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   5173篇
  免费   1014篇
  国内免费   1348篇
测绘学   191篇
大气科学   1239篇
地球物理   1576篇
地质学   2378篇
海洋学   627篇
天文学   352篇
综合类   542篇
自然地理   630篇
  2024年   19篇
  2023年   86篇
  2022年   206篇
  2021年   271篇
  2020年   173篇
  2019年   257篇
  2018年   283篇
  2017年   244篇
  2016年   293篇
  2015年   250篇
  2014年   331篇
  2013年   260篇
  2012年   293篇
  2011年   349篇
  2010年   333篇
  2009年   289篇
  2008年   275篇
  2007年   269篇
  2006年   239篇
  2005年   214篇
  2004年   201篇
  2003年   214篇
  2002年   179篇
  2001年   149篇
  2000年   188篇
  1999年   201篇
  1998年   197篇
  1997年   177篇
  1996年   157篇
  1995年   170篇
  1994年   137篇
  1993年   141篇
  1992年   102篇
  1991年   61篇
  1990年   51篇
  1989年   52篇
  1988年   47篇
  1987年   44篇
  1986年   28篇
  1985年   15篇
  1984年   11篇
  1983年   8篇
  1982年   10篇
  1981年   7篇
  1980年   6篇
  1977年   4篇
  1958年   4篇
  1937年   4篇
  1933年   9篇
  1924年   6篇
排序方式: 共有7535条查询结果,搜索用时 15 毫秒
61.
通过分析市县地震部门的地震应急组织机构、主要职能、关键流程,设计了地震应急工作助手的需求方案。并应用ASP.NET MVC、HTML5等开发技术,基于企业微信,开发了包括震情服务、灾情服务、应急服务、工作报告、资料查询、通知公告、通信录等7大功能的地震应急工作助手软件。该软件具有相对机动灵活、成本低、易于推广使用的优势。多次地震应急演练使用表明,软件能在较大程度上提高地震应急工作的规范化程度和处置效率。  相似文献   
62.
数字地图生产流程中质量的数据控制   总被引:1,自引:0,他引:1  
本文将着重讨论在ISO9001质量控制体系下、数字地图生产流程中影响地图质量的各类数据文件的设定与控制,从而进一步探讨在电子出版系统及CTP系统中,数字地图生产规范化、标准化、程序化作业的质量控制内容及方法。  相似文献   
63.
通过编制区域性1:300万“青藏高原板块构造图”和1:50万“柴达木北缘板块构造地质图”的体会,并结合国内学者对这两份图件的审查意见及建议,重点讨论有关区域性板块构造图编制的理论与方法,供区域地质工作者参考。  相似文献   
64.
Northeast China has been reported as having serious air pollution in China with increasing occurrences of severe haze episodes. Changchun City, as the center of Northeast China, has longstanding industry and is an important agricultural base. Additionally, Changchun City has a long winter requiring heating of buildings emitting pollution into the air. These factors contribute to the complexity of haze pollution in this area. In order to analyze the causes of heavy haze, surface air quality has been monitored from 2013 to 2015. By using satellite and meteorological data, atmospheric pollution status, spatio-temporal variations and formation have been analyzed. Results indicated that the air quality in 88.9% of days exceeding air quality index(AQI) level-1 standard(AQI 50) according to the National Ambient Air Quality Standard(NAAQS) of China. Conversely, 33.7% of the days showed a higher level with AQI 100. Extreme haze events(AQI 300) occurred frequently during agricultural harvesting period(from October 10 to November 10), intensive winter heating period(from Late-December to February) and period of spring windblown dust(April and May). Most daily concentrations of gaseous pollutants, i.e., NO_2(43.8 μg/m~3), CO(0.9 mg/m~3), SO_2(37.9 μg/m~3), and O_3(74.9 μg/m~3) were evaluated within level-1 concentration limits of NAAQS standards. However, particulate matter(PM_(2.5) and PM10) concentrations(67.3 μg/m~3 and 115.2 μg/m~3, respectively) were significantly higher than their level-1 limits. Severe haze in spring was caused by offsite transported dust and windblown surface soil. Heavy haze periods during fall and winter were mainly formed by intensive emissions of atmospheric pollutants and steady weather conditions(i.e., low wind speed and inversion layer). The overlay emissions of widespread straw burning and coal combustion for heating were the dominant factors contributing to haze in autumn, while intensive coal burning during the coldest time was the primary component of total emissions. In addition, general emissions including automobile exhaust, road and construction dust, residential and industrial activities, have significantly increased in recent years, making heavy haze a more frequent occurrence. Therefore, both improved technological strategies and optimized pollution management on a regional scale are necessary to minimize emissions in specified seasons in Changchun City, as well as comprehensive control measures in Northeast China.  相似文献   
65.
