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111.
An analytical model is presented for the analysis of constant flux tests conducted in a phreatic aquifer having a partially penetrating well with a finite thickness skin. The solution is derived in the Laplace transform domain for the drawdown in the pumping well, skin and formation regions. The time-domain solution in terms of the aquifer drawdown is then obtained from the numerical inversion of the Laplace transform and presented as dimensionless drawdown–time curves. The derived solution is used to investigate the effects of the hydraulic conductivity contrast between the skin and formation, in addition to wellbore storage, skin thickness, delayed yield, partial penetration and distance to the observation well. The results of the developed solution were compared with those from an existing solution for the case of an infinitesimally thin skin. The latter solution can never approximate that for the developed finite skin. Dimensionless drawdown–time curves were compared with the other published results for a confined aquifer. Positive skin effects are reflected in the early time and disappear in the intermediate and late time aquifer responses. But in the case of negative skin this is reversed and the negative skin also tends to disguise the wellbore storage effect. A thick negative skin lowers the overall drawdown in the aquifer and leads to more persistent delayed drainage. Partial penetration increases the drawdown in the case of a positive skin; however its effect is masked by the negative skin. The influence of a negative skin is pronounced over a broad range of radial distances. At distant observation points the influence of a positive skin is too small to be reflected in early and intermediate time pumping test data and consequently the type curve takes its asymptotic form. 相似文献
112.
By implementing the moisture-based form of Richards’ equation into the geochemical modelling framework PHREEQC, a generic tool for the simulation of one-dimensional flow and solute transport in the vadose zone undergoing complex geochemical reactions was developed. A second-order, cell-centred, explicit finite difference scheme was employed for the numerical solution of the partial differential equations of flow and transport. In this scheme, the charge-balanced soil solution is treated as an assembly of elements, where changes in water and solute contents result from fluxes of elements across cell boundaries. Therefore, water flow is considered in terms of oxygen and hydrogen transport. 相似文献
113.
‘Snake River (SR)-type’ volcanism at the Yellowstone hotspot track: distinctive products from unusual,high-temperature silicic super-eruptions 总被引:1,自引:0,他引:1
M. J. Branney B. Bonnichsen G. D. M. Andrews B. Ellis T. L. Barry M. McCurry 《Bulletin of Volcanology》2008,70(3):293-314
A new category of large-scale volcanism, here termed Snake River (SR)-type volcanism, is defined with reference to a distinctive
volcanic facies association displayed by Miocene rocks in the central Snake River Plain area of southern Idaho and northern
Nevada, USA. The facies association contrasts with those typical of silicic volcanism elsewhere and records unusual, voluminous
and particularly environmentally devastating styles of eruption that remain poorly understood. It includes: (1) large-volume,
lithic-poor rhyolitic ignimbrites with scarce pumice lapilli; (2) extensive, parallel-laminated, medium to coarse-grained
ashfall deposits with large cuspate shards, crystals and a paucity of pumice lapilli; many are fused to black vitrophyre;
(3) unusually extensive, large-volume rhyolite lavas; (4) unusually intense welding, rheomorphism, and widespread development
of lava-like facies in the ignimbrites; (5) extensive, fines-rich ash deposits with abundant ash aggregates (pellets and accretionary
lapilli); (6) the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre; (7) a bimodal association
between the rhyolitic rocks and numerous, coalescing low-profile basalt lava shields; and (8) widespread evidence of emplacement
in lacustrine-alluvial environments, as revealed by intercalated lake sediments, ignimbrite peperites, rhyolitic and basaltic
hyaloclastites, basalt pillow-lava deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial sands and palaeosols. Many
rhyolitic eruptions were high mass-flux, large volume and explosive (VEI 6–8), and involved H2O-poor, low-δ18O, metaluminous rhyolite magmas with unusually low viscosities, partly due to high magmatic temperatures (900–1,050°C). SR-type
volcanism contrasts with silicic volcanism at many other volcanic fields, where the fall deposits are typically Plinian with
pumice lapilli, the ignimbrites are low to medium grade (non-welded to eutaxitic) with abundant pumice lapilli or fiamme,
and the rhyolite extrusions are small volume silicic domes and coulées. SR-type volcanism seems to have occurred at numerous
times in Earth history, because elements of the facies association occur within some other volcanic fields, including Trans-Pecos
Texas, Etendeka-Paraná, Lebombo, the English Lake District, the Proterozoic Keewanawan volcanics of Minnesota and the Yardea
Dacite of Australia.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic
rhyolites. 相似文献
114.
115.
116.
This numerical investigation was carried out to advance mechanistic understanding of sediment transport under sheet flow conditions. An Euler–Euler coupled two-phase flow model was developed to simulate fluid–sediment oscillatory sheet flow. Since the concentration of sediment particles is high in such flows, the kinematics of the fluid and sediment phases are strongly coupled. This model includes interaction forces, intergranular stresses and turbulent stress closure. Each phase was modeled via the Reynolds-Averaged Navier–Stokes equations, with interphase momentum conservation accounting for the interaction between the phases. The generation and transformation of turbulence was modeled using the two-equation k–ε turbulence model. Concentration and sediment flux profiles were compared with experimental data for sheet flow conditions considering both symmetric and asymmetric oscillatory flows. Sediment and fluid velocity variations, concentration profiles, sediment flux and turbulence parameters of wave-generated sheet flow were studied numerically with a focus on sediment transport characteristics. In all applications, the model predictions compared well with the experimental data. Unlike previous investigations in which the flow is driven by a horizontal pressure gradient, the present model solves the Navier–Stokes equations under propagating waves. The model’s ability to predict sediment transport under oscillatory sheet flow conditions underscores its potential for understanding the evolution of beach morphology. 相似文献
117.
