全文获取类型
收费全文 | 844篇 |
免费 | 26篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 30篇 |
大气科学 | 49篇 |
地球物理 | 216篇 |
地质学 | 225篇 |
海洋学 | 129篇 |
天文学 | 174篇 |
综合类 | 5篇 |
自然地理 | 47篇 |
出版年
2021年 | 9篇 |
2020年 | 12篇 |
2019年 | 5篇 |
2018年 | 15篇 |
2017年 | 17篇 |
2016年 | 17篇 |
2015年 | 15篇 |
2014年 | 21篇 |
2013年 | 42篇 |
2012年 | 19篇 |
2011年 | 30篇 |
2010年 | 32篇 |
2009年 | 40篇 |
2008年 | 32篇 |
2007年 | 32篇 |
2006年 | 39篇 |
2005年 | 19篇 |
2004年 | 27篇 |
2003年 | 26篇 |
2002年 | 20篇 |
2001年 | 18篇 |
2000年 | 20篇 |
1999年 | 14篇 |
1998年 | 20篇 |
1997年 | 13篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 13篇 |
1993年 | 9篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 13篇 |
1986年 | 7篇 |
1985年 | 21篇 |
1984年 | 32篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 22篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 7篇 |
1973年 | 11篇 |
1971年 | 5篇 |
1966年 | 3篇 |
排序方式: 共有875条查询结果,搜索用时 15 毫秒
61.
Robert M. Moore Stephen Punshon Claire Mahaffey David Karl 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(9):1449-1458
Fixed nitrogen is a key nutrient involved in regulating global marine productivity and hence the global oceanic carbon cycle. Oceanic nitrogen (N2) fixation is estimated to supply 8×1012 moles N y?1 to the ocean, approximately equal to current riverine and the atmospheric inputs of fixed N, and between 50 and 100% of current estimates of oceanic denitrification. However, the spatial and temporal variability of N2 fixation remains uncertain, mostly because of the normal low resolution sampling for diazotroph distribution and fixation rates. It is well established that N2 fixation, mediated by the enzyme nitrogenase, is a source of hydrogen (H2), but the extent to which it leads to supersaturation of H2 in oceanic waters is unresolved. Here, we present simultaneous measurements of upper ocean dissolved H2 concentration (nmol L?1), and rates of N2 fixation (μmol N m?3 d?1), determined using 15N2 tracer techniques (at 7 or 15 m), on a transect from Fiji to Hawaii. We find a significant correlation (r=0.98) between dissolved H2 and rates of N2 fixation, with the greatest supersaturation of H2 and highest rates of N2 fixation being observed in the subtropical gyres at the southern (~18°S) and northern (18°N) reaches of the transect. The lowest H2 saturation and N2 fixation were observed in the equatorial region between 8°S and 14°N. We propose that an empirical relationship between H2 supersaturations and N2 fixation measurements could be used to guide sampling for 15N fixation measurements or to aid the spatial interpolation of such measurements. 相似文献
62.
Samples of lake water and coastal seawater from Nova Scotia, Canada, were irradiated with natural or artificial sunlight to investigate the potential for photochemical hydrogen production. Hydrogen photo-production was observed in all natural water samples. Rates of hydrogen formation were highest in coloured lake water (range: 98–163 pmol L− 1h− 1) and lower in seawater (range: 19–45 pmol L− 1 h − 1). Dilutions of the most highly coloured lake sample (Kejimkujik Lake) showed a positive linear relationship between H2 production rates and CDOM concentration. Photo-production rates normalised to UV absorption coefficients at 350 nm indicated that the photochemical efficiency of hydrogen formation varied between samples, perhaps due to differences in the CDOM composition. Photochemical hydrogen formation was also seen in solutions of syringic acid and acetaldehyde: two low-molecular-weight carbonyl compounds found in natural waters. Photochemistry may therefore offer least a partial explanation for the persistently high levels of hydrogen observed in the low-latitude surface ocean. 相似文献
63.
