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511.
Contrasting transient and steady-state rivers crossing active normal faults: new field observations from the Central Apennines, Italy 总被引:2,自引:0,他引:2
Alexander C. Whittaker Patience A. Cowie Mikaël Attal Gregory E. Tucker† Gerald P. Roberts‡ 《Basin Research》2007,19(4):529-556
We present detailed data on channel morphology, valley width and grain size for three bedrock rivers crossing active normal faults which differ in their rate, history and spatial distribution of uplift. We evaluate the extent to which downstream changes in unit stream power correlate with footwall uplift, and use this information to identify which of the channels are likely to be undergoing a transient response to tectonics, and hence clarify the key geomorphic features associated with this signal. We demonstrate that rivers responding transiently to fault slip-rate increase are characterised by significant long-profile convexities (over-steepened reaches), a loss of hydraulic scaling, channel aspect ratios which are a strong non-linear function of slope, narrow valley widths, elevated coarse-fraction grain-sizes and reduced downstream variability in channel planform geometry. We are also able to quantify the steady-state configurations of channels, that have adjusted to differing spatial uplift fields. The results challenge the application of steady-state paradigms to transient settings and show that assumptions of power-law width scaling are inappropriate for rivers, that have not reached topographic steady state, whatever exponent is used. We also evaluate the likely evolution of bedrock channels responding transiently to fault acceleration and show that the headwaters are vulnerable to beheading if the rate of over-steepened reach migration is low. We estimate that in this setting the response timescale to eliminate long-profile convexity for these channels is ∼1 Myr, and that typical hydraulic scaling is regained within 3 Myr. 相似文献
512.
Ryszard Kryza Jan Zalasiewicz Stanisław Mazur Paweł Aleksandrowski Sergey Sergeev Alexander Larionov 《International Journal of Earth Sciences》2007,96(6):1153-1162
SHRIMP dating of detrital zircons from sandstones of the Gackowa Formation (Kaczawa Complex, Sudetes, SW Poland) indicates
input from late (550–750 Ma) and early Proterozoic to Archaean sources (∼2.0–3.4 Ga, the latter being the oldest recorded
age from the Sudetic region). These dates preclude within-terrane derivation from seemingly correlatory acid volcanic rocks
of early Palaeozoic age. Rather, they indicate provenance from Cadomian and older rocks that currently form part of other,
geographically distant terranes; the most likely source identified to date is the Lusatian Block in the Saxothuringian Zone.
Hence, the Gackowa Formation may be late Proterozoic rather than early Palaeozoic in depositional age, possibly coeval with
the late Proterozoic (pre-Cadomian) greywackes of Lusatia, being subsequently tectonically interleaved with early Palaeozoic
volcanic rocks into the Kaczawa accretionary prism during the Variscan orogeny. However, correlation with the lithologically
similar early Ordovician Dubrau Quartzite of Saxothuringia, and so assignation to the early Paleozoic (post-Cadomian) rift
succession deposited at the northern margin of Gondwana, cannot yet be precluded. 相似文献
513.
Alexander Likholyot 《Geochimica et cosmochimica acta》2007,71(10):2436-2447
We report on the thermochemistry of proton hydration by water in the gas phase both experimentally using high-pressure mass spectrometry (HPMS) and theoretically using multilevel G3, G3B3, CBS-Q, CBS-QB3, CBS/QCI-APNO as well as density functional theory (DFT) calculations. Gas phase hydration enthalpies and entropies for protonated water cluster equilibria with up to 7 waters (i.e., n ? 7H3O+·(H2O)n) were observed and exhibited non-monotonic behavior for successive hydration steps as well as enthalpy and entropy anomalies at higher cluster rank numbers. In particular, there is a significant jump in the stepwise enthalpies and entropies of cluster formation for n varying from 6 to 8. This behavior can be successfully interpreted using cluster geometries obtained from quantum chemical calculations by considering the number of additional hydrogen bonds formed at each hydration step and simultaneous weakening of ion-solvent interaction with increasing cluster size. The measured total hydration energy for the attachment of the first six water molecules around the hydronium ion was found to account for more than 60% of total bulk hydration free energy. 相似文献
514.
We compare eruptive dynamics, effects and deposits of the Bezymianny 1956 (BZ), Mount St Helens 1980 (MSH), and Soufrière
Hills volcano, Montserrat 1997 (SHV) eruptions, the key events of which included powerful directed blasts. Each blast subsequently
generated a high-energy stratified pyroclastic density current (PDC) with a high speed at onset. The blasts were triggered
by rapid unloading of an extruding or intruding shallow magma body (lava dome and/or cryptodome) of andesitic or dacitic composition.
