全文获取类型
收费全文 | 2602篇 |
免费 | 292篇 |
国内免费 | 314篇 |
专业分类
测绘学 | 157篇 |
大气科学 | 305篇 |
地球物理 | 675篇 |
地质学 | 1079篇 |
海洋学 | 291篇 |
天文学 | 363篇 |
综合类 | 89篇 |
自然地理 | 249篇 |
出版年
2024年 | 11篇 |
2023年 | 31篇 |
2022年 | 107篇 |
2021年 | 134篇 |
2020年 | 106篇 |
2019年 | 119篇 |
2018年 | 142篇 |
2017年 | 132篇 |
2016年 | 145篇 |
2015年 | 125篇 |
2014年 | 134篇 |
2013年 | 151篇 |
2012年 | 149篇 |
2011年 | 172篇 |
2010年 | 107篇 |
2009年 | 160篇 |
2008年 | 104篇 |
2007年 | 94篇 |
2006年 | 89篇 |
2005年 | 97篇 |
2004年 | 77篇 |
2003年 | 73篇 |
2002年 | 66篇 |
2001年 | 56篇 |
2000年 | 60篇 |
1999年 | 59篇 |
1998年 | 34篇 |
1997年 | 45篇 |
1996年 | 29篇 |
1995年 | 28篇 |
1994年 | 37篇 |
1993年 | 23篇 |
1992年 | 24篇 |
1991年 | 20篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 17篇 |
1984年 | 17篇 |
1983年 | 16篇 |
1982年 | 18篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 17篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1974年 | 12篇 |
1972年 | 10篇 |
排序方式: 共有3208条查询结果,搜索用时 46 毫秒
931.
J. A. Peacock M. Rowan-Robinson A. W. Blain J. S. Dunlop A. Efstathiou D. H. Hughes T. Jenness R. J. Ivison A. Lawrence M. S. Longair R. G. Mann S. J. Oliver S. Serjeant 《Monthly notices of the Royal Astronomical Society》2000,318(2):535-546
We use an 850-μm SCUBA map of the Hubble Deep Field (HDF) to study the dust properties of optically-selected starburst galaxies at high redshift. The optical/infrared (IR) data in the HDF allow a photometric redshift to be estimated for each galaxy, together with an estimate of the visible star-formation rate. The 850-μm flux density of each source provides the complementary information: the amount of hidden, dust-enshrouded star formation activity. Although the 850-μm map does not allow detection of the majority of individual sources, we show that the galaxies with the highest UV star-formation rates are detected statistically, with a flux density of about S 850 =0.2 mJy for an apparent UV star-formation rate of 1 h −2 M⊙ yr−1 . This level of submillimetre output indicates that the total star-forming activity is on average a factor of approximately 6 times larger than the rate inferred from the UV output of these galaxies. The general population of optical starbursts is then predicted to contribute at least 25 per cent of the 850-μm background. We carry out a power-spectrum analysis of the map, which yields some evidence for angular clustering of the background source population, but at a level lower than that seen in Lyman-break galaxies. Together with other lines of argument, particularly from the NICMOS HDF data, this suggests that the 850-μm background originates over an extremely wide range of redshifts – perhaps 1≲ z ≲6. 相似文献
932.
933.
S. W. Allen T. Di Matteo A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2000,311(3):493-506
We report the detection of hard X-ray emission components in the spectra of six nearby, giant elliptical galaxies observed with the ASCA satellite. The systems studied, which exhibit strong dynamical evidence for supermassive black holes in their nuclei, are M87, NGC 1399 and NGC 4696 (the dominant galaxies of the Virgo, Fornax and Centaurus clusters, respectively) and NGC 4472, 4636 and 4649 (three further giant ellipticals in the Virgo cluster). The ASCA data for all six sources provide clear evidence for hard emission components, which can be parametrized by power-law models with photon indices in the range Γ=0.6–1.5 (mean value 1.2) and intrinsic 1–10 keV luminosities of 2×1040 –2×1042 erg s−1 . Our results imply the identification of a new class of accreting X-ray source, with X-ray spectra significantly harder than those of binary X-ray sources, Seyfert nuclei or low-luminosity active galactic nuclei, and bolometric luminosities relatively dominated by their X-ray emission. We discuss various possible origins for the hard X-ray emission and argue that it is most likely to be due to accretion on to the central supermassive black holes, via low radiative efficiency accretion flows coupled with strong outflows. In the case of M87, our detected power-law flux is in good agreement with a previously reported measurement from ROSAT High Resolution Imager observations, which were able to resolve the jet from the nuclear X-ray emission components. We confirm previous results showing that the use of multiphase models in the analysis of the ASCA data leads to determinations of approximately solar emission-weighted metallicities for the X-ray gas in the galaxies. We also present results on the individual element abundances in NGC 4636. 相似文献
934.
935.
936.
A comparison between the single ring pressure infiltrometer and simplified falling head techniques 下载免费PDF全文
Testing the relative performances of the single ring pressure infiltrometer (PI) and simplified falling head (SFH) techniques to determine the field saturated soil hydraulic conductivity, Kfs, at the near point scale may help to better establish the usability of these techniques for interpreting and simulating hydrological processes. A sampling of 10 Sicilian sites showed that the measured Kfs was generally higher with the SFH technique than the PI one, with statistically significant differences by a factor varying from 3 to 192, depending on the site. A short experiment with the SFH technique yielded higher Kfs values because a longer experiment with the PI probably promoted short‐term swelling phenomena reducing macroporosity. Moreover, the PI device likely altered the infiltration surface at the beginning of the run, particularly in the less stable soils, where soil particle arrangement may be expected to vary upon wetting. This interpretation was supported by a soil structure stability index, SSI, and also by the hydraulic conductivity data obtained with the tension infiltrometer, i.e. with a practically negligible disturbance of the sampled soil surface. In particular, a statistically significant, increasing relationship with SSI and an unsaturated conductivity greater than the saturated one were only detected for the Kfs data obtained with the PI. The SFH and PI techniques should be expected to yield more similar results in relatively rigid porous media (low percentages of fine particles and structurally stable soils) than in soils that modify appreciably their particle arrangement upon wetting. The simultaneous use of the two techniques may allow to improve Kfs determination in soils that change their hydrodynamic behaviour during a runoff producing rainfall event. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
937.
