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   7616篇
  免费   1156篇
  国内免费   1866篇
测绘学   436篇
大气科学   1947篇
地球物理   1871篇
地质学   3492篇
海洋学   1026篇
天文学   372篇
综合类   709篇
自然地理   785篇
  2024年   28篇
  2023年   114篇
  2022年   309篇
  2021年   345篇
  2020年   282篇
  2019年   349篇
  2018年   366篇
  2017年   323篇
  2016年   422篇
  2015年   353篇
  2014年   426篇
  2013年   451篇
  2012年   346篇
  2011年   378篇
  2010年   349篇
  2009年   415篇
  2008年   329篇
  2007年   307篇
  2006年   277篇
  2005年   279篇
  2004年   208篇
  2003年   234篇
  2002年   205篇
  2001年   222篇
  2000年   278篇
  1999年   417篇
  1998年   322篇
  1997年   321篇
  1996年   321篇
  1995年   273篇
  1994年   226篇
  1993年   203篇
  1992年   163篇
  1991年   136篇
  1990年   113篇
  1989年   116篇
  1988年   101篇
  1987年   55篇
  1986年   60篇
  1985年   44篇
  1984年   39篇
  1983年   20篇
  1982年   20篇
  1981年   24篇
  1980年   18篇
  1979年   11篇
  1978年   8篇
  1976年   6篇
  1973年   7篇
  1958年   8篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
柴达木盆地东北部过去1437 a的降水变化重建   总被引:24,自引:0,他引:24       下载免费PDF全文
利用青海柴达木盆地东北缘山地11个地点的祁连圆柏树轮宽度序列,在早期分析祁连圆柏的生长对气候要素响应的基础上,重建了青海德令哈和乌兰地区过去1437 a的年降水量变化序列。重建方程能够解释校准期(1955-2002年)内66%的降水量变化方差,尤其是重建的降水序列的低频变化能够很好地代表器测资料的变化。从过去1437 a的时间尺度来看,近40 a来柴达木东北缘地区处于相对湿润的时期,但最显著的湿润期出现在16世纪晚期。此外,小冰期期间降水量的变幅较大,而中世纪暖期降水低频变化相对较小。另外,重建降水量体现了150~250 a尺度的低频变化特征。  相似文献   
992.
Densities of major meiofaunal taxa were investigated at 34 sampling stations during six cruises by R/V Beidou to the southern Yellow Sea, China from 2000 to 2004, and the community structure of free-living marine nematodes was studied during one of the cruises in 2003. Meiofauna abundance ranged from 487.4 to 1655.3 individuals per 10 cm2. Nematodes and harpacticoid copepods were the two most dominant groups, contributing 73.8–92.8% and 3.5–18.7%, respectively, to the total meiofauna abundance. One-way ANOVA showed no significant annual fluctuation of meiofauna and nematode abundances from 2000 to 2004 in the southern Yellow Sea. However, two-way ANOVA based on six stations sampled in 4 years (2001–2004) showed that there were significant differences among the six stations and the 4 sampling years for meiofauna, nematode and copepod abundance. Correlation analysis demonstrated that meiofauna abundance was mainly linked to chloroplastic pigments. Other environmental factors could not be ruled out, however. A total of 232 free-living marine nematode species, belonging to 149 genera, 35 families and 4 orders, were identified. The dominant species in the sampling area were the following: Dorylaimopsis rabalaisi, Microlaimus sp.1, Prochromadorella sp., Promonohystera sp., Cobbia sp.1, Daptonema sp.1, Leptolaimus sp.1, Halalaimus sp.2, Aegialoalaimus sp., Chromadorita sp., Parodontophora marina, Parasphaerolaimus paradoxus, Quadricoma sp.1, Campylaimus sp.1, Halalaimus gracilis, Paramesacanthion sp.1, Paramonohystera sp.1, and Metalinhomoeus longiseta. CLUSTER and SIMPROF analyses revealed three main types of nematode community (or station groups) in the sampling area, including I: coastal community, II: transitory community between coastal and YSCWM (Yellow Sea Cold Water Mass), and III: YSCWM community. Each community was indicated by a number of dominant nematode species. Bio-Env correlation analysis between the nematode community and environmental variables showed that water depth, sediment water content, organic matter, chlorophyll a (Chl-a) and phaeophorbide a (Pha-a) were the most important factors to determine the community structure.  相似文献   
993.