The unloading was caused by sector failures of the volcanic edifices, with respective volumes for BZ, MSH, and SHV c. 0.5,
2.5, and 0.05 km3. The blasts devastated approximately elliptical areas, axial directions of which coincided with the directions of sector
failures. We separate the transient directed blast phenomenon into three main parts, the burst phase, the collapse phase,
and the PDC phase. In the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into
the atmosphere, with much of the expansion having an initially lateral component. The erupted material fails to mix with sufficient
air to form a buoyant column, but in the collapse phase, falls beyond the source as an inclined fountain, and thereafter generates
a PDC moving parallel to the ground surface. It is possible for the burst phase to comprise an overpressured jet, which requires
injection of momentum from an orifice; however some exploding sources may have different geometry and a jet is not necessarily
formed. A major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be
attributed to shock waves within an overpressured jet, or alternatively to dynamic pressures and shocks within the energetic
collapse and PDC phases. Internal shock structures related to unsteady flow and compressibility effects can occur in each
phase. We withhold judgment about published shock models as a primary explanation for the damage sustained at MSH until modern
3D numerical modeling is accomplished, but argue that much of the damage observed in directed blasts can be reasonably interpreted
to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified PDC.
This view is reinforced by recent modeling cited for SHV. In distal and peripheral regions, solids concentration, maximum
particle size, current speed, and dynamic pressure are diminished, resulting in lesser damage and enhanced influence by local
topography on the PDC. Despite the different scales of the blasts (devastated areas were respectively 500, 600, and >10 km2 for BZ, MSH, and SHV), and some complexity involving retrogressive slide blocks and clusters of explosions, their pyroclastic
deposits demonstrate strong similarity. Juvenile material composes >50% of the deposits, implying for the blasts a dominantly
magmatic mechanism although hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highly
viscous, phenocryst-rich, variable microlite content) determined the bimodal distributions of juvenile clast density and vesicularity.
Thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater, and laterally
from the axial region. The proximal stratigraphy of the blast deposits comprises four layers named A, B, C, D from bottom
to top. Layer A is represented by very poorly sorted debris with admixtures of vegetation and soil, with a strongly erosive
ground contact; its appearance varies at different sites due to different ground conditions at the time of the blasts. The
layer reflects intense turbulent boundary shear between the basal part of the energetic head of the PDC and the substrate.
Layer B exhibits relatively well-sorted fines-depleted debris with some charred plant fragments; its deposition occurred by
rapid suspension sedimentation in rapidly waning, high-concentration conditions. Layer C is mainly a poorly sorted massive
layer enriched by fines with its uppermost part laminated, created by rapid sedimentation under moderate-concentration, weakly
tractive conditions, with the uppermost laminated part reflecting a dilute depositional regime with grain-by-grain traction
deposition. By analogy to laboratory experiments, mixing at the flow head of the PDC created a turbulent dilute wake above
the body of a gravity current, with layer B deposited by the flow body and layer C by the wake. The uppermost layer D of fines
and accretionary lapilli is an ash fallout deposit of the finest particles from the high-rising buoyant thermal plume derived
from the sediment-depleted pyroclastic density current. The strong similarity among these eruptions and their deposits suggests
that these cases represent similar source, transport and depositional phenomena. 相似文献
515.
Albin Hammerle Alois Haslwanter Michael Schmitt Michael Bahn Ulrike Tappeiner Alexander Cernusca Georg Wohlfahrt 《Boundary-Layer Meteorology》2007,122(2):397-416
Carbon dioxide, latent and sensible heat fluxes were measured by means of the eddy covariance method above a mountain meadow
situated on a steep slope in the Stubai Valley in Austria, based on the hypothesis that, due to the low canopy height, measurements
can be made in the shallow equilibrium layer where the wind field exhibits characteristics akin to level terrain. In order
to test the validity of this hypothesis and to identify effects of complex terrain in the turbulence measurements, data were
subjected to a rigorous testing procedure using a series of quality control measures established for surface-layer flows.
The resulting high quality dataset comprised 36% of the original observations, the substantial reduction being mainly due
to a change in surface roughness and associated fetch limitations in the wind sector dominating during nighttime and transition
periods. The validity of the high quality dataset was further assessed by two independent tests: (i) a comparison with the
net ecosystem carbon dioxide exchange measured by means of ecosystem chambers, and (ii) the ability of the eddy covariance
measurements to close the energy balance. The net ecosystem CO2 exchange measured by the eddy covariance method agreed reasonably well with ecosystem chamber measurements. The assessment
of the energy balance closure showed that there was no significant difference in the correspondence between the meadow on
the slope and another one situated on flat ground at the bottom of the Stubai Valley, available energy being underestimated
by 28% and 29%, respectively. We thus conclude that, appropriate quality control provided, the eddy covariance measurements
made above a mountain meadow on a steep slope are of similar quality as compared to flat terrain. 相似文献
516.
