全文获取类型
收费全文 | 90493篇 |
免费 | 15071篇 |
国内免费 | 20941篇 |
专业分类
测绘学 | 7678篇 |
大气科学 | 11478篇 |
地球物理 | 16484篇 |
地质学 | 49873篇 |
海洋学 | 12597篇 |
天文学 | 5409篇 |
综合类 | 6401篇 |
自然地理 | 16585篇 |
出版年
2024年 | 370篇 |
2023年 | 974篇 |
2022年 | 2788篇 |
2021年 | 3173篇 |
2020年 | 3208篇 |
2019年 | 3965篇 |
2018年 | 3046篇 |
2017年 | 3467篇 |
2016年 | 3651篇 |
2015年 | 3970篇 |
2014年 | 5121篇 |
2013年 | 5023篇 |
2012年 | 5484篇 |
2011年 | 5835篇 |
2010年 | 4840篇 |
2009年 | 6010篇 |
2008年 | 5911篇 |
2007年 | 6365篇 |
2006年 | 6291篇 |
2005年 | 5681篇 |
2004年 | 5265篇 |
2003年 | 4909篇 |
2002年 | 4241篇 |
2001年 | 3742篇 |
2000年 | 3543篇 |
1999年 | 3249篇 |
1998年 | 2793篇 |
1997年 | 2338篇 |
1996年 | 2036篇 |
1995年 | 1701篇 |
1994年 | 1697篇 |
1993年 | 1410篇 |
1992年 | 1095篇 |
1991年 | 803篇 |
1990年 | 637篇 |
1989年 | 549篇 |
1988年 | 388篇 |
1987年 | 262篇 |
1986年 | 167篇 |
1985年 | 118篇 |
1984年 | 54篇 |
1983年 | 42篇 |
1982年 | 44篇 |
1981年 | 35篇 |
1980年 | 31篇 |
1979年 | 37篇 |
1978年 | 50篇 |
1977年 | 27篇 |
1975年 | 6篇 |
1954年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
821.
Abstract. In situ feeding habits of the cyclopoid copepods Oncaea venusta, Corycaeus amazonicus, Oithona plumifera , and O. simplex were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Peilet contents were compared with assemblages of phytoplankton present in surface waters at times of copepod collection. All samples were from the northern Gulf of Mexico. All four copepods fed upon phytoplankton and O. venusta also ingested other crustaceans. Dominant components of fecal pellets generally did not mirror those of available phytoplankton assemblages. In some cases, O. venusta ingested primarily larger-sized particles even when these were not most abundant, and in other cases it did not ingest large cells even when they were present in bloom concentrations. The presence of small (< 2–5 urn diameter) centric diatoms in O. venusta pellets suggests the possibility of feeding by mechanisms other than suspension or raptorial feeding. Limited observations suggest that C. amazonicus and O. plumifera may feed raptorially on larger particles even when these are not particularly abundant, and that the small O. simplex (< 500 nm total length) feeds mainly upon nanoplankton. It appears that cyclopoid feeding mechanisms are complex, and likely more so than those of many calanoids. 相似文献
822.
基于近年来有关日本海的古海洋古环境研究成果,总结概述了日本海中新世以来的古气候和古海洋环境演化特征。日本海作为西北太平洋一个主要的边缘海,在全球气候变化过程中起着重要的作用。它的古气候古环境演化,与全球其他地区一样受控于地球轨道参数的变化,同时也发育一些由气候的不稳定性、海峡的开合与其他一些地球气候系统的非线性活动所驱动的突发性快速气候变化事件。日本海的古海平面、古海水温度、古洋流等因子的变化还受到冰盖扩张和退缩、构造运动、冰川性地壳均衡补偿、东亚季风等因素的影响,具有鲜明的区域特点。总体说来,目前对日本海的研究程度还不高,对它在全球气候变化中的作用过程、驱动和响应机制的了解还很有限,需要进一步深入研究。 相似文献
823.
824.
The purpose of this study is to validate and improve satellite-derived downward surface shortwave radiation (DSSR) over the northwestern Pacific Ocean using abundant in situ data. The DSSR derivation model used here assumes that the reduction of solar radiation by clouds is proportional to the product of satellite-measured albedo and a cloud attenuation coefficient. DSSR is calculated from Geostationary Meteorological Satellite-5/Visible Infrared Spin-Scan Radiometer data in 0.05° × 0.05° grids. The authors first compare the satellite DSSR derived with a cloud attenuation coefficient table determined in past research with in situ values. Although the hourly satellite DSSR agrees well with land in situ values in Japan, it has a bias of +13∼+34 W/m2 over the ocean and the bias is especially large in the low latitudes. The authors then improve the coefficient table using the ocean in situ data. Usage of the new table successfully reduces the bias of the satellite DSSR over the ocean. The cloud attenuation coefficient for low-albedo cases over the ocean needs to be larger in the low latitudes than past research has indicated. Daily and hourly DSSR can be evaluated from the satellite data with RMS errors of 11–14% and 30–33%, respectively, over a wide region of the ocean by this model. It is also shown that the cloud attenuation coefficient over land needs to be smaller than over the ocean because the effect of the radiation reflected by the land surface cannot be ignored. 相似文献
825.
Observations of fluid mud were made in the lower North Passage of the Yangtze Estuary in February 2000, on 10 -11 August 2000, on 30 - 31 August 2000 (after two strong typhoons), on 21 - 24 August 2000 (neap tide) and on 3 -6 September 2000 (mean tide) respectively. In situ data show that the fluid mud in this area consists of fine cohesive sediment (median size 7.23 μm). The formation and movement of fluid mud varied during the neap-spring and flood-ebb tidal cycle. Observations suggest that fluid mud phenomena in this area may be categorised in a three-fold manner as slack water, storm and saltwedge features. The thickness of the fluid mud layer of slack water during the neap tide ranged from 0.2 to 0.96 m, whereas during the mean tide, the thickness ranged from 0.17 to 0.73 m, and the thickness of the fluid mud layer was larger during slack water than at the flood peak. Shoals cover an area of 800 km^2 with a water depth smaller than 5 m. Erosion of these extensive intertidal mudflats due to storm action provides an abundant sediment source. This is particularly significant in this estuary when the tidal level is lower than 5 m. The lower North Passage is a typical zone of saltwater wedging, so the saltwedge fluid mud has the most extensive spatial range in the estuary. 相似文献
826.
827.
828.
利用2000年9月在杭州湾进行的原型水文泥沙测验所获取的648个悬沙粒度样品,进行了样品粒度统计分析,结果表明:在杭州湾测验区域,粒径为4.00~20.20μm的悬沙出现频率最高;悬沙的中值粒径值自东向西迅速增大;北岸海域悬沙的中值粒径值大于南岸海域悬沙的中值粒径值。 相似文献
829.
830.
据开发应用水产饲料膨化机实践,初步论述了膨化机生产能力与动力匹配,螺杆设计参数长径比,物料在腔内滞留时间,膨化腔设计及其温度调控,并提出改进设计的见解. 相似文献