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   2272篇
  免费   357篇
  国内免费   502篇
测绘学   98篇
大气科学   386篇
地球物理   661篇
地质学   1058篇
海洋学   308篇
天文学   149篇
综合类   212篇
自然地理   259篇
  2024年   11篇
  2023年   38篇
  2022年   98篇
  2021年   124篇
  2020年   98篇
  2019年   98篇
  2018年   114篇
  2017年   100篇
  2016年   144篇
  2015年   105篇
  2014年   174篇
  2013年   123篇
  2012年   149篇
  2011年   148篇
  2010年   127篇
  2009年   119篇
  2008年   99篇
  2007年   91篇
  2006年   75篇
  2005年   89篇
  2004年   65篇
  2003年   75篇
  2002年   64篇
  2001年   83篇
  2000年   71篇
  1999年   94篇
  1998年   70篇
  1997年   71篇
  1996年   67篇
  1995年   55篇
  1994年   42篇
  1993年   45篇
  1992年   43篇
  1991年   25篇
  1990年   14篇
  1989年   47篇
  1988年   22篇
  1987年   11篇
  1986年   9篇
  1985年   11篇
  1984年   2篇
  1983年   4篇
  1982年   4篇
  1981年   2篇
  1980年   3篇
  1979年   1篇
  1975年   1篇
  1958年   5篇
  1942年   1篇
排序方式: 共有3131条查询结果,搜索用时 31 毫秒
571.
过去2000年气候变化对中国经济与社会发展影响研究综述   总被引:1,自引:1,他引:1  
It is one of the important focuses of the Past Global Changes (PAGES) to investigate the long-term impacts of climate change on regional social and economic development over the past two millenniums. The past decades in China have witnessed great progresses in the study of past climatic influence on human society. In this paper, reviews have been made on the latest proceedings related with researches about the past 2000-year climatic impacts on Chinese history in terms of the following three aspects: economic fluctuations, social stability, and the rise and fall of dynasties in China. It is concluded that climate change and socio-economic fluctuation in historical China really temporarily demonstrated a good coincidence, which indicates a potential driving-response mechanism was likely embedded in the complicated relationship between climate change and human society. A warm climate provided relatively stable conditions of agricultural production and thus generally played a positive role in the healthy development of the economy and society. On the contrary, socio-economic adverseness triggered by a colder climate was preconditioned with social problems such as the intensification of the contradiction between people and land, as well as the gradual accumulation of social rigidity. These social problems accompanying with social development contributed higher vulnerability of society in the face of changing climate, which to some extent might amplify the effects of climatic deterioration. The authors emphasize that the future studies of the relationship between past climate change and human history in China should attach more attention to the following key problems: making deeper exploitation of the potential of Chinese historical documents, exploring the mechanism of climate-society interaction, and studying the differences of climatic effects on socio-economic development at the regional scale. This study from a historical perspective might enhance the understanding of human-environment relationship under a situation of global warming, and also provide the scientific basis for the sustainable social development in China.  相似文献   
572.
珊瑚是地球上最古老的原住民,具有近6×108年的发育史,弱势群居、喜温和原地长成是珊瑚的基本特征。作者介绍了珊瑚-珊瑚礁的基本特征,综述了跨十年调查的研究区珍贵照片资料和相关认识,指出中国是全球主要的珊瑚-珊瑚礁国家,地位举足轻重;珊瑚-珊瑚礁作为地球生物多样性的代表,造岛、固礁、护鱼、防护岛岸流失,形成南海四大群岛280余座岛、礁、滩、沙,所构建庞大海洋生态系统是无与伦比的海洋生态资源和寸土寸金的南海海洋国土。提出划分南海珊瑚-珊瑚礁为中央区和周缘区2个分布区,阐述了南海中央区珊瑚-珊瑚礁的基本特征,系统汇集报道了间隔10年2个科考航次调查在浅水礁盘浮潜、至20 m水深浅潜-深潜和礁盘及开展岛、礁、滩、沙地质调查的发现,包括科学定名46种六放石珊瑚和6种八放软珊瑚等成果,同时,收集了西沙、中沙、东沙和南沙群岛海域的相关调查航次珊瑚照片;进一步阐述了单体环礁和复合环礁的特征及分布,并进行了初步对比,指出永乐环礁是南海唯一一个真正的切合达尔文模式的环礁,也是环礁发展到最高阶段的产物,构成现代海洋珊瑚-珊瑚礁形成演化研究最好的天然实验室。  相似文献   
573.
