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   1968篇
  免费   337篇
  国内免费   421篇
测绘学   132篇
大气科学   400篇
地球物理   512篇
地质学   960篇
海洋学   195篇
天文学   52篇
综合类   198篇
自然地理   277篇
  2024年   10篇
  2023年   34篇
  2022年   96篇
  2021年   108篇
  2020年   100篇
  2019年   119篇
  2018年   109篇
  2017年   104篇
  2016年   114篇
  2015年   110篇
  2014年   123篇
  2013年   129篇
  2012年   118篇
  2011年   118篇
  2010年   124篇
  2009年   108篇
  2008年   91篇
  2007年   72篇
  2006年   77篇
  2005年   56篇
  2004年   51篇
  2003年   70篇
  2002年   99篇
  2001年   75篇
  2000年   81篇
  1999年   68篇
  1998年   53篇
  1997年   43篇
  1996年   49篇
  1995年   47篇
  1994年   40篇
  1993年   31篇
  1992年   22篇
  1991年   18篇
  1990年   11篇
  1989年   13篇
  1988年   14篇
  1987年   9篇
  1986年   3篇
  1984年   3篇
  1983年   1篇
  1981年   3篇
  1978年   1篇
  1958年   1篇
排序方式: 共有2726条查询结果,搜索用时 15 毫秒
571.
湘中下石炭统测水组沉积层序及幕式聚煤作用   总被引:11,自引:0,他引:11  
下石炭统测水组是我国南方主要含煤地层之一,煤系及煤层在湘中地区发育最佳,该区测水组形成于碎屑滨岸障壁-泻湖沉积体系中。文中对测水组沉积层序及其所反映的海平面变化历史进行了分析,提出测水组上段底部砂岩是海侵背景下形成的障壁砂坝或障璧岛,其底部侵蚀面是海侵冲刷面(ravinement surface)。同时提出测水组下段三、五煤层的形成与受海平面变化控制的幕式聚煤作用(episodie coal accumulation)有关,后者系指海平面下降阶段横跨不同相区的区域性聚煤作用。  相似文献   
572.
In this paper, on the basis of intensity data from 85 earthquakes occurred in China, the method of resolving overdetermined equations by using the damped least squares method is applied to inversing for the mean value of 1 s. S wave quality factor, the spreading factor and the S-wave acceleration at the foci of earthquakes in 7 regions of China. The relation between S-wave acceleration at the foci of earthquakes and magnitude is discussed. As an example, 2-DQ S distribution in Taiwan Province is obtained by inversion. It is found that there exists some corresponding relation between this distribution and Bouguer gravitational anomaly and seismicity. Preliminary analysis and discussion on results of the inversion, and estimation of error inQ S are made.  相似文献   
573.
风暴沉积(storm deposits)是由风暴浪引起的一种密度流沉积,主要发生在具开阔海面的浅海陆棚区。风暴岩(tempestite)一词是由凯林(G.Kelling,1973)提出,用以描述受风暴扰动后再沉积的浅海盆地的递变浊积岩。库马尔和桑德(N.Kumar和J.E.Sander,1978)以及艾格内尔(T.Aigner,1979)将其含义扩充为风暴沉积,泛指非正常天气风暴作用所形成的一系列沉积。近十余年来,世界上许多地方都有风暴沉积发现,古代地层记录中的有  相似文献   
574.
This study examines the technical efficiency (TE) differences among typical cropping systems of smallholder farmers in the purple-soiled hilly region of southwestern China. Household-, plot-, and crop- level data and community surveys were conducted to explore TE levels and determinants of typical cropping systems by using a translog stochastic frontier production function. Results indicate significant difference in TE and its determinants among cropping systems. The mean TEs of the rice cropping system (R), the rice-rape cropping system (RR), the rice-rape-potato cropping system (RRP), and the oil cropping system (O) are 0.86, 0.90, 0.84, and 0.85, respectively, which are over 1.17 times higher than those of the maize-sweet potato-other crop cropping system (MSO) and the maize-sweet potato-wheat cropping system (MSW) at 0.78 and 0.69, respectively. Moreover, Technical inefficiency (TIE) of different cropping systems is significantly affected by characteristics of the household as well as plot. However, the impact of land quality, mechanical cultivation conditions, crop structure, farming system, farm radius, household type, cultivated land area per capita, and annual household income per capital on TIE vary by cropping system. Additionally, output elasticity of land, labor, and capital, as a group, is greater than the one of agricultural machinery and irrigation. Finally, when household-owned effective agricultural labor is at full farming capacity, optimal plot sizes for the R, RR, RRP, MSO, MSW, and O cropping systems are 1.12 hm2, 0.35 hm2, 0.25 hm2, 2.82 hm2, 1.87 hm2, and 1.17 hm2, respectively.  相似文献   
575.
576.
