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   3607篇
  免费   2113篇
  国内免费   54篇
测绘学   97篇
大气科学   117篇
地球物理   2341篇
地质学   1321篇
海洋学   383篇
天文学   20篇
综合类   157篇
自然地理   1338篇
  2024年   9篇
  2023年   99篇
  2022年   174篇
  2021年   182篇
  2020年   178篇
  2019年   158篇
  2018年   173篇
  2017年   176篇
  2016年   191篇
  2015年   180篇
  2014年   243篇
  2013年   277篇
  2012年   239篇
  2011年   251篇
  2010年   194篇
  2009年   278篇
  2008年   259篇
  2007年   314篇
  2006年   275篇
  2005年   214篇
  2004年   212篇
  2003年   184篇
  2002年   161篇
  2001年   110篇
  2000年   124篇
  1999年   117篇
  1998年   164篇
  1997年   110篇
  1996年   103篇
  1995年   87篇
  1994年   88篇
  1993年   82篇
  1992年   66篇
  1991年   23篇
  1990年   25篇
  1989年   14篇
  1988年   12篇
  1987年   5篇
  1986年   7篇
  1985年   6篇
  1984年   6篇
  1983年   2篇
  1979年   1篇
  1977年   1篇
排序方式: 共有5774条查询结果,搜索用时 31 毫秒
1.
东太湖不同水域浮游生物周年变化及鲢、鳙食性的研究   总被引:1,自引:0,他引:1  
2012 年2~11 月对东太湖不同水域浮游生物组成、数量、水质情况及水生植物密度进行了调查, 并对9 月份太湖鲢、鳙的食性进行了研究。结果表明: 浮游动物和浮游植物多样性指数均为保护区>余山>老鼠山>漫山岛。水体浮游生物数量和综合营养状态指数均为漫山岛>老鼠山>余山>保护区。浮游植物总数量与蓝藻总数量、浮游动物总数量、富营养化因子呈正相关, 与水生植物密度呈负相关。9 月份太湖鲢、鳙的肠内含物中浮游植物和浮游动物分别占食物数量的89.1%、70.6%和10.9%、29.4%,其中蓝藻分别占74.5%、56%, 枝角类和轮虫分别占3.7%、13.4%和1.5%、11.5%; 鲢、鳙对蓝藻的选择性指数分别为1.42 和1.07, 对轮虫和原生动物的选择性指数分别为0.13、1.27 和1.30、0.40。本研究结果对揭示影响太湖浮游生物周年变化的环境因素以及通过鲢、鳙控制蓝藻具有一定的理论价值和指导意义。  相似文献   
2.
青海盐湖锂资源综合利用规模探讨   总被引:1,自引:1,他引:0       下载免费PDF全文
锂是战略新兴产业的“关键原料”,已入国家战略新兴产业矿产。当前和可预见的未来,新能源、人工智能、航空航天等都离不开锂原料。有机构预测全球锂需求量将从2017年24万吨增加到2025年的83万吨LCE(碳酸锂当量)。青海柴达木盆地是我国锂资源富集地,为探讨青海盐湖锂资源的合理开发规模,分析了锂盐开发的矿床特性、钾肥规模、开采方式、盐田精制浓缩工艺等影响因素,以察尔汗盐湖为例计算其开发规模,提出“先锂后钾”重点研发低含量卤水提锂技术、全流程工艺优化、构建锂资源产业集群等建议,将有利于科学规划青海盐湖锂资源开发,促进我国盐湖锂产业可持续发展。  相似文献   
3.
柴达木盆地盐尘暴及其资源生态环境影响   总被引:1,自引:1,他引:0       下载免费PDF全文
在干旱和半干旱地区,盐尘暴是一种常见的极端气象灾害。柴达木盆地频发的沙尘暴实质上就是盐尘暴。柴达木盆地盐尘暴已经对区域大气、土壤、植被、人类健康乃至全球气候变化都产生了不同程度的影响。由于柴达木盆地盐尘暴中可溶盐成分来源复杂,尽管目前对其形成过程和机制还缺乏深刻理解,但人类的资源开发活动对盐尘暴显然具有促进作用。有必要从系统的角度来考虑柴达木盆地盐尘暴对区域乃至全球的影响,这对于区域生态环境的正确评估和后期恢复治理具有重要意义。  相似文献   
4.
