全文获取类型
收费全文 | 62072篇 |
免费 | 1141篇 |
国内免费 | 537篇 |
专业分类
测绘学 | 1306篇 |
大气科学 | 4049篇 |
地球物理 | 11899篇 |
地质学 | 22897篇 |
海洋学 | 5831篇 |
天文学 | 14240篇 |
综合类 | 148篇 |
自然地理 | 3380篇 |
出版年
2022年 | 458篇 |
2021年 | 748篇 |
2020年 | 864篇 |
2019年 | 959篇 |
2018年 | 1812篇 |
2017年 | 1771篇 |
2016年 | 1951篇 |
2015年 | 1033篇 |
2014年 | 1852篇 |
2013年 | 3215篇 |
2012年 | 2078篇 |
2011年 | 2740篇 |
2010年 | 2445篇 |
2009年 | 3011篇 |
2008年 | 2669篇 |
2007年 | 2719篇 |
2006年 | 2560篇 |
2005年 | 1686篇 |
2004年 | 1649篇 |
2003年 | 1662篇 |
2002年 | 1551篇 |
2001年 | 1380篇 |
2000年 | 1284篇 |
1999年 | 1114篇 |
1998年 | 1111篇 |
1997年 | 1079篇 |
1996年 | 928篇 |
1995年 | 868篇 |
1994年 | 821篇 |
1993年 | 700篇 |
1992年 | 658篇 |
1991年 | 669篇 |
1990年 | 723篇 |
1989年 | 587篇 |
1988年 | 571篇 |
1987年 | 638篇 |
1986年 | 600篇 |
1985年 | 779篇 |
1984年 | 789篇 |
1983年 | 763篇 |
1982年 | 733篇 |
1981年 | 638篇 |
1980年 | 649篇 |
1979年 | 586篇 |
1978年 | 565篇 |
1977年 | 535篇 |
1976年 | 495篇 |
1975年 | 506篇 |
1974年 | 482篇 |
1973年 | 517篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
581.
Harilaos Loukos Bruce Frost D.E. Harrison James W. Murray 《Deep Sea Research Part II: Topical Studies in Oceanography》1997,44(9-10)
We construct a one-dimensional ecosystem model (nitrate, ammonium, phytoplankton, zooplnakton and detritus) with simple physics and biology in order to focus on the structural relations and intrinsic properties of the food web that characterizes the biological regime in the central equatorial Pacific at 140°W. When possible, data collected during the EgPac and other cruises were used to calibrate model parameters for two simulations that differ in the limiting nutrient, i.e. nitrogen or iron. Both simulations show annual results in good agreement with the data, but phytoplankton biomass and primary production show a more pronounced annual variability when iron is used as the limiting nutrient. This more realistically reproduces the variability of biological production and illustrates the greater coupling between vertical physical processes and biological production when the limiting nutrient is iron rather than nitrogen. The iron simulation also illustrates how iron supply controls primary production variability, how grazingbalances primary production and controls phytoplankton biomass, and how both iron supply and grazingcontrol primary production. These results suggest that it is not possible to capture primary production variability in the central equatorial Pacific with biological models using nitrogen as the limiting nutrient. Other indirect results of this modeling study were: (1) partitioning of export production between dissolved and particulate matter is almost equal, suggesting that the importance of DOC export may have been previously overestimated; (2) lateral export of live biomass has to be taken into account in order to balance the nitrogen budget on the equator at 140°W; and (3) preferential uptake of ammonium (i.e. nitrate uptake inhibition by ammonium) associated with high regeneration of nitrogen (low f ratio as a consequence of the food web structure imposed by iron limitation) largely accounts for the surface build-up of upwelled nitrate. 相似文献
582.
