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   4979篇
  免费   936篇
  国内免费   783篇
测绘学   272篇
大气科学   676篇
地球物理   1306篇
地质学   2219篇
海洋学   240篇
天文学   93篇
综合类   380篇
自然地理   1512篇
  2024年   21篇
  2023年   35篇
  2022年   141篇
  2021年   185篇
  2020年   152篇
  2019年   199篇
  2018年   174篇
  2017年   225篇
  2016年   194篇
  2015年   208篇
  2014年   255篇
  2013年   293篇
  2012年   318篇
  2011年   298篇
  2010年   296篇
  2009年   304篇
  2008年   294篇
  2007年   345篇
  2006年   358篇
  2005年   287篇
  2004年   309篇
  2003年   241篇
  2002年   202篇
  2001年   180篇
  2000年   170篇
  1999年   151篇
  1998年   149篇
  1997年   112篇
  1996年   112篇
  1995年   93篇
  1994年   115篇
  1993年   74篇
  1992年   60篇
  1991年   30篇
  1990年   27篇
  1989年   42篇
  1988年   19篇
  1987年   12篇
  1986年   3篇
  1985年   2篇
  1984年   2篇
  1983年   4篇
  1979年   3篇
  1978年   2篇
  1954年   2篇
排序方式: 共有6698条查询结果,搜索用时 742 毫秒
341.
从成功的区域合作经验看,区域间的差异性和联系性是选择经济合作区域必须考虑的两大核心因素.通过多种指标对两大因素进行测度和分析,最终正确选择合作区域.泛珠三角合作为云南经济发展提供难得机遇,云南如何立足自身优势,正确选择重点合作省区,关系到合作的最终成效.从云南与泛珠三角其他省区间的经济发展差异、产业结构差异、优劣势条件、综合距离系数以及历史联系等角度分析,提出了云南应把广东、广西、四川、香港作为重点合作省区.  相似文献   
342.
建设新农村与城镇化的关系初探   总被引:1,自引:0,他引:1  
如何正确处理新农村建设和城镇化的关系,是一个值得深入探讨的问题。中国城市化进程应是以区域一体化为中心的城市化道路,乡村发展也是城镇化的内容。从中国的实情出发,要重新认识“二元经济”理论,“二元经济”的模型是以粮食代表整个农业生产。城镇化和建设新农村在组织形式上存在差异区别,但不存在严重的资金竞争,并且在服务对象、功能、产业结构方面是互补的,城镇化和建设新农村是并行不悖的,城镇化与建设新农村可以在很长时间内共同发展。  相似文献   
343.
文化景观是人类活动的产物,文化景观研究是文化地理学的主要研究领域之一,而作为文化景观研究的重要组成部分的文化景观特性研究无疑将推动这一领域研究的进一步发展。运用比较分析方法,根据文化景观的定义,探讨文化景观具有时代性、继承性、叠加性、区域性、民族性等五大特性,并对这些特性进行科学的界定和详细的阐述,展示出这五大特性之间所存在的矛盾统一关系和其内在的紧密联系,进一步说明这些特性的实际意义和利用价值。  相似文献   
344.
Abstract:  This paper offers a geographical perspective on the way in which health regions have been defined and established in São Paulo as part of overall Brazilian health policy. To accomplish this, I first situate the Brazilian case in the international debate over the regionalization of health services. Second, I examine health regionalization in the case of São Paulo and show how the process is dynamic and complex, involving geographical scales that go well beyond the administrative boundaries of Brazilian municipalities.  相似文献   
345.
An overview of the crust and upper mantle structure of Central America and the Caribbean region is presented as a result of the processing of more than 200 seismograms recorded by digital broadband stations from SSSN and GSN seismic networks. Group velocity dispersion curves are obtained in the period range from 10s to 40s by FTAN analysis of the fundamental mode of the Rayleigh waves; the error of these measurements varies from 0.06 and 0.09 km/s. From the dispersion curve, seven tomographic maps at different periods and with average spatial resolution of 500 km are obtained. Using the logical combinatorial classification techniques, eight main groups of dispersion curves are determined from the tomographic maps and eleven main regions, each one characterized by one kind of dispersion curves, are identified. The average dispersion curves obtained for each region are extended to 150s by adding data from a larger-scale tomographic study (Vdovin et al., 1999) and inverted using a nonlinear procedure. A set of models of the S-wave velocity vs. depth in the crust and upper mantle is found as a result of the inversion process. In six regions we identify a typically oceanic crust and upper mantle structure, while in the other two the models are consistent with the presence of a continental structure. Two regions, located over the major geological zones of the accretionary crust of the Caribbean region, are characterized by a peculiar crust and upper mantle structure, indicating the presence of lithospheric roots reaching, at least, about 200 km of depth.  相似文献   
346.
The variation of Load/Unload Response Ratio (LURR) against time for earthquakes in Kerman Province, Iran, on February 22, 2005, M6.4 and in Lorestan Province, Iran, on March 31, 2006, M6.1, has been calculated and analysed in this paper. The tempo-spatial scanning of LURR in the region of Iran during January 1, 2003 to March 31, 2006 has been conducted, with 1 year as a time-window, 1 month as a time-step, and the comparison of the LURR anomalous regions in 2004 with the actual earthquakes with M≥5.0 in the next year (2005) is also given, which shows that 11 earthquakes with M≥5.0 occurred in LURR anomalous regions while 12 earthquakes with M≥5.0 in LURR regions in 2005. Furthermore, the seismicity in this region is studied by investigating the evolvement of the anomalous LURR regions.  相似文献   
347.
The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period -ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal (NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall, whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the years of increase (decrease) of convective activity over the NARA.  相似文献   
348.
乌兰布和沙漠北部地下水资源的环境同位素探讨   总被引:8,自引:4,他引:8       下载免费PDF全文
工作范围在乌兰布和沙漠北部,面积共约4200km2.年平均降水量85~140mm,由西南向东递增,降水同位素组成δD~δ18O恰与Craig线一致,并与阿拉善地区相同.测得地下水中同位素含量范围,δ18O为-74‰~121‰,氚为0~190TU,14C为17~97pMC.由地下水同位素组成区别出与降水线平行或相交的6种类型.从所有地下水水点,以及可能有补给关系的其它水点的各类同位素关系,包括δ18O,T,δ13C和pMC,识别出两类承压水的各3个补给源和潜水的3个补给源,并区别出一组氚含量极低的潜水,对不同位置的承压水和潜水,由其同位素关系估算出了各补给源的组成和变幅.  相似文献   
349.
攀枝花地区不同工业区表层土壤中重金属分布的特征   总被引:12,自引:0,他引:12  
作者在本文中研究了攀枝花地区不同工业区表层土壤中重金属的污染特征及元素组合。结果表明,煤矿区以Cd,Ni,Cu,Zn污染为主,攀钢工业区以Ni,Cu,Cd, Cr污染为主,其它工业区以Cd,Ni污染为主。不同工业区的重金属元素组合特征基本一致,即分为As-Cd-Cu-Ni-Pb和Cr-Zn两组。  相似文献   
350.
运用标志层、层间距、物性特征、煤层本身特征等方法,对霍东地区沁安矿区和沁源矿区山西组1号、2号、3号煤层进行对比,认为沁安矿区未沉积3号煤层,该矿区现有的两个煤与沁源矿区的1号、2号煤层可比,而不是2号、3号煤层,其中既有定性又有定量,较成功的解决了南北两大矿区山本组主要可采煤层间的对比问题。  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