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   2361篇
  免费   429篇
  国内免费   723篇
测绘学   141篇
大气科学   594篇
地球物理   539篇
地质学   1238篇
海洋学   281篇
天文学   94篇
综合类   325篇
自然地理   301篇
  2024年   9篇
  2023年   58篇
  2022年   134篇
  2021年   163篇
  2020年   127篇
  2019年   124篇
  2018年   161篇
  2017年   149篇
  2016年   162篇
  2015年   163篇
  2014年   181篇
  2013年   151篇
  2012年   156篇
  2011年   170篇
  2010年   180篇
  2009年   155篇
  2008年   126篇
  2007年   114篇
  2006年   107篇
  2005年   72篇
  2004年   71篇
  2003年   58篇
  2002年   56篇
  2001年   52篇
  2000年   63篇
  1999年   84篇
  1998年   75篇
  1997年   60篇
  1996年   44篇
  1995年   53篇
  1994年   43篇
  1993年   29篇
  1992年   33篇
  1991年   28篇
  1990年   25篇
  1989年   14篇
  1988年   17篇
  1987年   14篇
  1986年   12篇
  1985年   4篇
  1984年   3篇
  1983年   6篇
  1982年   3篇
  1981年   1篇
  1958年   3篇
排序方式: 共有3513条查询结果,搜索用时 31 毫秒
131.
2017年精河MS6.6地震震前,尼勒克、巴伦台和小泉沟台钻孔应变仪记录到显著的应变变化,经现场落实确认,异常是可靠的。根据这3个台的记录资料计算相对应变的变化,结果表明,地震前最大(小)主应变大小和方向分别出现明显加速和急剧偏转变化。此外还发现,相对应变场异常期间主方向与震源机制解P轴方位较为一致。  相似文献   
132.
Urban expansion is a phenomenon of urban space increase, and an important measuring index of the process of urbanization. Taking Shanghai as an example, the changes of urban average height and built-up area were studied to represent city’s vertical and horizontal increases respectively, and statistical methods were used to analyze the driving forces of urban expansion. The research drew following conclusions: 1) The urban expansion process of Shanghai from 1985 to 2006 had a clear periodic feature, and could be divided into three stages: vertical expansion in dominance, coordinated vertical and horizontal expansion, and horizontal expansion in dominance. 2) The average height and quantity of buildings in core city were significantly bigger than those in suburbs, but the changing speed of the latter was faster. And 3) urbanization process was the major driving force for the city’s horizontal expansion, while industrial structure improvement was the key driving factor for the vertical expansion. Those two driving forces were simultaneously affected by city’s political factors.  相似文献   
133.
临沂市土地整理项目后期管护的做法   总被引:2,自引:0,他引:2  
临沂市地处沂蒙山区,山区、丘陵、平原各占三分之一。近几年,临沂市政府高度重视土地开发整理工作,自2003年以来,共实施国家级项目17个、省级项目12个、市级项目177个、县级项目1130个,争取国家和省级资金5.3亿元,投入市、县级项目资金6.6亿多元,项目总规模9.8万hm^2,新增耕地2.2万余公顷,新建新修灌溉水渠347.89km,  相似文献   
134.
临沂市加强土地开发整理推动新农村建设   总被引:1,自引:1,他引:0  
土地开发整理与新农村建设相结合,是当前社会关注的热点之一。临沂市高度重视通过土地整理促进新农村建设,自2003年以来,共实施国家和省、市、县四级土地开发整理项目1335个,累计投入11.12亿元,项目总规模14.4万hm^,新增耕地2.2万hm^2。通过实施土地开发整理,新建灌溉渠系1930km,排水沟2315km,新上泵站313个,农用井661眼,新上输变电线路287km,  相似文献   
135.
以灰树花发酵菌丝体水不溶性多糖GF4A为原料,以三氧化硫三甲胺盐(SO3·Me3N)为酯化试剂,探讨了GF4A的硫酸酯化工艺.实验结果表明,当三氧化硫与GF4A中单糖摩尔比为5.8∶1、反应温度为80 ℃、反应时间6 h时,所得酯化产物(SGF4A)水溶性好、产率高.该产物硫酸酯取代度为1.6, 相对分子质量为88 kD.通过红外光谱及核磁共振碳谱分析表明,糖残基中所有C-6羟基以及部分C-4和C-2位羟基被酯化.体外实验表明,SGF4A具有较好的抗凝活性.  相似文献   
136.
