671 毫秒
1.
论文采用引文分析法,对陆大道院士所著《关于“点-轴”空间结构系统的形成机理分析》一文的349篇引文进行分析,从引文数量、引文作者、发文期刊、研究案例地、学科领域、研究主题、引用动机7个方面,探究“点-轴系统”理论的研究现状。结果表明:自2002年以来,“点-轴系统”理论的引文数量整体呈上升趋势,研究主题聚焦于区域经济和旅游发展;研究案例地以省级和市级中观尺度为主,多为单一型研究区,主要集中在西北丝绸之路经济带、东北辽宁经济圈、西南藏滇川渝、东部长三角四大热点区域;引文质量普遍较高,并且具有多学科交叉应用的特点,但批判性思考和创造性提升相对较少。基于此,论文从拓展研究和实践应用2个角度对“点-轴系统”理论未来发展进行了展望,以期为相关研究提供参考。 相似文献
2.
Time–space fractional governing equations of transient groundwater flow in confined aquifers: Numerical investigation 下载免费PDF全文
In order to model non‐Fickian transport behaviour in groundwater aquifers, various forms of the time–space fractional advection–dispersion equation have been developed and used by several researchers in the last decade. The solute transport in groundwater aquifers in fractional time–space takes place by means of an underlying groundwater flow field. However, the governing equations for such groundwater flow in fractional time–space are yet to be developed in a comprehensive framework. In this study, a finite difference numerical scheme based on Caputo fractional derivative is proposed to investigate the properties of a newly developed time–space fractional governing equations of transient groundwater flow in confined aquifers in terms of the time–space fractional mass conservation equation and the time–space fractional water flux equation. Here, we apply these time–space fractional governing equations numerically to transient groundwater flow in a confined aquifer for different boundary conditions to explore their behaviour in modelling groundwater flow in fractional time–space. The numerical results demonstrate that the proposed time–space fractional governing equation for groundwater flow in confined aquifers may provide a new perspective on modelling groundwater flow and on interpreting the dynamics of groundwater level fluctuations. Additionally, the numerical results may imply that the newly derived fractional groundwater governing equation may help explain the observed heavy‐tailed solute transport behaviour in groundwater flow by incorporating nonlocal or long‐range dependence of the underlying groundwater flow field. 相似文献
3.
Urbanization and eco-environment coupling is a research hotspot.Dynamic simulation of urbanization and eco-environment coupling needs to be improved because the processes of coupling are complex and statistical methods are limited.Systems science and cross-scale coupling allow us to define the coupled urbanization and eco-environment system as an open complex giant system with multiple feedback loops.We review the current state of dynamic simulation of urbanization and eco-environment coupling and find that:(1)The use of dynamic simulation is an increasing trend,the relevant theory is being developed,and modeling processes are being improved;(2)Dynamic simulation technology has become diversified,refined,intelligent and integrated;(3)Simulation is mainly performed for three aspects of the coupling,multiple regions and multiple elements,local coupling and telecoupling,and regional synergy.However,we also found some shortcomings:(1)Basic theories are inadequately developed and insufficiently integrated;(2)The methods of unifying systems and sharing data are behind the times;(3)Coupling relations and the dynamic characteristics of the main driving elements are not fully understood or completely identified.Additionally,simulation of telecoupling does not quantify parameters and is not systemically unified,and therefore cannot be used to represent spatial synergy.In the future,we must promote communication between research networks,technology integration and data sharing to identify the processes governing change in coupled relations and in the main driving elements in urban agglomerations.Finally,we must build decision support systems to plan and ensure regional sustainable urbanization. 相似文献
4.
5.
6.
试论新世纪地理信息科学的发展 总被引:1,自引:0,他引:1
本文重点分析了当前国内外地理信息科学的形成与发展趋势和地理信息科学不断发展的历程 ,提出近期发展的前沿 ,以及当前地理信息产业化面临的一些重要问题。最后提出对地理信息科学发展的意见。 相似文献
7.
Janis Britals 《Celestial Mechanics and Dynamical Astronomy》1992,54(4):305-316
It is shown in this paper how to build a canonical transformation of variables, so that the eccentric anomaly becomes the new independent variable. In the case of eccentric elliptical orbits it changes the equations of motion so, that they can be integrated analytically to any order of approximation comparatively easy. 相似文献
8.
9.
10.