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921.
Short-term otolith growth rates, residence times, and forage of two species of juvenile salmon (Oncorhynchus spp.) were compared in a created and natural estuarine slough on the Chehalis River in Washington to assess the functional equivalency of the created slough in providing suitable rearing habitat. Otolith microstructure, mark-recapture data, and forage of sub-yearling chinook, O. tshawytscha, and coho, O. kisutch, residing in both sloughs during the spring of 1991 and 1992 served as indicators of rearing habitat quality. No significant differences in the number or width of coho daily otolith increments were detected between the sloughs. Juvenile chinook residence and emigration times were also comparable. Aquatic and terrestrial insects composed the majority of all chinook and coho diets; however, the order of importance of main prey items did differ between sloughs, and salmon in the created slough had emptier stomachs, possibly because of reduced prey availability and/or foraging efficiency. 相似文献
922.
Charles L. Gallegos 《Estuaries and Coasts》2001,24(3):381-397
Submersed aquatic vegetation (SAV) is an important component of shallow water estuarine systems that has declined drastically in recent decades. SAV has particularly high light requirements, and losses of SAV have, in many cases, been attributed to increased light attenuation in the water column, frequently due to coastal eutrophication. The desire to restore these valuable habitats to their historical levels has created the need for a simple but accurate management tool for translating light requirements into water quality targets capable of supporting SAV communities. A procedure for calculating water quality targets for concentrations of chlorophyll and total suspended solids (TSS) is derived, based on representing the diffuse attenuation coefficient for photosynthetically active radiation, Kd(PAR), as a linear function of contributions due to water plus colored dissolved organic matter (CDOM), chlorophyll, and TSS. It is assumed that Kd(PAR) conforms to the Lambert-Beer law. Target concentrations are determined as the intersection of a line representing intended reduction of TSS and chlorophyll by management actions, with another line describing the dependence of TSS on chlorophyll at a constant value of Kd(PAR). The validity of applying the Lambert-Beer law to Kd(PAR) in estuarine waters was tested by comparing the performance of a linear model of Kd(PAR) with data simulated using a more realistic model of light attenuation. The linear regression model tended to underestimate Kd(PAR) at high light attenuation, resulting in erroneous predictions of target concentrations at shallow restoration depths. The errors result more from the wide spectral bandwidth of PAR, than from irrecoverable nonlinearities in the diffuse attenuation coefficient per se. In spite of the failure of the Lambert-Beer law applied to Kd(PAR), the variation of TSS with chlorophyll at constant Kd(PAR) determined by the more mechanistic attenuation model was, nevertheless, highly linear. Use of the management tool based on intersecting lines is still possible, but coefficients in the line describing the dependence of TSS on chlorophyll at constant Kd(PAR) must be determined empirically by application of an optical model suitably calibrated for the region of interest. An example application of the procedure to data from the Rhode River, Maryland, indicates that approximately 15% reduction in both TSS and chlorophyll concentrations, or 50% reduction in chlorophyll alone, will be needed to restore conditions for growth of SAV to levels that existed in the late 1960s. 相似文献
923.
John A. Westgate Shari J. Preece Duane G. Froese Robert C. Walter Amanjit S. Sandhu Charles E. Schweger 《Quaternary Research》2001,56(3)
The late Cenozoic deposits of central Yukon contain numerous distal tephra beds, derived from vents in the Wrangell Mountains and Aleutian arc–Alaska Peninsula region. We use a few of these tephra beds to gain a better understanding on the timing of extensive Pleistocene glaciations that affected this area. Exposures at Fort Selkirk show that the Cordilleran Ice Sheet advanced close to the outer limit of glaciation about 1.5 myr ago. At the Midnight Dome Terrace, near Dawson City, exposed outwash gravel, aeolian sand, and loess, related to valley glaciers in the adjacent Ogilvie Mountains, are of the same age. Reid glacial deposits at Ash Bend on the Stewart River are older than oxygen isotope stage (OIS) 6 and likely of OIS 8 age, that is, about 250,000 yr B.P. Supporting evidence for this chronology comes from major peaks in the rates of terrigeneous sediment input into the Gulf of Alaska at 1.5 and 0.25 myr B.P. 相似文献
924.
