全文获取类型
收费全文 | 97863篇 |
免费 | 1200篇 |
国内免费 | 1650篇 |
专业分类
测绘学 | 3254篇 |
大气科学 | 7262篇 |
地球物理 | 18576篇 |
地质学 | 37511篇 |
海洋学 | 7793篇 |
天文学 | 18801篇 |
综合类 | 2355篇 |
自然地理 | 5161篇 |
出版年
2022年 | 458篇 |
2021年 | 805篇 |
2020年 | 866篇 |
2019年 | 917篇 |
2018年 | 6447篇 |
2017年 | 5677篇 |
2016年 | 4832篇 |
2015年 | 1523篇 |
2014年 | 2381篇 |
2013年 | 4019篇 |
2012年 | 3356篇 |
2011年 | 5652篇 |
2010年 | 4634篇 |
2009年 | 5691篇 |
2008年 | 4856篇 |
2007年 | 5316篇 |
2006年 | 3041篇 |
2005年 | 2420篇 |
2004年 | 2597篇 |
2003年 | 2491篇 |
2002年 | 2259篇 |
2001年 | 1871篇 |
2000年 | 1808篇 |
1999年 | 1422篇 |
1998年 | 1443篇 |
1997年 | 1405篇 |
1996年 | 1184篇 |
1995年 | 1159篇 |
1994年 | 1058篇 |
1993年 | 905篇 |
1992年 | 907篇 |
1991年 | 869篇 |
1990年 | 924篇 |
1989年 | 779篇 |
1988年 | 787篇 |
1987年 | 833篇 |
1986年 | 750篇 |
1985年 | 962篇 |
1984年 | 1018篇 |
1983年 | 969篇 |
1982年 | 905篇 |
1981年 | 815篇 |
1980年 | 826篇 |
1979年 | 763篇 |
1978年 | 723篇 |
1977年 | 626篇 |
1976年 | 586篇 |
1975年 | 572篇 |
1974年 | 542篇 |
1973年 | 558篇 |
排序方式: 共有10000条查询结果,搜索用时 1 毫秒
141.
142.
143.
144.
145.
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
146.
This article examines the recent progress of geography education as an academic subfield of geography and as an area of activity in K–12 education. Challenges to both endeavors are described. A restructuring of geography education's research program that focuses on spatial thinking and reasoning and merges applied and theoretical research is proposed. 相似文献
147.
148.
Contamination of groundwater has become a major concern in recent years. Since testing of water quality of all domestic and irrigation wells within large watersheds is not economically feasible, one frequently used monitoring strategy is to develop contamination potential maps of groundwater, and then prioritize those wells located in the potentially highly contaminated areas for testing of contaminants. However, generation of contamination potential maps based on groundwater sensitivity and vulnerability is not an easy task due inherent uncertainty. Therefore, the overall goal of this research is to improve the methodology for the generation of contamination potential maps by using detailed landuse/pesticide and soil structure information in conjunction with selected parameters from the DRASTIC model. The specific objectives of this study are (i) to incorporate GIS, GPS, remote sensing and the fuzzy rule-based model to generate groundwater sensitivity maps, and (ii) compare the results of our new methodologies with the modified DRASTIC Index (DI) and field water quality data. In this study, three different models were developed (viz. DIfuzz, VIfuzz and VIfuzz_ped) and were compared to the DI. Once the preliminary fuzzy logic-based (DIfuzz) was generated using selected parameters from DI, the methodology was further refined through VIfuzz and VIfuzz_ped models that incorporated landuse/pesticide application and soil structure information, respectively. This study was conducted in Woodruff County of the Mississippi Delta region of Arkansas. Water quality data for 55 wells were used to evaluate the contamination potential maps. The sensitivity map generated by VIfuzz_ped with soil structure showed significantly better coincidence results when compared with the field data. 相似文献
149.
Sensitivity kernels for finite-frequency surface waves 总被引:1,自引:1,他引:1
150.