全文获取类型
收费全文 | 29457篇 |
免费 | 1224篇 |
国内免费 | 2130篇 |
专业分类
测绘学 | 1792篇 |
大气科学 | 3065篇 |
地球物理 | 5915篇 |
地质学 | 14156篇 |
海洋学 | 1595篇 |
天文学 | 1782篇 |
综合类 | 2676篇 |
自然地理 | 1830篇 |
出版年
2024年 | 17篇 |
2023年 | 78篇 |
2022年 | 240篇 |
2021年 | 266篇 |
2020年 | 207篇 |
2019年 | 269篇 |
2018年 | 4984篇 |
2017年 | 4223篇 |
2016年 | 2838篇 |
2015年 | 493篇 |
2014年 | 359篇 |
2013年 | 343篇 |
2012年 | 1242篇 |
2011年 | 2962篇 |
2010年 | 2276篇 |
2009年 | 2555篇 |
2008年 | 2110篇 |
2007年 | 2586篇 |
2006年 | 250篇 |
2005年 | 395篇 |
2004年 | 575篇 |
2003年 | 584篇 |
2002年 | 497篇 |
2001年 | 318篇 |
2000年 | 292篇 |
1999年 | 273篇 |
1998年 | 240篇 |
1997年 | 196篇 |
1996年 | 167篇 |
1995年 | 156篇 |
1994年 | 148篇 |
1993年 | 123篇 |
1992年 | 108篇 |
1991年 | 79篇 |
1990年 | 64篇 |
1989年 | 54篇 |
1988年 | 45篇 |
1987年 | 32篇 |
1986年 | 18篇 |
1985年 | 22篇 |
1984年 | 15篇 |
1983年 | 17篇 |
1982年 | 9篇 |
1981年 | 31篇 |
1980年 | 24篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 7篇 |
1958年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
现代黄河三角洲土地利用变化分析 总被引:3,自引:1,他引:3
人类对海岸带的开发利用影响着海岸的变迁。土地利用和土地覆被变化可以说是自然与人文过程交叉最为密切的问题,是人地关系的具体体现。通过对黄河三角洲土地利用变化的研究,可了解人为因素对三角洲海岸带的开发利用状况及其对海岸侵蚀的影响。根据1987年和2001年两期卫星遥感图像和2001年野外调查数据,运用地理信息系统方法及数理统计方法,分析了现代黄河三角洲地区1987年至2001年土地利用/覆被的数量变化和空间变化特征;揭示了该地区各类土地利用数量变化的幅度、速度和土地利用空间变化的主要类型;阐明了该区各类土地利用变化的区域特点。该地区土地利用变化速度比整个环渤海地区速度快,草地、滩地和不明地类这3种类型大量减少,其他类型增多。从变化率来看,养殖区、库塘、油田用地增长最快。库塘、海滩养殖区等类型的增加,滨海草地的减少,使海岸侵蚀加剧。 相似文献
162.
163.
结合近年来参与有关水下工程检测和项目监理的实践体验 ,从总结水下工程项目的基本特征着手 ,分析了水下工程项目监理的主要任务 ,并结合对于实施水下工程项目监理必备装备条件的探讨 ,介绍了国内外现有水下监测技术装备的开发应用状况 相似文献
164.
There is a low nutrient level in the Aoshan Bay. In June 1999, the chemical adjustmentand control experiment was made in the Aoshan Bay. Following tracts investigation was carried out before the experiment and on the 1st, 2nd, 4th, 5th, 6th and 45th day/after the experiment. While the variance of amount of phytoplankton, the replacement of superior species and the species composition of phytoplankton were researched. The results show that the amount of phytoplankton in the Aoshan Bay rises gradually after the experiment. Ceratium macroceros Cleve of pyrophyta was the dominant species before the experiment, its dominant index was 37.7%. Six days after the experiment, its dominant index dropped to 17.6% . Meanwhile the dominant index of Asterionella japanics Cleve rose from 7.1% to 39.2% , it became the first dominant species. Forty-five days after the experiment, the amount of phytoplankton in the Aoshan Bay was 5.15 to 137.32 times more than that in 1997. 相似文献
165.
