全文获取类型
收费全文 | 41789篇 |
免费 | 586篇 |
国内免费 | 354篇 |
专业分类
测绘学 | 1287篇 |
大气科学 | 3053篇 |
地球物理 | 8495篇 |
地质学 | 13781篇 |
海洋学 | 3607篇 |
天文学 | 10230篇 |
综合类 | 114篇 |
自然地理 | 2162篇 |
出版年
2021年 | 328篇 |
2020年 | 362篇 |
2019年 | 448篇 |
2018年 | 972篇 |
2017年 | 898篇 |
2016年 | 1172篇 |
2015年 | 662篇 |
2014年 | 1112篇 |
2013年 | 2010篇 |
2012年 | 1218篇 |
2011年 | 1610篇 |
2010年 | 1468篇 |
2009年 | 2024篇 |
2008年 | 1780篇 |
2007年 | 1802篇 |
2006年 | 1690篇 |
2005年 | 1271篇 |
2004年 | 1297篇 |
2003年 | 1184篇 |
2002年 | 1226篇 |
2001年 | 1086篇 |
2000年 | 1016篇 |
1999年 | 893篇 |
1998年 | 884篇 |
1997年 | 888篇 |
1996年 | 711篇 |
1995年 | 666篇 |
1994年 | 625篇 |
1993年 | 543篇 |
1992年 | 475篇 |
1991年 | 486篇 |
1990年 | 466篇 |
1989年 | 485篇 |
1988年 | 434篇 |
1987年 | 503篇 |
1986年 | 456篇 |
1985年 | 535篇 |
1984年 | 658篇 |
1983年 | 568篇 |
1982年 | 557篇 |
1981年 | 505篇 |
1980年 | 440篇 |
1979年 | 432篇 |
1978年 | 445篇 |
1977年 | 381篇 |
1976年 | 341篇 |
1975年 | 349篇 |
1974年 | 324篇 |
1973年 | 362篇 |
1972年 | 260篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
在冬小麦主要生育期(2002年4月初到5月底),对不灌溉的冬小麦测定了冠层温度、地温、气温以及土壤含水量,计算了冠气温差且分析了冠层温度和冠气温差与不同土层厚度的土壤含水量相关关系。结果表明:14:00的冠层温度能较好地反映20cm土层的土壤含水量变化,但与其它各土层相关性有较大的波动性;14:00的冠气温差能较好地反映40cm以上土层的土壤含水量变化,二者的相关性很高,在20cm、40cm土层,两者相关系数R2分别为0.98866、0.99389,这为用区域遥感数据反演主要生育期冬小麦的冠气温差进而监测区域40cm土壤含水量提供了实验性的依据;拔节期和灌浆期,用14:00冠气温差来拟合各土壤层的土壤含水量有较高的精度,从而为用区域遥感数据监测区域土壤含水量提供了经验性的模型。 相似文献
82.
83.
84.
85.
86.
Authigenic iron and titanium oxides occur in three main textural varieties in the St. Bees Sandstone, a Triassic red bed succession in Cumbria. These are: (a) overgrowths of haematite, titaniferous haematite, anatase, and rutile which generally occur on detrital host grains of similar compositions. The overgrowths may occur as syntaxial rim cements or rhombohedral and prismatic projections and are always in optical continuity, (b) discrete crystals of anatase (including octahedrite) and haematite 10-40 μm in size occur in pore spaces, and (c) fine grained ( < 1 μm) pigmentary haematite occurs as grain coatings around detrital grains and in the interstitial matrix. The euhedral nature and pore-filling habit clearly indicate the authigenic origin of these iron- and titanium oxides. All three textural varieties are believed to have been precipitated from ground waters containing dissolved ferrous and titanium ions. These ions were derived from the intrastratal solution of detrital iron and titanium bearing grains including ferromagnesian silicates and iron-titanium oxides. The precipitation of iron oxides from migrating ground waters in the manner described here goes some way to explaining how thick, uniformly red successions can be formed. 相似文献
87.
In the present paper revised distances of optical objects from radio sources of the 5C2 survey are given. A statistical investigation of the data is given for blue objects and galaxies by the statistical method of the “first neighbour”. The identification rate on blue plates for both the blue objects and galaxies amounts to about 40% out of the total number of 26 identifications. For every blue object and galaxy which are proposed as an identification the statistical reliability is given. 相似文献
88.
Plates of the Tautenburg Schmidt can be calibrated with the aid of a beam-splitting prisma only if Schraffierkassette plates are used. The effects of image structure, scintillation and intermittence on in-focus images are different for the main component and the prismatic companion. Therefore, such plates cannot be used to set up a photometric scale. 相似文献
89.
P. Notni 《Astronomische Nachrichten》1976,297(3):147-154
Optical quasar candidate counts in the far reaching radio surveys B2 and 5C are consistent either with a luminosity function containing a high percentage of low luminosity objects and a cut off in quasar density or, more probably, with a normal number of quasars at high redshifts and a less steep luminosity function. The absence of high redshifted objects in currently available samples is to be expected of q0 ≈︂ o and if some of the few quasars observed at z>2.2 are exceptionally bright intrinsically and not typical for the bulk. 相似文献
90.
K. R. Muraleedharan P. K. Dinesh Kumar S. Prasanna Kumar Sebin John B. Srijith K. Anil Kumar K. Naveen Kumar S. Gautham V. Samiksha 《Estuaries and Coasts》2018,41(4):1021-1035
Mud bank formation during the southwest monsoon along the southwest coast of India remains an enigma to the researchers and coastal community in spite of several earlier studies. The present study attempts to unravel the mystery through a high-frequency, season-long time-series observation at Alappuzha, located at the southern part of the west coast of India, a region of frequent occurrence of mud bank. Using 7-month-long weekly time-series observation, we identified strong winds and high waves associated with onset of the southwest monsoon and subsequent three episodic atmospheric low-pressure events (LPEs).With the help of in situ time-series data, we show that the strong winds and high waves associated with southwest monsoon pre-conditions the near shore bottom sediment to bring it into suspension. The high amplitude waves associated with the southwest monsoon, while propagating from the deep water to shallow water region, interact with the bottom initiating bottom-sediment movement and its suspension due to wave refraction and shoaling. The sporadic occurrence of the atmospheric LPEs enhances the process of suspension of bottom sediment in the near shore region leading to the formation of fluid mud. Simulations with a cohesive sediment transport model yielded realistic estimates of sediment transport, in the presence of an onshore current, a pre-requisite for transporting the fluid mud toward the coast. The prevailing onshore upwelling current during the southwest monsoon provides the favorable pre-requisite conditions for transporting the fluid mud through depression channel network towards the coast. Once sufficient quantity and thickness of fluid mud is accumulated in the near shore region, it acts as a wave damper for subsequent high monsoon waves, as indicated by the time-series wave data, leading to the formation of tranquil mud bank region. Depression channel networks extending from the shelf to the coast off Alappuzha, Kochi, Ponnani, Beypore, and Ullal were found in the bathymetric charts, thus explaining why mud banks occur only at few locations in spite of the prevalence of similar monsoon conditions. 相似文献