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   871篇
  免费   222篇
  国内免费   161篇
测绘学   32篇
大气科学   22篇
地球物理   329篇
地质学   209篇
海洋学   533篇
天文学   1篇
综合类   55篇
自然地理   73篇
  2024年   1篇
  2023年   9篇
  2022年   20篇
  2021年   31篇
  2020年   33篇
  2019年   34篇
  2018年   36篇
  2017年   37篇
  2016年   34篇
  2015年   40篇
  2014年   49篇
  2013年   70篇
  2012年   48篇
  2011年   59篇
  2010年   66篇
  2009年   71篇
  2008年   70篇
  2007年   66篇
  2006年   80篇
  2005年   48篇
  2004年   51篇
  2003年   45篇
  2002年   44篇
  2001年   38篇
  2000年   24篇
  1999年   24篇
  1998年   14篇
  1997年   16篇
  1996年   8篇
  1995年   14篇
  1994年   11篇
  1993年   10篇
  1992年   8篇
  1991年   8篇
  1990年   3篇
  1989年   4篇
  1988年   1篇
  1987年   3篇
  1985年   4篇
  1984年   9篇
  1983年   4篇
  1982年   3篇
  1981年   5篇
  1976年   1篇
排序方式: 共有1254条查询结果,搜索用时 16 毫秒
1.
本文首先提出了河流泥沙输移过程中泥沙中的钙镁矿物溶蚀消耗水体中的CO2并具有碳汇功能的观点。基于前人长江干流从源头到入海口和支流2003~2007年期间4次河流悬移质泥沙的化学元素组成和矿物组成资料,分析悬移质中CaO、MgO含量和方解石、白云石含量变化特征,定量计算了这些取样点悬移质泥沙的CO2总碳汇能力和非永久性、永久性碳汇能力,分析了不同碳汇能力沿程变化规律及其原因。碳汇计算结果表明:寸滩—大通河段1956~2000年期间泥沙输移过程中钙镁矿物溶蚀产生的总碳汇量、非永久性和永久性碳汇量分别为2572万t/a、1700万t/a和872万t/a。由于输沙量减少,寸滩站—大通站河段的总碳汇量、非永久性和永久性碳汇量2006~2019年期间较1956~2000年期间相应分别减少了1852万t、1224万t和872万t。三峡水库年均淤积量1. 145亿t,损失总碳汇量675. 6万t,相当于三峡电站减排二氧化碳8580万t的7. 9%。全球河流入海年输沙量126. 1亿t,以寸滩- 吴淞口河段碳汇功能0. 060 t/t计,总碳汇量7. 57亿t相当于全球岩石风化碳汇总量10. 56亿t CO2的71. 6%。河流泥沙输移过程中钙镁矿物溶蚀的碳汇量具有重要的作用,其溶蚀速率大于原地风化。  相似文献   
2.
邵晨晨 《探矿工程》2021,48(6):102-108
上海某国际金融大厦深大基坑项目北侧紧邻运营轨道交通8号线区间隧道,最近处仅7.87 m,南侧与现有世博共同管沟净距2.0~3.0 m,基坑变形控制要求高,周边环境敏感。同时,本项目位于黄浦江畔,场地浅层为典型的淤泥质软土地层,下部为富含承压水粉(砂)土层,且微承压水层与第一、第二承压水层互为连通,止水帷幕无法隔断承压水层,地质条件复杂。设计采用分坑施工、被动区加固、预应力伺服钢支撑系统、抽灌一体化降水方案、超深地下连续墙、跟踪注浆、型钢垫层等技术方案。实测结果表明,区间隧道的最大变形6.52 mm,共同管沟的最大变形15.3 mm,其最大变形均满足变形控制要求,确保了运营区间隧道和共同管沟的安全。  相似文献   
3.
