全文获取类型
收费全文 | 2313篇 |
免费 | 374篇 |
国内免费 | 397篇 |
专业分类
测绘学 | 175篇 |
大气科学 | 280篇 |
地球物理 | 439篇 |
地质学 | 1136篇 |
海洋学 | 286篇 |
天文学 | 34篇 |
综合类 | 128篇 |
自然地理 | 606篇 |
出版年
2024年 | 7篇 |
2023年 | 22篇 |
2022年 | 83篇 |
2021年 | 82篇 |
2020年 | 96篇 |
2019年 | 104篇 |
2018年 | 88篇 |
2017年 | 106篇 |
2016年 | 108篇 |
2015年 | 93篇 |
2014年 | 131篇 |
2013年 | 157篇 |
2012年 | 109篇 |
2011年 | 149篇 |
2010年 | 132篇 |
2009年 | 149篇 |
2008年 | 120篇 |
2007年 | 133篇 |
2006年 | 170篇 |
2005年 | 120篇 |
2004年 | 108篇 |
2003年 | 132篇 |
2002年 | 89篇 |
2001年 | 79篇 |
2000年 | 67篇 |
1999年 | 77篇 |
1998年 | 50篇 |
1997年 | 37篇 |
1996年 | 36篇 |
1995年 | 40篇 |
1994年 | 29篇 |
1993年 | 22篇 |
1992年 | 22篇 |
1991年 | 6篇 |
1990年 | 14篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 15篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 4篇 |
排序方式: 共有3084条查询结果,搜索用时 31 毫秒
951.
综合的可持续评价是对地区经济发展的准确衡量,也是科学指导地区可持续发展的重要凭借。构建了一个基于IPAT等式改进的恒等式评价模型——IPADSEFÍÁ等式,以水资源在农业种植业方面的利用现状作为环境影响因素,综合考虑人口规模、经济增长、农业依赖度、农业产业结构、生产效益和先进农业技术应用等人文因素,结合1990-2014年的地区发展历程,对张掖的发展进程进行了分解和归因,并对发展的可持续性进行评价,得出张掖当前的发展模式对自然环境存在较强的压迫,尤其在水资源利用方面不是可持续的。而进行合理的产业结构调整、发展高附加值农业产业、继续加大应用先进农业技术可以缓解目前的水资源压力,让张掖的发展更具可持续性。 相似文献
952.
中国玛珥湖及其研究意义 总被引:4,自引:2,他引:2
随着全球变化研究的深入,科学家们认识到古气候变化除轨道尺度的冰期/间冰期旋回之外还存在年代际-千年尺度的高频变化和突变事件。要认知这些变化,古气候记录的时间分辨率要达到"年-年代际"。因此,寻求时间跨度长、连续性好、信息丰富的高分辨率记录是过去全球变化研究面临的主要任务之一。玛珥湖为火山射汽喷发形成的封闭湖泊,由于其独特的形成机制及水文背景,使其能够提供数万年乃至几十万年连续稳定的沉积记录,是高分辨率古气候、古环境变化研究的重要对象。中国玛珥湖得天独厚,从热带到寒温带均有分布,为系统研究中国不同气候区各种时间尺度古气候变化规律提供了理想材料。本文基于中国玛珥湖的分布及沉积特征,探讨玛珥湖沉积记录能够为古全球变化研究做出怎样的贡献,解决什么科学问题,具体包括以下三个方面的内容:(1)在轨道尺度上,精确定位现代气候在地质历史中的位置是未来最具挑战性的科学问题之一。我国热带地区玛珥湖沉积物跨越了至少4个冰期-间冰期旋回,能够为理解冰期驱动机制、下一次冰期来临、高低纬关联等关键科学问题提供多学科数据;(2)千年尺度古气候变化,其成因可能源于不同的动力学机制,是气候系统对各种外部和内部驱动因子响应的结果,因此千年震荡可能存在明显的时空差异。集成不同气候带的玛珥湖沉积记录将为理解千年震荡规律及其驱动因子做出贡献;(3)年-年代际气候变化是预测未来气候变化的基础,PAGES、IPCC以及PAGES-Asia 2K均将过去2千年来气候变化作为预测未来几十年至百年尺度上重大全球变化的背景,并为此构建全球数据体系,玛珥湖沉积特别是纹层沉积记录能够填补某些地区高分辨率数据的空白。 相似文献
953.