A novel self-contained navigation system has been devised for underwater vehicles operating in and around offshore installations. This system matches data from a sector-scanning sonar device to a computer model of the installation. The paper begins by highlighting the existing approaches to subsea navigation before outlining the main features of the proposed system. It then concentrates on a key component of this system which is a method for calculating the position and heading of an underwater vehicle navigating in the vincinity of tubular steel structures. An iterative solution method is presented which incorporates six degree of freedom vehicle motions and this is verified in a series of laboratory experiments with various arrangements of structural members and using a commercial sonar device. The key features, applications and performance of this method are discussed. The main conclusion is that the proposed method for calculating the position and heading of an underwater vehicle contributes towards achieving an accurate and reliable subsea navigation capability. 相似文献
583.
Predation by the big brackish-water isopod Saduria entomon on two age-classes (juveniles and adults) of the small deposit-feeding amphipod Monoporeia (syn. Pontoporeia) affinis was studied at different oxygen conditions (normoxia>11 mg/l O2, and moderate hypoxia 4 mg/l O2). Vulnerability of juvenile and adult M. affinis was studied in single and mixed age-class treatments. The proportions of juveniles and adults were varied at one total numerical density. The predatory effects were measured as numbers of individuals consumed and of total biomass intake (mg ash-free dry weight). The highest predation rate (in numbers) was detected in the single-prey treatment with juvenile M. affinis, both in normoxia and moderate hypoxia. No preference for any of the two size classes was found in the treatment with equal numbers of adults and juveniles. In normoxia, the total number of prey consumed decreased with increasing proportions of adults, while intake increased in terms of biomass. In moderate hypoxia, high proportions of adults reduced the predation rate of S. entomon to such an extent that also the intake in terms of biomass was significantly negatively affected. 相似文献
584.
Morphodynamics of a bar-trough surf zone 总被引:3,自引:0,他引:3
A field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar-trough topography (the “longshore-bar-trough” and “rhytmic-bar-and-beach” states as described by Wright and Short, 1984). Characteristic features of the morphology include a shallow bar with a steep shoreward face, a deep trough, and a steep beach face. This morphology, which is favored by moderate breaker heights and small tidal ranges, strongly controls the coupled suite of hydrodynamic processes. In contrast to fully dissipative surf zones, the bar-trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach. The degree of incident wave groupiness does not change appreciably across the surf zone. Infragravity standing waves which, in dissipative surf zones, dominate the inshore energy, remain energetically secondary and occur at higher frequencies in the bar trough surf zone. Analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal—antinodal positions over observed surf-zone profiles, indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough. Standing waves which would have nodes over the bar are suppressed. Sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress. In addition, the extra near-bottom eddy viscosity provided by the reformed, non-breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current. Whereas the bar is highly mobile in terms of onshore—offshore migration rates, the beach face and inner regions of the trough are remarkably stable over time. 相似文献
585.
Low-frequency damping of a moored semisubmersible drilling platform was obtained from numerical extinction tests simulated in still water and in regular waves and from mean wave drift forces calculated at zero forward speed. The influence of drag forces was represented by the modified Morison equation. The platform as used for the 18th ITTC Comparative Mooring Study was analyzed in irregular beam waves. The computed time series of sway as well as the corresponding sway response spectrum compared favourably with model test measurements, demonstrating that this procedure to determine low-frequency damping can be effective. 相似文献
586.
