全文获取类型
收费全文 | 54876篇 |
免费 | 21666篇 |
国内免费 | 45602篇 |
专业分类
测绘学 | 3800篇 |
大气科学 | 24430篇 |
地球物理 | 17448篇 |
地质学 | 45058篇 |
海洋学 | 17625篇 |
天文学 | 2012篇 |
综合类 | 6759篇 |
自然地理 | 5012篇 |
出版年
2024年 | 208篇 |
2023年 | 598篇 |
2022年 | 1449篇 |
2021年 | 1854篇 |
2020年 | 2837篇 |
2019年 | 6282篇 |
2018年 | 6712篇 |
2017年 | 6322篇 |
2016年 | 6591篇 |
2015年 | 5584篇 |
2014年 | 5421篇 |
2013年 | 5993篇 |
2012年 | 5599篇 |
2011年 | 5476篇 |
2010年 | 5236篇 |
2009年 | 4654篇 |
2008年 | 3788篇 |
2007年 | 3860篇 |
2006年 | 3336篇 |
2005年 | 3238篇 |
2004年 | 3294篇 |
2003年 | 3014篇 |
2002年 | 2744篇 |
2001年 | 2542篇 |
2000年 | 2305篇 |
1999年 | 2653篇 |
1998年 | 2529篇 |
1997年 | 2566篇 |
1996年 | 2168篇 |
1995年 | 1988篇 |
1994年 | 1795篇 |
1993年 | 1670篇 |
1992年 | 1359篇 |
1991年 | 1019篇 |
1990年 | 938篇 |
1989年 | 747篇 |
1988年 | 689篇 |
1987年 | 514篇 |
1986年 | 418篇 |
1985年 | 327篇 |
1984年 | 333篇 |
1983年 | 212篇 |
1982年 | 240篇 |
1981年 | 168篇 |
1980年 | 134篇 |
1979年 | 133篇 |
1978年 | 61篇 |
1977年 | 53篇 |
1971年 | 54篇 |
1970年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 466 毫秒
991.
本文综述了国内外酸雨的研究现状;包括酸雨的来源、成因、模式、影响及防治对策;介绍了酸雨的研究趋势;并着重就厦门地区今后酸雨的研究方向提出也作者的看法。 相似文献
992.
993.
994.
995.
996.
在洋浦近岸海域计算潮流场基础上,计算预选排污口附近海水质点运动轨迹及预测污染物浓度分布,最后,从环保角度出发,推荐了洋浦地区各开发区的排污口位置,为洋浦近岸海域污染控制规划方案的制定及其优化提供了依据。 相似文献
997.
天津港南部海区水体富营养化水平评价 总被引:1,自引:0,他引:1
根据2006年3月和7月的调查数据,对天津港南部海区海水的营养水平及动态变化进行了评价。考虑到水质状况和水体营养水平是水体中多项指标分析评价的综合结果.本文先通过单项指标评价法和营养指数法进行评价。然后又通过集对分析方法,选择化学需氧量(COD)、无机磷(DIP)、无机氮(DIN)、叶绿素a(Ch1—a)等作为评价指标,对该区海水富营养化水平进行了综合评价。评价结果表明集对分析方法更能反映海水的实际富营养化状况,计算结果显示天津港南部海域两期调查中除21,26.27和29站位在7月呈富营养化状态其余站位海水均在中营养(Ⅳ)及以下水平状况,未达到富营养化水平,水质状况较好。 相似文献
998.