Oyster reefs and their spatial patterns are deemed to change the local hydrodynamic condition and exert profound impacts on the grain size, concentration and transportation of suspended sediments. Meanwhile, high suspended sediment concentration often results in excess mortality among oysters. Oyster reefs are rare and vital ecosystem in Liyashan national marine park, Jiangsu Coast, China. However, urgent conservation efforts should be made on account of the drastic reduction in reef areas. To investigate the sediment dynamics and the geomorphology, two tripod observation systems were deployed and UAV aerial surveys with elevation measurement using Real Time Kinematic (RTK) were also carried out. High mud content (60%) was found in the bed sediment at the reef ridge, causing much lower drag coefficient than other recorded values of living oyster reefs, indicating the death of oysters and the degradation of reefs in Liyashan. Ridgelines of the string reefs at 45° to the current direction and high suspended sediment concentration in the water body (50–370 mg/L) that exceeds the threshold (200 mg/L), which would affect nutrient uptake efficiency and further result in gill saturation, decrease of clearance rate and associated deposition, were probably crucial causes of the death of oysters. The findings are useful for restoring natural oyster reefs and designing artificial reefs for nature-based coastal defense.  相似文献   
4.
为实现对用海项目施工期悬浮泥沙扩散的实时跟踪监测,尝试采用无人机携带可见光、多光谱传感器对古雷围填海项目的围堰区进行遥感探测。利用Pix4d软件对无人机携带的可见光及多光谱照片进行处理,结果表明,可见光及多光谱影像对水体中的悬浮泥沙分布均具有一定的识别能力。通过研究分析多光谱影像的光谱特征发现,近红外波段对悬浮泥沙含量的升高较为敏感,运用波段比值公式计算结果显示,围堰内含沙水体经溢流口排入海域后浓度明显降低,并可向南继续扩散约300 m。该方法可克服遥感卫星影像时空分辨率低的特点,为用海项目资源环境影响跟踪监测提供新的技术手段。  相似文献   
5.
Laboratory experiments on the New Zealand freshwater mussel Echyridella menziesii were used to investigate the short-term effects (7–8 days) of food type on rates of biodeposition and benthic substrate respiration. Post-feeding biodeposition rates ranged from 0.34 to 1.52?mg?g?1?h?1 (mean?=?0.50?mg g?1?h?1) and were unaffected by the addition of toxin-producing Microcystis. Addition of suspended sediment (30?mg?L?1) visibly altered substrate composition, and increased total and inorganic biodeposit production rates by 24–33% compared to mussels fed commercial phytoplankton stock. Biodeposition rates of mussels in lake bed substrates were 38% higher than those in silica sand for identical feeding regimes, suggesting that a significant proportion of material produced in this experiment could have been derived from feeding on organic matter in the lake bed sediments. Respiration rates were higher in treatments with Microcystis but were unaffected by the presence of mussels. This laboratory study suggests that biodeposition by E. menziesii is resilient to short-term exposure to Microcystis, and highlights the ability of mussels to alter benthic substrate composition by incorporating suspended sediment into substrates.  相似文献   
6.
The restoration of meadowland using the pond and plug technique of gully elimination was performed in a 9‐mile segment along Last Chance Creek, Feather River Basin, California, in order to rehabilitate floodplain functions such as mitigating floods, retaining groundwater, and reducing sediment yield associated with bank erosion and to significantly alter the hydrologic regime. However, because the atmospheric and hydrological conditions have evolved over the restoration period, it was difficult to obtain a comprehensible evaluation of the impact of restoration activities by means of field measurements. In this paper, a new use of physically based models for environmental assessment is described. The atmospheric conditions over the sparsely gauged Last Chance Creek watershed (which does not have any precipitation or weather stations) during the combined historical critical dry and wet period (1982–1993) were reconstructed over the whole watershed using the atmospheric fifth‐generation mesoscale model driven with the US National Center for Atmospheric Research and US National Center for Environmental Prediction reanalysis data. Using the downscaled atmospheric data as its input, the watershed environmental hydrology (WEHY) model was applied to this watershed. All physical parameters of the WEHY model were derived from the existing geographic information system and satellite‐driven data sets. By comparing the prerestoration and postrestoration simulation results under the identical atmospheric conditions, a more complete environmental assessment of the restoration project was made. Model results indicate that the flood peak may be reduced by 10–20% during the wet year and the baseflow may be enhanced by 10–20% during the following dry seasons (summer to fall) in the postrestoration condition. The model results also showed that the hydrologic impact of the land management associated with the restoration mitigates bank erosion and sediment discharge during winter storm events. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
7.
