全文获取类型
收费全文 | 59046篇 |
免费 | 1293篇 |
国内免费 | 1282篇 |
专业分类
测绘学 | 1502篇 |
大气科学 | 4482篇 |
地球物理 | 12364篇 |
地质学 | 21491篇 |
海洋学 | 5048篇 |
天文学 | 12553篇 |
综合类 | 274篇 |
自然地理 | 3907篇 |
出版年
2021年 | 568篇 |
2020年 | 566篇 |
2019年 | 602篇 |
2018年 | 1154篇 |
2017年 | 1093篇 |
2016年 | 1397篇 |
2015年 | 976篇 |
2014年 | 1400篇 |
2013年 | 2878篇 |
2012年 | 1754篇 |
2011年 | 2426篇 |
2010年 | 2102篇 |
2009年 | 2732篇 |
2008年 | 2400篇 |
2007年 | 2412篇 |
2006年 | 2370篇 |
2005年 | 1816篇 |
2004年 | 1750篇 |
2003年 | 1635篇 |
2002年 | 1596篇 |
2001年 | 1326篇 |
2000年 | 1348篇 |
1999年 | 1165篇 |
1998年 | 1174篇 |
1997年 | 1164篇 |
1996年 | 968篇 |
1995年 | 952篇 |
1994年 | 861篇 |
1993年 | 785篇 |
1992年 | 770篇 |
1991年 | 672篇 |
1990年 | 789篇 |
1989年 | 632篇 |
1988年 | 644篇 |
1987年 | 764篇 |
1986年 | 629篇 |
1985年 | 853篇 |
1984年 | 995篇 |
1983年 | 925篇 |
1982年 | 818篇 |
1981年 | 786篇 |
1980年 | 795篇 |
1979年 | 708篇 |
1978年 | 697篇 |
1977年 | 653篇 |
1976年 | 645篇 |
1975年 | 601篇 |
1974年 | 659篇 |
1973年 | 668篇 |
1972年 | 406篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
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
82.
83.
84.
A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143 总被引:1,自引:0,他引:1
K. A. Pounds J. N. Reeves A. R. King K. L. Page P. T. O'Brien M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713
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
85.
High‐resolution pollen, plant macrofossil and sedimentary analyses from early Holocene lacustrine sediments on the Faroe Islands have detected a significant vegetation perturbation suggesting a rapid change in climate between ca. 10 380 cal. yr BP and the Saksunarvatn ash (10 240±60 cal. yr BP). This episode may be synchronous with the decline in δ18O values in the Greenland ice‐cores. It also correlates with a short, cold event detected in marine cores from the North Atlantic that has been ascribed to a weakening of thermohaline circulation associated with the sudden drainage of Lake Agassiz into the northwest Atlantic, or, alternatively, a period with distinctly decreased solar forcing. The vegetation sequence begins at ca. 10 500 cal. yr BP with a succession from tundra to shrub‐tundra and increasing lake productivity. Rapid population increases of aquatic plants suggest high summer temperatures between 10 450 and 10 380 cal. yr BP. High pollen percentages, concentrations and influx of Betula, Juniperus and Salix together with macrofossil leaves indicate shrub growth around the site during the initial phases of vegetation colonisation. Unstable conditions followed ca. 10 380 cal. yr BP that changed both the upland vegetation and the aquatic plant communities. A decrease in percentage values of shrub pollen is recorded, with replacement of both aquatics and herbaceous plants by pioneer plant communities. An increase in total pollen accumulation rates not seen in the concentration data suggests increased sediment delivery. The catchment changes are consistent with less seasonal, moister conditions. Subsequent climatic amelioration reinitiated a warmth‐driven succession and catchment stabilisation, but retained high precipitation levels influencing the composition of the post‐event communities. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
86.
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
87.
E. Tóth F. Deak C. S. Gyurkócza Z. S. Kasztovszky R. Kuczi G. Marx B. Nagy S. Oberstedt L. Sajó-Bohus C. S. Sükösd G. Toth N. Vajda 《Environmental Geology》1997,31(1-2):123-127
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
88.
89.