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951.
952.
Sabine Hilt Elisabeth M. Gross Michael Hupfer Harald Morscheid Jens Mhlmann Arnulf Melzer Jens Poltz Stefan Sandrock Eva-Maria Scharf Susanne Schneider Klaus van de Weyer 《Limnologica》2006,36(3):155-171
One of the most serious problems caused by eutrophication of shallow lakes is the disappearance of submerged macrophytes and the switch to a turbid, phytoplankton-dominated state. The reduction of external nutrient loads often does not result in a change back to the macrophyte-dominated state because stabilising mechanisms that cause resilience may delay a response. Additional internal lake restoration measures may therefore be needed to decrease the concentration of total phosphorus and increase water clarity. The re-establishment of submerged macrophytes required for a long-term stability of clear water conditions, however, may still fail, or mass developments of tall-growing species may cause nuisance for recreational use. Both cases are often not taken into account when restoration measures are planned in Germany, and existing schemes to reduce eutrophication consider the topic inadequately. Here we develop a step-by-step guideline to assess the chances of submerged macrophyte re-establishment in shallow lakes. We reviewed and rated the existing literature and case studies with special regard on (1) the impact of different internal lake restoration methods on the development of submerged macrophytes, (2) methods for the assessment of natural re-establishment, (3) requirements and methods for artificial support of submerged macrophyte development and (4) management options of macrophyte species diversity and abundance in Germany. This guideline is intended to help lake managers aiming to restore shallow lakes in Germany to critically asses and predict the potential development of submerged vegetation, taking into account the complex factors and interrelations that determine their occurrence, abundance and diversity. 相似文献
953.
Carlos L. Aiken Mark E. Ander Mauricio F. de la Fuente 《Journal of Volcanology and Geothermal Research》1991,45(1-2)
Geophysical studies were carried out in Platanares in Western Honduras, as part of an investigation of the geothermal resource potential of Honduras by Los Alamos National Laboratory in cooperation with the Honduran government and the U.S. Geological Survey. Geological and geochemical studies later followed by exploratory bore-holes indicate this area of rugged topography has the best geothermal potential in Honduras. In order to better understand the geology of the area, gravity, audiomagnetotelluric and telluric surveys were carried out. The site is in a graben of Tertiary volcanics and Cretaceous red beds, with numerous hot springs along faults in the center of the graben. Gravity mapped the thickness of low-density tuffs and the position of associated normal faults. It also indicated the presence of persuasive, shallow, highly silicified, high-density tuffs and gravels. The electrical surveys mapped shallow, high-resistivity silicified zones and faults as low-resistivity zones. The red beds below the tuffs are considered to contain the geothermal reservoir, but their maximum thickness could not be determined by these surveys due to the lack of density contrast and the high surface electrical conductivities. Three geologic cross-sections constructed by others have been re-evaluated using gravity modeling and analysis of eletrical anomalies, showing general agreement but in detail significant differences. 相似文献
954.
955.
956.
The ecological tidal model simulates the cycling of carbon, nitrogen, phosphate, and silicate and describes the tidal, diurnal,
and annual dynamics of the back barrier area of the island Spiekeroog in the German Bight. The region is characterized by
strong tidal currents and extensive tidal flats. It is strongly influenced by the conditions in the southern part of the German
Bight. This model study investigates the dependence of the model behavior on the boundary conditions and the forcing. The
effect of short- and long-term sea level rise on nutrient and plankton dynamics is analyzed. As the model is set up as semi-Lagrangian
with only a coarse approximation of the hydrodynamics, the seasonal and intratidal variability in the biogeochemical cycling
can only be reproduced qualitatively. By varying the boundary conditions, the intrinsic dynamics of the back barrier area
can be separated from boundary condition effects. This study shows that any agreement between model results and field data
cannot be expected without correct boundary conditions. The seasonal variability is of major importance, while higher-frequency
variability only plays a minor role. 相似文献
957.
Stefan Jansen Eva Walpersdorf Ursula Werner Markus Billerbeck Michael E Böttcher Dirk de Beer 《Ocean Dynamics》2009,59(2):317-332
In this article, we describe the dynamics of pH, O2 and H2S in the top 5–10 cm of an intertidal flat consisting of permeable sand. These dynamics were measured at the low water line
and higher up the flat and during several seasons. Together with pore water nutrient data, the dynamics confirm that two types
of transport act as driving forces for the cycling of elements (Billerbeck et al. 2006b): Fast surface dynamics of pore water chemistry occur only during inundation. Thus, they must be driven by hydraulics (tidal
and wave action) and are highly dependent on weather conditions. This was demonstrated clearly by quick variation in oxygen
penetration depth: Seeps are active at low tide only, indicating that the pore water flow in them is driven by a pressure
head developing at low tide. The seeps are fed by slow transport of pore water over long distances in the deeper sediment.
