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   40105篇
  免费   3202篇
  国内免费   5118篇
测绘学   2798篇
大气科学   5319篇
地球物理   8511篇
地质学   19594篇
海洋学   3404篇
天文学   2253篇
综合类   3503篇
自然地理   3043篇
  2024年   95篇
  2023年   347篇
  2022年   657篇
  2021年   855篇
  2020年   656篇
  2019年   769篇
  2018年   5385篇
  2017年   4611篇
  2016年   3396篇
  2015年   1013篇
  2014年   1087篇
  2013年   997篇
  2012年   1890篇
  2011年   3665篇
  2010年   2938篇
  2009年   3233篇
  2008年   2698篇
  2007年   3181篇
  2006年   778篇
  2005年   927篇
  2004年   977篇
  2003年   930篇
  2002年   957篇
  2001年   702篇
  2000年   648篇
  1999年   740篇
  1998年   584篇
  1997年   536篇
  1996年   499篇
  1995年   418篇
  1994年   396篇
  1993年   332篇
  1992年   299篇
  1991年   241篇
  1990年   202篇
  1989年   159篇
  1988年   139篇
  1987年   77篇
  1986年   81篇
  1985年   53篇
  1984年   49篇
  1983年   26篇
  1982年   46篇
  1981年   41篇
  1980年   45篇
  1979年   14篇
  1978年   7篇
  1977年   7篇
  1976年   12篇
  1958年   13篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
71.
IntroductionIn color reproduction and communication, met-amer exists inherently when the conventionalcondition matching used. Therefore , the colordifference introduced makes the accuracy of colorrepresentation be lowered.In order to eli minatemetamer CIE developed the scheme so-calledspectra matching in Publication 15 .2 and speci-fied that using band-pass filters with 5 nminter-val to sample and then reconstruct the originalcolor signal ,thus the color difference will disap-pear[1].Spectra…  相似文献   
72.
A wide-field time-series CCD photometric survey of variable stars in the field of the open cluster NGC 2168 was carried out using the BATC Schmidt telescope. In total 13 new variable stars are discovered with three W UMa systems, one EA type and two EB type eclipsing binaries (one of them could be a W UMa system), and seven pulsating stars including three candidates of δScuti stars.  相似文献   
73.
A numerical simulation of Otsuchi Bay located on the northeast coast of the Honshu, the largest island of Japan, is conducted, using an ocean general circulation model (OGCM) with a nested-grid system in order to illustrate seasonal variability of the circulation in the bay. Through a year, an anticlockwise circulation is dominant in the bay, as observational studies have implied, although it is modified in the bay-mouth-half of the bay in winter. In addition, there is an intense outflow at the surface layer during spring to autumn, influenced by river water discharge. Intrusion of the Pacific water into the bay is influened by mean circulations, but it is also influenced by baroclinic tides from spring to autumn. Pacific water intrusions affected by baroclinic tides may have an impact on the environment in Otsuchi Bay.  相似文献   
74.
柘林湾生态信息系统以ArcGIS为开发平台,采用客户端/服务器的体系结构,基于ArcObjects的组件式二次开发而成,实现了柘林湾的三维实景模拟、生态数学模型与GIS的有效集成、分析评价结果与地理空间数据相结合等功能,具有可视化、智能化、易扩展、安全实用等特点.该系统的建成.极大地提高了柘林湾生态信息的现代化管理水平,并为制定柘林湾的污染防治与生态环境保护策略提供了强有力的支持.  相似文献   
75.
鲈鱼的鳃寄生车轮虫病及扫描电镜观察   总被引:1,自引:0,他引:1  
报道鲈鱼的鳃寄生车轮虫病及病理组织的扫描电镜观察。病原鉴定为 :直钩车轮虫、杜氏车轮虫、日本车轮虫、劳牧小车轮虫及简单两分虫 5种车轮虫科纤毛虫 ,其中数量上占优势的为直钩车轮虫。扫描电镜观察结果表明 ,鳃寄生车轮虫可对宿主的上皮组织产生明显的机械性损伤并造成鳃丝粘液分泌增多及炎症反应。文中就海水养殖鱼类及野生鱼类由车轮虫寄生造成的危害及防治途径做了探讨。  相似文献   
76.
