全文获取类型
收费全文 | 1138篇 |
免费 | 173篇 |
国内免费 | 99篇 |
专业分类
测绘学 | 124篇 |
大气科学 | 273篇 |
地球物理 | 283篇 |
地质学 | 290篇 |
海洋学 | 115篇 |
天文学 | 35篇 |
综合类 | 87篇 |
自然地理 | 203篇 |
出版年
2023年 | 6篇 |
2022年 | 13篇 |
2021年 | 24篇 |
2020年 | 33篇 |
2019年 | 28篇 |
2018年 | 25篇 |
2017年 | 32篇 |
2016年 | 37篇 |
2015年 | 42篇 |
2014年 | 60篇 |
2013年 | 63篇 |
2012年 | 60篇 |
2011年 | 63篇 |
2010年 | 58篇 |
2009年 | 87篇 |
2008年 | 81篇 |
2007年 | 71篇 |
2006年 | 82篇 |
2005年 | 65篇 |
2004年 | 31篇 |
2003年 | 54篇 |
2002年 | 41篇 |
2001年 | 34篇 |
2000年 | 42篇 |
1999年 | 31篇 |
1998年 | 32篇 |
1997年 | 33篇 |
1996年 | 19篇 |
1995年 | 27篇 |
1994年 | 12篇 |
1993年 | 23篇 |
1992年 | 21篇 |
1991年 | 17篇 |
1990年 | 25篇 |
1989年 | 15篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 3篇 |
排序方式: 共有1410条查询结果,搜索用时 62 毫秒
41.
《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
42.
43.
中国高铁网络结构特征及其组织模式 总被引:2,自引:4,他引:2
基于2018年高铁网络OD数据,运用社会网络分析方法从高铁网络、城市节点等方面探讨中国城市高铁网络结构特征及其地域组织模式,结果表明:①中国高铁网络整体较为松散,东北地区网络密度最高,东部和中部地区作为整体网络的中介作用明显;②重要高铁线路的“廊道效应”突出,中心度呈现出以京广、京沪和沪昆高铁组成的“三角旗状”空间格局并向两侧城市呈不规则递减的态势;③多层级网络识别出紧密关联高铁线路和四横四纵向八横八纵格局的转变;④高铁网络的地域组织形式表现为点-轴串珠模式、双核组团模式和极核模式,高铁网络的完善使组织模式由单核趋向于网络化转变。 相似文献
44.
45.
针对使用传统的编程方式进行空间回归分析过于繁杂的问题,本文提出将Python函数式编程引入空间回归分析方法,分析和探讨脚本语言+函数式编程方式在GIS数据分析中的推广应用前景。编程实践证明,采用Python函数式编程,结合既有的开源软件包,可以有效提高编程效率,减少代码量,将实践人员的注意力集中在问题本身。 相似文献
46.
47.
The quantitative evaluation of key factors of soil variability together with the spatial pattern of underlying variables are important steps in understanding the functioning of soilscapes. Fifteen physical and chemical variables describing 24 pedons were used to elucidate processes of soil differentiation in a humid forest environment in Southern Cameroon. Landscape positions delineated on the basis of remote-sensed imageries included flat convex summits (FCS), linear valley slopes (LVS), colluvial inland valleys (CIV), and alluvial floodplain. Ordination by principal component analysis (PCA) resulted in a three-factor model, which efficiently summarized the dataset explaining 81.4% of total soil variance (TSV). The pedogenetic factor of ferralitization emerged as the main factor accounting for 43.7% of TSV. Identified underlying processes included basic cation leaching, solum acidification, and in situ clay build up. Cation exchange properties as influenced by organic constituents and soil available P were the second and third most important factors associated with 26.7 and 11.4% of TSV respectively. Organic constituents and related biological processes had strong control on nutrient availability and cation-exchange properties, while soil available phosphorus (AP) showed a strong dependence on potential acidity and organic matter mineralization status. The analysis of variance highlighted significant differences for most soil properties across landscape position and soil depth except for organic carbon (OC), total nitrogen (TN), and total phosphorus. All the soils, however, have low pH, low basic cation saturation, and correlatively high exchangeable Al irrespective of their position in the landscape. 相似文献
48.
As a widely used approach for feature extraction and data reduction, Principal Components Analysis (PCA) suffers from high computational cost, large memory requirement and low efficacy in dealing with large dimensional datasets such as Hyperspectral Imaging (HSI). Consequently, a novel Folded-PCA is proposed, where the spectral vector is folded into a matrix to allow the covariance matrix to be determined more efficiently. With this matrix-based representation, both global and local structures are extracted to provide additional information for data classification. Moreover, both the computational cost and the memory requirement have been significantly reduced. Using Support Vector Machine (SVM) for classification on two well-known HSI datasets and one Synthetic Aperture Radar (SAR) dataset in remote sensing, quantitative results are generated for objective evaluations. Comprehensive results have indicated that the proposed Folded-PCA approach not only outperforms the conventional PCA but also the baseline approach where the whole feature sets are used. 相似文献
49.
基于1961-2012年NCEP/NCAR月平均再分析资料和云南地区124个观测站月降水资料,利用相关分析法分析夏季南亚高压与云南地区降水的关系。结果表明:1961-2012年夏季滇西南地区降水与南亚高压主中心经度呈较显著负相关,滇南地区降水与南亚高压面积呈较显著负相关;6月滇西北和滇南地区降水与南亚高压脊线位置、高压主中心纬度呈显著正相关,滇西南地区降水与南亚高压主中心强度呈显著正相关,而与南亚高压主中心经度呈显著负相关,滇中地区降水与南亚高压主中心纬度呈显著正相关;7月滇西南、滇西北的西南部和滇西的北部地区降水与南亚高压脊线位置呈较显著正相关,滇西地区降水与南亚高压主中心强度呈较显著负相关,滇中和滇东地区降水与南亚高压主中心经度呈较显著负相关;8月滇西南、滇中、滇南和滇东地区降水与南亚高压面积呈显著负相关。 相似文献
50.
苏北沿岸流对浒苔暴发及漂移过程的影响 总被引:4,自引:0,他引:4
根据2009年5月的温度、盐度和溶解氧观测数据,应用"对应分析法"对水团的分布范围和特性进行了分析,并探讨了苏北沿岸流对浒苔(Enteromorpha prolifera)暴发及漂移过程的影响。结果表明,春季苏北沿岸流向东南流动过程中,可以将海水中悬浮的浒苔繁殖体向南输运至长江冲淡水与苏北沿岸流交汇处,此处的浒苔繁殖体在适宜的条件下生长繁殖。在夏季风的作用下,长江口北部的浒苔与苏北沿岸海域的浒苔,随表层海流向北漂移,在地球偏转力作用下,漂移路径不断向右偏转,并最终在山东半岛南部海域聚集堆积。苏北沿岸流在浒苔暴发初期将繁殖体输送到条件适宜的生长区,并对浒苔向北漂移起到了重要作用。长江冲淡水向北扩展范围的增大对于浒苔的生长和繁殖起到了促进作用。查明浒苔扩散路径及输运机制不仅具有重要环境意义,而且对深入了解沿岸和河口区泥沙及污染物的扩散和输运机制具有指示作用。 相似文献