全文获取类型
收费全文 | 1123篇 |
免费 | 346篇 |
国内免费 | 247篇 |
专业分类
测绘学 | 52篇 |
大气科学 | 207篇 |
地球物理 | 554篇 |
地质学 | 483篇 |
海洋学 | 172篇 |
天文学 | 50篇 |
综合类 | 37篇 |
自然地理 | 161篇 |
出版年
2023年 | 12篇 |
2022年 | 38篇 |
2021年 | 37篇 |
2020年 | 39篇 |
2019年 | 49篇 |
2018年 | 29篇 |
2017年 | 49篇 |
2016年 | 45篇 |
2015年 | 46篇 |
2014年 | 56篇 |
2013年 | 53篇 |
2012年 | 47篇 |
2011年 | 79篇 |
2010年 | 50篇 |
2009年 | 80篇 |
2008年 | 92篇 |
2007年 | 98篇 |
2006年 | 81篇 |
2005年 | 84篇 |
2004年 | 56篇 |
2003年 | 48篇 |
2002年 | 49篇 |
2001年 | 69篇 |
2000年 | 47篇 |
1999年 | 49篇 |
1998年 | 50篇 |
1997年 | 40篇 |
1996年 | 31篇 |
1995年 | 32篇 |
1994年 | 38篇 |
1993年 | 40篇 |
1992年 | 26篇 |
1991年 | 14篇 |
1990年 | 15篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1716条查询结果,搜索用时 404 毫秒
51.
52.
Marcus Schulz Jan Priegnitz Jörg Klasmeier Stefan Heller Stefan Meinecke Michael Feibicke 《水文研究》2012,26(2):272-280
In several empirical and modelling studies on river hydraulics, dispersion was negatively correlated to surface roughness. In this study, it was aimed to investigate the influence of surface roughness on longitudinal dispersion under controlled conditions. In artificial flow channels with a length of 104 m, tracer experiments with variations in channel bed material were performed. By use of measured tracer breakthrough curves, average flow velocity, mean longitudinal dispersion, and mean longitudinal dispersivity were calculated. Longitudinal dispersion coefficients ranged from 0·018 m2 s?1 in channels with smooth bed surface up to 0·209 m2 s?1 in channels with coarse gravel as bed material. Longitudinal dispersion was linearly related to mean flow velocity. Accordingly, longitudinal dispersivities ranged between 0·152 ± 0·017 m in channels with smooth bed surface and 0·584 ± 0·015 m in identical channels with a coarse gravel substrate. Grain size and surface roughness of the channel bed were found to correlate positively to longitudinal dispersion. This finding contradicts several existing relations between surface roughness and dispersion. Future studies should include further variation in surface roughness to derive a better‐founded empirical equation forecasting longitudinal dispersion from surface roughness. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
53.
A microscale model for air pollutant dispersion simulation in urban areas: Presentation of the model and performance over a single building 总被引:1,自引:0,他引:1
A microscale air pollutant dispersion model system is developed for emergency response purposes. The model includes a diagnostic wind field model to simulate the wind field and a random-walk air pollutant dispersion model to simulate the pollutant concentration through consideration of the influence of urban buildings. Numerical experiments are designed to evaluate the model's performance, using CEDVAL(Compilation of Experimental Data for Validation of Microscale Dispersion Models) wind tunnel experiment data, including wind fields and air pollutant dispersion around a single building. The results show that the wind model can reproduce the vortexes triggered by urban buildings and the dispersion model simulates the pollutant concentration around buildings well. Typically, the simulation errors come from the determination of the key zones around a building or building cluster. This model has the potential for multiple applications; for example, the prediction of air pollutant dispersion and the evaluation of environmental impacts in emergency situations; urban planning scenarios;and the assessment of microscale air quality in urban areas. 相似文献
54.
55.
56.
57.
模式内部变率是模拟结果不确定性的重要来源,然而它对于1.5℃和2℃升温阈值出现时间不确定性的影响尚不清楚。因此,基于耦合模式比较计划第五阶段(CMIP5)的多模式数据研究了模式内部变率对1.5℃和2℃升温阈值出现时间不确定性的影响以及对未来排放情景的敏感性。结果表明,模式内部变率对升温阈值出现时间模拟的影响与外强迫的影响相当,单个模式内部不同成员达到全球平均1.5℃或2℃增温的年份相差2~12年;其影响具有明显的空间差异,影响极大值出现在欧亚大陆以北洋面、白令海峡周围区域、北美东北部及其与格陵兰岛之间的海域、南半球高纬地区等;低排放情景下模式内部变率的影响大于高排放情景。 相似文献
58.
In designing the coastal structures, the accurate estimation of the wave forces on them is of great importance. In this paper, the influences of the phase difference on wave pressure acting on a composite breakwater installed in the three-dimensional (3-D) wave field are studied numerically. We extend the earlier model [Hur, D.S., Mizutani, N., 2003. Coastal Engineering 47, 329–345] to simulate 3-D wave fields by introducing 3-D Navier–Stokes solver with the Smagorinsky's sub-grid scale (SGS) model. For the validation of the model, the wave field around a 3-D asymmetrical structure installed on a submerged breakwater, in which the complex wave deformations generate, is simulated, and the numerical solutions are compared to the experimental data reported by Hur, Mizutani, Kim [2004. Coastal Engineering (51, 407–420)]. The model is then adopted to investigate 3-D characteristics of wave pressure and force on a caisson of composite breakwater, and the numerical solutions were discussed with respect to the phase difference between harbor and seaward sides induced by the transmitted wave through the rubble mound or the diffraction. The numerical results reveal that wave forces acting on the composite breakwater are significantly different at each cross-section under influence of wave diffraction that is important parameter on 3-D wave interaction with coastal structures. 相似文献
59.
大量的数值模拟表明,SVD和LSQRD在面波频散网格反演两步法中的应用效果都很好.但SVD可以用分辨矩阵、信息矩阵和协方差矩阵对解估计进行数学上客观有效的评价;而对于大型稀疏方程组的求解,LSQRD确是一种内存需求小、计算速度快以及分辨抗噪能力都较强的算法 .在现有计算机运算速度较快、内存可以扩充较大的条件下,实测数据量不很大时,应采用SVD算法进行线性反演. 相似文献
60.
采用经验模式分解技术研究了从第12到第24太阳活动周黑子面积数据中的Rieger-type周期,以及它们在南北半球上的相位关系。研究发现:(1)Rieger-type周期尺度以及时间尺度在两个半球上是不一样的,从而导致了半球分布的不对称性;(2)平均Rieger-type周期在北半球上的周期长度略大于南半球上的;(3)占主导地位半球的Rieger-type周期的相位不一定是超前的,半球Rieger-type周期尺度相近的也并不一定是相关性强的。 相似文献