Respiration and calcification rates of the Pacific oyster Crassostrea gigas were measured in a laboratory experiment in the air and underwater, accounting for seasonal variations and individual size, to estimate the effects of this exotic species on annual carbon budgets in the Bay of Brest, France. Respiration and calcification rates changed significantly with season and size. Mean underwater respiration rates, deducted from changes in dissolved inorganic carbon (DIC), were 11.4 μmol DIC g−1 ash-free dry weight (AFDW) h−1 (standard deviation (SD), 4.6) and 32.3 μmol DIC g−1 AFDW h−1 (SD 4.1) for adults (80–110 mm shell length) and juveniles (30–60 mm), respectively. The mean daily contribution of C. gigas underwater respiration (with 14 h per day of immersion on average) to DIC averaged over the Bay of Brest population was 7.0 mmol DIC m−2 day−1 (SD 8.1). Mean aerial CO2 respiration rate, estimated using an infrared gas analyzer, was 0.7 μmol CO2 g−1 AFDW h−1 (SD 0.1) for adults and 1.1 μmol CO2 g−1 AFDW h−1 (SD 0.2) for juveniles, corresponding to a mean daily contribution of 0.4 mmol CO2 m−2 day−1 (SD 0.50) averaged over the Bay of Brest population (with 10 h per day of emersion on average). Mean CaCO3 uptake rates for adults and juveniles were 4.5 μmol CaCO3 g−1 AFDW h−1 (SD 1.7) and 46.9 μmol CaCO3 g−1 AFDW h−1 (SD 29.2), respectively. The mean daily contribution of net calcification in the Bay of Brest C. gigas population to CO2 fluxes during immersion was estimated to be 2.5 mmol CO2 m−2 day−1 (SD 2.9). Total carbon release by this C. gigas population was 39 g C m−2 year−1 and reached 334 g C m−2 year−1 for densely colonized areas with relative contributions by underwater respiration, net calcification, and aerial respiration of 71%, 25%, and 4%, respectively. These observations emphasize the substantial influence of this invasive species on the carbon cycle, including biogenic carbonate production, in coastal ecosystems.  相似文献   
77.
徐彬  王占阁  吴军  许正文  薛昆  吴健 《极地研究》2010,22(4):334-347
2009年夏季,中国在挪威特罗姆瑟(Tromsφ)利用非相干散射雷达进行电离层加热实验。实验中发现了处在150km附近的小范围温度增强事件和150—400km之间的大范围温度增强事件。前者的相对增幅明显,后者的绝对增量更大。对这两类加热事件来说,温度增量百分比均随加热功率的增加线性增长,但对比两类加热事件来看,增长速度随加热频率增大而减小。加热效应存在显著的二维分布特征,加热最强方向处在场向附近。加热效应受加热天线波束指向与地磁场的夹角影响明显,随夹角增大温度增强效应减弱。  相似文献   
78.
中国对虾新病害—肠结节病的观察   总被引:1,自引:0,他引:1  
1991年7~8月浙江省宁波、舟山地区一些中国对虾Penaeruschinensis的养虾场中爆发了一种肠结节新病害,危害十分严重。本文报导该病发生与危害的情况以及症状、病原、组织病理观察的结果。  相似文献   
79.
Although the circulation of intermediate water masses in the eastern North Atlantic remains poorly defined, the presence of fresher intermediate waters, the Sub-Artic (SAIW) and the Antarctic Intermediate Water (AAIW), as well the saline intermediate Mediterranean Water (MW), has been tracked using biogeochemical properties. Here we assess the hydrographic and chemical structures of intermediate waters along the western Portuguese margin by examining the vertical distributions and property-property plots of chemical tracers (oxygen and nutrients). AAIW was traced by low oxygen and high nutrients, while SAIW was recognized by low nutrients. The Mediterranean Water (MW) undercurrent is shown to spread towards the eastern flank of Gorringe bank. Concurrently, the fresher waters gained salt by direct incorporation of MW, while this water was enriched in nutrients on its way northward and westward owing, to a great extent, to the entrainment of an AAIW branch. The distributions of nutrients and apparent oxygen utilization are discussed in terms of regional ocean circulation. Our analysis suggests a circulation pattern of the various intermediate waters along the western Portuguese margin: MW extends all over the area, but its presence is more pronounced around cape St. Vincent; SAIW apparently moves southward, reaching the Gorringe bank region, and AAIW flows northward along the coast and around the bank.  相似文献   
80.
Subsidence of organic soils in the Sacramento-San Joaquin Delta threatens sustainability of the California (USA) water supply system and agriculture. Land-surface elevation data were collected to assess present-day subsidence rates and evaluate rice as a land use for subsidence mitigation. To depict Delta-wide present-day rates of subsidence, the previously developed SUBCALC model was refined and calibrated using recent data for CO2 emissions and land-surface elevation changes measured at extensometers. Land-surface elevation change data were evaluated relative to indirect estimates of subsidence and accretion using carbon and nitrogen flux data for rice cultivation. Extensometer and leveling data demonstrate seasonal variations in land-surface elevations associated with groundwater-level fluctuations and inelastic subsidence rates of 0.5–0.8 cm yr–1. Calibration of the SUBCALC model indicated accuracy of ±0.10 cm yr–1 where depth to groundwater, soil organic matter content and temperature are known. Regional estimates of subsidence range from <0.3 to >1.8 cm yr–1. The primary uncertainty is the distribution of soil organic matter content which results in spatial averaging in the mapping of subsidence rates. Analysis of leveling and extensometer data in rice fields resulted in an estimated accretion rate of 0.02–0.8 cm yr–1. These values generally agreed with indirect estimates based on carbon fluxes and nitrogen mineralization, thus preliminarily demonstrating that rice will stop or greatly reduce subsidence. Areas below elevations of –2 m are candidate areas for implementation of mitigation measures such as rice because there is active subsidence occurring at rates greater than 0.4 cm yr–1.  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