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   28546篇
  免费   6190篇
  国内免费   7346篇
测绘学   2282篇
大气科学   6957篇
地球物理   4626篇
地质学   11614篇
海洋学   6017篇
天文学   5707篇
综合类   1941篇
自然地理   2938篇
  2024年   39篇
  2023年   347篇
  2022年   903篇
  2021年   1024篇
  2020年   1285篇
  2019年   1197篇
  2018年   1062篇
  2017年   1214篇
  2016年   1127篇
  2015年   1300篇
  2014年   1688篇
  2013年   1944篇
  2012年   1850篇
  2011年   1971篇
  2010年   1742篇
  2009年   2281篇
  2008年   2311篇
  2007年   2454篇
  2006年   2372篇
  2005年   2000篇
  2004年   1698篇
  2003年   1490篇
  2002年   1310篇
  2001年   1057篇
  2000年   1091篇
  1999年   1041篇
  1998年   837篇
  1997年   577篇
  1996年   495篇
  1995年   437篇
  1994年   387篇
  1993年   380篇
  1992年   268篇
  1991年   205篇
  1990年   116篇
  1989年   144篇
  1988年   93篇
  1987年   64篇
  1986年   62篇
  1985年   49篇
  1984年   36篇
  1983年   24篇
  1982年   32篇
  1981年   11篇
  1980年   19篇
  1979年   5篇
  1978年   8篇
  1977年   19篇
  1954年   7篇
  1877年   2篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
基于15 d的精密卫星钟差数据,从不同角度全面分析6种常用钟差预报模型(LP模型、QP模型、GM模型、SA模型、ARIMA模型、KF模型)基于钟差一次差分预报原理的预报效果,得到以下结论:1)采用钟差一次差分预报原理,可以提高LP模型、SA模型、GM模型及KF模型对于GPS卫星钟差的3 h预报精度,提高QP模型和ARIMA模型对于ⅡF Rb钟的3 h预报精度,提高LP模型和GM模型在6 h和12 h预报中的精度,提高ARIMA模型在6 h、12 h和24 h预报中的精度;2)基于钟差一次差分预报原理的预报结果与卫星及其星载钟类型有关,对于GPS BLOCK ⅡF Rb钟,该预报原理可以提高6种模型的短期预报精度,特别是对GM模型、LP模型和ARIMA模型预报效果的改善最为显著;3)对于3 h和6 h的预报,采用钟差一次差分预报原理的LP模型(DLP模型)对应的RMS值都最小,即DLP模型的预报精度最高,说明钟差一次差分数据更适合一次多项式模型的短期预报。  相似文献   
2.
利用地震前后的Landsat-8和Sentinel-2光学遥感影像数据,基于频率域互相关算法提取加利福尼亚州MW7.1及MW6.4地震的同震形变场,同时针对形变数据易受轨道误差、条带误差及时间失相干的影响,分别采用最小二乘多项式曲线拟合、改进均值相减等方法去除系统误差项。研究表明,MW7.1主震以右旋走滑为主,地表形变的断层迹线呈NNW走向,长度达55 km,最大滑移量约为2.82 m;MW6.4地震的发震断层迹线呈NE走向,长度达15 km,最大滑移量约为1.05 m,推测2次地震的发震断层分别为NNW向右旋走滑断裂及NE向左旋走滑断裂,二者形成典型的共轭关系。  相似文献   
3.
We report on a hexactinellid sponge new to science, Walteria demeterae sp. nov., which was collected from the northwestern Pacific seamounts at depths of 1271–1703 m. Its tubular and basiphytous body, extensive lateral processes, numerous oval lateral oscula which are irregularly situated in the body wall, the presence of microscleres with oxyoidal, discoidal and onychoidal outer ends, and the absence of anchorate discohexasters, indicate it belongs to the genus Walteria of family Euplectellidae, which is also supported by molecular phylogenetic evidence from 18S, 28S, 16S rRNA and cytochrome c oxidase subunit I (COI) gene sequences. The unique morphotype, which is structured by a thin and rigid framework of body wall and lateral processes consisting of diactins, characterizes it as a new species. Local aggregations of individuals of this new species coupled with their associated macrofauna in the Suda Seamount are reported, highlighting its functional significance in providing biogenic microhabitats in the deep sea.  相似文献   
4.