Subsurface flow processes in sloping cropland of purple soil   总被引:2,自引:1,他引:2  
Subsurface flow is a prominent runoff process in sloping lands of purple soil in the upper Yangtze River basin.However,it remains difficult to identify and quantify.In this study,in situ runoff experimental plots were used to measure soil moisture dynamics using an array of time domain reflectometry(TDR) together with overland flow and subsurface flow using isolated collecting troughs.Frequency of preferential flow during rainfall events and the controls of subsurface flow processes were investigated through combined analysis of soil properties,topography,rainfall intensity,initial wetness,and tillage.Results showed that subsurface flow was ubiquitous in purple soil profiles due to welldeveloped macropores,especially in surface soils while frequency of preferential flow occurrence was very low(only 2 cases in plot C) during all 22 rainfall events.Dry antecedent moisture conditions promoted the occurrence of preferential flow.However,consecutive real-time monitoring of soil moisture at different depths and various slope positions implied the possible occurrence of multiple subsurface lateral flows during intensive storms.Rainfall intensity,tillage operation,and soil properties were recognized as main controls of subsurface flow in the study area,which allows the optimization of management practices for alleviating adverse environmental effects of subsurface flow in the region.  相似文献   
66.
Monthly changes in sedimentation and sediment properties were studied for three different culture treatments: sea cucumber monoculture (Mc), sea cucumber and scallop polyculture (Ps-c) and scallop monoculture (Ms). Results indicated that the survival rate of sea cucumber was significantly higher in Ps-c cultures than in Mc cultures. Sea cucumber yield was 69.6% higher in Ps-c culture than in Mc culture. No significant differences in body weight and scallop shell length were found between Ps-c and Ms cultures. The mean sedimentation rate of total particulate matter (TPM) was 72.2 g/(m2 d) in Ps-c cultures, with a maximum of 119.7 g/(m2 d), which was markedly higher than that of Mc (mean value). Sedimentation rates of organic matter (OM), total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) in Ps-c cultures were also significantly higher than those in Mc cultures. TOC and TN contents of sediment increased rapidly in the first 5 months in Ms cultures and remained at a high level. TOC and TN contents in Mc and Ps-c cultures decreased during sea cucumber feeding seasons and increased during sea cucumber dormancy periods (summer and winter). The study demonstrates that co-culture of sea cucumber and scallop in earthen ponds is an alternative way to alleviate nutrient loads and improve water quality in coastal aquaculture systems. Moreover, it provides the additional benefit of an increased sea cucumber yield.  相似文献   
67.
A numerical study of flow around two tandem cylinders with unequal diameters was carried out. The upstream larger cylinder was fixed and the downstream smaller cylinder was allowed to oscillate in the transverse direction only. Comparisons of the experimental and numerical results were made to investigate the effects of the gap ratio on the maximum vibration amplitude and vortex shedding frequency. The results showed that the vibration response of the smaller cylinder was significantly affected by the presence of the upstream larger cylinder, and resulted in greatly reduced vibration amplitudes. With an increasing gap ratio, the vibration amplitude increased. However, the magnitude was lower than that corresponding to a single cylinder (with the same diameter as that of the downstream smaller cylinder) under the same flow conditions.  相似文献   
68.
The wave crest is an important factor for the design of both fixed and floating marine structures. Wave crest height is a dominant parameter in assessing the likelihood of wave-in-deck impact and resultant severe damage. Many empirical and theoretical distribution functions for wave crest heights have been proposed, but there is a lack of agreement between them. It is of significance to develop a better new nonlinear wave crest height distribution model. The progress in the research of wave crest heights is reviewed in this paper. Based on Stokes’ wave theory, an approximate nonlinear wave crest-height distribution formula with simple parameters is derived. Two sets of measured data are presented and compared with various theoretical distributions of wave crests obtained from nonlinear wave models and analysis of the comparison is given in detail. The new crest-height distribution model agrees well with observations. Also, the new theoretical distribution is more accurate than the other methods cited in this paper and has a greater range of applications.  相似文献   
69.
????????????﹫????????????ζ??????????????????????????????????????????????λ????????μ?????????衢????????????????????裬??????????????????????????????????????????????????????????????????????????????????????????????????????侫??????ó???????????????????????ε????????????????????????????????????????????淶????????????????????????????????????????  相似文献   
70.
??TOPEX/Poseidon????????????Χ???????ν??з??????ó?????:?????????????Χ???????????ε?????;????????????????????ζ??????????,?????????????????????????????????????????????  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