全文获取类型
收费全文 | 2933篇 |
免费 | 137篇 |
国内免费 | 32篇 |
专业分类
测绘学 | 69篇 |
大气科学 | 227篇 |
地球物理 | 728篇 |
地质学 | 986篇 |
海洋学 | 263篇 |
天文学 | 442篇 |
综合类 | 14篇 |
自然地理 | 373篇 |
出版年
2024年 | 5篇 |
2023年 | 6篇 |
2022年 | 8篇 |
2021年 | 54篇 |
2020年 | 47篇 |
2019年 | 55篇 |
2018年 | 64篇 |
2017年 | 66篇 |
2016年 | 89篇 |
2015年 | 89篇 |
2014年 | 91篇 |
2013年 | 180篇 |
2012年 | 139篇 |
2011年 | 185篇 |
2010年 | 141篇 |
2009年 | 168篇 |
2008年 | 154篇 |
2007年 | 161篇 |
2006年 | 144篇 |
2005年 | 125篇 |
2004年 | 124篇 |
2003年 | 108篇 |
2002年 | 99篇 |
2001年 | 71篇 |
2000年 | 64篇 |
1999年 | 59篇 |
1998年 | 53篇 |
1997年 | 37篇 |
1996年 | 43篇 |
1995年 | 29篇 |
1994年 | 34篇 |
1993年 | 32篇 |
1992年 | 28篇 |
1991年 | 26篇 |
1990年 | 24篇 |
1989年 | 23篇 |
1988年 | 25篇 |
1987年 | 29篇 |
1986年 | 24篇 |
1985年 | 31篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 23篇 |
1980年 | 18篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1974年 | 4篇 |
1971年 | 3篇 |
排序方式: 共有3102条查询结果,搜索用时 33 毫秒
831.
Mark L. Mallory Kerry Woo Anthony J. Gaston W. Eric Davies Pierre Mineau 《Polar research》2004,23(1):111-114
Although the diet of walrus ( Odobenus rosmarus ) is considered varied, records of walrus consuming marine birds are rare in the published literature. In 2001 and 2002 we observed walrus foraging on adult thick-billed murres ( Uria lomvia ) at Coats Island in Nunavut, Canada. Approximately 46% of the attacks on murres were successful, and as many as 67 murres may have been killed in one day in August 2002. All soft parts of the murres were sucked out, with carcasses of only bones, feathers and skin left floating on the water. The extent to which predation on seabirds by walrus occurs across the eastern Arctic is unknown, but it could represent an important source of mortality for murres at some breeding colonies. 相似文献
832.
The radiological environment of Svalbard 总被引:1,自引:0,他引:1
Justin P. Gwynn Mark Dowdall Corine Davids Øyvind G. Selnæs Bjørn Lind 《Polar research》2004,23(2):167-180
833.
834.
Mark A. Bishop 《地球表面变化过程与地形》2004,29(1):125-131
Distribution‐free statistical methods of comparative data analysis have identi?ed subtle granulometric differences attributed to the evolution of barchan form at Gurra‐Gurra waterhole. Geomorphic locations on the barchan dunes display statistically signi?cant grain‐size differences that assist in the interpretation of aeolian processes. In summer, very ?ne sands mantle the dunescape and are the fraction that most affects the parameters of sorting and skewness. The sur?cial sedimentological character is one of subtle contrasts between the processes of grain winnowing and intergranular protection. The second and third moment measures are parameters that best demonstrate the spatial granulometric differences. Dune‐forming processes at Gurra‐Gurra have produced dune sands that have a very narrow range of grain size, which, in turn, re?ects textural and mineralogical maturity, and hence an extensive transport history. The statistical techniques employed in this study can also be used for the comparison of temporal (seasonal) sedimentological change, and for the granulometric analysis and association of process for dunes of different morpho‐types. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
835.
This paper examines the use of multi‐agent simulations (MAS) to model the petrol market. The development of a purely agent based model for petrol prices is presented. Failings within this model are discussed and an alternative strategy for controlling the price of each petrol station based on population of customers is considered. Individual level modelling of customers is too computationally intensive; therefore a spatial interaction model was used to estimate the sales and linked to the agent system to create a hybrid model. To evaluate how effective this hybrid model was, a comparison was made with an existing data set of real petrol prices collected over a two month period. This was achieved both statistically and visually with the use of a Geographical Information System (GIS). Experimentation revealed that the hybrid model outperformed the agent model. Investigation into the behaviour and sensitivity of the system (for example, how prices diffuse spatially) was undertaken by means of idealised simulations. 相似文献
836.
837.
There are very few experimental studies identifying hydrological pathways within rain forest slopes. Such knowledge is, however, necessary to understand why forest disturbance affects rainfall–riverflow response and nutrient migration. This study examines flow pathways within lowland rain forest slopes comprising Udults of the Ultisol soil order. Experimentation was conducted on four SE Asian hillslope units (each 5 × 5 m in plan) in the Bukit Timah catchment (Singapore Island), and in the W8S5 catchment (Sabah, Borneo Island). The flow pathways were identified by artificial tracer experiments. We evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns. The tracer work indicated much faster subsurface flows at Bukit Timah than W8S5 for the storms studied. Some explanation of the greater subsurface waterflows at Bukit Timah in comparison to W8S5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries. Vertical flow of the tracer through the upper 1 m of soil predominated (>90 per cent of percolation) in the Bukit Timah slopes. In some contrast, a major component (approximately 60 per cent) of the tracer percolation was directed laterally within the W8S5 slopes. The flow vectors calculated using the hydrometric methods did, however, grossly under‐estimate the degree of lateral deflection of waterflow generated at W8S5 and to a lesser extent over‐estimated it at Bukit Timah. In part, these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small ‘natural soil pipes’ present within both catchments. In conclusion, the study indicates that marked variations in flow vectors exist within the Udult great group of SE Asian soils and hydrometric calculations may be poor predictors of these dominant pathways. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
838.