Charles I. Scaife Jonathan M. Duncan Laurence Lin Christina Tague Colin D. Bell Lawrence Band 《水文研究》2021,35(9):e14313
The spatial variation of soil moisture over very small areas (<100 m2) can have nonlinear impacts on cycling and flux rates resulting in bias if it is not considered, but measuring this variation is difficult over extensive temporal and spatial scales. Most studies examining spatial variation of soil moisture were conducted at hillslope (0.01 km2) to multi-catchment spatial scales (1000 km2). They found the greatest variation at mid wetness levels and the smallest variation at wet and dry wetness levels forming a concave down relationship. There is growing evidence that concave down relationships formed between spatial variation of soil moisture and average soil moisture are consistent across spatial scales spanning several orders of magnitude, but more research is needed at very small, plot scales (<100 m2). The goal of this study was to characterise spatial variation in shallow soil moisture at the plot scale by relating the mean of measurements collected in a plot to the standard deviation (SD). We combined data from a previous study with thousands of new soil moisture measurements from 212 plots in eight catchments distributed across the US Mid-Atlantic Region to (1) test for a generalisable mean–SD relationship at plot scales, (2) characterise how landcover, land use, season, and hillslope position contribute to differences in mean–SD relationships, and (3) use these generalised mean–SD relationships to quantify their impacts on catchment scale nitrification and denitrification potential. Our study found that 98% of all measurements formed a generalised mean–SD relationship like those observed at hillslope and catchment spatial scales. The remaining 2% of data comprised a mean–SD relationship with greater spatial variation that originated from two riparian plots reported in a previous study. Incorporating the generalised mean–SD relationship into estimates of nitrification and denitrification potential revealed strong bias that was even greater when incorporating mean–SD observations from the two riparian plots with significantly greater spatial variation. 相似文献
938.
Testing the use of an image‐based technique to measure gully erosion at Sparacia experimental area 下载免费PDF全文
Costanza Di Stefano Vito Ferro Vincenzo Palmeri Vincenzo Pampalone Fabrizio Agnello 《水文研究》2017,31(3):573-585
The first part of this investigation was aimed at testing the use of a three‐dimensional (3D) digital terrain model and a quasi‐tridimensional (2.5D) digital elevation model obtained by a large series of oblique images of eroded channels taken from consumer un‐calibrated and non‐metric cameras. For two closed earth channels having a different sinuosity, the ground measurement of some cross sections by a profilometer (P) was carried out and their real volume was also measured. The comparison among the three methods (3D, 2.5D, and P) pointed out that a limited underestimation of the total volume always occurs and that the 3D method is characterized by the minimum difference between measured and real volume. For this reason, 3D model can be used as benchmark. In the subsequent part of the investigation, the three ground measurement methods were applied for surveying of an ephemeral gully (EG) channel at the Sparacia area. The morphological and hydraulic variable values of the 24 surveyed cross sections determined by both 2.5D model and profilometer were compared. This comparison showed that the estimate error is generally less than ±10%. The EG measurements carried out by the three methods supported the applicability both of the empirical relationship between EG length and its eroded volume and the theoretical dimensionless relationship among the morphological variables describing the channelized erosion process. Finally, it was demonstrated that the effect of the distance interval on the EG volume measurement by 3D and 2.5D models is negligible for the investigated EG. 相似文献
939.
A review of low‐cost space‐borne data for flood modelling: topography,flood extent and water level 下载免费PDF全文
Kun Yan Giuliano Di Baldassarre Dimitri P. Solomatine Guy J.‐P. Schumann 《水文研究》2015,29(15):3368-3387
During the last two decades, remote sensing data have led to tremendous progress in advancing flood inundation modelling. In particular, low‐cost space‐borne data can be invaluable for large‐scale flood studies in data‐scarce areas. Various satellite products yield valuable information such as land surface elevation, flood extent and water level, which could potentially contribute to various flood studies. An increasing number of research studies have been dedicated to exploring those low‐cost data towards building, calibration and evaluation, and remote‐sensed information assimilation into hydraulic models. This paper aims at reviewing these recent scientific efforts on the integration of low‐cost space‐borne remote sensing data with flood modelling. Potentials and limitations of those data in flood modelling are discussed. This paper also introduces the future satellite missions and anticipates their likely impacts in flood modelling. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
940.
According to the article named “Shear Strength of an Accumulation Soil from Direct Shear Test” by J. Wang, H. Zhang, H. Wen, and Y. Liang, this paper proposes a more reasonable expression to characterize shear strength in the original paper. The angle of shearing resistance is expressed by the combination of initial and incremental angles of shearing resistance, the relative compactions and water content were used to analyze the effects. The initial and incremental angles of shearing resistance are generally increasing with the increment of the relative compaction, increasing then decreases with the increment of the water content. The finial expressions which combined by the series of simulation equation was given to precisely predict the accumulation soil's angle of shearing resistance in relation to the relative compactions and water content. 相似文献