Effects of water temperature (17, 21, 25, 30 and 35℃) and body size (14.75-281.41 g initial body weight) on food consumption, growth, feed conversion, and dry matter content in orange-spotted grouper fed to satiation were investigated. The combined effect of temperature (T, ℃) and body weight (W, g) on maximum food consumption (Cmax, g/day) was described as: InCmax=-7.411+0.828 InW+0.317T4).004 7T2, and the optimum feeding temperature was 33.9℃. The combined effect of temperature and body weight on growth (G) was described as: lnG=-4.461-0.2081nW+0.394T-0.006 3T^2. The optimum growth temperature was 31.4℃, whereas overall growth rates were high at 25, 30 and 35 ℃. Feed conversion efficiencies (FCE, %), increasing first and then decreasing with increasing temperature, averaged from 1.8 to 2.1 in terms of dry weight of food fish. The optimum temperature for FCE tended to be lower than that for growth or feeding. Dry matter content increased with both increasing water temperature (17, 25, 30 and 35℃) and body weight, and the combined effect of temperature and body weight on dry matter content (DM, %) was described as: lnDM =3.232+0.01 4 lnW-0.004 4T+0.001 2TInW.  相似文献   
137.
Based on the sequence data of the nuclear ribosomal DNA internal transcribed spacer (ITS) 1, 5.8 S, and ITS 2, the molecular phylogeny was analyzed on Ulvaceae species collected from Qingdao coasts in summer of 2007, including 15 attached Ulva and Enteromorpha samples from 10 locations and 10 free-floating Enteromorpha samples from seven locations. The result supported the monophyly of all free-floating Enteromorpha samples, implying the unialgal composition of the free-floating Enteromorpha, and the attached Ulvaceae species from Qingdao coasts were grouped into other five clades, suggesting that they were not the biogeographic origin of the free-floating Enteromorpha in that season. Supported by NSFC (40506030); the Innovative Key Project of the Chinese Academy of Sciences (KZCX2-YW-209); Science & Technology Project of Qingdao City (06-2-2-12-JCH)  相似文献   
138.
A non-dimensional relative sensitivity coefficient was employed to predict the responses of reference crop evapotranspiration (ET0) to perturbation of four climate variables in Tao'er River Basin of the northeastern China. Mean monthly ET0 and yearly ET0 from 1961 to 2005 were estimated with the FAO-56 Penman-Monteith Equation. A 45-year historical dataset of average monthly maximum/minimum air temperature, mean air temperature, wind speed, sunshine hours and relative humidity from 15 meteorological stations was used in the analysis. Results show that: 1) Sensitivity coefficients of wind speed, air temperature and sunshine hours were positive except for those of air temperature of Arxan Meteorological Station, while those of relative humidity were all negative. Relative humidity was the most sensitive variable in general for the Tao'er River Basin, followed by sunshine hours, wind speed and air temperature. 2) Similar to climate variable, monthly sensitivity coefficients exhibit large annual fluctuations. 3) Sensitivity coefficients for four climate variables all showed significant trends in seasonal/yearly series. Also, sensitivity coefficients of air temperature, sunshine hours and wind speed all showed significant trends in spring. 4) Among all sensitivity coefficients, the average yearly sensitivity coefficient of relative humidity was highest throughout the basin and showed largest spatial variability. Longitudinal distribution of sensitivity coefficients for air temperature, relative humidity and sunshine hours was also found, which was similar to the distribution of the three climate variables.  相似文献   
139.
GPS���ٶ�λ�в�̬��������򻯿���ⷨ   总被引:1,自引:1,他引:0  
???GPS?????λ??????????????????????????????????????????????M??????????????μ??????????????????????????????????????????????е????????????????????????÷??????п???????????????????????????  相似文献   
140.
为获知不同时期封闭式海水循环水养殖系统中水质因子、浮游植物和细菌群落结构对大西洋鲑(Salmo salar)体内土腥味物质含量变化的影响,于2018—2019年4次采集封闭式海水循环水养殖系统的水样和大西洋鲑。测定水体的氨氮、亚硝氮、硝氮、活性磷酸盐等指标,分析水体浮游植物群落结构并通过16S高通量测序测定水体中细菌群落结构,通过微波蒸馏-固相微萃取-气相色谱-质谱联用(MD-SPME-GC-MS)方法,分析大西洋鲑背肌中土臭素(Geosmin,GSM)和二甲基异冰片(2-methylisoborneol,MIB)的含量。研究发现:测定水质指标中温度、溶解氧、盐度、pH、氨氮、亚硝氮、硝氮、活性磷酸盐、总氮、总磷、总悬浮物和溶解性总有机碳在不同时期差异显著。共鉴定出浮游植物7门、13目、26属,蓝藻门(Cyanophyta)、硅藻门(Bacillariophyta)和绿藻门(Chlorophyta)为优势门;不同时期细菌群落中的优势菌门均为变形菌门(Proteobacteria)和拟杆菌门(Bacteroidetes),但细菌群落结构存在差异,这主要是由水质差异引起的。在不同时期,鱼体MIB含量均高于GSM含量,MIB为鱼体中主要的土腥味物质,在7和10月MIB含量较高。鱼体土腥味物质含量受浮游植物和细菌群落影响,而水质因子可能通过影响浮游植物和细菌群落的结构,来影响养殖鱼体内土腥味物质的含量。  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