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   3376篇
  免费   777篇
  国内免费   1333篇
测绘学   120篇
大气科学   1742篇
地球物理   740篇
地质学   1192篇
海洋学   838篇
天文学   83篇
综合类   233篇
自然地理   538篇
  2024年   26篇
  2023年   66篇
  2022年   124篇
  2021年   153篇
  2020年   164篇
  2019年   189篇
  2018年   156篇
  2017年   159篇
  2016年   170篇
  2015年   167篇
  2014年   246篇
  2013年   242篇
  2012年   250篇
  2011年   268篇
  2010年   195篇
  2009年   270篇
  2008年   257篇
  2007年   298篇
  2006年   270篇
  2005年   202篇
  2004年   208篇
  2003年   180篇
  2002年   156篇
  2001年   148篇
  2000年   117篇
  1999年   97篇
  1998年   109篇
  1997年   108篇
  1996年   95篇
  1995年   75篇
  1994年   76篇
  1993年   54篇
  1992年   38篇
  1991年   33篇
  1990年   28篇
  1989年   19篇
  1988年   35篇
  1987年   8篇
  1986年   8篇
  1985年   5篇
  1984年   5篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1980年   2篇
  1978年   6篇
  1973年   1篇
排序方式: 共有5486条查询结果,搜索用时 359 毫秒
71.
用气象站地温资料计算多年平均土壤热流的初步结果   总被引:11,自引:3,他引:8  
董文杰  汤懋苍 《高原气象》1992,11(2):115-125
  相似文献   
72.
漳州热田的对流热流和传导热流的研究   总被引:5,自引:1,他引:4       下载免费PDF全文
漳州地热系统属对流型地热系统.漳州热田是我国东南沿海地区目前所见热田中温度最高(121.5℃)的一个.地表热流值的研究表明,热田中心具有最大的实测热流值(359mw/m2).本文根据热田内152个钻孔的测温资料和56块岩石样品的热导率数据,采用三种不同方法计算出漳州热田及其邻近地区的大地热流值,并讨论了热流值分布的特点.  相似文献   
73.
Daily measurements of atmospheric concentrations of dimethylsulfide (DMS) were carried out for two years in a marine site at remote area: the Amsterdam Island (37°50S–77°31E) located in the southern Indian Ocean. DMS concentrations were also measured in seawater. A seasonal variation is observed for both DMS in the atmosphere and in the sea-surface. The monthly averages of DMS concentrations in the surface coastal seawater and in the atmosphere ranged, respectively, from 0.3 to 2.0 nmol l-1 and from 1.4 to 11.3 nmol m-3 (34 to 274 pptv), with the highest values in summer. The monthly variation of sea-to-air flux of DMS from the southern Indian Ocean ranges from 0.7 to 4.4 mol m-2 d-1. A factor of 2.3 is observed between summer and winter with mean DMS fluxes of 3.0 and 1.3 mol m-2 d-1, respectively.  相似文献   
74.
Mt. Amiata (Southern Tuscany, Central Italy) is an extinct Quaternary volcano located in an area still marked by high heat-flow that is caused by deep seated (6-10 km) hot masses related to Pliocene magmatic activity. The anomalous geothermal gradient gives rise, within the Mesozoic limestone formation (Tuscan series), to geothermal systems that fed the Ca-SO4 thermal springs characterizing this area. Besides of thermal fluids, several cold, dry CO2-rich gas emissions seep out on the NE flank of the volcano. These gas vents mostly consist of large sub-circular craters at variable depth and diameter (5-15 m and 10-50 m, respectively), and represent a serious hazard for the local population, as testified by the several asphyxia casualties that have been repeatedly occurred within these morphological depressions. In this work, the chemical and isotopic compositions of the Mt. Amiata "CO2-rich gas vents" and the estimation of both the CO2 flux from the soil and the CO2 distribution in air of their surroundings, has been carried out in order to: (1) assess the origin of gases, (2) recognize the mechanism of formation for these gas emissions and their relationship with local tectonics, and (3) to evaluate the CO2 hazard in the high flux emanations. The chemical composition of the gases is largely dominated by CO2 (up to 98 % by vol) and shows relatively high concentrations of N2, CH4 and H2S (up to 1.1%, 0.9% and 3.9 % by vol, respectively). These features, coupled with the carbon and nitrogen isotopic signatures, suggest that the origin of the main gas compounds may be related to the contribution of deep (i.e., thermometamorphic processes on carbonate formations for CO2) and shallow (i.e. thermal decomposition of organic material for CH4, N2 and H2S) sources.  相似文献   
75.
