全文获取类型
收费全文 | 15838篇 |
免费 | 2834篇 |
国内免费 | 3376篇 |
专业分类
测绘学 | 700篇 |
大气科学 | 1626篇 |
地球物理 | 4127篇 |
地质学 | 8895篇 |
海洋学 | 2843篇 |
天文学 | 43篇 |
综合类 | 1102篇 |
自然地理 | 2712篇 |
出版年
2024年 | 132篇 |
2023年 | 305篇 |
2022年 | 497篇 |
2021年 | 742篇 |
2020年 | 697篇 |
2019年 | 774篇 |
2018年 | 646篇 |
2017年 | 700篇 |
2016年 | 679篇 |
2015年 | 761篇 |
2014年 | 971篇 |
2013年 | 1217篇 |
2012年 | 936篇 |
2011年 | 1033篇 |
2010年 | 950篇 |
2009年 | 955篇 |
2008年 | 991篇 |
2007年 | 1008篇 |
2006年 | 1052篇 |
2005年 | 887篇 |
2004年 | 837篇 |
2003年 | 726篇 |
2002年 | 675篇 |
2001年 | 575篇 |
2000年 | 520篇 |
1999年 | 441篇 |
1998年 | 404篇 |
1997年 | 361篇 |
1996年 | 285篇 |
1995年 | 265篇 |
1994年 | 242篇 |
1993年 | 186篇 |
1992年 | 139篇 |
1991年 | 105篇 |
1990年 | 72篇 |
1989年 | 84篇 |
1988年 | 46篇 |
1987年 | 37篇 |
1986年 | 23篇 |
1985年 | 28篇 |
1984年 | 14篇 |
1983年 | 15篇 |
1982年 | 3篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1978年 | 9篇 |
1973年 | 1篇 |
1971年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
11.
本文分析了秦山核电站邻近水域生态零点调查四个航次的浮游植物样品,结果表明:调查区浮游植物的种数和细胞密度在时间尺度上均存在明显的季节变化趋势:夏大于秋大于春大于冬,并且与环境因子的变化密切相关,其中最主要的影响因子是温度、盐度和径流,而影响日变化的环境因子主要是潮汐。 相似文献
12.
根据聚类方法,对闽南-台湾浅滩渔场变性水团进行划分,本海区共有6个变性水团。讨论各个变性水团在不同季节消长变化规律。 相似文献
13.
The interstitial water composition (
, alkalinity, Ca2+, Mg2+, Sr2+, Na+, K+) and the cation exchange capacity (CEC) were determined for the muddy sediments of the continental shelf off the Gironde Estuary (France), in the area where the sediment represents the deposit of the muddy suspension of the river. In comparison with seawater concentrations, the pore waters below 10 cm depth, show depletions of
and Ca2+ and below a 30 cm depth show depletions of Mg2+. Inversely, the upper 10 cm an enrichment of Ca2+ concentration, and an increase of K+ concentration to a 40 cm depth. High values of
are observed at the top 4 cm. Alkalinity enrichment is observed along the length of the core. Applying the alkalinity models for the sediment below a 10 cm depth demonstrates generally that calculated alkalinities are higher than the measured ones. Ca2+ dissolution occurs at the first 10 cm and authigenic carbonate precipitation starts beneath that level. Mg2+ depletion is accompanied by bicarbonate loss. This proves that Mg2+ depletion is due to a Mg-silicate reaction. The result of the CEC does not confirm the Mg2+ uptake by clay minerals in exchangeable site, under reducing conditions. Diffusion and bioturbation play an important role in the pore water concentration at the top of the core. 相似文献
14.
15.
Frequency-selective attenuation of sound propagaion and reverberation in shallow waterTXFrequency-selectiveattenuationofsoundp... 相似文献
16.
Salt-water inflows into the Baltic Sea are important events for renewing the deep and bottom waters of the deep basins of the Baltic Sea. These events occur only at irregular intervals. The last strong event was in January 1993 followed by minor inflows in winter 1993/1994. As a result of these inflows, the deep water of the central Baltic basins was completely renewed.Based on extensive observations of polycyclic aromatic hydrocarbons (PAHs) in water, fluffy layer material and surface sediments between 1992 and 1998, the transformation of PAHs and the modification of their distribution in the Baltic deep water is discussed in connection with the spreading of the inflowing highly saline and oxygen-rich water along its pathway from the sills into the central basins. In the course of the inflows in 1993/1994, the PAH concentration in the deep water of the different basins increased significantly. The concentrations were elevated, at least by a factor of 2 and as much as seven to eight times (for the four-ring PAHs) compared to the previous and the following years. Two hypotheses for the causes were discussed: the inflowing salt water may have entrained more highly polluted surface water in the western Baltic Sea, or it may have entrained contaminated fluffy layer material or sediment particles along the route of transport. 相似文献
17.
基于神经网络的平底结构砰击压力预报 总被引:1,自引:0,他引:1
对利用神经网络预报平底结构入水砰击压力的方法进行了探讨。首先利用仿真软件计算各种情况下平底结构入水所产生的砰击压力,以此形成训练神经网络的数据集。其次利用数据集对三层反馈式网络进行了训练,讨论了不同隐含层节点数对该非线性系统的拟合能力,并且对梯度下降法、动量修正法和基于优化的LM算法的有效性和精度进行了比较,最后得出了适合平底结构入水砰击系统的网络结构。 相似文献
18.
Historical data of total dissolved inorganic carbon (CT), together with nitrate and phosphate, have been used to model the evolution of these constituents over the year in the Atlantic water of the Norwegian Sea. Changes in nutrient concentration in the upper layer of the ocean are largely related to biological activity, but vertical mixing with the underlying water will also have an impact. A mixing factor is estimated and used to compute the entrainment of these constituents into the surface water from below. After taking the mixing contribution into account, the resulting nutrient concentration changes are attributed to biological production or decay. The results of the model show that the change in CT by vertical mixing and by biological activity based on nutrient equivalents needs another sink to balance the carbon budget. It cannot be the atmosphere as the surface water is undersaturated with respect to carbon dioxide and is, thus, a source of CT in this region. Inasmuch as the peak deficit of carbon is more than a month later than for the nutrients, the most plausible explanation is that other nitrogen and phosphate sources than the inorganic salts are used together with dissolved inorganic carbon during this period. As nitrate and phosphate show a similar trend, it is unlikely that the explanation is the use of ammonia or nitrogen fixation but rather dissolved organic nitrogen and phosphate, while dissolved organic carbon is accumulating in the water. 相似文献
19.
A preliminary optical classification of lakes in Estonia and south Finland which can also be used for small bays of the Baltic Sea is elaborated. The classification is based on the optical properties of water (diffuse attenuation coefficient, diffuse reflectance) and parameters that are routinely monitored in water bodies (Secchi depth, concentration of chlorophyll-a, total suspended matter and yellow substance). The data complex used for our classification covers different types of water ecosystems (ranging from oligotrophic to hypertrophic) and the variability of water constituent concentrations in the ice-free period in Estonia and south Finland. Using cluster analysis, we found 5 optical classes of waters: clear (C), moderate (M), turbid (T), very turbid (V) and brown (B). There is satisfactory correspondence between class of water, shape of diffuse attenuation coefficient and diffuse reflectance spectra and trophic state of the lakes. 相似文献
20.