邢台隆东井田煤变质情况复杂,直接影响煤的工业利用。利用X-射线衍射(XRD)法探讨了区内不同变质程度煤的基本结构单元特征及煤变质规律。结果显示:垂向上,随着煤层埋藏深度的增加,层片间距d002值逐渐减小,层片堆积高度Lc和层片直径La值逐渐增大;平面上,离岩浆侵入体越近,煤层的层间距d002值越小,反之d002值越大。在煤种分布上,因受岩浆热源的影响,井田内煤种以旧城为中心向外呈带状分布,旧城附近为无烟煤,向外逐渐过渡为贫煤、瘦煤、焦煤、肥煤和气煤。区内煤变质作用主要受区域变质作用影响,并叠加了岩浆热变质作用。   相似文献   
574.
基于2012年6~8月的实测水汽同位素数据及相关气象数据,对黑河中游夏季昼夜的同位素基本特征、水汽来源方向及潜在蒸发源地进行了研究。结果表明:空气水汽线斜率白天大于夜晚和水汽过量氘值白天大于夜晚,综合说明白天局地蒸发较夜晚强烈;夏季受西风水汽影响显著。其中,6月主要受西风水汽和北冰洋水汽影响,7、8月主要受西风水汽和东南方向水汽影响,且8月受东南方向水汽影响最为明显;水汽运移路径上下垫面地形和气压带移动会影响水汽后向轨迹高度,西北方向上水汽输送通道较顺畅,风速较大,有利于水汽的输送;水汽蒸发源地主要集中在研究区周围及以东、以北部,其次是西北部。绿洲是主要的水汽蒸发源地,其次是城市和河流,白天较夜晚局地蒸发强烈且面积大。  相似文献   
575.
Kelan River is a branch of the Ertix River, originating in the Altay Mountains in Xinjiang, northwestern China. The upper streams of the Kelan River are located on the southern slope of the Altay Mountains; they arise from small glacial lakes at an elevation of more than 2,500 m. The total water-collection area of the studied basin, from 988 to 3,480 m, is about 1,655 km2. Almost 95 percent of the basin area is covered with snow in winter. The westerly air masses deplete nearly all the moisture that comes in the form of snow during the winter months in the upper and middle reaches of the basin. That annual flow from the basin is about 382 mm, about 45 percent of which is contributed by snowmelt. The mean annual precipitation in the basin is about 620 mm, which is primarily concentrated in the upper and middle basin. The Kelan River system could be vulnerable to climate change because of substantial contribution from snowmelt runoff. The hydrological system could be altered significantly because of a warming of the climate. The impact of climate change on the hydrological cycle and events would pose an additional threat to the Altay region. The Kelan River, a typical snow-dominated watershed, has more area at higher elevations and accumulates snow during the winter. The peak flow occurs as a result of snow-melting during the late spring or early summer. Stream flow varies strongly throughout the year because of seasonal cycles of precipitation, snowpack, temperature, and groundwater. Changes in the temperature and precipitation affect the timing and volume of stream-flow. The stream-flow consists of contributions from meltwater of snow and ice and from runoff of rainfall. Therefore, it has low flow in winter, high flow during the spring and early summer as the snowpack melts, and less flows during the late summer. Because of the warming of the current climate change, hydrology processes of the Kelan River have undergone marked changes, as evidenced by the shift of the maximum flood peak discharge from May to June  相似文献   
576.
Climate condition over a region is mostly determined by the changes in precipitation, temperature and evaporation as the key climate variables. The countries belong to the Belt and Road region are subjected to face strong changes in future climate. In this paper, we used five global climate models from the latest Sixth Phase of Coupled Model Intercomparison Project (CMIP6) to evaluate future climate changes under seven combined scenarios of the Shared Socioeconomic Pathways and the Representative Concentration Pathways (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0 and SSP5-8.5) across the Belt and Road region. This study focuses on undertaking a climate change assessment in terms of future changes in precipitation, air temperature and actual evaporation for the three distinct periods as near-term period (2021-2040), mid-term period (2041-2060) and long-term period (2081-2100). To discern spatial structure, K?ppen-Geiger Climate Classification method has been used in this study. In relative terms, the results indicate an evidence of increasing tendency in all the studied variables, where significant changes are anticipated mostly in the long-term period. In addition to, though it is projected to increase under all the SSP-RCP scenarios, greater increases will be happened under higher emission scenarios (SSP5-8.5 and SSP3-7.0). For temperature, robust increases in annual mean temperature is found to be 5.2 °C under SSP3-7.0, and highest 7.0 °C under SSP5-8.5 scenario relative to present day. The northern part especially Cold and Polar region will be even more warmer (+6.1 °C) in the long-term (2081-2100) period under SSP5-8.5. Similarly, at the end of the twenty-first century, annual mean precipitation is inclined to increase largely with a rate of 2.1% and 2.8% per decade under SSP3-7.0 and SSP5-8.5 respectively. Spatial distribution demonstrates that the largest precipitation increases are to be pronounced in the Polar and Arid regions. Precipitation is projected to increase with response to increasing warming most of the regions. Finally, the actual evaporation is projected to increase significantly with rate of 20.3% under SSP3-7.0 and greatest 27.0% for SSP5-8.5 by the end of the century. It is important to note that the changes in evaporation respond to global mean temperature rise consistently in terms of similar spatial pattern for all the scenarios where stronger increase found in the Cold and Polar regions. The increase in precipitation is overruled by enhanced evaporation over the region. However, this study reveals that the CMIP6 models can simulate temperature better than precipitation over the Belt and Road region. Findings of this study could be the reliable basis for initiating policies against further climate induced impacts in the regional scale.  相似文献   
577.
