全文获取类型
收费全文 | 792篇 |
免费 | 19篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 23篇 |
大气科学 | 46篇 |
地球物理 | 154篇 |
地质学 | 241篇 |
海洋学 | 65篇 |
天文学 | 234篇 |
综合类 | 5篇 |
自然地理 | 46篇 |
出版年
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 13篇 |
2018年 | 23篇 |
2017年 | 12篇 |
2016年 | 24篇 |
2015年 | 11篇 |
2014年 | 20篇 |
2013年 | 42篇 |
2012年 | 26篇 |
2011年 | 27篇 |
2010年 | 30篇 |
2009年 | 34篇 |
2008年 | 42篇 |
2007年 | 41篇 |
2006年 | 43篇 |
2005年 | 26篇 |
2004年 | 32篇 |
2003年 | 24篇 |
2002年 | 26篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 20篇 |
1998年 | 20篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 11篇 |
1994年 | 7篇 |
1993年 | 11篇 |
1992年 | 4篇 |
1990年 | 12篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 12篇 |
1984年 | 6篇 |
1983年 | 9篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1973年 | 9篇 |
1971年 | 4篇 |
1959年 | 3篇 |
1950年 | 3篇 |
1949年 | 3篇 |
1910年 | 4篇 |
排序方式: 共有814条查询结果,搜索用时 15 毫秒
741.
R.D. Hill 《Singapore journal of tropical geography》1998,19(1):1-25
Agricultural change in Southeast Asia over the last three decades is reviewed within the context of structural change of the region's economies. An agricultural typology is outlined and the degree to which agricultural systems have remained static is examined leading to the identification of various kinds and degrees of change amongst the systems – extensification, intensification and disintensification. The paper concludes that while macro-level structural change is the basic process driving changes in agriculture, there remain important issues of long-term sustainability, poverty and distribution of benefits. 相似文献
742.
Scattering parameters of the lithosphere below the Massif Central, France, from teleseismic wavefield records 总被引:1,自引:0,他引:1
Joachim R. R. Ritter Sergei A. Shapiro & Barbara Schechinger 《Geophysical Journal International》1998,134(1):187-198
Teleseismic P -wave recordings are analysed in the frequency range 0.3–6 Hz to derive structural (statistical) parameters of the lithosphere underneath the French Massif Central. For this we analyse differences in frequency-dependent intensities of the mean wavefield and the fluctuation wavefield. It is possible to discriminate a weak fluctuation regime of the wavefield in the frequency range below 1 Hz and a strong fluctuation regime starting above 1 Hz and continuing to higher frequencies. The observed wavefield fluctuations in the frequency range 0.3–3 Hz can be explained by scattering of the teleseismic P wave front at elastic inhomogeneities in the lithosphere. A statistical distribution of the inhomogeneities is assumed and the concept of random media is applied. The lithospheric structure under the Massif Central can be described as a 70 km thick heterogeneous layer with velocity fluctuations of 3–7 per cent and correlation lengths of the heterogeneities of 1–16 km. 相似文献
743.
Antony A. Hill 《Dynamics of Atmospheres and Oceans》2004,38(1):441
A model for convection induced by the selective absorption of radiation in a fluid layer is investigated. The concentration based internal heat source is modelled quadratically. A linear analysis and conditional nonlinear analysis are performed. Due to the presence of significantly large regions of potential subcritical instabilities, the results indicate that linear theory may only be accurate enough to predict the onset of convective motion when the model for the internal heat source is predominantly linear. This lends much credence to the use of a linearly modelled internal heat source as used by Krishnamurti [Dyn. Atmos. Oceans 27 (1997) 367], within the constraints of the parameter ranges. 相似文献
744.