Sublittoral meiofauna with particular reference to nematodes in the southern Yellow Sea, China 总被引:6,自引:0,他引:6
Densities of major meiofaunal taxa were investigated at 34 sampling stations during six cruises by R/V Beidou to the southern Yellow Sea, China from 2000 to 2004, and the community structure of free-living marine nematodes was studied during one of the cruises in 2003. Meiofauna abundance ranged from 487.4 to 1655.3 individuals per 10 cm2. Nematodes and harpacticoid copepods were the two most dominant groups, contributing 73.8–92.8% and 3.5–18.7%, respectively, to the total meiofauna abundance. One-way ANOVA showed no significant annual fluctuation of meiofauna and nematode abundances from 2000 to 2004 in the southern Yellow Sea. However, two-way ANOVA based on six stations sampled in 4 years (2001–2004) showed that there were significant differences among the six stations and the 4 sampling years for meiofauna, nematode and copepod abundance. Correlation analysis demonstrated that meiofauna abundance was mainly linked to chloroplastic pigments. Other environmental factors could not be ruled out, however. A total of 232 free-living marine nematode species, belonging to 149 genera, 35 families and 4 orders, were identified. The dominant species in the sampling area were the following: Dorylaimopsis rabalaisi, Microlaimus sp.1, Prochromadorella sp., Promonohystera sp., Cobbia sp.1, Daptonema sp.1, Leptolaimus sp.1, Halalaimus sp.2, Aegialoalaimus sp., Chromadorita sp., Parodontophora marina, Parasphaerolaimus paradoxus, Quadricoma sp.1, Campylaimus sp.1, Halalaimus gracilis, Paramesacanthion sp.1, Paramonohystera sp.1, and Metalinhomoeus longiseta. CLUSTER and SIMPROF analyses revealed three main types of nematode community (or station groups) in the sampling area, including I: coastal community, II: transitory community between coastal and YSCWM (Yellow Sea Cold Water Mass), and III: YSCWM community. Each community was indicated by a number of dominant nematode species. Bio-Env correlation analysis between the nematode community and environmental variables showed that water depth, sediment water content, organic matter, chlorophyll a (Chl-a) and phaeophorbide a (Pha-a) were the most important factors to determine the community structure. 相似文献
999.
The algorithms of extracting chlorophyll-a(Chl-a) concentration have been established for Chinese moderate resolution imaging spectrometer(CMODIS) mounted on Shenzhou-3 spaceship launched on 25 March 2002.The CMODIS is an ocean color sensor with 30 visible channels and 4 infrared channels,much different from other ocean color satellites and needs new algorithms to process data.Three models of Chl-a concentration were established based on Chl-a data retrieved from sea-viewing wide field-of-view sensor(SeaWiFS),with the average relative errors of 26.6%,24%.0% and 33.5%,respectively.This practical and economic approach can be used for developing the algorithms of Chinese ocean color and temperature sensor(COCTS) on the satellite Haiyang-1 to derive the Chl-a concentration concentration distribution.The applicability of the algorithms was analyzed using some in situ measurements.Suspended sediment is the main factor influencing the accuracy of the spectral ratio algorithms of Chl-a concentration.The algorithms are suitable to using in the regions where suspended sediment concentrations(SSC) are less than 5 g/m3 under the condition of relative error of Chl-a concentration retrieval within 35%.High concentration of suspended sediment leads to the overestimate remote sensing retrieval of concentration of Chl-a,while low-middle SSCs lead to the low Chl-a concentration values using the spectral ratio algorithms.Since the accuracy of Chl-a concentration by the spectral ratio algorithms is limited to waters of Case 2,it is necessary to develop semi-analytical models to improve the performance of satellite ocean color remote sensing in turbid coastal waters. 相似文献
1000.
海洋环境荷载下输液立管的静、动力特性研究 总被引:1,自引:1,他引:1
考虑管内流动流体和管外海洋环境荷载共同作用 ,建立海洋立管侧向运动微分方程。用Hermite插值函数离散 ,在微机上编写海洋立管静、动力分析程序 ,通过计算分析研究管内流体对立管侧向变形和应力的作用 ;另外 ,探讨管内流体的流动速度和立管顶端的预张力对立管动力特性的影响。结果表明 ,立管变形和应力均随管内流体流动速度增加而增大 ,同时内流速度的增大会降低立管的固有频率 ,但适当增大立管顶端预张力会抵消内流流速增加引起的固有频率下降。 相似文献