全文获取类型
收费全文 | 108848篇 |
免费 | 1264篇 |
国内免费 | 664篇 |
专业分类
测绘学 | 2519篇 |
大气科学 | 7122篇 |
地球物理 | 20427篇 |
地质学 | 40932篇 |
海洋学 | 9424篇 |
天文学 | 24343篇 |
综合类 | 366篇 |
自然地理 | 5643篇 |
出版年
2022年 | 710篇 |
2021年 | 1182篇 |
2020年 | 1236篇 |
2019年 | 1396篇 |
2018年 | 4442篇 |
2017年 | 4016篇 |
2016年 | 3884篇 |
2015年 | 1591篇 |
2014年 | 2972篇 |
2013年 | 5110篇 |
2012年 | 3811篇 |
2011年 | 5658篇 |
2010年 | 5052篇 |
2009年 | 6224篇 |
2008年 | 5268篇 |
2007年 | 5756篇 |
2006年 | 4038篇 |
2005年 | 3018篇 |
2004年 | 2937篇 |
2003年 | 2836篇 |
2002年 | 2764篇 |
2001年 | 2282篇 |
2000年 | 2202篇 |
1999年 | 1736篇 |
1998年 | 1776篇 |
1997年 | 1659篇 |
1996年 | 1420篇 |
1995年 | 1401篇 |
1994年 | 1214篇 |
1993年 | 1129篇 |
1992年 | 1073篇 |
1991年 | 1080篇 |
1990年 | 1061篇 |
1989年 | 943篇 |
1988年 | 855篇 |
1987年 | 1026篇 |
1986年 | 896篇 |
1985年 | 1092篇 |
1984年 | 1212篇 |
1983年 | 1177篇 |
1982年 | 1083篇 |
1981年 | 1017篇 |
1980年 | 924篇 |
1979年 | 886篇 |
1978年 | 845篇 |
1977年 | 728篇 |
1976年 | 717篇 |
1975年 | 716篇 |
1974年 | 685篇 |
1973年 | 774篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Analysis of objective functions used in urban runoff models 总被引:1,自引:0,他引:1
The objective functions used in parameter estimation in urban runoff models are compared by using a method proposed by Diskin and Simon1 and the urban runoff model ILLUDAS. Two sets of objective functions, the first one used by Diskin and Simon in their study and a second one which includes other objective functions are used. Rainfall-runoff data from urban watersheds in the US are used in the study. The results indicate that the least squares criterion is the best among those studied. 相似文献
942.
Krakatau Volcano is located along a N35E volcanic lineament running through the Sunda straits (Indonesia). Its last activity has been characterized by successive phases, each beginning with the construction of a cone, and ending with its destruction and the formation of a caldera. The two last (pre- and post-1883) cycles are well known, but the more ancient ones are not so clearly defined.Lavas of Krakatau belong to an andesitic series, in which fractional crystallization plays the most important role. The petrologic evolution is characterized by a cyclicity in good agreement with the structural evolution: the succession is regular: basalts, basic andesites, acid andesites, dacites. A gap between acid and basic andesites occurs in each cycle. The destructive stages correspond to the occurrence of dacitic terms.The Anak cycle was characterized from 1927 to 1979 by basalts and basic andesites; the 1981 eruption involved a more differentiated magma (close to dacitic). Detailed study of the petrologic evolution since 1883 emphasizes the predominant role of fractional crystallization. This process occurred during a very short period, between 1979 and 1981. Separation of labradorite, augite, olivine and magnetite from parental basic andesite may generate the dacitic descendant, in a shallow reservoir (PH2O estimated about 0.5 kbar). Implications for a future activity are considered. 相似文献
943.
H. Burtscher P. Cohn L. Scherrer H.C. Siegmann G. Faraci A.R. Pennisi V. Privitera R. Cristofolini V. Scribano 《Journal of Volcanology and Geothermal Research》1987,33(4)
Volcanic aerosol emissions have been studied for the first time by in situ photoelectric charging. Explorative studies on Mt Etna reveal large concentrations of particles below 1000 nm with the spectrum peaking in the size range of 100–1000 nm diameter. Although a large fraction of the particles is already charged upon emission, the net electrical charge carried by the aerosol turned out to be close to zero.Particles with high photoelectric yield vary greatly in their relative abundance and seem to occur mainly at active points of the volcano. 相似文献
944.
945.
Use of Rhodamine water tracer in the marshland upwelling system 总被引:1,自引:0,他引:1
Rhodamine water tracer (RWT) was used to characterize the migration of waste water within the saline subsurface of a marshland upwelling system (MUS), which is an alternative on-site waste water treatment system for coastal areas. Field tracer studies were performed to investigate the fresh waste water plume movement within the saline ground water. Pore velocities were calculated using first detection times and ranged from 0.68 to 10.7 x 10(-4) cm/sec for the loamy sandy soil matrix present at the site. Use of RWT in the field also allowed determination of main and preferential flowpaths. One- and two-dimensional laboratory experiments were performed using silica sand to investigate the interactions of the organically rich waste water with RWT within the zone surrounding the point of injection (one-dimensional) and the impact of background salinity on plume movement (two-dimensional). The results from these studies were used to help explain the field data. One-dimensional breakthrough curves revealed retardation factors for the RWT in the waste water mixture of 1.73 to 1.90. These results were similar to other researchers, indicating little interaction between the waste water and RWT. Variations in pore water salinity (5, 15, 25, and 35 ppt) were found to have a significant effect on pore water velocities of the fresh water plume (two-dimensional), indicating the need to incorporate background salinities into the design process for MUS. 相似文献
946.
