全文获取类型
收费全文 | 1630篇 |
免费 | 42篇 |
国内免费 | 21篇 |
专业分类
测绘学 | 52篇 |
大气科学 | 94篇 |
地球物理 | 345篇 |
地质学 | 581篇 |
海洋学 | 109篇 |
天文学 | 343篇 |
综合类 | 16篇 |
自然地理 | 153篇 |
出版年
2022年 | 8篇 |
2021年 | 8篇 |
2020年 | 23篇 |
2019年 | 22篇 |
2018年 | 31篇 |
2017年 | 23篇 |
2016年 | 31篇 |
2015年 | 25篇 |
2014年 | 37篇 |
2013年 | 72篇 |
2012年 | 49篇 |
2011年 | 68篇 |
2010年 | 87篇 |
2009年 | 91篇 |
2008年 | 78篇 |
2007年 | 63篇 |
2006年 | 58篇 |
2005年 | 57篇 |
2004年 | 50篇 |
2003年 | 63篇 |
2002年 | 52篇 |
2001年 | 27篇 |
2000年 | 39篇 |
1999年 | 25篇 |
1998年 | 20篇 |
1997年 | 22篇 |
1996年 | 21篇 |
1995年 | 23篇 |
1994年 | 27篇 |
1993年 | 25篇 |
1992年 | 23篇 |
1990年 | 21篇 |
1989年 | 23篇 |
1988年 | 26篇 |
1987年 | 21篇 |
1986年 | 20篇 |
1985年 | 34篇 |
1984年 | 32篇 |
1983年 | 27篇 |
1982年 | 22篇 |
1981年 | 36篇 |
1980年 | 24篇 |
1979年 | 16篇 |
1978年 | 15篇 |
1977年 | 20篇 |
1976年 | 14篇 |
1975年 | 11篇 |
1974年 | 19篇 |
1973年 | 16篇 |
1971年 | 8篇 |
排序方式: 共有1693条查询结果,搜索用时 15 毫秒
971.
Direct numerical simulation (DNS) is applied to investigate properties of katabatic and anabatic flows along thermally perturbed
(in terms of surface buoyancy flux) sloping surfaces in the absence of rotation. Numerical experiments are conducted for homogeneous
surface forcings over infinite planar slopes. The simulated flows are the turbulent analogs of the Prandtl (1942) one-dimensional
laminar slope flow. The simulated flows achieve quasi-steady periodic regimes at large times, with turbulent fluctuations
being modified by persistent low-frequency oscillatory motions with frequency equal to the product of the ambient buoyancy
frequency and the sine of the slope angle. These oscillatory wave-type motions result from interactions between turbulence
and ambient stable stratification despite the temporal constancy of the surface buoyant forcing. The structure of the mean-flow
fields and turbulence statistics in simulated slope flows is analyzed. An integral dynamic similarity constraint for steady
slope/wall flows forced by surface buoyancy flux is derived and quantitatively verified against the DNS data. 相似文献
972.
In Ethiopia and Sierra Leone, recent social, political and environmental transformations have precipitated the intensification of wetland use, as local people have sought to safeguard and strengthen their livelihoods. Concurrent decentralization policies in both countries have also seen the government strengthen its position at the local level. Drawing upon recent field-based evidence from Ethiopia and Sierra Leone, this paper examines the compatibility between community-based local institutions for wetland use, and the process of decentralization. It argues that decentralization has in fact restricted the development of mature local institutional arrangements, due to its intrinsically political interventionist nature. 相似文献
973.
Sorptive barrier technologies have emerged as useful tools for addressing a wide range of remediation problems. When simulating barrier performance, numerous isotherm expressions are available for relating aqueous and sorbed concentrations. However, isotherm selection is non-trivial because alternative expressions may yield comparable fits to experimental data. Additionally, concentration data collected for parameter fitting is often outside the range of concentrations relevant to simulating barrier performance. This incompatibility necessitates extrapolation of isotherm behavior during simulation–optimization. Consequently, equally plausible isotherms may predict significantly different barrier performance.Numerical experiments involving organic contaminants were performed to examine the influence of isotherm selection and extrapolation on optimal barrier design. Ten isotherms were calibrated to existing experimental data and evaluated using information-theoretic selection criteria. When incorporated into simulation–optimization, extrapolation effects were clearly evident and optimal designs varied according to the chosen isotherm. To ensure robust barrier design in the presence of such variability, a simple methodology is proposed that utilizes a piecewise-minimum isotherm concept. By favoring plausible isotherms that predict the least amount of sorption, the methodology encourages conservative barrier design while respecting available data. 相似文献
974.
David A. Osleger Alan C. Heyvaert Joseph S. Stoner Kenneth L. Verosub 《Journal of Paleolimnology》2009,42(1):103-122
Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those
generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic,
and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident
trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ13Corg and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from
the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a
synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents
into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic
and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites.
Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis
that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad
patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the
western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap
suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term
regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms
between ~3000 and ~900 cal yr B.P. in the Tahoe watershed. 相似文献
975.
Depositional history of the Miocene Lake Sinj (Dinaride Lake System,Croatia): a long-lived hard-water lake in a pull-apart tectonic setting 总被引:1,自引:0,他引:1
Oleg Mandic Davor Pavelić Mathias Harzhauser Jožica Zupanič Doris Reischenbacher Reinhard F. Sachsenhofer Neven Tadej Alan Vranjković 《Journal of Paleolimnology》2009,41(3):431-452
Early Miocene transpressional wrenching yielded a series of NW–SE-elongated pull-apart basins in the Dinarides of Croatia
and Bosnia and Herzegovina. They accommodated a huge lake system that gave rise to spectacular endemic mollusk radiation.
Lake Sinj, moderately sized at 342 km2, flooded the south-westernmost basin of this system. Due to the karstic environment, the hard-water, alkaline, long-lived
lake developed a sediment infill with an average thickness of 370 m, dominated by authigenic limestone. The studied section
represents the upper third of the basinal infill and provides detailed insights into the critical period of the lake and of
the basinal evolution during the final stages of its filling. It comprises two large-scale, shallowing-upward cycles, both
starting with fossil-poor limestones, gradually passing into coal-bearing carbonate rocks and coal seams. The fossil-poor
intervals are interpreted as phases of repetitive acidification events due to changing lake level, which induced periodic
drying and flooding of the uppermost littoral zone inhabited by starfruit (Damasonium) meadows. The flooding of the aerated, limy mud plain introduced H+ ions from organic-matter decay reactions into the shallow
lake. This decreased its pH level, with catastrophic consequences for its biota. The ecosystem then stabilized during the
orbitally-forced, dry climate phases. Based on the mollusk record, streams still influenced the marginal lake environment
and rich organic-matter production created swamps and mires. The onset of mollusk radiation in the section correlates with
stabilized lake alkalinity, as indicated by the disappearance of starfruits, ongoing authigenic carbonate production and by
coal seams representing textbook examples for coal formation in alkaline environments. The inferred basinal setting fits well
with the pull-apart basin model, pointing to the presence of an extended shallow ramp in front of a steep, fault-induced hillside
of the hinterland. 相似文献
976.
977.
978.
The viscosity of hydrous dacitic liquids: implications for the rheology of evolving silicic magmas 总被引:2,自引:1,他引:1
Alan G. Whittington Bridget M. Hellwig Harald Behrens Bastian Joachim André Stechern Francesco Vetere 《Bulletin of Volcanology》2009,71(2):185-199
The viscosity of a series of six synthetic dacitic liquids, containing up to 5.04 wt% dissolved water, was measured above
the glass transition range by parallel-plate viscometry. The temperature of the 1011 Pa s isokom decreases from 1065 K for the anhydrous liquid, to 864 K and 680 K for water contents of 0.97 and 5.04 wt% H2O. Including additional measurements at high temperatures by concentric-cylinder and falling-sphere viscometry, the viscosity
(η) can be expressed as a function of temperature and water content w according to: where η is in Pa s, T is temperature in K, and w is in weight percent. Within the conditions of measurement, this parameterization reproduces the 76 viscosity data with a
root-mean square deviation (RMSD) of 0.16 log units in viscosity, or 7.8 K in temperature. The measurements show that water
decreases the viscosity of the dacitic liquids more than for andesitic liquids, but less than for rhyolites. At low temperatures
and high water contents, andesitic liquids are more viscous than the dacitic liquids, which are in turn more viscous than
rhyolitic liquids, reversing the trend seen for high temperatures and low water contents. This suggests that the relative
viscosity of different melts depends on temperature and water content as much as on bulk melt composition and structure. At
magmatic temperatures, rhyolites are orders of magnitude more viscous than dacites, which are slightly more viscous than andesites.
During degassing, all three liquids undergo a rapid viscosity increase at low water contents, and both dacitic and andesitic
liquids will degas more efficiently than rhyolitic liquids. During cooling and differentiation, changing melt chemistry, decreasing
temperature and increasing crystal content all lead to increases in the viscosity of magma (melt plus crystals). Under closed
system conditions, where melt water content can increase during crystallization, viscosity increases may be small. Conversely,
viscosity increases are very abrupt during ascent and degassing-induced crystallization. 相似文献
979.
980.
Alan Latham 《Area》1999,31(2):161-168
Summary This paper explores the dynamics of engagement and disengagement within urban space. It argues that a certain capacity to encounter the world around the self in an active, creative way is central to the self's ability to recognize and care about the places it inhabits and the people encountered within those places. Drawing on the work of Donald Winnicott and Jessica Benjamin, it outlines a framework of self-other psycho-dynamics through which this active, creative encountering may be understood, and examines the violence of indifference that occurs when this everyday creativity is absent. 相似文献