Low-frequency current and temperature variability on the southeast US continental shelf during summer conditions of weak wind forcing and vertical stratification was found to be similar in many aspects to previous findings for winter, when stronger wind forcing and vertical homogeneity prevails. Subtidal variability in the outer shelf is dominated by the weekly occurrence of Gulf Stream frontal eddies and meanders. These baroclinic events strongly affect the balance of momentum in the outer shelf, but not at mid-shelf. A negative alongshore sea level slope of order −10−7 is required to balance mean along-shelf momentum at the shelf edge, similar to oceanic estimates, and can contribute to the observed northward mean flow over the shelf.Low-frequency flow at mid-shelf and coastal sea level fluctuations appear to occur as a forced wave response to local alongshore wind stress events that are coherent over the shelf domain. Momentum balances indicate a trapped wave response similar to the arrested topographic wave found in the mid-Atlantic Bight (CSANADY, 1978). Density driven currents from river discharge do not appear to be significant at mid-shelf. Cold, subsurface intrusions of deeper, nutrient rich Gulf Stream waters can occasionally penetrate to mid- and inner-shelf regions north of Cape Canaveral, causing strong phytoplankton and zooplankton responses. These events were observed following the simultaneous occurrence of upwellings from northward winds and Gulf Stream frontal eddies at the shelf break during periods when the Stream was in an onshore position. Subsurface Gulf Stream intrusions to mid-shelf occur only during the summer, when the shelf is vertically stratified and cross-shelf density gradients do not present a barrier as in winter. 相似文献
587.
The paper analyses results of the numerical simulation of upwelling events in the north-western part of the Black Sea, mostly
near the South Crimea. The calculations were performed using a numerical model based on primitive hydrodynamics equations.
Emphasis is laid on the case when a salinity front simulating the Black Sea rim current is prescribed in the initial conditions.
The interaction of the Black Sea rim current's stream with the coastline and bottom topography leads to the development of
an upwelling near the Crimea's coast, even in the absence of wind forcing. The paper discusses the structure of the three-dimensional
circulation of waters in the shelf area of the NW Black Sea. Numerical modelling results are matched up with the satellite
data obtained by the HRPT receiving station.
Translated by Vladimir A. Puchkin. 相似文献
588.
E. Va˚gnes 《Geo-Marine Letters》1997,17(1):100-109
Estimates of uplift at ocean–continent transform margins vary significantly between models incorporating both thermal conduction
and viscous coupling across the transform and models taking only one of these processes into account. More heat is predicted
to be conducted and advected into the continental lithosphere in combined models than in conductive models. Nevertheless,
tectonic uplift predicted by the combined models is only half that predicted conductive models. This is because viscous coupling
implies crustal thinning close to the transform. The form and amplitude of uplift in the combined models agree well with erosion
estimates along a seismic transect across the Senja shear margin, in the southwestern Barents Sea.
Received: 31 January 1995 / Revision received: 20 July 1995 相似文献
589.
In the frame of the linear theory for long waves, the paper studies long waves generated by a barotropic tide running at an
arbitrary angle over a bottom ridge. The ocean is assumed to be two-layered. In the area of a ridge with a rectangular cross
section, geostrophic flows are considered, coupled with the inclinations of the free surface and interface. Wave amplitudes
are shown to depend on the angle of an onrunning tide and this allows us to reach a conclusion about the influence of a geostrophic
current on the generation of internal waves.
Translated by Vladimir A. Puchkin. 相似文献
590.
Kern E. Kenyon 《Journal of Oceanography》1997,53(1):89-92
The effects of the gravity torques acting on the angular momentum of surface gravity waves are calculated theoretically. For
short crested waves the gravity torque is caused by the force of gravity on the orbiting fluid particles acting down the slopes
of the crests and troughs and in the direction parallel to the crests and troughs. The gravity torque tries to rotate the
angular momentum vectors, and thus the waves themselves, counterclockwise in the horizontal plane, as viewed from above, in
both hemispheres. The amount of rotation per unit time is computed to be significant assuming reasonable values for the along-crest
and trough slopes for waves in a storm area. The gravity torque has a frequency which is double the frequency of the waves.
For long crested waves the gravity torque acts in the vertical plane of the orbit and tries to decelerate the particles when
they rise and accelerate them when they fall. By disrupting the horizontal cyclostrophic balance of forces on the fluid particles
(centrifugal force versus pressure force) the gravity torque accounts qualitatively for the three characteristics of breaking
waves: that they break at the surface, that they break at the crest, and that the crest breaks in the direction of wave propagation. 相似文献