全文获取类型
收费全文 | 12119篇 |
免费 | 2762篇 |
国内免费 | 3173篇 |
专业分类
测绘学 | 404篇 |
大气科学 | 542篇 |
地球物理 | 3189篇 |
地质学 | 10138篇 |
海洋学 | 1257篇 |
天文学 | 531篇 |
综合类 | 851篇 |
自然地理 | 1142篇 |
出版年
2024年 | 44篇 |
2023年 | 140篇 |
2022年 | 348篇 |
2021年 | 445篇 |
2020年 | 459篇 |
2019年 | 530篇 |
2018年 | 486篇 |
2017年 | 496篇 |
2016年 | 576篇 |
2015年 | 604篇 |
2014年 | 741篇 |
2013年 | 729篇 |
2012年 | 752篇 |
2011年 | 761篇 |
2010年 | 650篇 |
2009年 | 841篇 |
2008年 | 807篇 |
2007年 | 835篇 |
2006年 | 819篇 |
2005年 | 712篇 |
2004年 | 714篇 |
2003年 | 648篇 |
2002年 | 562篇 |
2001年 | 537篇 |
2000年 | 559篇 |
1999年 | 464篇 |
1998年 | 454篇 |
1997年 | 388篇 |
1996年 | 347篇 |
1995年 | 334篇 |
1994年 | 263篇 |
1993年 | 239篇 |
1992年 | 185篇 |
1991年 | 144篇 |
1990年 | 108篇 |
1989年 | 106篇 |
1988年 | 67篇 |
1987年 | 54篇 |
1986年 | 33篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 624 毫秒
41.
42.
43.
Starting with analysis on the evolving course of oasis and the characteristics and evolution of transitional zone between oasis and desert, in consideration of ecological elements including plant stomata resistance, area covered by vegetation, and physical elements including albedo of vegetation and bare soil, atmosphere temperature, and humidity, under the condition of the balance among net radiation flux, latent heat flux, and sensible heat flux, the following are calculated: temperatures of vegetation and bare soil in different conditions, as well as the evapotranspiration rate of ecosystem. Analysis on evapotranspiration rate indicates that it depends on both the climate of environment and the physiological and ecological conditions of plants. On certain conditions, the evapotranspiration rate of transitional zone between oasis and desert (i.e. area covered by vegetation less than 20%), in some parameter domains, appears in bifurcation or multiequilibrium state. Meanwhile, in such area, ecosystem is extremely unstable. Any minor change to the balance will cause either increase or reduction of area covered by vegetation in ecosystem, on the basis of discussion on the emergency of these phenomena. This paper is attempting to propose an effective way of destruction and rebuilt ecosystem in transitional zone. The way is to control the evaporation of plant through selecting anti-drought country plant with big stomata resistance, and modify the roughness of the underlying surface in ecosystem by establishing rational interspace structure of plant community, so as to put the degenerative ecosystem into the natural succession track. This primary theory is being verified through observation and analysis on historical data. 相似文献
44.
45.
46.
47.
A new model of albedo and emissivity of planetary surfaces covered by volatile ices in the form of porous slab-like deposits is described. In the model, a radiative transfer model is coupled with a microphysical model in order to link changes in albedo and emissivity to changes in porosity caused by ice metamorphism. Preliminary results for Triton, Pluto, and Io are presented (the martian CO2 caps will be the subject of a separate publication). The coupled model will aid in the interpretation of ground-based and spacecraft observations and should lead to advances in surface and atmospheric modeling. 相似文献
48.
Z. Q. Qu C. L. Xu X. Y. Zhang X. L. Yan C. L. Jin 《Monthly notices of the Royal Astronomical Society》2006,370(4):1790-1796
The influence of magnetic fields on the energy level populations of atoms is firstly studied by analysing the Stokes profiles of Fe i 6302.5 forming in the solar magnetized atmosphere, with the aid of a departure factor defined to evaluate the deviation from the normal Boltzmann distribution without a magnetic field. This factor is directly related to the ratio of line-source function to the continuum one. The relationship between the departure and the magnetic field reveals an effect that the magnetic field induces an exponential increase in the level population of the lower level of Fe i 6302.5 (Landé factor g = 2.5 ) with the field strength. This indicates that the magnetic field can cause the redistribution of populations of those levels whose Landé factors are non-zero. Therefore, this effect should be included in the calculation of the statistical equilibrium. Secondly, an experiment utilizing the Hg5461 line in the laboratory on the Earth is carried out to reveal that the exponential relation is independent of variations in temperature, and the excitation is completely magneto-induced. Finally, the exponential relation is explained by taking account of the magnetic energy in the Boltzmann distribution. 相似文献
49.
本文探讨了利用地震矩反演断裂形变带运动学参数的基本理论和方法,将其初步应用于鲜水河断裂形变带变形分析和运动机制的研究。结果表明,鲜水河断裂带呈现出走向拉伸、倾向压缩的形变格局,由地震矩反演的断裂带剪切形变速率(10.9mm/a)与用地质学估算方法(17mm/a)和现今地壳形变测量(8mm/a)的结果相当。同时,反演出的应变主方向能解释鲜水河断裂现今活动分段性特征以及多种滑动方式共存的现状,从而证明该方法是目前研究区域运动学问题切实有效的手段之一。 相似文献
50.
太子河流域西部竹蜓类动物群,以本溪—牛心台地区为最发育,一般可划分为一个共存延限带,一个组合带,后者又可进一步划分为3个组合亚带。ⅡFusulina-Fusulinela组合带3Fusulinacylindrica-F.quasicylindrica组合亚带2Fusulinelaprovectu-Fusulinapseudokonnoi组合亚带1Fusulinaschelwieni-F.mayiensis组合亚带ⅠProfusulinelaparva-Eostafelasubsolana共存延限带Profusulinelaparva系作者等研究辽南竹蜓类并建带的带分子,Eostuffelasubsolana为盛金章教授建立的辽东本溪群下部竹蜓带的带分子,时代为中石炭世早期。Fusulina-Fusulinela组合带代表本区及东北南部最发育的竹蜓类动物群。该带属于中石炭世中—晚期,也是太子河流域最大海侵期。以上二个竹蜓类生物带总体应属于Moscovian阶,时代为中石炭世。 相似文献