全文获取类型
收费全文 | 29758篇 |
免费 | 4224篇 |
国内免费 | 6385篇 |
专业分类
测绘学 | 3571篇 |
大气科学 | 4424篇 |
地球物理 | 5308篇 |
地质学 | 10658篇 |
海洋学 | 4152篇 |
天文学 | 6310篇 |
综合类 | 2296篇 |
自然地理 | 3648篇 |
出版年
2024年 | 64篇 |
2023年 | 281篇 |
2022年 | 841篇 |
2021年 | 950篇 |
2020年 | 1171篇 |
2019年 | 1240篇 |
2018年 | 1022篇 |
2017年 | 1226篇 |
2016年 | 1202篇 |
2015年 | 1426篇 |
2014年 | 1667篇 |
2013年 | 1890篇 |
2012年 | 1917篇 |
2011年 | 2029篇 |
2010年 | 1789篇 |
2009年 | 2199篇 |
2008年 | 2183篇 |
2007年 | 2354篇 |
2006年 | 2288篇 |
2005年 | 1910篇 |
2004年 | 1596篇 |
2003年 | 1374篇 |
2002年 | 1178篇 |
2001年 | 954篇 |
2000年 | 1011篇 |
1999年 | 927篇 |
1998年 | 772篇 |
1997年 | 471篇 |
1996年 | 403篇 |
1995年 | 342篇 |
1994年 | 329篇 |
1993年 | 318篇 |
1992年 | 217篇 |
1991年 | 156篇 |
1990年 | 107篇 |
1989年 | 141篇 |
1988年 | 91篇 |
1987年 | 60篇 |
1986年 | 60篇 |
1985年 | 49篇 |
1984年 | 42篇 |
1983年 | 20篇 |
1982年 | 25篇 |
1981年 | 12篇 |
1980年 | 17篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 16篇 |
1954年 | 7篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
以标准化降水蒸散指数(SPEI)作为评估指标,基于渭河流域28个气象站点1961—2017年实测降水量和气温数据,采用Mann-Kendall(M-K)趋势检验、经验正交函数以及小波变换等方法分析渭河流域干旱时空变化特征,并研究渭河流域干旱与6种大尺度气候因子之间的相关关系,进一步探讨主要气候因子对流域干旱时空分布特征的潜在影响。研究表明:渭河流域在1961—2017年间整体呈现出变旱的趋势。通过经验正交函数分解,渭河流域干旱分布场主要有3种典型模态类型,分别为全局型、西北—东南反向分布型以及东—西反向分布型;同时,大尺度气候因子南方涛动指数SOI与流域干旱分布场具有更好的相关关系,对该区域内干旱变化有较强的影响。 相似文献
22.
本文回顾了自然和自然贡献情景模型发展的背景、历史和内容,概括总结了自然和自然贡献情景模型的发展进程以及联合国生物多样性与生态系统服务政府间科学—政策平台(IPBES)情景模型的概念框架,讨论了自然和自然贡献情景模型存在的问题和发展方向。为了在全球层面解决现有综合集成模型存在的问题,根据地球表层建模基本定理和生态环境曲面建模基本定理,提出了具有中国原创特点的自然与自然贡献情景模型概念框架。 相似文献
23.
Sunyong Eom Tsutomu Suzuki Myeong-Hun Lee 《International journal of geographical information science》2020,34(5):899-923
ABSTRACT Land-Use Mix (LUM) refers to the strategy of integrating complementary functions within a building or area. While LUM has become a dominant approach in urban planning, its actual benefits and vision for spatial planning remain unclear. To clarify this issue, this study discerns the spatial features of land-use patterns depending on the compatibilities among land-use categories. Accordingly, this study introduces three LUM measures – adjacency, intensity, and proximity – to identify differences in the spatial distribution of land-use categories. Based on these measures, a land-use allocation model is developed to specify spatial patterns satisfying the given compatibilities. This model is tested by applying the concept of the neighborhood unit on a case study of normative land-use patterns subject to specified compatibilities. The results describe spatial features of four compatibility sets, including a set exhibiting a compatibility conflict between the same land-use pair and LUM measures when, for example, a given land-use pair is compatible in terms of intensity but incompatible in terms of proximity. Understanding the spatial features of a normative land-use pattern that satisfies various possible compatibilities will facilitate the incorporation of the LUM approach into local planning guidance and zoning ordinances. 相似文献
24.
Microbial degradation technologies have been developed to restore ground water quality in aquifers polluted by organic contaminants effectively in recent years. However, in course of the degradation, the formation of biofilms in ground water remediation technology can be detrimental to the effectiveness of a ground water remediation project. Several alternatives are available to a remedial design engineer, such as Permeable Reactive Barriers (PRBs) and in -situ bioremediation, Hydrogen Releasing Compounds (HRCs) barrier, Oxygen Releasing Compounds (ORCs) barrier etc. which are efficient and cost- effective technologies. Excessive biomass formation renders a barrier ineffective in degrading the contaminants, Efforts are made to develop kinetics models which accurately determine bio - fouling and bio - filn formation and to control excessive biomass formation. 相似文献
25.
《Astroparticle Physics》2002,16(4):183-386
Frequency distributions of local muon densities in high-energy extensive air showers (EAS) are presented as signature of the primary cosmic ray energy spectrum in the knee region. Together with the gross shower variables like shower core position, angle of incidence, and the shower sizes, the KASCADE experiment is able to measure local muon densities for two different muon energy thresholds. The spectra have been reconstructed for various core distances, as well as for particular subsamples, classified on the basis of the shower size ratio Nμ/Ne. The measured density spectra of the total sample exhibit clear kinks reflecting the knee of the primary energy spectrum. While relatively sharp changes of the slopes are observed in the spectrum of EAS with small values of the shower size ratio, no such feature is detected at EAS of large Nμ/Ne ratio in the energy range of 1–10 PeV. Comparing the spectra for various thresholds and core distances with detailed Monte Carlo simulations the validity of EAS simulations is discussed. 相似文献
26.
27.
28.
29.
30.
The kinetics of the reactions of C2H radical with ethane (k1), propane (k2), and n-butane (k3) are studied over the temperature range of T = 96-296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis-chemiluminescence technique. The C2H decay profiles in the presence of both the alkane reactant and O2 are monitored by the CH(A2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k1(T) = (0.51 ± 0.06) × 10−10 exp[(−76 ± 30)K/T] cm3 molecule−1 s−1 (T = 96-800 K), k2(T) = (0.98 ± 0.32) × 10−10exp[(−71 ± 60)K/T] cm3 molecule−1 s−1 (T = 96-361 K), and k3(T) = (1.23 ± 0.26) × 10−10 cm3 molecule−1 s−1 (T = 96-297 K). At T = 296 K, k1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed. 相似文献