Zhejiang Province, located in the Yangtze River Delta region, is representative of China's economically developed areas. It enjoys superior natural conditions and a long history of agriculture, and is a comprehensive agricultural area with integrated development of agriculture, forestry, animal husbandry and fishery. It has nurtured the farming culture represented by Hemudu culture and Liangzhu culture, which have given rise to numerous precious Agricultural Heritage Systems. At present, Zhejiang Province has three of the world’s Globally Important Agricultural Heritage Systems (GIAHS) and 12 China Nationally Important Agricultural Heritage Systems (China-NIAHS), so it not only has the largest number of heritages in China, but it has also attained remarkable achievements in heritage conservation. Taking Zhejiang Province as an example in combination with the rural revitalization strategy, this paper summarizes the achievements in the protection of Important Agricultural Heritage Systems (IAHS) in Zhejiang Province during the past 15 years from the aspects of increasing farmers’ income, cultural Inheritance and industrial upgrading, as well as the conservation experiences in government promotion, community initiative, enterprise participation, technology driving and social linkage. Further, in view of the problems that exist in the current heritage protection,such as imperfect management of heritage sites, low participation of community residents, lack of special protection funds, and imperfect provincial management system,the following countermeasures and suggestions are put forward: (1) Improve the management mechanism for the conservation and development of Agricultural Heritage Systems; (2) Develop regional public branding of agricultural products in Agricultural Heritage System sites; (3) Increase the Agricultural Heritage System science education as well as cultural and creative product development; (4) Carry out the evaluation and recognition of IAHS at the provincial level; and (5) Provide substantial support and input to the conservation and utilization of IAHS. This study can provide some guidance for the conservation of IAHS in Zhejiang Province and it provides important reference for IAHS in the economically developed areas in China.  相似文献   
5.
基于2018年12月至2020年3月喀左、沈阳、辽阳、满洲里4个国家级地面气象站人工冻土器与测温式冻土自动观测仪观测的资料,对人工冻土观测获得的冻点与测温式冻土自动观测仪获得的相应深度的温度进行对比分析。结果表明:人工冻土器获取的冻点对应的土壤温度与0℃总体一致,又不完全重合;0—35 cm深度范围,冻点对应的温度变化范围为-2~6℃,呈现跳跃性变化。35 cm以下深度范围,冻土冻点对应的温度变化范围为-0.5~1.0℃;融化过程冻点对应的平均温度高于冻结过程冻点对应的平均温度。从完全融化时间上来看,人工冻土器观测到的完全融化时间晚于测温式冻土仪0℃线完全消失的时间。人工冻土观测的实质是获得土壤温度0℃点所在位置。灌注不同台站水的冻土器内管在相同的温度环境下,冻结与融化状态无明显区别;人工冻土器内管冻结过程是温度和持续时间双重作用的结果,深层土壤温度变化缓慢,使得内管中的水冻结和融化需要的时间长。另外,作为接触式测温设备,减小外因产生的时滞是提高其灵敏度的重要环节,建议测温式冻土仪的外管壁使用温度滞后效应更小的金属外管。  相似文献   
6.
Soil resources in parts of Tanzania are rapidly being depleted by increased rates of soil erosion and downstream sediment transport, threatening ecosystem health, water and livelihood security in the region. However, incomplete understanding to what effect the dynamics of soil erosion and sediment transport are responding to land-use changes and climatic variability are hindering the actions needed to future-proof Tanzanian land-use practices. Complementary environmental diagnostic tools were applied to reconstruct the rates and sources of sedimentation over time in three Tanzanian river systems that have experienced changing land use and climatic conditions. Detailed historical analysis of sediment deposits revealed drastic changes in sediment yield and source contributions. Quantitative sedimentation reconstruction using radionuclide dating showed a 20-fold increase in sediment yield over the past 120 years. The observed dramatic increase in sediment yield is most likely driven by increasing land-use pressures. Deforestation, cropland expansion and increasing grazing pressures resulted into accelerating rates of sheet erosion. A regime shift after years of progressive soil degradation and convergence of surface flows resulted into a highly incised landscape, where high amounts of eroded soil from throughout the catchment are rapidly transported downstream by strongly connected ephemeral drainage networks. By integrating complementary spatial and temporal evidence bases, this study demonstrated links between land-use change, increased soil erosion and downstream sedimentation. Such evidence can guide stakeholders and policy makers in the design of targeted management interventions to safeguard future soil health and water quality.  相似文献   
7.