María Charco José Fernández Francisco Luzón Kristy F. Tiampo John B. Rundle 《Pure and Applied Geophysics》2007,164(4):865-878
Surface displacements and gravity changes due to volcanic sources are influenced by medium properties. We investigate topographic,
elastic and self-gravitation interaction in order to outline the major factors that are significant in data modelling. While
elastic-gravitational models can provide a suitable approximation to problems of volcanic loading in areas where topographic
relief is negligible, for prominent volcanoes the rough topography could affect deformation and gravity changes to a greater
extent than self-gravitation. This fact requires the selection, depending on local relief, of a suitable model for use in
the interpretation of surface precursors of volcanic activity. We use the three-dimensional Indirect Boundary Element Method
to examine the effects of topography on deformation and gravity changes in models of magma chamber inflation/deflation. Topography
has a significant effect on predicted surface deformation and gravity changes. Both the magnitude and pattern of the geodetic
signals are significantly different compared to half-space solutions. Thus, failure to account for topographic effects in
areas of prominent relief can bias the estimate of volcanic source parameters, since the magnitude and pattern of deformation
and gravity changes depend on such effects. 相似文献
947.
Effects of trace metal and exposure to air on serotonin and dopamine levels in tissues of the mussel Perna perna 总被引:1,自引:0,他引:1
We evaluated levels of serotonin (5HT) and dopamine (DOPA) in muscle and digestive glands of the mussel, Perna perna, collected at different times of day; exposed to air for 24 h, followed by re-submersion; and after exposure to different metals. Mussels collected at different periods of day showed little oscillation in 5HT and DOPA levels. Mussels exposed to metals showed significant changes in 5HT and DOPA levels in digestive gland and muscle, as did mussels exposed to air. Our data suggest that analyses of 5HT and DOPA in tissues of mussels could serve as a tool to evaluate the presence and effects of heavy metal contamination in mussels. Care in data interpretation is required, however, since other environmental factors such as exposure of mussels to air (i.e. at low tides) can also cause changes in DOPA and 5HT levels. Additional research is necessary to separate such natural environmental effects from effects of contaminants. 相似文献
948.
The wavefield in, and at the surface of, a homogeneous, isotropic, perfectly elastic half-space, excited by a traction distribution at the surface of the medium is investigated. The emitted wavefield is a spatial convolution of the surface tractions and the spatial impulse response. The properties of the wavefield in the far-field of the medium are derived and it is shown that the far-field particle velocity is essentially equal to a weighted sum of the time derivative of the integrated surface tractions, that is, of the components of the ‘ground force’. The theory is valid for an arbitrary geometry and orientation of the surface tractions, and is independent of the boundary conditions at the surface of the medium. The surface tractions are related to a source that consists of a mass distribution with an arbitrary force distribution imposed upon it. A boundary condition is introduced that accounts for the mass load and the forces applied to it but neglects vibrations within the mass. The boundary condition follows from the equation of motion of the surface mass load. The theory is applied to the Vibroseis configuration, using a P-wave vibrator model with a uniformly distributed force imposed on top of the baseplate, and assuming that horizontal surface traction components are absent. The distribution of displacement and stress directly underneath the baseplate of a single vibrator and an array of vibrators is investigated. Three different boundary conditions are used: (1) assuming uniform pressure, (2) assuming uniform displacement, (3) using the equation of motion of the baseplate as a boundary condition. The calculations of the distribution of stress and displacement over the plate for different elastic media and several frequencies of operation show that only the results obtained with the mixed boundary condition agree with measurements made in the field. The accuracy of three different phase-feedback signals is compared using synthetic data. Baseplate velocity phase-feedback leads to huge deviations in the determination of the far-field wavelet; reaction mass acceleration phase-feedback looks stable but neglects the differentiating earth filter; and phase-feedback to a weighted sum of baseplate and reaction mass accelerations becomes unstable with increasing frequency. The instability can be overcome using measurements over the whole baseplate. The model can be extended to a lossy layered earth. 相似文献
949.
Discussed are the results of studying an evolution of tropical cyclones (TCs) in the Pacific Ocean using the data of computation of ETA and WRF NMM mesoscale numerical atmospheric models. Computed are the trajectories of TCs and the fields of meteorological variables in the typhoons, of the wind speed and kinetic energy in the subtropical jet stream during the development of Parma, Melor, and Lupit typhoons. Carried out are the analysis and comparison of computed fields of pressure, wind, kinetic energy, and trajectories of TCs obtained using these models and their comparison with the actual fields. It is demonstrated that both models computed rather well the complex trajectories and the fields of wind and kinetic energy varying in the course of the interaction. Proposed is an explanation of processes taking place during the interaction between the vortices and the subtropical jet stream and the polar front. 相似文献
950.
The climate–population relationship has long been conceived. Although the topic has been repeatedly investigated, most of
the related works are Eurocentric or qualitative. Consequently, the relationship between climate and population remains ambiguous.
In this study, fine-grained temperature reconstructions and historical population data sets have been employed to statistically
test a hypothesized relationship between temperature change and population growth (i.e., cooling associated with below average
population growth) in China over the past millennium. The important results were: (1) Long-term temperature change significantly
determined the population growth dynamics of China. However, spatial variation existed, whilst population growth in Central
China was shown to be responsive to both long- and short-term temperature changes; in marginal areas, population growth was
only sensitive to short-term temperature fluctuations. (2) Temporally, the temperature–population relationship was obscured
in some periods, which was attributable to the factors of drought and social buffers. In summary, a temperature–population
relationship was mediated by geographic factors, the aridity threshold, and social factors. Given the upcoming threat posed
by climate change to human societies, this study seeks to improve our knowledge and understanding of the climate–society relationship. 相似文献