印支造山带对华南地质演化具有重要影响,南盘江盆地作为华南与印支造山带相关的最大的盆地,其构造演化过程备受关注,然而对于盆地由伸展到挤压的构造转换时限还存在争议。西林县位于南盘江盆地的中部,古生界和中-下三叠统出露良好,是进行沉积大地构造学研究的理想地区。为进一步明确南盘江盆地的构造演化过程,本文选取南盘江盆地中部西林县一带中-下三叠统进行了详细的沉积相分布和演化、古水流以及碎屑组成分析。结果表明,空间上,下三叠统自下而上自北向南依次出露:(1)泥岩、泥灰岩、砂屑灰岩和凝灰岩构成的潮坪-泻湖相沉积;(2)砾岩、含砾砂岩、细砂岩和粉砂岩构成的块体搬运沉积(MTD,Mass-transport deposit)与浊积扇相沉积互层;(3)中粗砂岩、粉砂岩、泥岩组成的浊积扇相沉积,构成向上水体急剧加深的沉积序列。中三叠统为一套浊积扇相沉积,由粗砂岩、细砂岩、粉砂岩和泥岩组成。古水流和碎屑组成分析结果显示,西林县一带下三叠统碎屑物主要来自北侧西林-隆林一带的碳酸盐岩台地,而中三叠统碎屑除来自北侧相邻碳酸盐岩台地外,还可能来自江南造山带、康滇古陆、云开地块和峨眉山玄武岩。沉积相的垂向变化记录了西林一带由古生代孤立碳酸盐岩台地向三叠纪半深海浊积岩盆地的演化过程,而早三叠世MTD是盆地张开过程中的直接沉积记录,以上地质事实暗示了南盘江盆地在早三叠世受控于区域伸展体制,西林一带的古生代孤立碳酸盐岩台地则是伸展过程中形成的断隆。盆地内的基性岩墙侵位(258~248Ma)以及西林一带早三叠世晶屑沉凝灰岩的锆石U-Pb年代学分析结果(249.4±1.2Ma)表明南盘江盆地在晚二叠世至早三叠世处于伸展构造背景之下,中三叠统为半深海浊积岩沉积则表明这一伸展过程至少持续至中三叠世。由于在时间上和空间上,南盘江盆地的张开都与古特提斯分支洋盆的俯冲消减有关,因而我们认为南盘江盆地也是晚二叠世到中三叠世古特提斯洋岩石圈俯冲体系的一部分。由于南盘江盆地及其邻近区缺失上三叠统沉积,这可能说明古特提斯洋俯冲结束于中三叠世。 相似文献
954.
美国内布拉斯加州林肯市犯罪行为的聚类及热点分布分析 总被引:1,自引:0,他引:1
抢劫和入室盗窃犯罪行为是日常生活中最长见的违法行为,而这两种违法行为的发生有一定的规律可循。例如抢劫行为主要发生在市中心、CBD和商业中心的人群、商铺密集区域,而入室盗窃则多发生于生活社区及其周边地带。然而,想要防范这两种犯罪行为需要根据实际情况进行分析和比对,结合分析结果提出准确合理的警力部署。美国内布拉斯加州林肯市警察局针对该市抢劫和入室盗窃犯罪行为高发的情况,需要针对具体分析结果对于警力进行部署,主要针对徒步巡逻和是否需要对住户进行入户警示进行分析,根据分析结果进行具体部署。本文中使用的分析方法为紧邻层空间聚类分析(NNH)和针对犯罪的空间和时间分析法(SPAC),分别对林肯市入室行窃和抢劫的数据进行分析并尝试将两种方法进行对比以便给出更加科学的分析报告。 相似文献
955.
张然和 《测绘与空间地理信息》2015,(3):163-165
在矿业权设置方案中,矿业权现状图及矿业权设置平面图分别是一定区域内矿业权现状分布和矿业权设置方案的直观体现。本文详细介绍MAPGIS二次开发软件Section和Microsoft Office Excel软件在编辑制作该类图件的交互应用和体会。 相似文献
956.
Trine Agervig Carstensen Anton Stahl Olafsson Nynne Marie Bech Thea Schmidt Poulsen Chunli Zhao 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2015,115(2):142-156
Cycling plays an important role in low-carbon transitions. Around the globe, cities are constructing bicycle infrastructure. The city of Copenhagen has a bicycle-friendly infrastructure celebrated for its fine-meshed network. This study documents the spatio-temporal development of Copenhagen’s bicycle infrastructure and explores how the development corresponds to other processes of urban transformation. The study builds on historical maps of bicycle infrastructure that are digitised into geographical information, which allows for a comprehensive analysis of the formation of the network. In search for identifying drivers, the study analyses the city’s spatial growth pattern, migration pattern, development of road network and changes in the transport culture. Analyses reveal that the bicycle infrastructure expanded at a relatively constant pace during distinct periods of urban transformation, including periods when the city suffered from spatial, economic and demographic decline, and dominance of car traffic. By discussing reasons and demands for constructing bicycle infrastructure, the study identifies four distinct periods in which bicycle infrastructure was constructed to enhance comfort and safety (first cycling city); the flow for cars (car city); urban liveability for soft transport (liveable city); and, finally, to improve the flow for cyclists as part a strategic re-design of urban space (liveable cycling city). 相似文献
957.