渤海小型底栖生物的丰度和生物量 总被引:9,自引:1,他引:9
该文是渤海 1997年 6月、1998年 9月和 1999年 4月 3个航次小型底栖生物调查结果。结果表明 ,3个航次小型底栖生物的平均丰度分别为 :(2 30 0± 12 0 6 ) ind/ (10 cm2 )、(86 9± 5 10 ) ind/(10 cm2 )和 (6 32± 4 0 0 ) ind/ (10 cm2 )。平均生物量分别为 :(15 2 1± 6 34) μg(dwt) / (10 cm2 )、(72 5±35 4 )μg (dwt) / (10 cm2 )和 (5 17± 393)μg (dwt) / (10 cm2 )。共鉴定出 14个小型底栖生物类群 ,其中自由生活海洋线虫丰度占绝对优势 ,桡足类丰度居第 2位 ,这两个类群总和占小型底栖生物总丰度的 94 .8%~ 97.5 %。在生物量中所占比例列前 4位的类群依次为线虫、多毛类、桡足类、双壳类 ,加起来超过小型底栖生物总生物量的 80 %。小型底栖生物的 74 %分布于 2 cm以浅表层中。小型底栖生物的丰度和生物量在渤海海峡和渤海中东部较高 ,与环境因子的相关分析表明小型底栖生物的丰度与水深呈极显著的正相关 ,与沉积物的中值粒径呈显著的负相关 相似文献
166.
167.
168.
Seasonal evolution of surface mixed layer in the Northern Arabian Sea (NAS) between 17° N–20.5° N and 59° E-69° E was observed
by using Argo float daily data for about 9 months, from April 2002 through December 2002. Results showed that during April
- May mixed layer shoaled due to light winds, clear sky and intense solar insolation. Sea surface temperature (SST) rose by
2.3 °C and ocean gained an average of 99.8 Wm−2. Mixed layer reached maximum depth of about 71 m during June - September owing to strong winds and cloudy skies. Ocean gained
abnormally low ∼18 Wm−2 and SST dropped by 3.4 °C. During the inter monsoon period, October, mixed layer shoaled and maintained a depth of 20 to
30 m. November - December was accompanied by moderate winds, dropping of SST by 1.5 °C and ocean lost an average of 52.5 Wm−2. Mixed layer deepened gradually reaching a maximum of 62 m in December. Analysis of surface fluxes and winds suggested that
winds and fluxes are the dominating factors causing deepening of mixed layer during summer and winter monsoon periods respectively.
Relatively high correlation between MLD, net heat flux and wind speed revealed that short term variability of MLD coincided
well with short term variability of surface forcing. 相似文献
169.
Spatial and Temporal Variations of Sound Speed at the PN Section 总被引:3,自引:0,他引:3
Gridded sound speed data were calculated using Del Grosso's formulation from the temperature and salinity data at the PN section
in the East China Sea covering 92 cruises between February 1978 and October 2000. The vertical gradients of sound speed are
mainly related to the seasonal variations, and the strong horizontal gradients are mainly related to the Kuroshio and the
upwelling. The standard deviations show that great variations of sound speed exist in the upper layer and in the slope zone.
Empirical orthogonal function analysis shows that contributions of surface heating and the Kuroshio to sound speed variance
are almost equivalent.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
170.
Ana Luisa Rosa Yutaka Isoda Kazuyuki Uehara Tomokazu Aiki 《Journal of Oceanography》2007,63(4):573-588
Hydrographic data and composite current velocity data (ADCP and GEK) were used to examine the seasonal variations of upper-ocean
flow in the southern sea area of Hokkaido, which includes the “off-Doto” and “Hidaka Bay” areas separated by Cape Erimo. During
the heating season (April–September), the outflow of the Tsugaru Warm Current (TWC) from the Tsugaru Strait first extends
north-eastward, and then one branch of TWC turns to the west along the shelf slope after it approaches the Hidaka Shelf. The
main flow of TWC evolves continuously, extending eastward as far as the area off Cape Erimo. In the late cooling season (January–March),
part of the Oyashio enters Hidaka Bay along the shallower part of the shelf slope through the area off Cape Erimo, replacing
almost all of the TWC water, and hence the TWC devolves. It is suggested that the bottom-controlled barotropic flow of the
Oyashio, which may be caused by the small density difference between the Oyashio and the TWC waters and the southward migration
of main front of TWC, permits the Oyashio water to intrude along the Hidaka shelf slope. 相似文献