全文获取类型
收费全文 | 74338篇 |
免费 | 982篇 |
国内免费 | 644篇 |
专业分类
测绘学 | 2251篇 |
大气科学 | 5716篇 |
地球物理 | 14307篇 |
地质学 | 25371篇 |
海洋学 | 6143篇 |
天文学 | 17583篇 |
综合类 | 231篇 |
自然地理 | 4362篇 |
出版年
2021年 | 528篇 |
2020年 | 565篇 |
2019年 | 611篇 |
2018年 | 1496篇 |
2017年 | 1432篇 |
2016年 | 1878篇 |
2015年 | 1166篇 |
2014年 | 1915篇 |
2013年 | 3767篇 |
2012年 | 1937篇 |
2011年 | 2577篇 |
2010年 | 2304篇 |
2009年 | 3014篇 |
2008年 | 2706篇 |
2007年 | 2666篇 |
2006年 | 2582篇 |
2005年 | 2288篇 |
2004年 | 2168篇 |
2003年 | 2076篇 |
2002年 | 1983篇 |
2001年 | 1800篇 |
2000年 | 1724篇 |
1999年 | 1608篇 |
1998年 | 1524篇 |
1997年 | 1524篇 |
1996年 | 1304篇 |
1995年 | 1239篇 |
1994年 | 1189篇 |
1993年 | 1084篇 |
1992年 | 1008篇 |
1991年 | 977篇 |
1990年 | 1083篇 |
1989年 | 978篇 |
1988年 | 912篇 |
1987年 | 1053篇 |
1986年 | 896篇 |
1985年 | 1177篇 |
1984年 | 1306篇 |
1983年 | 1250篇 |
1982年 | 1210篇 |
1981年 | 1089篇 |
1980年 | 997篇 |
1979年 | 946篇 |
1978年 | 924篇 |
1977年 | 856篇 |
1976年 | 804篇 |
1975年 | 721篇 |
1974年 | 825篇 |
1973年 | 826篇 |
1972年 | 522篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
142.
143.
144.
145.
146.
Isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the Atacama Desert in northern Chile has shown anomalous 17O enrichments in both minerals. Δ17O values of 14-21 ‰ in nitrate and 0.4 to 4 ‰ in sulfate are the most positive found in terrestrial minerals to date. Modeling of atmospheric processes indicates that the Δ17O signatures are the result of photochemical reactions in the troposphere and stratosphere. We conclude that the bulk of the nitrate, sulfate and other soluble salts in some parts of the Atacama Desert must be the result of atmospheric deposition of particles produced by gas to particle conversion, with minor but varying amounts from sea spray and local terrestrial sources. Flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200,000 to 2.0 M years during hyper-arid conditions similar to those currently found in the Atacama Desert. Correlations between Δ17O and δ18O in nitrate salts from the Atacama Desert and Mojave Desert, California, indicate varying fractions of microbial and photochemical end-member sources. The photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless, whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture, biologic activity, and nitrogen cycling. These isotopic signatures have important implications for paleoclimate, astrobiology, and N cycling studies. 相似文献
147.
The paper presents the first quantitative results of a laboratory study of the velocity field in a two-arm spiral-wave pattern generated in a steady-state fashion by a hydrodynamical instability in a differentially rotating, thin layer of liquid. The liquid layer has a free surface, and the rotational profile includes an interval where the velocity drops abruptly, as in the gaseous disks of spiral galaxies. The properties of anticyclonic vortices observed between the arms of this pattern at the corotation radius are considered. 相似文献
148.
149.
150.
Rainfall thresholds for landsliding in the Himalayas of Nepal 总被引:5,自引:0,他引:5
Emmanuel J. Gabet Douglas W. Burbank Jaakko K. Putkonen Beth A. Pratt-Sitaula Tank Ojha 《Geomorphology》2004,63(3-4):131-143
Landsliding of the hillslope regolith is an important source of sediment to the fluvial network in the unglaciated portions of the Himalayas of Nepal. These landslides can produce abrupt increases of up to three orders of magnitude in the fluvial sediment load in less than a day. An analysis of 3 years of daily sediment load and daily rainfall data defines a relationship between monsoonal rainfall and the triggering of landslides in the Annapurna region of Nepal. Two distinct rainfall thresholds, a seasonal accumulation and a daily total, must be overcome before landslides are initiated. To explore the geomorphological controls on these thresholds, we develop a slope stability model, driven by daily rainfall data, which accounts for changes in regolith moisture. The pattern of rainfall thresholds predicted by the model is similar to the field data, including the decrease in the daily rainfall threshold as the seasonal rainfall accumulation increases. Results from the model suggest that, for a given hillslope, regolith thickness determines the seasonal rainfall necessary for failure, whereas slope angle controls the daily rainfall required for failure. 相似文献