西藏自治区扎仓茶卡盐湖是我国重要的超大型固体硼镁矿产地,经多年开采,高品位硼矿资源(B_2O_3≥28%)已采掘殆尽,但矿区内仍堆有大量低品位矿石,这不仅是对环境的威胁,也是对资源的浪费。在采集样品的基础上,开展了基于X射线衍射与扫描电镜的矿物组成的鉴定和化学组分分析,为低品位矿石的综合利用提供理论依据。研究结果表明,扎仓茶卡盐湖低品位硼矿的矿物种类有库水硼镁石、柱硼镁石、石英、钠长石、微斜长石、方解石、白云母、文石、水菱镁矿、石膏和锰热臭石共11种,其中仅有库水硼镁石和柱硼镁石两种硼矿物,平均含量分别为21.14%与8.99%,且硼矿样品中B_2O_3平均含量高达13.50%,有巨大的经济利用价值。考虑到扎仓茶卡有着丰富的卤水资源,且低品位硼矿主要为库水硼镁石,可利用卤水及矿物的密度差来进行重选,该库水硼镁石为易选型。这种方法不仅能实现矿区内低品位矿的回收利用,而且生产成本低廉、绿色环保,将进一步促进区域经济发展。  相似文献   
5.
三峡建库后东洞庭湖适宜生态水位需求分析   总被引:1,自引:0,他引:1  
三峡水库的修建改变了水库下游的水沙条件,影响了洞庭湖湖区的生态平衡,进而引发相关生态问题本文以城陵矶站水位代表东洞庭湖水位,基于其1953 2018年的逐日水位资料,采用滑动t检验法对年平均水位序列进行突变检验,发现因强人类活动导致城陵矶水位发生突变的时间为2004年,考虑为三峡蓄水的影响借鉴IHA(Indicators of Hydrological Alteration,水文变化指标)及RVA(Range of Variability Approach,变化范围法)方法提出了一种同时考虑年内月平均水位过程、水位波动范围、高低水位发生情况以及水位涨落情况的适宜生态水位计算指标体系,能够直观和全面地描述生态系统健康发展对水位的要求,包括1 12月水位分别为:17.07~18.34、17.15~18.89、17.65~22.23、20.25~22.15、22.85~24.90、24.31~26.44、26.88~29.16、25.79~28.32、25.12~27.56、23.59~25.88、20.65~22.81、18.58~19.88 m;年最低水位:16.21~17.86 m,发生时间为第16~51天(年积日);年最高水位:28.54~31.48 m,发生时间为第187~211天(年积日);高水位平均持续时间为32.62~81.32 d/次,低水位平均持续时间为52.13~107.65 d/次;涨水次数为21.9~26.45次,涨水速率为0.17~0.21 m/d;落水次数为23.17~27.6次,落水速率为0.12~0.14 m/d基于上述结果分析三峡建库后城陵矶水位发现,其在1、2月月平均水位分别较适宜生态水位需求高0.83、0.27 m; 10月月平均水位较需求低0.83 m;年最低水位高出需求0.39 m,发生时间先于需求6天;涨水次数高于阈值要求4次,涨水速率低于阈值要求0.01 m/d;落水次数高于阈值要求2次研究成果可为三峡及上游梯级水库群联合调度提供依据.  相似文献   
6.
湖泊水情遥感研究进展   总被引:1,自引:0,他引:1  
宋春桥  詹鹏飞  马荣华 《湖泊科学》2020,32(5):1406-1420
湖泊作为最直接的淡水资源之一,在人类的生产、生活各方面都占据至关重要的地位.受到全球气候变化与人类活动的影响,湖泊正在发生急剧变化,因而有必要对其进行快速、准确的时空变化监测,从而为水资源管理与保护、未来气候变化预警提供依据.遥感技术的产生与发展为大范围、实时动态的湖泊变化监测提供了难得的契机,它克服了人类对湖泊实地考察的局限性.本文对现有国内外湖泊水情遥感监测技术与方法进行了综合梳理,主要综述了国内外在湖泊水域范围提取、湖泊水位提取、湖泊水量估算、流域水文过程等方面的遥感研究进展情况,重点总结了该领域近年来提出的新方法和新技术.最后,结合当前遥感技术的发展,对未来遥感在湖泊动态变化监测中的应用潜力和趋势进行了简要论述,并对多源遥感数据融合与云计算平台的结合在地表水体连续变化监测中的应用进行了展望.  相似文献   
7.
The objective of this research study is to assess the capability of time-series of MODIS imagery to provide information suitable for enhancing the understanding of the temporal cycles shown by the abnormal growth of the floating macrophytes in order to support monitoring and management action of Lake Victoria water resources.The proliferation of invasive plants and aquatic weeds is of growing concern. Starting from 1989, Lake Victoria has been interested by the high infestation of water hyacinth with significant socio-economic impact on riparian populations.In this paper, we describe an approach based on the time-series of MODIS to derive the temporal behaviour, the abundance and distribution of the floating macrophytes in the Winam Gulf (Kenyan portion of the Lake Victoria) and its possible links to the concentrations of the main water constituencies.To this end, we consider the NDVI values computed from the MODIS imagery time-series from 2000 to 2009 to identify the floating macrophytes cover and an appropriate bio-optical model to retrieve, by means of an inverse procedure, the concentrations of chlorophyll a, coloured dissolved organic matter and total suspended solid.The maps of the floating vegetation based on the NDVI values allow us to assess the spatial and temporal dynamics of the weeds with high time resolution.A floating vegetation index (FVI) has been introduced for describing the weeds pollution level.The results of the analysis show a consistent temporal relation between the water constituent concentrations within the Winam Gulf and the FVI, especially in the proximity of the greatest proliferation of floating vegetation in the last 10 years that occurred between the second half of 2006 and the first half of 2007.The adopted approach will be useful to implement an automatic system for monitoring and predicting the floating macrophytes proliferation in Lake Victoria.  相似文献   
8.
