全文获取类型
收费全文 | 7251篇 |
免费 | 279篇 |
国内免费 | 98篇 |
专业分类
测绘学 | 192篇 |
大气科学 | 646篇 |
地球物理 | 1759篇 |
地质学 | 2489篇 |
海洋学 | 661篇 |
天文学 | 1100篇 |
综合类 | 23篇 |
自然地理 | 758篇 |
出版年
2021年 | 67篇 |
2020年 | 76篇 |
2019年 | 101篇 |
2018年 | 149篇 |
2017年 | 151篇 |
2016年 | 184篇 |
2015年 | 168篇 |
2014年 | 227篇 |
2013年 | 395篇 |
2012年 | 269篇 |
2011年 | 301篇 |
2010年 | 225篇 |
2009年 | 377篇 |
2008年 | 351篇 |
2007年 | 312篇 |
2006年 | 278篇 |
2005年 | 233篇 |
2004年 | 257篇 |
2003年 | 240篇 |
2002年 | 232篇 |
2001年 | 170篇 |
2000年 | 176篇 |
1999年 | 139篇 |
1998年 | 142篇 |
1997年 | 111篇 |
1996年 | 106篇 |
1995年 | 111篇 |
1994年 | 98篇 |
1993年 | 87篇 |
1992年 | 75篇 |
1991年 | 89篇 |
1990年 | 86篇 |
1989年 | 81篇 |
1988年 | 77篇 |
1987年 | 82篇 |
1986年 | 69篇 |
1985年 | 103篇 |
1984年 | 103篇 |
1983年 | 106篇 |
1982年 | 96篇 |
1981年 | 102篇 |
1980年 | 93篇 |
1979年 | 78篇 |
1978年 | 50篇 |
1977年 | 72篇 |
1976年 | 87篇 |
1975年 | 65篇 |
1974年 | 73篇 |
1973年 | 63篇 |
1972年 | 34篇 |
排序方式: 共有7628条查询结果,搜索用时 281 毫秒
21.
22.
We develop a general formalism for analysing parameter information from non-Gaussian cosmic fields. The method can be adapted to include the non-linear effects in galaxy redshift surveys, weak lensing surveys and cosmic velocity field surveys as part of parameter estimation. It can also be used as a test of non-Gaussianity of the cosmic microwave background. Generalizing maximum-likelihood analysis to second order, we calculate the non-linear Fisher information matrix and likelihood surfaces in parameter space. To this order we find that the information content is always increased by including non-linearity. Our methods are applied to a realistic model of a galaxy redshift survey, including non-linear evolution, galaxy bias, shot-noise and redshift-space distortions to second order. We find that including non-linearities allows all of the degeneracies between parameters to be lifted. Marginalized parameter uncertainties of a few per cent will then be obtainable using forthcoming galaxy redshift surveys. 相似文献
23.
24.
25.
Donald C. Gordon Peter J. Cranford Con Desplanque 《Estuarine, Coastal and Shelf Science》1985,20(2):205-227
The Cumberland Basin, a 118 km2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin. 相似文献
26.
Peter Nielsen 《Coastal Engineering》1985,9(3):261-276
An empirical analysis is performed on the most detailed, recent measurements of turbulent oscillatory boundary layer flow. The measurements show that throughout elevations where the flow can be considered horizontally uniform, one deficit model is sufficient for describing the fundamental mode. Some general properties of the non dimensional velocity deficit D1(z) appear with striking consistency. First of all the identity , which is a theoretical result for smooth laminar flow, seems to hold with great accuracy for a large range of turbulent flow conditions as well. This is of principal theoretical interest because all previous analytical eddy viscosity models as well as numerical mixing length models predict a consistent and fairly large difference between Arg D1 and . If the identity between and Arg D1 extends all the way to the bed, it means that the bed shear stress leads the free stream velocity by 45 degrees. It is also found that the structure of smooth turbulent oscillatory flows as measured by Kalkanis (1964) corresponds to a sharp maximum in the normalized energy dissipation rate. 相似文献
27.
Alan M Bond Julie R Bradbury Henry A Hudson John S Garnham Peter J Hanna Stanley Strother 《Marine Chemistry》1985,16(1):1-9
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent. 相似文献
28.
29.
30.