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   5303篇
  免费   1021篇
  国内免费   1363篇
测绘学   352篇
大气科学   1206篇
地球物理   1268篇
地质学   2937篇
海洋学   558篇
天文学   200篇
综合类   496篇
自然地理   670篇
  2024年   28篇
  2023年   112篇
  2022年   270篇
  2021年   320篇
  2020年   235篇
  2019年   289篇
  2018年   360篇
  2017年   321篇
  2016年   323篇
  2015年   270篇
  2014年   318篇
  2013年   278篇
  2012年   303篇
  2011年   352篇
  2010年   340篇
  2009年   286篇
  2008年   220篇
  2007年   252篇
  2006年   233篇
  2005年   201篇
  2004年   134篇
  2003年   125篇
  2002年   154篇
  2001年   129篇
  2000年   167篇
  1999年   238篇
  1998年   162篇
  1997年   185篇
  1996年   165篇
  1995年   164篇
  1994年   133篇
  1993年   165篇
  1992年   100篇
  1991年   78篇
  1990年   56篇
  1989年   50篇
  1988年   41篇
  1987年   33篇
  1986年   21篇
  1985年   15篇
  1984年   8篇
  1983年   8篇
  1982年   12篇
  1981年   3篇
  1980年   6篇
  1979年   5篇
  1963年   1篇
  1958年   14篇
  1954年   1篇
  1947年   1篇
排序方式: 共有7687条查询结果,搜索用时 15 毫秒
51.
有机碳沉积作用在一定时间尺度上形成海洋碳汇作用的最终净效应,北冰洋陆架的有机碳沉积作用在全球碳循环中有着尤为重要的地位。为了测定楚科奇海陆架的有机碳沉积通量,本文应用210Pb测年法对中国第3次北极考察R17站位的沉积物样品的年龄及沉积速率进行了分析测定,并结合表层沉积物中有机碳的含量计算得到了楚科奇海陆架北缘的有机碳沉积通量。结果显示,该站位的沉积速率为0.6 mm/a,表观沉积质量通量为0.72 kg/m2/a,有机碳沉积通量为517 mmol C/m2/a。经估算,真光层输出的有机碳中约有29 %被长期封存在沉积物中,远高于中低纬度的正常值(~10 %),说明楚科奇海陆架区具有高效的碳汇作用。  相似文献   
52.
Based on the meteorological data of 20 stations in the Hengduan Mountains region during 1961-2009, the annual and seasonal variation of potential evapotranspiration was analyzed in combination with the Penman-Monteith model. With the method of Spline interpolation under ArcGIS, the spatial distribution of potential evapotranspiration was presented to research the regional difference, and the correlation analysis was used to discuss the dominant factor affecting the potential evapotranspiration. The results indicated that the an-nual potential evapotranspiration showed a decreasing tendency since the 1960s, especially from the 1980s to 1990s, while it showed an increasing tendency since 2000. Regional potential evapotranspiration showed a rate of -0.17 mm a?1. Potential evapotranspiration in north, middle and south of the Hengduan Mountains exhibited decreasing trends over the studied period, and its regional trend was on the decline from southwest to northeast.  相似文献   
53.
Soil sediment samples of 10 layers with a spacing of 10 cm each were collected in different floodplain zones adjacent to Huolin River in the Xianghai Nature Reserve, and contents of total N, total P and organic matters were analyzed. The results showed that contents of total N, total P and organic matters were generally decreasing with the increase of distance from sample locations to the river channel, and contents of the three items were generally higher in the upper soil layer than that in the lower soil layer. The content variations displayed how flooding functions influenced nutrient matter content variations in floodplain soils since the flood inundation frequencies of the sample locations varied. The correlation analysis displayed that there were remarkable relativities between total N, total P and organic matters within definite spatial distance from the Huolin River channel.  相似文献   
54.
??????????????????????С????????λ??????????????????????????????????????????????????????????????λ?????????α??/??λ?????????????????μ???????????????????????????????????????÷???????????????????????????÷???????????????GEO??IGSO??????????????????????????????С???????????????MEO?????????????????  相似文献   
55.
河北唐山地区中寒武统张夏组鲕粒灰岩的岩石学研究   总被引:1,自引:0,他引:1  
本区张夏组鲕粒灰岩内,广泛见到细径放射鲕和较粗的同心-放射鲕.放射鲕中存在连生体,我们称之“连生鲕”.与赵震等(1984)人工合成的静水鲕对比,区内放射鲕亦系静水条件下生成;因未见长串连生体,仅呈现数鲕(2—4个)连生,表明鲕粒生成后遭到搅动水拆开.由此可以认为,本组鲕粒灰岩属间歇高能成因模式的产物。又按该类岩石显示的成岩特征,可知本区鲕粒灰岩经历过多种成岩环境,至今仍保留较为明显的大气成岩环境的标志.  相似文献   
56.