The El Niño-Southern Oscillation (ENSO) effect has been found to be associated with regional climate variations in many regions of the world, and, in turn, with variation in crop yields. Previous studies have found that early releases of ENSO phase information could permit agricultural producers to make adjustments in their decisions and in turn generate an increase in agricultural sector welfare. This study examines whether the value of the agricultural responses can be enhanced by releasing more detailed ENSO information. Namely we evaluate the implications for projected agricultural welfare under release and adaptation to the Stone and Auliciems five phase definition of ENSO states as opposed to the more standard three phase definition. This value is estimated using a stochastic, U.S./global agricultural model representing 22 climate years. The results indicate that the release and exploitation of the more detailed ENSO phase definition almost doubles the welfare impact. The results also indicate that there is room for up to another doubling of information value through further refinements. 相似文献
745.
The revised empirical model for in- and outflow calculation of Upper Lake Constance has provided satisfying results supported by measured values. The given model was implemented to simulate total water inputs of the lake during the period from 1941 to 2000 with emphasis on the flood conditions of 1999. Analysis of annual water input development reveals a tendency toward slight increases until the 1960s. Thereafter, a reduction in inputs can be noted. This trend probably continues to hold true to present. Weather conditions of given individual years have caused distinct fluctuations to the water budget.Unusual meteorological conditions led to extreme flooding in early May of 1999. Daily water inputs of up to 200 mio m3 generated the highest water levels ever observed for this time of the year. Continual extraordinarily high water inputs occurring from February until July and then again from September until the end of 1999 resulted in the second largest annual total water input recorded since 1941. 相似文献
746.
Joachim R.R. Ritter 《Journal of Seismology》2002,6(4):477-486
The mechanical, automatically recording seismological instruments developed by Emil Wiechert in Göttingen at the beginning of the 20th century are still in use today. I comparethe recording characteristics of these seismographswith a modern, electrodynamic MARK L4-3D instrument. The frequency-dependent transfer functions are determined and used to restitute the`true' ground displacement. The displacement waveforms at the different instruments are well correlated at frequencies between 0.05 and 4.0 Hz.Amplitudes differ by a factor of less than two. This is sufficientfor the determination of seismological magnitudes which are defined on a logarithmicbasis. The timing accuracy is less favourable due to theslow recording speed on the smoked paper. However, relativetime differences within one second may be resolved, allowing the use of variousmodern seismological interpretation techniques. 相似文献
747.
Creep behaviour of the so called Emscher-Marl is investigated in uniaxial compression tests under a temperature of −10°C. Similar to the results of creep tests published elsewhere, it is shown that the secondary creep controls the deformation. In cases of stationary creep the material behaves like a nonlinear elasto-viscous Maxwell fluid. Assuming no hydrostatic pressure effect, the power law applied to the uniaxial case can be transformed to multiaxial states of stress. For structures of frozen soil with a long-term deformation process Norton's power relationship between stress and rate of steady flow leads to reasonable results. As a numerical solution a finite-element computer program in connection with incremental procedure is developed for treatment of creep problems in groundfreezing technology. The calculation of a simply supported beam demonstrates the important influence of the high nonlinearity in stress and strain for frozen soils. 相似文献
748.
749.
Earthquake swarms in California are often localized to areas within dextral offsets in the linear trend in active fault strands, suggesting a relation between earthquake swarms and local crustal spreading. Local crustal spereading is required by the geometry of dextral offsets when, as in the San Andreas system, faults have dominantly strike-slip motion with right-lateral displacement. Three clear examples of this relation occur in the Imperial Valley, Coso Hot Springs, and the Danville region, all in California. The first two of these areas are known for their Holocene volcanism and geothermal potential, which is consistent with crustal spreading and magmatic intrusion. The third example, however, shows no evidence for volcanism or geothermal activity at the surface. 相似文献
750.
Joachim Von Löhr 《Aquatic Sciences - Research Across Boundaries》1985,47(2):179-196
Measured oxygen concentrations over the depth in an impounded river showed considerable vertical gradients. Their numerical calculation based on concepts to be found in the literature was yet not possible. For assessing the signification of the mixing over the depth for the phytoplanktic oxygen production a mathematical concept will be developped. This enables the quantification of the influence of the turbulence dependent permanent light-dark changes upon the phytoplanktic production. The results lead to the conclusion that the mixing processes may have a stimulating effect upon the algae which rises their photosynthetic production by a factor from about 2 up to 4. 相似文献