Recent decades have seen the rural areas of developing and emerging countries undergo significant structural changes. They are the source of several pertinent international concerns, including extreme poverty and hunger, and rising spatial and interpersonal disparities, challenges that national governments and the international community have made limited headway in alleviating to date. By analysing the range of rural development approaches implemented in recent decades, we develop a picture in which territorial approaches have become more mainstream. Since the turn of the century in particular they have gradually supplanted more traditional place-neutral approaches, which, we argue, have served to increase rural-urban disparities and exasperate the incidence of poverty in rural areas. Rural territorial development approaches, where able to mobilise sufficient participation and coordination between local stakeholders, civil society, and various multi-level actors, offer the most favourable means of gaining a better understanding of the many social, economic, institutional assets within a region. They can be harnessed to drive brands of regional development that are not only sustainable, but also more equitable and inclusive across different segments of the population and territories. 相似文献
958.
Informed by a growing literature on space-time patterns of repeat and near repeat burglary victimization, a crime location choice model was used to test whether burglars are attracted to areas they previously targeted. Using data in 3337 detected burglaries from one UK police force, and accounting for the distance to the offender's residence, and for other factors that make target areas attractive to burglars, it was demonstrated that burglars were more likely to commit a burglary in an area they had targeted before. This was particularly the case if the prior burglary was (very) recent. Areas near to those in which burglaries had been committed were also more likely to be selected. 相似文献
959.
《Geoforum》2015
The purpose of this article is to analyze the advances of, and obstacles to, progressive attempts at modifying the balance of power over water through the use of the law as a tool for social transformation and to characterize the alternatives based on the ways in which they challenge neoliberal principles. The study of such dynamics using a broad perspective on the law which includes not only written laws, but also historical moments in which water laws were created and used, provides a platform to discern alternatives that in many cases might otherwise remain invisible.We critically analyze the major legal contradictions and tensions in Bolivia, Colombia and Peru that suggest post- and counter-neoliberal alternatives. First, certain alternatives indicate a break from neoliberal principles based on relational ontologies (that avoid divisions between nature and culture as well as between individuals and communities). Such breaks include the statement that water is a common good or a common resource for the benefit of all, the goal to re-establish the human connection with nature, and support for a water provision system based on reciprocity, community and cooperative principles. Second, additional proposals that can be characterized as counter-neoliberal oppose neoliberal means or outcomes, such as proposals for more equitable water allocation or inclusive participation mechanisms for decision making. Both counter- and post-neoliberal alternatives also find much resistance in water laws; however, they do represent progressive alternatives that reach state laws and challenge neoliberal conceptions of nature. 相似文献
960.
Recurrent spring‐fed rivers in a Middle to Late Pleistocene semi‐arid grassland: Implications for environments of early humans in the Lake Victoria Basin,Kenya 下载免费PDF全文
Emily J. Beverly Steven G. Driese Daniel J. Peppe Cara R. Johnson Lauren A. Michel J. Tyler Faith Christian A. Tryon Warren D. Sharp 《Sedimentology》2015,62(6):1611-1635
The effect of changing palaeoclimate and palaeoenvironment on human evolution during the Pleistocene is debated, but hampered by few East African records directly associated with archaeological sites prior to the Last Glacial Maximum. Middle to Late Pleistocene deposits on the shoreline of eastern Lake Victoria preserve abundant vertebrate fossils and Middle Stone Age arte‐facts associated with riverine tufas at the base of the deposits, which are ideal for palaeoenvironmental reconstructions. New data from tufas identified on Rusinga Island and on the mainland near Karungu, Kenya are provided from outcrop, thin sections, mineralogical, stable isotopic and U‐series dating analyses. Tufa is identified in four sites: Nyamita (94·0 ± 3·3 and 111·4 ± 4·2 ka); Kisaaka, Aringo (455 ± 45 ka); and Obware. The age ranges of these tufa deposits demonstrate that spring‐fed rivers were a recurrent, variably preserved feature on the Pleistocene landscape for ca 360 kyr. Poor sorting of clastic facies from all sites indicates flashy, ephemeral discharge, but these facies are commonly associated with barrage tufas, paludal environments with δ13C values of ca 10‰ indicative of C3 plants and fossil Hippopotamus, all of which indicate a perennial water source. Other tufa deposits from Nyamita, Obware and Aringo have a mixed C3/C4 signature consistent with a semi‐arid C4 grassland surrounding these spring‐fed rivers. The δ18O values of tufa from Nyamita are on average ca 1‰ more negative than calcite precipitated from modern rainfall in the region, suggesting greater contribution of depleted monsoonal input, similar to the Last Glacial Maximum. Microdebitage and surface‐collected artefacts indicate that early modern humans were utilizing these spring‐fed rivers. The presence of spring?fed rivers would have afforded animals a reliable water source, sustaining a diverse plant and animal community in an otherwise arid environment. 相似文献