A total of 12 indices of temperature extremes and 11 indices of precipitation ex-tremes at 111 stations in southwestern China at altitudes of 285-4700 m were examined for the period 1961-2008. Significant correlations of temperature extremes and elevation in-cluded the trends of diurnal temperature range, frost days, ice days, cold night frequency and cold day frequency. Regional trends of growing season length, warm night frequency, coldest night and warmest night displayed a statistically significant positive correlation with altitude. These characteristics indicated the obvious warming with altitude. For precipitation extreme indices, only the trends of consecutive dry days, consecutive wet days, wet day precipitation and the number of heavy precipitation days had significant correlations with increasing alti-tude owing to the complex influence of atmospheric circulation. It also indicated the increased precipitation mainly at higher altitude areas, whereas the increase of extreme precipitation events mainly at lowers altitude. In addition, the clearly local influences are also crucial on climate extremes. The analysis revealed an enhanced sensitivity of climate extremes to ele-vation in southwestern China in the context of recent warming.  相似文献   
57.
天津市地震前兆台网的运行监控与维护管理   总被引:8,自引:0,他引:8  
介绍天津市地震前兆台网的基本概况以及前兆台网部的职责任务;阐述仪器设备运行监控、观测数据质量监控的方式与方法;总结在仪器设备、技术系统维护过程中解决的一些关键技术问题;最后提出了关于仪器和软件系统的一些问题与建议。  相似文献   
58.
Pco2 of air and seawater samples from the East China Sea(ECS) were measured in situ in autumn, 1994,Ocean currents,terrestrial fluviation,biological activities,etc.,Pco2 char-acters in air and seawater were investigated,CO2 flux and its character in the East China Sea are discussed on the basis of the Pco2 profiles of air and seawater,It was clear that the nearshore was the source of CO2;and tht the oulter sea area was the sink of CO2; and that the shelf area of the EXS is a net sink for atmospheric CO2 in autumn.  相似文献   
59.
Yan  Taiming  Hu  Jiaxiang  Cai  Yueping  Xiong  Sen  Yang  Shiyong  Wang  Xiongyan  He  Zhi 《中国海洋湖沼学报》2017,35(5):1197-1204
Laboratory-reared Schizothorax davidi larvae and juveniles were examined to assess the formation and characteristics of David’s schizothoracin otoliths.Otolith development was observed and their formation period was verifi ed by monitoring larvae and juveniles of knownage.The results revealed that lapilli and sagittae developed before hatching,and the first otolith increment was identified at 2 days post hatching in both.The shape of lapilli was relatively stable during development compared with that of sagittae;however,growth of four sagittae and lapilli areas was consistent,but the posterior area grew faster than the anterior area and the ventral surface grew faster than the dorsal surface.Similarly,the sum length of the radius of the anterior and posterior areas on sagittae and lapilli were linearly and binomially related to total fish length,respectively.Moreover,daily deposition rates were validated by monitoring knownage larvae and juveniles.The increase in lapilli width was 1.88±0.080 0μm at the ninth increment,which reached a maximum and the decreased gradually toward the otolith edge,whereas that of sagittae increased more slowly.These results illustrate the developmental biology of S.davidi,which will aid in population conservation and fish stock management.  相似文献   
60.
北宋中期耕地面积及其空间分布格局重建(英文)   总被引:2,自引:1,他引:2  
To understand historical human-induced land cover change and its climatic effects, it is necessary to create historical land use datasets with explicit spatial information. Using the taxes-cropland area and number of families compiled from historical documents, we esti-mated the real cropland area and populations within each Lu (a province-level political region in the Northern Song Dynasty) in the mid-Northern Song Dynasty (AD1004-1085). The es-timations were accomplished through analyzing the contemporary policies of tax, population and agricultural development. Then, we converted the political region-based cropland area to geographically explicit grid cell-based fractional cropland at the cell size of 60 km by 60 km. The conversion was based on calculating cultivation suitability of each grid cell using the topographic slope, altitude and population density as the independent variables. As a result, the total area of cropland within the Northern Song territory in the 1070s was estimated to be about 720 million mu (Chinese area unit, 1 mu = 666.7 m2), of which 40.1% and 59.9% oc-curred in the north and south respectively. The population was estimated to be about 87.2 million, of which 38.7% and 61.3% were in the north and south respectively, and per capita cropland area was about 8.2 mu. The national mean reclamation ratio (i.e. ratio of cropland area to total land area; RRA hereafter for short) was bout 16.6%. The plain areas, such as the North China Plain, the middle and lower reaches of the Yangtze River, Guanzhong Plain, plains surrounding the Dongting Lake and Poyang Lake and Sichuan Basin, had a higher RRA, being mostly over 40%; while the hilly and mountainous areas, such as south of Nanling Mountains, the southwest regions (excluding the Chengdu Plain), Loess Plateau and south-east coastal regions, had a lower RRA, being less than 20%. Moreover, RRA varied with topographic slope and altitude. In the areas of low altitude (≤250 m), middle altitude (250-100 m) and high altitude (1000-3500 m), there were 443 million, 215 million and 64 million mu of cropland respectively and their regional mean RRAs were 27.5%, 12.6% and 7.2% respectively. In the areas of flat slope, gentle slope, medium slope and steep slope, there were 116 million, 456 million, 144 million and 2 million mu of cropland respectively and their regional mean RRAs were 34.6%, 20.7%, 8.5% and 2.3% respectively.  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