Submersed aquatic vegetation (SAV) communities have undergone declines worldwide, exposing them to invasions from non-native
species. Over the past decade, the invasive species Hydrilla verticillata has been documented in several tributaries of the lower Chesapeake Bay, Virginia. We used annual aerial mapping surveys from
1998 to 2007, integrated with spatial analyses of water quality data, to analyze the patterns and rates of change of a H. verticillata-dominated SAV community and relate them to varying salinity and light conditions. Periods of declining SAV coverage corresponded
to periods where salinities exceeded 7 and early growing season (April to May) Secchi depths were <0.4 m. Increases were driven
by the expansion of H. verticillata along with several other species into the upper estuary, where some areas experienced an 80% increase in cover. Field investigations
revealed H. verticillata dominance to be limited to the upper estuary where total suspended solid concentrations during the early growing season were
<15 mg l−1 and salinity remained <3. The effect of poor early growing season water clarity on annual SAV growth highlights the importance
of water quality during this critical life stage. Periods of low clarity combined with periodic salinity intrusions may limit
the dominance of H. verticillata in these types of estuarine systems. This study shows the importance of the use of these types of biologically relevant episodic
events to supplement seasonal habitat requirements and also provides evidence for the potential important role of invasive
species in SAV community recovery. 相似文献
64.
65.
A geopotential error analysis for a non planar satellite to satellite tracking mission 总被引:1,自引:0,他引:1
This study is concerned with gravity field recovery from low-low satellite to satellite range rate data. Compared against
a coplanar mission an improvement is predicted in errors associated with certain parts of the geopotential by the separation
of the orbital planes of the two satellites. Using Hill's equations an analytical scheme is developed to model the range rate
residuals. It is flexible enough to model the residuals between pairs of satellites in the same orbital plane or whose planes
are separated in right ascension. This scheme should allow the possibility of larger planar separations than previously since
no small angle approximation is made. The effects of such an orientation on gravity field recovery can therefore be analysed
by means of an extensive error analysis. The results of this analysis indicate that a significant improvement in the errors
of the near sectorial coefficients are obtained when the satellite's orbital planes are separated.
Received: 9 April 1996; Accepted: 26 September 1996 相似文献
66.
Brian J. Harshburger Karen S. Humes Von P. Walden Troy R. Blandford Brandon C. Moore Raymond J. Dezzani 《水文研究》2010,24(10):1285-1295
As demand for water continues to escalate in the western Unites States, so does the need for accurate monitoring of the snowpack in mountainous areas. In this study, we describe a simple methodology for generating gridded‐estimates of snow water equivalency (SWE) using both surface observations of SWE and remotely sensed estimates of snow‐covered area (SCA). Multiple regression was used to quantify the relationship between physiographic variables (elevation, slope, aspect, clear‐sky solar radiation, etc.) and SWE as measured at a number of sites in a mountainous basin in south‐central Idaho (Big Wood River Basin). The elevation of the snowline, obtained from the SCA estimates, was used to constrain the predicted SWE values. The results from the analysis are encouraging and compare well to those found in previous studies, which often utilized more sophisticated spatial interpolation techniques. Cross‐validation results indicate that the spatial interpolation method produces accurate SWE estimates [mean R2 = 0·82, mean mean absolute error (MAE) = 4·34 cm, mean root mean squared error (RMSE) = 5·29 cm]. The basin examined in this study is typical of many mid‐elevation mountainous basins throughout the western United States, in terms of the distribution of topographic variables, as well as the number and characteristics of sites at which the necessary ground data are available. Thus, there is high potential for this methodology to be successfully applied to other mountainous basins. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
67.
68.
D. Heath Jones Will Saunders Matthew Colless Mike A. Read Quentin A. Parker Fred G. Watson Lachlan A. Campbell Daniel Burkey Thomas Mauch Lesa Moore Malcolm Hartley Paul Cass Dionne James Ken Russell Kristin Fiegert John Dawe John Huchra Tom Jarrett Ofer Lahav John Lucey Gary A. Mamon Dominique Proust Elaine M. Sadler Ken-ichi Wakamatsu 《Monthly notices of the Royal Astronomical Society》2004,355(3):747-763
69.
We report on initial observations of coronal holestructure and evolution by the Coronal Diagnostic Spectrometer (CDS) instrument
on board the Solar and Heliospheric Observatory (SOHO). The data show that there is coronal activity on time scales of tens
of minutes, manifested as brightenings at chromospheric network cell junctions in extreme ultraviolet (EUV) wavelengths. There
are also significant differences in structure seen in different wavelengths in coronal holes. Finally, we show coronal hole
EUV spectra and compare them to quiet-Sun spectra, also taken by CDS. 相似文献
70.
Colin G. Moore Scot Mathieson David J. L. Mills Brian J. Bett 《Marine pollution bulletin》1987,18(12):646-649
A methodology is presented for the estimation of nematode diversity using Cairns' technique of sequential comparisons. The accuracy and precision of the method are examined and a test presented for the comparison of diversity estimates. 相似文献