全文获取类型
收费全文 | 2265篇 |
免费 | 304篇 |
国内免费 | 444篇 |
专业分类
测绘学 | 469篇 |
大气科学 | 227篇 |
地球物理 | 583篇 |
地质学 | 906篇 |
海洋学 | 250篇 |
天文学 | 30篇 |
综合类 | 212篇 |
自然地理 | 336篇 |
出版年
2024年 | 5篇 |
2023年 | 19篇 |
2022年 | 54篇 |
2021年 | 71篇 |
2020年 | 78篇 |
2019年 | 102篇 |
2018年 | 77篇 |
2017年 | 103篇 |
2016年 | 104篇 |
2015年 | 103篇 |
2014年 | 111篇 |
2013年 | 139篇 |
2012年 | 123篇 |
2011年 | 134篇 |
2010年 | 134篇 |
2009年 | 156篇 |
2008年 | 135篇 |
2007年 | 157篇 |
2006年 | 123篇 |
2005年 | 127篇 |
2004年 | 127篇 |
2003年 | 102篇 |
2002年 | 88篇 |
2001年 | 68篇 |
2000年 | 67篇 |
1999年 | 63篇 |
1998年 | 71篇 |
1997年 | 64篇 |
1996年 | 49篇 |
1995年 | 43篇 |
1994年 | 33篇 |
1993年 | 39篇 |
1992年 | 31篇 |
1991年 | 31篇 |
1990年 | 17篇 |
1989年 | 9篇 |
1988年 | 15篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1973年 | 1篇 |
1971年 | 3篇 |
1954年 | 2篇 |
排序方式: 共有3013条查询结果,搜索用时 875 毫秒
241.
242.
The annual peak flow series of Polish rivers are mixtures of summer and winter flows. As Part II of a sequence of two papers, practical aspects of applicability of seasonal approach to flood frequency analysis (FFA) of Polish rivers are discussed. Taking A Two‐Component Extreme Value (TCEV1) model as an example it was shown in the first part that regardless of estimation method, the seasonal approach can give profit in terms of upper quantile estimation accuracy that rises with the return period of the quantile and is the greatest for no seasonal variation. In this part, an assessment of annual maxima (AM) versus seasonal maxima (SM) approach to FFA was carried out with respect to seasonal and annual peak flow series of 38 Polish gauging stations. First, the assumption of mutual independence of the seasonal maxima has been tested. The smoothness of SM and AM empirical probability distribution functions was analysed and compared. The TCEV1 model with seasonally estimated parameters was found to be not appropriate for most Polish data as it considerably underrates the skewness of AM distributions and upper quantile values as well. Consequently, the discrepancies between the SM and AM estimates of TCEV1 are observed. Taking SM and TCEV1 distribution, the dominating season in AM series was confronted with predominant season for extreme floods. The key argument for presumptive superiority of SM approach that SM samples are more statistically homogeneous than AM samples has not been confirmed by the data. An analysis of fitness to SM and AM of Polish datasets made for seven distributions pointed to Pearson (3) distribution as the best for AM and Summer Maxima, whereas it was impossible to select a single best model for winter samples. In the multi‐model approach to FFA, the tree functions, i.e., Pe(3), CD3 and LN3, should be involved for both SM and AM. As the case study, Warsaw gauge on the Vistula River was selected. While most of AM elements are here from winter season, the prevailing majority of extreme annual floods are the summer maxima. The upper quantile estimates got by means of classical annual and two‐season methods happen to be fairly close; what's more they are nearly equal to the quantiles calculated just for the season of dominating extreme floods. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
243.
土压力滑楔理论的统一解 总被引:3,自引:0,他引:3
基于滑楔理论计算土压力的基本原理,运用考虑中间主应力效应的统一强度理论,对传统的土压力公式不能反映土体材料强度的中间主应力效应的公式进行了改进,推导了基于滑楔理论的土压力统一解公式,Coulomb和Rankine土压力公式为其特例.通过变换统一强度参数,可获得一系列的解.在工程运用中可根据土质和工程的实际情况,适当选择统一强度参数和应力状态来确定土压力的大小. 相似文献
244.