Barry L. Roberts Sean A. McKenna 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(2):227-236
The identification and characterization of target areas at former bombing ranges is the first step in investigating these
sites for residual unexploded ordnance. Traditionally, magnetometer surveys along transects are used in identifying areas
with high densities of magnetic anomalies, which are likely former target areas. Combining magnetometer survey data with other
data sources may reduce the level of survey data required for site characterization, increasing characterization efficiency.
Here, several techniques for incorporating secondary information into kriging estimates of magnetic anomaly density are investigated
for a former bombing range located near Pueblo, Colorado. In particular, kriging with external drift, collocated ordinary
cokriging, and simple kriging with local means (SKLM) are used to incorporate information from a secondary variable. The secondary
variable consists of a grid of crater density values derived from a topographic light detection and ranging (LIDAR) analysis.
The craters, which are clearly identifiable in the LIDAR data, were generated through munitions use at the site and are therefore
related to the target locations. The results from this study indicate that the inclusion of the secondary information in the
kriging estimates does benefit target area characterization and provides a means of elucidating target area details from only
limited magnetometer transect data. For the Pueblo site, the use of SKLM with the crater density as a secondary variable and
only limited magnetometer transect data, provided results comparable to those obtained from using much larger magnetometer
transect data sets. 相似文献
118.
L. Li D. A. Barry C. Cunningham F. Stagnitti J. -Y. Parlange 《Advances in water resources》2000,23(8):176
Previous studies on tidal dynamics of coastal aquifers have focussed on the inland propagation of oceanic tides in the cross-shore direction, a configuration that is essentially one-dimensional. Aquifers at natural coasts can also be influenced by tidal waves in nearby estuaries, resulting in a more complex behaviour of head fluctuations in the aquifers. We present an analytical solution to the two-dimensional depth-averaged groundwater flow equation for a semi-infinite aquifer subject to oscillating head conditions at the boundaries. The solution describes the tidal dynamics of a coastal aquifer that is adjacent to a cross-shore estuary. Both the effects of oceanic and estuarine tides on the aquifer are included in the solution. The analytical prediction of the head fluctuations is verified by comparison with numerical solutions computed using a standard finite-difference method. An essential feature of the present analytical solution is the interaction between the cross- and along-shore tidal waves in the aquifer area near the estuary’s entry. As the distance from the estuary or coastline increases, the wave interaction is weakened and the aquifer response is reduced, respectively, to the one-dimensional solution for oceanic tides or the solution of Sun (Sun H. A two-dimensional analytical solution of groundwater response to tidal loading in an estuary, Water Resour Res 1997;33:1429–35) for two-dimensional non-interacting tidal waves. 相似文献
119.
Spectroradiometers using photodiode array detectors (PDAs) are increasingly applied for airborne and ground-based atmospheric measurements of spectral actinic flux densities due to their high time resolution (less than one second). However they have limited sensitivity of ultraviolet (UV) radiation for wavelengths less than about 305 nm. This results in uncertainties of ozone photolysis frequencies derived from spectral actinic flux density measurements using PDA spectrometers. To overcome this limitation a parameterization method is introduced which extrapolates the data towards the wavelength range of limited sensitivity of the PDA spectrometers (less than about 305 nm). The parameterization is based on radiative transfer simulations and is valid for measurements in the lower troposphere. The components of the suggested parameterization are the lower threshold wavelength of the PDA spectrometer, the slant ozone column (ratio of the total ozone column and the cosine of the solar zenith angle), and the ambient temperature. Tests of the parameterization with simulated actinic flux density spectra have revealed an uncertainty of the derived ozone photolysis frequency of ±5%. Field comparisons of the parameterization results with independent measurements of the ozone photolysis frequency were within ±10% for solar zenith angles less than 70^∘. Finally the parameterization was applied to airborne measurements to emphasize the advantage of high time resolution of PDA spectrometers to study ozone photolysis frequency fields in inhomogeneous cloud condtitions. 相似文献
120.
An Anatomy of Adaptation to Climate Change and Variability 总被引:29,自引:7,他引:29
Adaptation to climate variability and change is important both for impact assessment (to estimate adaptations which are likely to occur) and for policy development (to advise on or prescribe adaptations). This paper proposes an "anatomy of adaptation" to systematically specify and differentiate adaptations, based upon three questions: (i) adapt to what? (ii) who or what adapts? and (iii) how does adaptation occur? Climatic stimuli include changes in long-term mean conditions and variability about means, both current and future, and including extremes. Adaptation depends fundamentally on the characteristics of the system of interest, including its sensitivities and vulnerabilities. The nature of adaptation processes and forms can be distinguished by numerous attributes including timing, purposefulness, and effect. The paper notes the contribution of conceptual and numerical models and empirical studies to the understanding of adaptation, and outlines approaches to the normative evaluation of adaptation measures and strategies. 相似文献