In the seeps, high concentrations of degradation products such as nutrients and sulphide were found, showing them to be the
outlets of deep-seated degradation processes. The degradation products appear toxic for bioturbating/bioirrigating organisms,
as a consequence of which, these were absent in the wider seep areas. These two mechanisms driving advection determine oxygen
dynamics in these flats, whereas bioirrigation plays a minor role. The deep circulation causes a characteristic distribution
of strongly reduced pore water near the low water line and rather more oxidised sediments in the centre of the flats. The
two combined transport phenomena determine the fluxes of solutes and gases from the sediment to the surface water and in this
way create specific niches for various types of microorganisms. 相似文献
958.
临界转换的早期预警信号 总被引:1,自引:0,他引:1
Marten Scheffer George Sugihara Jordi Bascompte Victor Brovkin Vasilis Dakos Hermann Held Max Rietkerk 李迎春 闫伟 蒋长胜 《国际地震动态》2009,(9):1-14
从生态系统到金融市场和气候在内的很多复杂动力系统,都会有临界点,在这样的点上系统可能会发生突变,从而演变到一个对立的动力模式上。在这样的临界点到达之前对其进行预测极为困难,但是现在,不同科学领域的研究工作表明,普遍性的早期预警信号有迹可循。对一系列不同类型的系统来说,这些信号会指示是否正在趋近一个临界阈值。 相似文献
959.
The influence of wave–bedform feedbacks on both the initial formation of shoreface-connected sand ridges (sfcr) and on grain
size sorting over these ridges on micro-tidal inner shelves is studied. Also, the effect of sediment sorting on the growth
and the migration of sfcr is investigated. This is done by applying a linear stability analysis to an idealized process-based
morphodynamic model, which simulates the initial growth of sfcr due to the positive coupling between waves, currents, and
an erodible bed. The sediment consists of sand grains with two different sizes. New elements with respect to earlier studies
on grain sorting over sfcr are that wave-topography interactions are explicitly accounted for, entrainment of sediment depends
on bottom roughness, and transport of suspended sediment involves settling lag effects. The results of the model indicate
that sediment sorting causes a reduction of the growth rate and migration speed of sfcr, whereas the wavelength is only slightly
affected. In the case where the entrainment of suspended sediment depends on bottom roughness, the coarsest sediment is found
in the troughs; otherwise, the finest sediment occurs in the troughs. Compared to previous work, modeled maximum variations
in the mean grain size over the topography are in better agreement with field observations. Settling lag effects are important
for the damping of high-wavenumber mode instabilities such that a preferred wavelength of the bedforms is obtained. 相似文献
960.
Benjamin Bernard Benjamin van Wyk de Vries Hervé Leyrit 《Bulletin of Volcanology》2009,71(9):1041-1056
Debris avalanches associated with volcanic sector collapse are usually high-volume high-mobility phenomena. Debris avalanche
deposit remobilisation by cohesive debris flows and landslides is common, so they can share textural characteristics such
as hummocks and jigsaw cracks. Distinguishing original deposits from reworked products is critical for geological understanding
and hazard assessment because of their different origin, frequency and environmental impact. We present a methodology based
on field evidence to differentiate such epiclastic breccias. Basal contact mapping constrained by accurate altitude and location
data allows the reconstruction of deposit stratigraphy and geometry. Lithological analysis helps to distinguish the different
units. Incorporation structures, kinematic indicators and component mingling textures are used to characterise erosion and
transport mechanisms. We apply this method to the enigmatic sequence at Perrier (French Massif Central), where four units
(U1–U4) have been interpreted either as debris flow or debris avalanche deposits. The sequence results from activity on the
Monts Dore Volcano about 2 Ma ago. The epiclastic units are matrix supported with an almost flat top. U2 and U3 have clear
debris flow deposit affinities such as rounded clasts and intact blocks (no jigsaw cracks). U1 and U4 have jigsaw cracked
blocks with matrix injection and stretched sediment blocks. U1 lacks large blocks (>10 m wide) and has a homogenous matrix
with an upward increase of trapped air vesicle content and size. This unit is interpreted as a cohesive debris flow deposit
spawned from a debris avalanche upstream. In contrast, U4 has large mega-blocks (up to 40 m wide), sharp contacts between
mixed facies zones with different colours and numerous jigsaw fit blocks (open jigsaw cracks filled by monogenic intra-clast
matrix). Mega-blocks are concentrated near the deposit base and are spatially associated with major substratum erosion. This
deposit has a debris avalanche distal facies with local debris flow affinities due to partial water saturation. We also identify
two landslide deposits (L1 and L2) resulting from recent reworking that has produced a similar facies to U1 and U4. These
are distinguishable from the original deposits, as they contain blocks of mixed U1/U4 facies, a distinctly less consolidated
and more porous matrix and a fresh hummocky topography. This work shows how to differentiate epiclastic deposits with similar
characteristics, but different origins. In doing so, we improve understanding of present and past instability of the Monts
Dore and identify present landslide hazards at Perrier. 相似文献