崔笃信 《大地测量与地球动力学》1994,(1)
地壳纯变形分为剪切变形和扩张变形。在前人研究的基础上提出了仅利用复测水准资料求出描述地壳剪切变形和扩张变形指标的方法和步骤,所得的定量指标不受基准的影响,受测量累积误差的影响亦很小,能客观地反映地壳变形的特征。通过理论和实例证明:以一点周围变形分量的最大值代表该点周围垂直面的变形程度比其平均值更能突出变形信息。最后根据实例计算结果分析了剪切变形分量、扩张变形分量与形变速率、构造地震的关系。 相似文献
245.
The author puts forward the proposal in this paper that all the terrestrial planets (Venus, the Earth, and Mars) as well as
the Moon deviate from hydrostatic equilibrium to some degree. The Earth's level of deviation of these four celestial bodies
is minimum, and that of Mars is maximum. Moreover, the author estimates Martian nonhydrostatic components of the principal
moments-of-inertia using five models for the interior of Mars. Comparison with other terrestrial planets shows that setting
the range of mean moment-of-inertia ratio, I/MR2, in 0.345 ~ 0.355for Mars is reasonable.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
246.
银河系厚盘的本地空间密度 总被引:1,自引:0,他引:1
本文利用北银极天区的现代恒星计数资料,采用标准的银河系模型,通过恒星统计积分方程对参数值组合模型的亮度函数进行计数分析,证明了银河系厚盘恒星成份存在的必要性,并进一步得出太阳附近厚盘成份的空间密度的最佳期望值为薄盘密度的0.041. 相似文献
247.
248.
常州市是长江三角洲地区典型的工业化城市,多年来的快速发展对地下水环境产生了系列影响,地下水化学组分受到天然条件和人为活动的双重因素控制,现状地下水化学成因和影响因素亟需深入研究。本文在地下水赋存条件分析的基础上,综合采用统计分析、离子比值、主成分分析法对常州各层地下水化学特征和成因开展分析。结果表明,区域潜水水化学类型以HCO3-—Ca2+·Mg2+、HCO3-—Ca2+和HCO3-·Cl-—Na+·Ca2+型为主,第Ⅰ承压水以HCO3-—Na+·Ca2+和HCO3-—Ca2+型为主、第Ⅱ承压水以HCO3-—Na+ 相似文献
249.
《地学前缘(英文版)》2022,13(3):101365
The study evaluated the sources and controlling factors of the groundwater contaminants in an agroeconomic region of Lower Ganga Basin using principal component analysis (PCA), multivariable linear regressions (MLR), correlation analysis, and hierarchical cluster analysis, and evaluated the public health risks using the Latin Hypercube Sampling, goodness-of-fit statistics, Monte Carlo simulation and Sobol sensitivity analysis based on the 1000 samples collected in two sampling cycles (N = 1000). The study reveals that the dissolution of fluoride-bearing minerals and semi-arid climate regulate the fluoride concentrations (0.10–18.25 mg/L) in groundwater. Extensive application of inorganic nitrogenous fertilizers and livestock manure mainly contributed to elevated nitrate levels (up to 435.0 mg/L) in groundwater. The health risks analysis indicates that fluoride exposure is more prevalent in the residents of each age group than the nitrate and both contaminants exhibited higher non-carcinogenic health risks on the infant and child (minor) age groups compared to adolescents and adults. Based on the cokriging interpolation mapping, the minor residents of 17.88%–23.15% of the total area (4545.0 km2) are vulnerable to methemoglobinemia whereas the residents of all age-groups in 38.47%–44.45% of the total area are susceptible to mild to severe dental/skeletal fluorosis owing to consumption of untreated nitrate and fluoride enriched groundwater. The Sobol sensitivity indices revealed contaminant levels, groundwater intake rate and their collective effects are the most influential factors to pose potential health risks on the residents. Artificial recharge and rainwater harvesting practices should be adopted to improve the groundwater quality and the residents are advised to drink purified groundwater. 相似文献
250.