首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   252821篇
  免费   5542篇
  国内免费   3776篇
测绘学   6999篇
大气科学   19263篇
地球物理   52720篇
地质学   87947篇
海洋学   21474篇
天文学   54612篇
综合类   1152篇
自然地理   17972篇
  2021年   2257篇
  2020年   2630篇
  2019年   2878篇
  2018年   3388篇
  2017年   3066篇
  2016年   5655篇
  2015年   4262篇
  2014年   6978篇
  2013年   14305篇
  2012年   6626篇
  2011年   7916篇
  2010年   7009篇
  2009年   9640篇
  2008年   8452篇
  2007年   7873篇
  2006年   9791篇
  2005年   7777篇
  2004年   7599篇
  2003年   7037篇
  2002年   6574篇
  2001年   5892篇
  2000年   5583篇
  1999年   4816篇
  1998年   4858篇
  1997年   4631篇
  1996年   4197篇
  1995年   4298篇
  1994年   3984篇
  1993年   3738篇
  1992年   3484篇
  1991年   3523篇
  1990年   3623篇
  1989年   3329篇
  1988年   3159篇
  1987年   3713篇
  1986年   3250篇
  1985年   4108篇
  1984年   4626篇
  1983年   4305篇
  1982年   4217篇
  1981年   3840篇
  1980年   3587篇
  1979年   3433篇
  1978年   3426篇
  1977年   3216篇
  1976年   2960篇
  1975年   2902篇
  1974年   2866篇
  1973年   3062篇
  1972年   1993篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
41.
Sharma  M. K.  Sharma  M.  Chandra  S. 《Astronomy Reports》2020,64(9):769-777
Astronomy Reports - Though accurate laboratory studies of malononitrile (CH2(CN)2), a dinitrile molecule, have been carried out from time to time, and it has large electric dipole moment $$\mu =...  相似文献   
42.
Astronomy Reports - The initial form of present-day space optical observations contain considerable geometrical and brightness distortions. This problem can be solved based on geometrical...  相似文献   
43.
Geology of Ore Deposits - The recognition of the Fouta Djallon–Mandingo territory as an independent bauxite province is substantiated. It is confined to the morphostructure of the same name...  相似文献   
44.
Doklady Earth Sciences - The results of an analysis of changes in the atmospheric air quality in Moscow during the lockdown period and the decline in business activity caused by the COVID-19...  相似文献   
45.
Geomagnetism and Aeronomy - The paper considers a special class of artifacts that appears in the useful signal when the source moves along a discrete matrix of the receiver. A parasitic component...  相似文献   
46.
Taseiko  O. V.  Moskvichev  V. V.  Chernykh  D. A. 《Water Resources》2019,46(6):983-991
Water Resources - The basic indicators of regional water use have been identified and assessed as a component of a single social–natural–technogenic system. These indicators have been...  相似文献   
47.
Ever increasing pressures on tropical forests worldwide due to anthropogenic disturbances have greatly affected both above-and belowground functioning of these forests.While fine roots play major ecological roles in forests through assisting in nutrient and water uptake and returning elements to the soil environment,coarse roots play an important role in C sequestration.We studied changes in fine and coarse root biomass,production,turnover and carbon and nitrogen return to the soil in two regenerating forest stands(RFs)following stonemining that were 5 years(RF-5)and 15 years(RF-15)post-disturbance compared with a natural forest stand(NF)in Mizoram,North-east India.Fine(2mm)and coarse root(2-10 mm)biomass differed significantly among the forest stands and ranged from239(RF-5)to 415(NF)and 230(RF-5)to 436(NF)g m 2,respectively.Total root(fine+coarse)biomass increased during stand development but the proportion of very fine root(0.5 mm)to total root production decreased.Fine root biomass decreased with increasing soil depth.Fine and total root biomass showed strong seasonal correlations with soil moisture,more so than for rainfall and temperature,whereas these relationships were less clear for the coarse root biomass.The amount of N(25-55 kg ha~(-1))and C(1.9-3.6t ha~(-1))stored in root biomass increased with stand age with a corresponding increase in production and turnover of C and N to the soil.Disturbance to these tropical forests negatively affected root dynamics,influenced their spatiotemporal patterns,and reduced the production,amount and availability of nutrients returned to the soil along with a strong reduction in the root biomass carbon pool and sequestration in carbon residence time.We observed that root growth,especially fine roots,is dependent on abiotic variables,and plays a significant role in early stages of secondary succession by adding organic matter and nutrients through high turnover rates in these forests.  相似文献   
48.
华雨淋  吕彦 《地球物理学报》2019,62(8):2982-2990
利用云南腾冲火山地区15个固定台站记录到的7923次地震的P波到时资料,采用双差层析成像方法,反演得到腾冲火山及周边地区地壳及上地幔顶部三维P波速度结构和地震重定位结果.研究发现,腾冲火山区域地壳内存在明显的地震波低速区,P波速度低于整个区域地壳速度平均值超过15%,上地幔顶部存在规模较大的低速异常区.推测腾冲火山地区存在较大规模的地幔热物质上涌以及向地壳的侵入,热物质在地壳内以岩浆囊形式存储,并且壳内岩浆囊之间可能存在岩浆通道.通过联合反演获得的地震重定位结果显示,丛集地震位置更加集中,其展布特征与断裂构造具有显著的对应关系,表明研究区域断裂构造比较活跃.获得的高分辨率三维P波层析成像结果,为进一步认识火山地区岩浆存储特征以及地震分布与区域构造之间的关系提供了新的地震学依据.  相似文献   
49.
Geomagnetism and Aeronomy - The history of solar observations in the radio range at the Mountain Astronomical Station, State Astronomical Observatory, Russian Academy of Sciences (MAS RAS) since...  相似文献   
50.
River deltas and associated turbidity current systems produce some of the largest and most rapid sediment accumulations on our planet. These systems bury globally significant volumes of organic carbon and determine the runout distance of potentially hazardous sediment flows and the shape of their deposits. Here we seek to understand the main factors that determine the morphology of turbidity current systems linked to deltas in fjords, and why some locations have well developed submarine channels while others do not. Deltas and associated turbidity current systems are analysed initially in five fjord systems from British Columbia in Canada, and then more widely. This provides the basis for a general classification of delta and turbidity current system types, where rivers enter relatively deep (>200 m) water. Fjord-delta area is found to be strongly bimodal. Avalanching of coarse-grained bedload delivered by steep mountainous rivers produces small Gilbert-type fan deltas, whose steep gradient (11°–25°) approaches the sediment's angle of repose. Bigger fjord-head deltas are associated with much larger and finer-grained rivers. These deltas have much lower gradients (1.5°–10°) that decrease offshore in a near exponential fashion. The lengths of turbidity current channels are highly variable, even in settings fed by rivers with similar discharges. This may be due to resetting of channel systems by delta-top channel avulsions or major offshore landslides, as well as the amount and rate of sediment supplied to the delta front by rivers. © 2018 John Wiley & Sons, Ltd.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号