天山公路沿线年降水量分布特征及其变化趋势   总被引:1,自引:0,他引:1  
以天山公路沿线9个气象水文站1956~2005年年降水量资料为依据,采用均值统计参数和线性方程分析了天山公路沿线年降水量分布特征以及变化趋势,在此基础上,应用小波分析方法对天山公路沿线各桩号代表站年降水量做周期分析,结果表明:天山公路沿线多数路段的年降水时间序列包含了多个不同时间尺度的周期变化和演变特征,具有10~17a、8~9a以及5a以下的周期变化,10~30a时间尺度突变点多出现于20世纪80年代熏  相似文献   
994.
This work presents a novel procedure for identifying the dynamic characteristics of a building and diagnosing whether the building has been damaged by earthquakes, using a back‐propagation neural network approach. The dynamic characteristics are directly evaluated from the weighting matrices of the neural network trained by observed acceleration responses and input base excitations. Whether the building is damaged under a large earthquake is assessed by comparing the modal parameters and responses for this large earthquake with those for a small earthquake that has not caused this building any damage. The feasibility of the approach is demonstrated through processing the dynamic responses of a five‐storey steel frame, subjected to different strengths of the Kobe earthquake, in shaking table tests. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
995.
利用2010—2011年江西省有代表性的12个县(市)共34个观测点的双季早稻农业气象观测资料,采用频数分析方法,研究建立了江西省双季早稻长势观测定量指标。结果表明,在各主要生育期,不同苗情的早稻,其叶面积指数与单株干物质重量差异明显,可用于早稻长势的定量评价。产量分析表明,研究建立的双季早稻长势定量等级指标可用于实际。  相似文献   
996.
本文分析了1987年12月至1988年11月在闽南-台湾浅滩渔场6个航次采集的浮游软体动物,初步鉴定44种,以马蹄(?)螺、棒笔帽螺、尖笔帽螺和蝴蝶螺为该海区的优势种。本文研究了它们的季节变化,平面分布和垂直分布,并讨论了它们与上升流及其他水系的关系。  相似文献   
997.
从气候、地形等方面分析了凌云县2009年秋至2010年干旱灾害的成因,并提出了相应的抗旱措施,尽可能减少干旱灾害造成的损失。  相似文献   
998.
Typhoon Herb in 1996 caused widespread debris flows in central Taiwan. The 7.3 Chi-Chi earthquake on September 21, 1999, which also took place in central Taiwan, induced many landslides in the region. These landslides turned into debris flows when Typhoon Toraji struck Taiwan in 2001. This research selects three regions which suffered a ground motion class of 5, 6, and 7 on the Richter scale during the Chi-Chi earthquake as study areas. Air photos from 1997 and 2001 of these regions are used to map the gully-type debris flows that took place after Typhoons Herb and Toraji, respectively. The gullies adjacent to the debris flow, but without a trace of debris flows, are also mapped as the non-debris flow data. The topography, hydrogeology, and rainfall factors – where debris flow occurred and in which there was no occurrence of debris flows in these gullies were retrieved from DTM, geological maps, and iso-countour maps, and of rainfall through GIS processing. These characteristic are introduced into a probabilistic neural network to build a predicting model for the probability of the occurrence of debris flows. Three series of cross analyses are conducted to compare the probability of the occurrence of debris flows of the same dataset predicted by different prediction models. The results reveal that the susceptibility of debris flows was elevated after the Chi-Chi earthquake struck. The upsurge of susceptibility was more obvious for the regions that received a higher class of ground motion.  相似文献   
999.
彭荔红  洪丽玉 《台湾海峡》1995,14(4):334-337
本文综述了国内外酸雨的研究现状;包括酸雨的来源、成因、模式、影响及防治对策;介绍了酸雨的研究趋势;并着重就厦门地区今后酸雨的研究方向提出也作者的看法。  相似文献   
1000.
We analyze the time-longitude structure of composite cases from model-assimilated ocean data in the period 1958–1998, following on from earlier work by Huang and Kinter (J. Geophys. Res. 107(C11) (2002) 3199) that studied east–west thermocline variability in the Indian Ocean. Our analysis focuses on the Rossby wave signal along the thermocline ridge in the tropical SW Indian Ocean (10°S, 60–80°E), where wind stress curl is important. Anomalous winds in the equatorial east Indian Ocean force successive Rossby waves westward at speeds of 0.1 m s−1±30%. With a wavelength of 7000 km, the period of oscillation is in the range 1.9–5.2 years. The Indian Ocean Rossby wave is partially resonant with the global influence of the El Nino–Southern Oscillation, except during quasi-biennial rhythm. The presence of the Rossby wave offers potential predictability for east–west atmospheric circulation systems and climate that affect resources in countries surrounding the Indian Ocean.  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