Xingzhong Shi Richard T. McNider M. P. Singh David E. England Mark J. Friedman William M. Lapenta William B. Norris 《Pure and Applied Geophysics》2005,162(10):1811-1829
Previous studies of the stable atmospheric boundary layer using techniques of nonlinear dynamical systems (MCNIDER et al., 1995) have shown that the equations support multiple solutions in certain parameter spaces. When geostrophic speed is used as a bifurcation parameter, two stable equilibria are found—a warm solution corresponding to the high-wind regime where the surface layer of the atmosphere stays coupled to the outer layer, and a cold solution corresponding to the low-wind, decoupled case. Between the stable equilibria is an unstable region where multiple solutions exist. The bifurcation diagram is a classic S shape with the foldback region showing the multiple solutions. These studies were carried out using a simple two-layer model of the atmosphere with a fairly complete surface energy budget. This allowed the dynamical analysis to be carried out on a coupled set of four ordinary differential equations. The present paper extends this work by examining additional bifurcation parameters and, more importantly, analyzing a set of partial differential equations with full vertical dependence. Simple mathematical representations of classical problems in dynamical analysis often exhibit interesting behavior, such as multiple solutions, that is not retained in the behavior of more complete representations. In the present case the S-shaped bifurcation diagram remains with only slight variations from the two-layer model. For the parameter space in the foldback region, the evolution of the boundary layer may be dramatically affected by the initial conditions at sunset. An eigenvalue analysis carried out to determine whether the system might support pure limit-cycle behavior showed that purely complex eigenvalues are not found. Thus, any cyclic behavior is likely to be transient. 相似文献
839.
Maria Schönbächler Der-Chuen Lee Mark Rehkämper Bodo Hattendorf 《Geochimica et cosmochimica acta》2005,69(3):775-785
A comprehensive Zr isotopic study was conducted on eleven lunar basalts and highland rocks to search for evidence of the extinct nuclide 92Nb, which decays to 92Zr with a half-life of 36 Ma. Internal isochrons were determined for two early highland rocks, 77215 and 60025. No resolvable Zr isotopic variations were detected in this wide range of lunar samples and thus there is no evidence for the former existence of live 92Nb on the Moon. The Nb/Zr ratios of lunar ilmenites and bulk rock samples vary by only a factor of two to three relative to the chondritic Nb/Zr ratio. No evidence for larger Nb/Zr fractionation was found. This limited fractionation and late isotopic closure of the source region prevents the formation of measurable 92Zr anomalies in high-Ti mare basalts. As a consequence, it is not possible to draw conclusions from the 92Nb-92Zr chronometer about the timing of early lunar differentiation and to constrain the role of ilmenite in the source region of high-Ti mare basalts. However, the fractionation is still sufficient to deduce an upper limit for the initial 92Nb/93Nb ratio of the solar system of <5 × 10−4. 相似文献
840.
Sune G. Nielsen Mark Rehkämper Per Andersson Peter W. Swarzenski Detlef Günther 《Geochimica et cosmochimica acta》2005,69(8):2007-2019
The thallium (Tl) concentrations and isotope compositions of various river and estuarine waters, suspended riverine particulates and loess have been determined. These data are used to evaluate whether weathering reactions are associated with significant Tl isotope fractionation and to estimate the average Tl isotope composition of the upper continental crust as well as the mean Tl concentration and isotope composition of river water. Such parameters provide key constraints on the dissolved Tl fluxes to the oceans from rivers and mineral aerosols.The Tl isotope data for loess and suspended riverine detritus are relatively uniform with a mean of ε205Tl = −2.0 ± 0.3 (ε205Tl represents the deviation of the 205Tl/203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 104). For waters from four major and eight smaller rivers, the majority were found to have Tl concentrations between 1 and 7 ng/kg. Most have Tl isotope compositions very similar (within ±1.5 ε205Tl) to that deduced for the upper continental crust, which indicates that no significant Tl isotope fractionation occurs during weathering. Based on these results, it is estimated that rivers have a mean natural Tl concentration and isotope composition of 6 ± 4 ng/kg and ε205Tl = −2.5 ± 1.0, respectively.In the Amazon estuary, both additions and losses of Tl were observed, and these correlate with variations in Fe and Mn contents. The changes in Tl concentrations have much lower amplitudes, however, and are not associated with significant Tl isotope effects. In the Kalix estuary, the Tl concentrations and isotope compositions can be explained by two-component mixing between river water and a high-salinity end member that is enriched in Tl relative to seawater. These results indicate that Tl can display variable behavior in estuarine systems but large additions and losses of Tl were not observed in the present study. 相似文献