Kenneth Broad Alexander Pfaff Renzo Taddei A. Sankarasubramanian Upmanu Lall Franciso de Assis de Souza Filho 《Climatic change》2007,84(2):217-239
We assess the potential benefits from innovative forecasts of the stream flows that replenish reservoirs in the semi-arid
state of Ceará, Brazil. Such forecasts have many potential applications. In Ceará, they matter for both water-allocation and
participatory-governance issues that echo global debates. Our qualitative analysis, based upon extensive fieldwork with farmers,
agencies, politicians and other key actors in the water sector, stresses that forecast value changes as a society shifts.
In the case of Ceará, current constraints on the use of these forecasts are likely to be reduced by shifts in water demand,
water allocation in the agricultural Jaguaribe Valley, participatory processes for water allocation between this valley and
the capital city of Fortaleza, and risk perception. Such changes in the water sector can also have major distributional impacts.
Broad, Pfaff and Taddei equally share lead authorship. 相似文献
517.
Ralph M. Gailis Alexander Hill Eugene Yee Trevor Hilderman 《Boundary-Layer Meteorology》2007,122(3):577-607
Fluctuating plume models provide a useful conceptual paradigm in the understanding of plume dispersion in a turbulent flow.
In particular, these models have enabled analytical predictions of higher-order concentration moments, and the form of the
one-point concentration probability density function (PDF). In this paper, we extend the traditional formalism of these models,
grounded in the theory of homogeneous and isotropic turbulent flow, to two cases: namely, a simple sheared boundary layer
and a large array of regular obstacles. Some very high-resolution measurements of plume dispersion in a water channel, obtained
using laser-induced fluorescence (LIF) line-scan techniques are utilised. These data enable us to extract time series of plume
centroid position (plume meander) and dispersion in the relative frame of reference in unprecedented detail. Consequently,
experimentally extracted PDFs are able to be directly compared with various theoretical forms proposed in the literature.
This includes the PDF of plume centroid motion, the PDF of concentration in the relative frame, and a variety of concentration
moments in the absolute and relative frames of reference. The analysis confirms the accuracy of some previously proposed functional
forms of model components used in fluctuating plume models, as well as suggesting some new forms necessary to deal with the
complex boundary conditions in the spatial domain. 相似文献
518.
Hwa-Lung Yu Alexander Kolovos George Christakos Jiu-Chiuan Chen Steve Warmerdam Boris Dev 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(5):647-572
This paper describes the spatiotemporal epistematics knowledge synthesis and graphical user interface (SEKS–GUI) framework
and its application in medical geography problems. Based on sound theoretical reasoning, the interactive software library
of SEKS–GUI explores heterogeneous (spatially non-homogeneous and temporally non-stationary) health attribute distributions
(disease incidence, mortality, human exposure, epidemic propagation etc.); expresses the health system’s dependence structure
using (ordinary and generalized) spatiotemporal covariance models; synthesizes core knowledge bases, empirical evidence and
multi-sourced system uncertainty; and generates a meaningful picture of the real-world system using space–time dependent probability
functions and associated maps of health attributes. The implementation stages of the SEKS–GUI library are described in considerable
detail using appropriate screens. The wide applicability of SEKS–GUI is demonstrated by reviewing a selection of real-world
case studies.
An erratum to this article can be found at 相似文献
519.
520.
H.C. Thomas L. Dunne M.S. Clemens P. Alexander S. Eales D.A. Green 《Monthly notices of the Royal Astronomical Society》2002,329(4):747-758
We present matched-resolution VLA H i and SCUBA 850-μm maps of 20 IRAS -bright galaxies. Of the galaxies observed, two were not detected in H i and two were detected in absorption. The H i distributions of the galaxies have a range of morphologies. Some of the systems appear H i deficient in the central regions which could be due to a high conversion rate of H i into molecules or H i absorption. In contrast to the H i , the 850-μm emission has a smooth distribution which is concentrated towards the optical centre of each galaxy. We also find evidence for 850-μm emission extending to the periphery of the optical disc in some of the galaxies. Finally, we note that the relative lack of 850-μm emission when compared with H i does not necessarily mean that the atomic gas and dust do not have similar mass distributions. 相似文献