黄土高原土壤干层研究进展与展望   总被引:23,自引:0,他引:23  
土壤干层发生机理及其生态环境效应是土壤物理学和生态学及其交叉领域的重要科学问题,也是我国黄土高原开展生态文明建设,实现可持续发展面临的重大生态环境问题之一。通过回顾土壤干层研究取得的重要进展,包括土壤干层的概念与特征、评价指标、形成原因与危害、不同尺度时空分异特征及调控对策等,针对黄土高原水资源短缺和植被退化现状,提出土壤干层研究面临的机遇和挑战,包括建立统一的土壤干层动力学量化指标、干层内土壤水分对植物生长的有效性、土壤干层强度对土壤—植被—大气传输体(SVATs)中水分运移的影响、土壤干层发生发展过程和时空演变规律、土壤干层预测模型及其调控机制。此外,土壤干层的研究应结合当前植被恢复与生态文明建设的国家需求。在气候变化和剧烈人类活动背景下,开展黄土高原土壤干燥化与土壤水资源影响及其调控对策研究,将是水科学与生态学领域长期面临的科学问题之一。  相似文献   
577.
The length-weight relationship and allometric growth patterns of hatchery-reared Chinese loach, Paramisgurnus dabryanus (Dabry de Thiersant, 1872), were determined from hatching to 60 days after hatching. A best power model was estimated for the length-weight relationship during the early life stages. Positive allometric growth for the head segment, trunk length, tail length and eye diameter was also found in the early life stages, while body depth, tail depth, tail fin length, pectoral fin length and barbel length displayed a negative coefficient. During the subsequent early developmental stage, the growth coefficients showed a clear and common tendency towards isometry for all measured body ratios. The allometric growth changes in Chinese loach during the early stage are possibly the result of selective organogenesis directed towards survival priorities.  相似文献   
578.
A cultured female half-smooth tongue sole (Cynoglossus semilaevis) was crossed with a wild male, yielding the first filial generation of pseudo-testcrossing from which 200 fish were randomly selected to locate the Vibrio anguillarum resistance trait in half-smooth tongue sole at its microsatellite linkage map. In total, 129 microsatellites were arrayed into 18 linkage groups, ≥4 each. The map reconstructed was 852.85 cM in length with an average spacing of 7.68 cM, covering 72.07% of that expected (1 183.35 cM). The V. anguillarum resistance trait was a composite rather than a unit trait, which was tentatively partitioned into Survival time in Hours After V. anguillarum Infection (SHAVI) and Immunity of V. Anguillarum Infection (IVAI). Above a logarithm of the odds (LOD) threshold of 2.5, 18 loci relative to SHAVI and 3 relative to IVAI were identified. The 3 loci relative to IVAI explained 18.78%, 5.87% and 6.50% of the total phenotypic variation in immunity. The microsatellites bounding the 3 quantitative trait loci (QTLs) of IVAI may in future aid to the selection of V. anguillarum-immune half-smooth tongue sole varieties, and facilitate cloning the gene(s) controlling such immunity.  相似文献   
579.
月池内流体存在活塞和晃荡两类振荡现象。基于线性势流理论,推导了波浪斜向入射下,直墙前矩形月池辐射和绕射问题的解析解。通过分离变量法和特征函数展开法求解了速度势函数,根据边界条件来确定速度势函数中的未知系数,由速度势函数计算斜向波与矩形月池相互作用的水动力系数和波浪激励力,对它们的变化规律进行了分析讨论,研究了底部开口大小、波浪入射角度对矩形月池水动力特性的影响以及直墙远近对波浪力的影响。结果表明,月池底部开口大小对流体水平作用的影响较小,而对流体垂直作用的影响较大;波浪入射角度的变化对矩形月池横荡和横摇运动时的水动力特性有一定的影响;在一定条件下,直墙的存在会使得月池在水平方向所受到的波浪力比开敞水域中的要大。  相似文献   
580.
Many ecosystems extend across national or political boundaries. The consistent and effective protection of these ecosystems in transboundary areas(ETAs) is an important global research focus. Previous research on the protection of such areas can be categorized into seven themes: 1) ecological conservation of a single ETA; 2) investigation of the effects of a single conservation measure on a specific ETA; 3) determination of species-level effects due to ETA conservation; 4) comparison of the same protection measures between different ETAs; 5) introduction of a single conservation measure to a specific ETA; 6) understanding the relationship between conservation and sustainable development; and 7) generalization across multiple ETA conservation cases. The protection of ETAs involves various considerations, including funding support, demand and will for collaboration, community and public participation, historical and cultural factors, political backgrounds, uniqueness of biological resources, formulation of laws and regulations, founding of specialized administrative departments, non-governmental organizations, and fairness. Here, we briefly explain the research themes and considerations related to ETA conservation. The most important finding is that most major research themes do not focus on the challenges of ETA conservation. We use two nature reserves located between China and North Korea as examples to identify specific ways to improve ETA conservation on Changbai Mountains. The efficiency of ETA administration still remains low. The study of ETA conservation should focus on concrete regional information and aim to improve existing measures through the accumulation of experience.  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