In order to understand the differences in the suspended sediment and total dissolved solid (TDS) yield patterns between the glacial and non‐glacial catchments at the headwaters of Urumqi River, northwestern China, water samples were collected from a glacier catchment and an empty cirque catchment within the region, during three melting seasons from 2006 to 2008. These samples were analyzed to estimate suspended sediment and TDS concentrations, fluxes and erosion rates in the two adjoining catchments. There were remarked differences in suspended sediment and TDS yield patterns between the two catchments. Suspended sediment concentrations were controlled mainly by the sediment source, whereas TDS concentrations were primarily related to the hydrologic interaction with soil minerals. Generally, the glacial catchment had much higher suspended sediment and TDS yields, together with higher denudation rates, than the non‐glacial catchment. Overall, glacial catchment was mainly dominated by physical denudation process, whereas the non‐glacial catchment was jointly influenced by physical and chemical denudation processes. The observed differences in material delivery patterns were mainly controlled by the runoff source and the glacial processes. The melting periods of glacier and snow were typically the most important time for the suspended sediment and TDS yields. Meanwhile, episodic precipitation events could generate disproportionately large yields. Subglacial hydrology dynamics, glaciers pluck and grind processes could affect erodibility, and the large quantities of dust stored on the glacier surface provided additional sources for suspended sediment transport in the glacial catchment. These mechanisms imply that, in response to climate change, the catchment behaviour will be modified significantly in this region, in terms of material flux. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
8.
海水中光合作用有效光强(PAR)的变化受多种因素影响,其中分布在海水中的悬浮颗粒物是造成PAR在水中衰减的重要因素之一,尤其是在风与潮混合作用较强的近海。已有的研究表明:PAR在水中是呈指数衰减的,并且在悬浮物颗粒物分布比较稳定时,光衰减系数k与悬浮物体积浓度线性相关。在生态模型中,往往需要利用k获得水下光强,而衰减系数k很难直接通过观测得到,因此本文目的就是建立衰减系数与悬浮颗粒物体积浓度的关系,后者可通过观测得到。经过对2个航次与2个定点观测数据的处理,得到了PAR衰减系数分别与所有粒径悬浮物颗粒物总体积浓度以及3种粒级(黏土、粉砂、细砂)悬浮物体积浓度之间的线性关系式,并且发现相对于细砂,较小粒径的黏土、粉砂的浓度能更好的表征PAR的衰减特性。  相似文献   
9.
龙小志  王珍岩 《海洋与湖沼》2022,53(6):1322-1337
台风作为事件性的强动力因素,其对河口海域沉积环境的影响目前研究较少,开展典型台风事件对长江口外海域悬浮体分布的影响研究对于深入理解长江径流挟带陆源物质向东海陆架扩散机制等具有重要意义。利用2015年09号台风"灿鸿"过境前后长江口外海域现场调查数据,分析台风前后长江口外水体结构和悬浮体粒度分布变化,结合同期环境观测数据与开源数据,阐明台风对河口外海域悬浮体分布的影响机制。结果表明:长江口外海域悬浮体中细颗粒组分(≤ 128μm)主要为无机矿物颗粒,而粗颗粒组分(>128μm)主要是生源有机颗粒。生源有机粗颗粒主要分布于中上层水体,而无机细颗粒主要分布于底层水体,使得长江口外海域悬浮体平均粒径分布呈双层结构。台风前后悬浮体粒度分布变化反映了台风对长江口外海域物理和生物过程的双重影响,其中物理过程主要影响无机细颗粒分布变化,生物过程主要影响有机粗颗粒。台风期间强烈的偏北风使得长江冲淡水在口门外海域由东北输运转为往东输运,与长江冲淡水输运方向一致的表层无机细颗粒在台风后输运方向同样往东。另外,台风作用在河口区产生的下降流将12250-4站位底部再悬浮的泥质沉积物向东搬运至12300-4站,导致12300-4站底层悬浮体浓度增加、粒度变细。台风过境还造成长江口外海域初级生产力提高,而浮游植物生长对悬浮体中有机粗颗粒的形成有促进作用,使得口门外海域上层水体中有机粗颗粒体积浓度升高。长江口外海域由于台风过境导致悬浮体中无机细颗粒和生源有机颗粒含量均增加,使得其平均粒径整体变化不大。  相似文献   
10.
杨少鹏  拾兵 《海洋科学》2018,42(3):121-130
海底管线作为海洋工程的重要组成部分,其安全性受到广泛关注。本文简单回顾了国内外学者通过物理实验、理论分析与数值模拟等方法研究海底管线冲刷悬空及防护问题的成果,并对工程中应用较广泛的防护方法的优缺点进行了简单对比,为该领域的进一步研究提供参考。  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