首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   39033篇
  免费   1004篇
  国内免费   1065篇
测绘学   1002篇
大气科学   3070篇
地球物理   7966篇
地质学   14269篇
海洋学   3504篇
天文学   8457篇
综合类   242篇
自然地理   2592篇
  2022年   293篇
  2021年   487篇
  2020年   473篇
  2019年   508篇
  2018年   930篇
  2017年   899篇
  2016年   1096篇
  2015年   759篇
  2014年   1072篇
  2013年   1990篇
  2012年   1399篇
  2011年   1848篇
  2010年   1599篇
  2009年   2089篇
  2008年   1770篇
  2007年   1833篇
  2006年   1766篇
  2005年   1277篇
  2004年   1194篇
  2003年   1099篇
  2002年   1064篇
  2001年   894篇
  2000年   858篇
  1999年   711篇
  1998年   746篇
  1997年   716篇
  1996年   591篇
  1995年   586篇
  1994年   499篇
  1993年   444篇
  1992年   438篇
  1991年   401篇
  1990年   473篇
  1989年   392篇
  1988年   365篇
  1987年   458篇
  1986年   359篇
  1985年   455篇
  1984年   564篇
  1983年   474篇
  1982年   470篇
  1981年   425篇
  1980年   434篇
  1979年   381篇
  1978年   367篇
  1977年   358篇
  1976年   324篇
  1975年   319篇
  1974年   333篇
  1973年   358篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
OSCR is an HF radar system that has been developed for high spatial resolution coastal surface current measurement. This paper describes preliminary results that demonstrate that wave measurement can be successfully obtained from suitably processed OSCR data. Comparisons with data from a WAVEC directional buoy are presented and show encouraging agreement. Some of the limitations to the measurement process are discussed and indicate a maximum range of about 20 km. Surface current variability on short time scales presents the most serious obstacle to wave measurement. This appears to be more of a problem when the mean currents are large, in that in these circumstances the data fail initial quality control criteria. However, in lower mean currents, the effect is often still present and leads to errors in long wave measurement  相似文献   
52.
Infiltration experiments have been performed at three sites along a well-known catena under virgin tropical rain forest using a portable sprinkling infiltrometer. Experimentally determined infiltration curves are presented. Infiltration curves are also simulated on the basis of the Mein-Larson equation. The parameters for this model have been obtained from the infiltration curves (saturated conductivity) and simple soil moisture determinations (fillable porosity). The agreement between experimentally determined and modelled infiltration is reasonable, provided (a) saturated conductivity as derived from the experimental data is corrected, (b) a storage parameter, also derived from the experimental data, is added to the Mein-Larson model, and (c) the decline in soil porosity with depth is either small or occurs abruptly at shallow depth. Comparison of observed infiltration rates with rainfall intensity shows that Horton Overland Flow has to occur naturally at least on the middle and lower section of the catena. Despite the fact that most parameters can be estimated in principle from basic soil data, it remains advisable to obtain sprinkling infiltrometer field measurements, because of soil variability due to dynamic surface conditions, macroporosity, air entrapment, and irregularity of the wetting front.  相似文献   
53.
54.
55.
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1. The highest energy lines require a column density of   N H∼ 5 × 1023 cm−2  , at an ionization parameter of  log ξ∼ 3.4  . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143.  相似文献   
56.
The evolution of a homogeneous, isotropic cosmological model driven by a nonminimally coupled scalar field is studied. The potential for the quintessential inflation model proposed by Peebles and Vilenkin is selected as a scalar potential. Possible scenarios for the cosmological dynamics are described in the conformal Einstein and Jordan representations. It is shown that, unlike in models with a minimal scalar field, here a class of solutions exists for which the scalar field is fixed at finite values during cosmological expansion. __________ Translated from Astrofizika, Vol. 49, No. 3, pp. 487–498 (August 2006).  相似文献   
57.
 A steady radon exhalation is assumed in most publications. In a village of North-East Hungary, however, high radon concentrations have been measured, differing strongly in neighbouring houses and varying in time, due to the interplay of geochemical phenomena. Received: 20 November 1995 · Accepted: 18 June 1996  相似文献   
58.
59.
60.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号