Seamounts are vulnerable ecosystems in the deep sea and can be heavily impacted by human activities, such as bottom fishing and deep-sea mining. The species composition and distribution patterns of benthic fauna is key information for the designation of marine protected areas and environmental management plans. Three contracts for cobalt-rich crust exploration have been granted to China, Japan and Korea in the Northwest Pacific Ocean by the International Seabed Authority. However, our knowledge of benthic biodiversity in this area is extremely insufficient. During 2013–2020, eight Chinese Ocean Mineral Resources R&D Association (COMRA) cruises were conducted to investigate the benthic assemblages of nine seamounts in this region. In this study, 191 ophiuroids collected from seamounts in the Northwest Pacific were identified into 29 species in 11 families. Ophiacanthidae and Euryalidae were the two most dominant families with 12 and 6 species, respectively. Ophiotomidae and Ophiopyrgidae were represented by two species each, while seven families were represented by only one species. Four species were widely distributed among 4–5 seamounts, and 17 species were found only at a single site. An integrated regional taxonomic dataset of Ophiuroidea was generated and analyzed. A total of 23 and 14 species were obtained from the Magellan Seamount Chain (MSC) and the Marcus-Wake seamounts (MWS), respectively, with 8 species shared between the two seamount groups. The individual-based rarefaction curves did not reach an asymptote, suggesting that the sampling effort was inadequate for either the entire region or each single seamount. Most species distributed in a narrow depth range, and the species composition was different between water depths above and below 2000 m. Our results greatly improve the understanding of megafaunal biodiversity from seamounts in the Northwest Pacific Ocean, and highlight the necessity of further surveys to provide more robust information for environmental protection and management in this region.  相似文献   
8.
人类活动和自然因素共同但有区别的作用引起了长江口及邻近海域富营养化,造成夏季底层水体低氧现象加剧,成为近海生态健康恶化的重要征兆。本文梳理了国内外学者在该海域低氧研究中获得的重要认识,分析了底层水体溶解氧的潮周期尺度、事件尺度和年际尺度的变化特征,重点从层化与物质输运角度,介绍了长江冲淡水、台湾暖流、海洋锋面、风和潮等过程影响底层水体中氧气消耗或补充的机制,揭示了本海域主要低氧现象分别位于长江口和浙江近海的特征,对比了两处低氧区形成与演变的异同机制。目前,对低氧形成机制的定性认识和多尺度变化特征的了解已经有较好的基础,未来需要从多学科交叉角度加强现场试验和定量研究,掌握低氧的长期演变趋势,研发底层水体低氧的预测预警技术,支撑我国河口近海的生态预警监测工作。  相似文献   
9.
Engineering Nanoparticles(ENPs)’superior characteristics of adsorption depends on their dispersion in the medium.In this study,multi-walled carbon nanotubes(nonmetal),iron nanoparticles and silver nanoparticles(metallic simple substance),and Nano-TiO2,Nano-Fe2O3 and Nano-ZnO(metal oxide)were selected and respectively added into pure water and aqueous solution with 1%Sodium dodecyl benzene sulfonate(SDBS)surfactant.The dispersion effects were compared by leaving the solutions standing at room temperature under ultrasound.The results show that the dispersion of iron nanoparticles is the lowestamong the six ENPs,and that of multi-walled carbon nanotubes(MWCTS)is the highest.Adding anionic surfactants(SDBS)can obviously improve the dispersion performance of ENPs.The concentration of solution decreases by only 5%in 10 daysafter adding 1%SDBS for ultrasonic dispersion.  相似文献   
10.
为评估CWRF模式的降尺度能力和其热带气旋模拟对物理参数化方案的敏感性,本文利用ERI再分析资料驱动CWRF在30km网格上对1982-2016年中国近海热带气旋开展了一次集合模拟.结果表明:CWRF与ERI均能模拟出热带气旋的季节变化和年际变化形势且均存在低估,但相较ERI,CWRF的降尺度技术和集合模拟可以再现更多的热带气旋,显著减少低估.年际变化结果提升最为明显,它对积云方案最为敏感,其次是边界层,陆面和辐射方案,对云和微物理方案较弱.该研究为应用CWRF理解和预报热带气旋提供了参考.  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