全文获取类型
收费全文 | 109311篇 |
免费 | 1593篇 |
国内免费 | 716篇 |
专业分类
测绘学 | 2362篇 |
大气科学 | 7293篇 |
地球物理 | 21238篇 |
地质学 | 39147篇 |
海洋学 | 9869篇 |
天文学 | 24974篇 |
综合类 | 289篇 |
自然地理 | 6448篇 |
出版年
2022年 | 711篇 |
2021年 | 1209篇 |
2020年 | 1341篇 |
2019年 | 1487篇 |
2018年 | 2964篇 |
2017年 | 2793篇 |
2016年 | 3250篇 |
2015年 | 1714篇 |
2014年 | 3167篇 |
2013年 | 5591篇 |
2012年 | 3473篇 |
2011年 | 4576篇 |
2010年 | 4149篇 |
2009年 | 5244篇 |
2008年 | 4630篇 |
2007年 | 4769篇 |
2006年 | 4374篇 |
2005年 | 3174篇 |
2004年 | 3132篇 |
2003年 | 2990篇 |
2002年 | 2885篇 |
2001年 | 2502篇 |
2000年 | 2457篇 |
1999年 | 1978篇 |
1998年 | 2061篇 |
1997年 | 1932篇 |
1996年 | 1678篇 |
1995年 | 1629篇 |
1994年 | 1397篇 |
1993年 | 1343篇 |
1992年 | 1230篇 |
1991年 | 1277篇 |
1990年 | 1269篇 |
1989年 | 1100篇 |
1988年 | 1023篇 |
1987年 | 1173篇 |
1986年 | 1085篇 |
1985年 | 1330篇 |
1984年 | 1481篇 |
1983年 | 1437篇 |
1982年 | 1330篇 |
1981年 | 1247篇 |
1980年 | 1122篇 |
1979年 | 1064篇 |
1978年 | 1010篇 |
1977年 | 933篇 |
1976年 | 884篇 |
1975年 | 878篇 |
1974年 | 855篇 |
1973年 | 946篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers. 相似文献
83.
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
84.
The existence of prolonged periods of abnormally low solar activity (such as the Maunder minimum) is explained within the framework of Leighton's model of a solar cycle with a hypothetical internal magnetic field of the Sun taken into account. 相似文献
85.
86.
Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x
l
). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t). 相似文献
87.
G. Djurašević P. Rovithis H. Rovithis-Livaniou E. Fragoulopoulou 《Astrophysics and Space Science》2005,296(1-4):311-314
BV light curves of the eclipsing binary UV Leo obtained at the Kryonerion Astronomical Station of the National Observatory of
Athens, Greece, are analyzed. The analysis is based on a Roche configuration with two spots on the secondary surface. The
elements of the two components of the system are calculated and the spot characteristics are given. 相似文献
88.
The spectra of active galaxies and their nuclei are rich in emission and absorption line features. A major aim of present
research is the development of self-consistent hydrodynamic models for the production of the line-forming regions. We here
review such modelling and stress the central role played by shock phenomena induced by winds and explosions on scales ranging
from the circumstellar to the intergalactic. 相似文献
89.
90.
Structural crust is a thin layer formed on the soil surface after a rainstorm. The crust is the result of a physical segregation and rearrangement of soil particles in a way that affects some of the soil properties, such as infiltration, runoff and soil erosion. In practice, there is no rapid, in situ method for monitoring, assessing and mapping crust intensity and quality. In this study, a controlled spectral investigation of the structural crust across the NIR–SWIR spectral region was conducted on three selected Israeli soils, to study the potential of reflectance radiation to detect structural crust in soils. Two major factors served as the driving forces for this study: (1) there is no valid method for in situ assessment of the crust's characteristics in the agriculture field, and (2) the crust might bias thematic remote sensing of soils, because the thin layer of crust blocks photon–matter interaction, which represents the relevant soil body. Through the use of a laboratory rainfall simulator and a sensitive spectrometer, it was revealed that for three selected soils, significant spectral differences occurred between the crust and its bulk soil. The spectral information was found to be related to changes in particle size distribution and texture at the surface of the soil. This conclusion was based on indications of absorption of OH in clay lattice, OH in adsorbed water and CO3 in carbonates. It was concluded that the structural crust is a phenomenon that should not be ignored by remote-sensing users. In fact, in the field of agriculture, the spectral properties of crust can be used as tools for estimating the crust's intensity. 相似文献