Reference Minerals for the Microanalysis of Light Elements 总被引:2,自引:0,他引:2
M. Darby Dyar Michael Wiedenbeck David Robertson Laura R. Cross Jeremy S. Delaney Kurt Ferguson Carl A. Francis Edward S. Grew Charles V. Guidotti Richard L. Hervig John M. Hughes John Husler William Leeman Anne V. McGuire Dieter Rhede Heike Rothe Rick L. Paul Ian Richards Martin Yates 《Geostandards and Geoanalytical Research》2001,25(2-3):441-463
The quantitative determination of light element concentrations in geological specimens represents a major analytical challenge as the electron probe is generally not suited to this task. With the development of new in situ analytical techniques, and in particular the increasing use of secondary ion mass spectrometry, the routine determination of Li, Be and B contents has become a realistic goal. However, a major obstacle to the development of this research field is the critical dependence of SIMS on the availability of well characterized, homogeneous reference materials that are closely matched in matrix (composition and structure) to the sample being studied. Here we report the first results from a suite of large, gem crystals which cover a broad spectrum of minerals in which light elements are major constituents. We have characterized these materials using both in situ and wet chemical techniques. The samples described here are intended for distribution to geochemical laboratories active in the study of light elements. Further work is needed before reference values for these materials can be finalized, but the availability of this suite of materials represents a major step toward the routine analysis of the light element contents of geological specimens. 相似文献
925.
926.
Self-diffusion coefficients for Si and O in Di58An42 liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in 18O and 28Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of 18O/16O and 29,30Si/28Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10−11 m2/s and D(Si) = 3.65 ± 0.37 × 10−11 m2/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm3/mol and −2.3 ± 0.4 cm3/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di58An42 liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen. 相似文献
927.
Phil Charles and Andy Fabian review the changes brought to X-ray astronomy by the two orbiting observatories, XMM-Newton and Chandra.
We describe here the changes to X-ray astronomy that are being brought about by the two new orbiting X-ray observatories, NASA's Chandra and ESA's XMM-Newton. Between them they bring a dramatic gain in sensitivity, spatial resolution (now comparable to ground-based optical and infrared telescopes) and spectral resolution. The superb Chandra mirrors rapidly resolved the diffuse X-ray background and have produced stunning detailed images of a wide variety of cosmic objects. Large gains have been made by both missions in spectroscopic performance, with individual X-ray emission lines now detectable in hot stellar coronae, supernova remnants, X-ray binaries, active galactic nuclei and galaxy clusters. 相似文献
We describe here the changes to X-ray astronomy that are being brought about by the two new orbiting X-ray observatories, NASA's Chandra and ESA's XMM-Newton. Between them they bring a dramatic gain in sensitivity, spatial resolution (now comparable to ground-based optical and infrared telescopes) and spectral resolution. The superb Chandra mirrors rapidly resolved the diffuse X-ray background and have produced stunning detailed images of a wide variety of cosmic objects. Large gains have been made by both missions in spectroscopic performance, with individual X-ray emission lines now detectable in hot stellar coronae, supernova remnants, X-ray binaries, active galactic nuclei and galaxy clusters. 相似文献
928.
929.
Impact of imperfect rainfall knowledge on the efficiency and the parameters of watershed models 总被引:1,自引:0,他引:1
Vazken Andrassian Charles Perrin Claude Michel Iolanda Usart-Sanchez Jacques Lavabre 《Journal of Hydrology》2001,250(1-4):206-223
It is crucial to analyze the sensitivity of watershed (rainfall-runoff) models to imperfect knowledge of rainfall input, in order to judge whether or not they are reliable and robust, especially if they are to be used for operational purposes. In this paper, a new approach to sensitivity analysis is proposed, based on a comparison between the efficiency ratings and parameter values of the models and the quality of rainfall input estimate (GORE and BALANCE indexes, assessing the quality of rainfall time distribution and the total depth respectively). Data from three watersheds of increasing size (71, 1120, and 10700 km2), are used to test three watershed models of varying complexity (three-parameter GR3J model and six-parameter modified versions of TOPMODEL and IHACRES).
These models are able to cope with imperfect rainfall input estimates, and react to improvements in rainfall input accuracy by better performance and reduced variability of efficiency. Two different types of model behavior were identified: the models either benefit from improved rainfall data by producing more consistent parameter values, or they are unable to take advantage of the improvements. Although the watershed size seems to be immaterial, the smaller watersheds appear to need more precise areal rainfall estimates (a higher concentration of raingages) to ensure good modeling results. 相似文献
930.
Tide gauge measurements are used for a variety of scientific purposes, not least of which are the definition of vertical data
and the detection of long-term variations in mean sea level. GPS measurements at tide gauge sites provide a means of separating
local verticl motions from sea level rise, and a means of unifying vertical data in a single reference system. This paper
describes a GPS survey to determine the positions and heights of reference stations at South African tide gauge sites. The
data were processed in baseline mode using a commercial software package.
The heights of the tide gauge stations relative to the fixed ITRF reference station HRAO were determined at a precision of
around 3 cm – better than 0.1 ppm. Analysis of the error sources showes that use of the precise ephemeris contributed to a
substantial improvement in accuracy, as did the use of ionosphere-free fixed integer baseline solutions. Variations in the
antenna phase centers also contributed significant changes in height. ? 2001 John Wiley & Sons, Inc. 相似文献