9.
In this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3D seismic reflection dataset from the Suez Rift, Egypt. Our analysis indicates that the October Fault Zone is composed of two fault systems that are locally decoupled across a salt-bearing interval of Late Miocene (Messinian) age. The sub-salt system offsets pre-rift crystalline basement, and was active during the Late Oligocene-early Middle Miocene. It is composed of four, planar, NW–SE-striking segments that are hard- linked by N–S-striking segments, and up to 2 km of displacement occurs at top basement, suggesting that this fault system nucleated at or, more likely, below this structural level. The supra-salt system was active during the Pliocene-Holocene, and is composed of four, NW–SE-striking, listric fault segments, which are soft-linked by unbreached relay zones. Segments in the supra-salt fault system nucleated within Pliocene strata and have maximum throws of up to 482 m. Locally, the segments of the supra-salt fault system breach the Messinian salt to hard-link downwards with the underlying, sub-salt fault system, thus forming the upper part of a fault zone composed of: (i) a single, amalgamated fault system below the salt and (ii) a fault system composed of multiple soft-linked segments above the salt. Analysis of throw-distance (T-x) and throw-depth (T-z) plots for the supra-salt fault system, isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history. The fault tips were then effectively ‘pinned’ and the faults accumulated displacement via predominantly downward propagation. We interpret that the October Fault Zone had the following evolutionary trend; (i) growth of the sub-salt fault system during the Oligocene-to-early Middle Miocene; (ii) cessation of activity on the sub-salt fault system during the Middle Miocene-to-?Early Pliocene; (iii) stretching of the sub- and supra-salt intervals during Pliocene regional extension, which resulted in mild reactivation of the sub-salt fault system and nucleation of the segmented supra-salt fault system, which at this time was geometrically decoupled from the sub-salt fault system; and (iv) Pliocene-to-Holocene growth of the supra-salt fault system by downwards vertical tip line propagation, which resulted in downward breaching of the salt and dip-linkage with the sub-salt fault system. The structure of the October Fault Zone and the rapid establishment of supra-salt fault lengths are compatible with the predictions of the coherent fault model, although we note that individual segments in the supra-salt array grew in accordance with the isolated fault model. Our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre-scale, basin-bounding faults. Furthermore, we highlight the role that fault reactivation and dip-linkage in mechanically layered sequences can play in controlling the three-dimensional geometry of normal faults.  相似文献   
10.
Displacement ratio (Dr) is the ratio between salt thickness (Tv) and sub-salt normal fault displacement (D) (Dr = Tv/D), and it is typically used to predict the degree of geometric and kinematic linkage between sub- and supra-salt fault populations, and the overall structural style in salt-influenced extensional settings. However, we currently lack natural examples of how Dr and the underlying geological controls vary, and how these may control the three-dimensional geometry and evolution of salt-influenced normal fault systems. Furthermore, it is currently unknown if kinematic coherence in salt-influenced extensional settings can be maintained over relatively long length-scales (101–103 m) and for relatively long timeframes, and how this may impact the growth and geometry of large-throw (>500 m), salt-influenced normal fault systems. In this paper we use a 3600 km2, high-quality 3D seismic reflection dataset and borehole data from the Stavanger Fault System (SFS), Egersund Basin, eastern North Sea Basin to investigate; (i) how pre-rift salt thickness (Tv) and sub-salt fault throw (T) control the structural style and evolution of a basin-bounding, salt-influenced normal fault system; and (ii) the role salt plays in maintaining kinematic coherence in normal fault systems. We demonstrate that; (i) pre-rift salt distribution (Tv), specifically its presence in the proto-footwall (i.e., when Tv > 0), is the primary control on partitioning of faulting and (forced) folding along the fault system, and the style of linkage (i.e., hard- or soft-linkage) between sub- and supra-salt fault populations; and (ii) sub- and supra-salt fault populations represent brittle elements of a single, geometrically and kinematically coherent structure, the geometry and evolution of which is related to the ductile translation of strain on a scale (up to 8 km) and duration (c. 65 Myr) that believe is significantly greater and longer than previously documented.  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