Guizhou is located in the Circum-Pacific Global Mercuriferous Belt, and mercury concentrations in soil in this area are enriched. In-situ total gaseous mercury (TGM) exchange fluxes between air and soil surface were intensively measured at four sampling sites in Guiyang from 21 May to 16 June, 2003, and five sites in the Lanmuchang mercury mining area in December 2002 and May 2003, respectively. The in-situ Hg flux measurement was conducted with a dynamic flux chamber (DFC) of quartz. Overall, net emissions were obtained from all sampling sites. Soil mercury concentration and solar radiation have been proved to be the two most important parameters to control mercury emissions from soil. Meanwhile, rain events can enhance mercury emission rate significantly.  相似文献   
76.
77.
Palaeovariations of the atmospheric 210Pb flux in Northeast China (Long Gang area, Jilin province), quantified by high resolution 210Pb measurements on seasonally laminated sediments of Lake Sihailongwan are presented on decadal scale. The mean flux of unsupported 210Pb between 1790 and 1970 for the centre of the lake basin is 517 Bq m−2 yr−1 with maximum deviations between −23% and +27% of this value. Flux rates above this average were found between 1783 and 1813, around 1836, and between 1860 and 1901. The mean 210Pbexc flux rates derived were clearly lower around 1821, 1908, 1930, and 1953. The 210Pbexc flux reached its minimum of 344 Bq m−2 yr−1 during the period 1977–1982. The atmospheric flux of unsupported 210Pb is correlated with the precipitation frequency during the summer monsoon and shows coinciding variability with geochemical proxies that document the groundwater inflow into the lake. Al2O3-rich dust of remote provenance scavenged by wet-deposition in the rainy season is the major carrier of the atmospheric 210Pbexc flux.  相似文献   
78.
79.
The varved sediments of the dimictic Lake Sihailongwan (Long Gang mountain area, Jilin Province, Northeast China) represent a palaeoclimatic archive which documents the local precipitation frequency during the summer monsoon, and variations in the aeolian flux of dust with their remote sources in the arid and semi-arid regions of inner Asia. Based on a detailed discussion of sediment genesis in Lake SHL, dust flux rates and palaeohydrological conditions were reconstructed on a decadal scale over the past 220 years. The aeolian influx by dry and wet deposition was quantified and characterised in its chemical composition. Photosynthetic production in the lake is positively correlated with the inflow of nutrient-rich groundwater. The groundwater discharge largely reflects the strength of the summer monsoon. Net accumulation rates for biogenic silica were determined for annually laminated sediments from the centre of the U-shaped lake basin based on sediment data. In a Si-balance model of the modern lake, the depositional flux of biogenic silica could be independently quantified on the base of hydrochemical monitoring data. Comparison of the both estimates allowed to asses the focussing of the particle flux in the lake. Though water retention in Lake SHL is rather high (ca. 30 years), changes in the hydrological conditions are sensitively recorded in the sediments because (i) nutrient-rich groundwater discharges into the productive zone of the lake, (ii) a substantial proportion of the total dissolved Si-inventory of the mixed lake (ca. 30%) is annually consumed by diatom growth, and (3) sediment accumulation is substantially focussed towards the flat bottom of the lake basin. The bulk siliciclastic sediment fraction (ca. 75 wt.%) largely originates from influx of dust of remote provenance. In sediment thin-sections, the dry-deposited dust fraction is microscopically identifiable as seasonal silt layer. Aeolian input by wet-deposition shows a distinctly higher variability than the influx of dust by dry-deposition. As diatom production, wet-deposition of dust is positively correlated with the rainfall during the summer monsoon. The inferred positive correlation between rainfall and dust flux during the summer monsoon implies that dust deposition is determined by the out-wash efficiency of mineral particles for a permanent high atmospheric dust concentration over Northeast China in the last 220 years.  相似文献   
80.
胞外聚合物对膜生物反应器运行性能的影响   总被引:3,自引:1,他引:3  
膜生物反应器处理污水过程中,依据胞外聚合物(EPS)、蛋白质、多糖、粘度等参数随时间的变化趋势以及对膜通量的影响,求出上述参数及蛋白质/多糖的比值与膜通量间的相关系数分别为0.28、0.57、0.38、0.14、0.70。结果表明:蛋白质/多糖的比值与膜通量的衰减速度两者相关系数最大。  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