基于露头、钻井岩心和测录井资料,采用层序地层学理论与方法,将四川盆地西北部二叠系梁山组+栖霞组作为一个整体予以解剖,识别出梁山组底界面(Ⅰ型)、栖一段与栖二段界面(Ⅱ型)和栖霞组与茅口组界面(Ⅰ型)3个三级层序界面,将中二叠统栖霞阶划分为2个三级层序: 下部的SQ1层序对应梁山组+栖一段,上部的SQ2层序对应栖二段。通过栖霞阶层序地层格架分析,发现栖霞阶地层存在“底超顶削”的充填规律,统计并绘制SQ1与SQ2层序地层厚度等值线图,对研究区栖霞阶SQ1初期和SQ2期构造—古地理格局进行了恢复,在明确SQ2期为栖霞阶主要成滩期的基础上,结合岩溶发育单元及白云岩展布特征,讨论栖霞阶油气储集意义。结果表明:区内栖霞阶存在北西、北东向隆坳分异,汉南隆起、川北隆起、北缘隆起与广元—旺苍凹陷始终控制了区内沉积格局,指出广元—旺苍海槽雏形始于二叠系栖霞阶。研究区岩溶白云岩孔洞型储集层区与环广元—旺苍凹陷周缘的灰岩岩溶型储集层区为有利的勘探区。  相似文献   
578.
针对准噶尔盆地玛湖凹陷西斜坡风南地区三叠系百口泉组扇三角洲砂岩物性空间变化大、优质储集层(孔隙度大于7.4%,渗透率大于0.05×10-3μm2)预测难的问题,在沉积岩石学、地震沉积学以及地震反演和解释理论指导下,综合利用测井、岩心和三维地震等资料开展了高精度层序地层划分、沉积微相描述和优质储集层地震反演研究。建立了风南井区四级层序地层格架,明确了扇三角洲多期水进水退的充填过程,指出SSQ3和SSQ5是优质储集层的发育层系;识别出扇三角洲平原分流河道、河道间和扇三角洲前缘水下分流水道、河口坝、席状砂等沉积微相,指出扇三角洲平原是优质储集层发育相带;通过应用高分辨层序地层纵向边界和沉积相横向边界约束,进行分层相控叠后地震波阻抗反演,提升储集层预测精度,在SSQ3和SSQ5预测5个优质储集层发育区,提出3口井的井位建议,钻探均获工业油流。  相似文献   
579.
鄱阳湖成因与演变的历史论证   总被引:5,自引:2,他引:5  
苏守德 《湖泊科学》1992,4(1):40-47
本文通过大量史料分析和地质钻孔证据,认为鄱阳湖大水面形成于公元400年前后,为距今约1600年的年青湖泊。鄱阳湖形成的直接和主导因素是长江主泓道南移到湖口一带,因江水阻碍赣江水的下泄,使湖泊水域向南扩张,到唐初面积最大时曾达6000km~2。之后,鄱阳湖水位和面积的变化主要取决于湖口处长江水位的变化。  相似文献   
580.
水分盈亏指数及其在农业干旱监测中的应用   总被引:6,自引:2,他引:6  
苏永秀  李政  吕厚荃 《气象科技》2008,36(5):592-595
从广西农业生产特点和农业干旱监测业务实际需要出发,建立了综合考虑降雨量和同期作物需水量的基于作物水分盈亏原理的农业干旱监测模型,农业干旱监测等级指标,并用该指标分析了1961~2004年广西双季稻、甘蔗生育期间农业干旱的时空分布特征,用自治区农业厅同期旱情资料对2004年广西春旱发生情况进行监测验证.结果表明:水分盈亏指数模型能很好地反映旱情,是一种实时的干旱监测方法,能运用于广西农业干旱监测业务,在每季干旱发生频率的时空分布特征分析上得到的结果也与实际情况相符合.  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