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   4728篇
  免费   973篇
  国内免费   1398篇
测绘学   382篇
大气科学   960篇
地球物理   996篇
地质学   2647篇
海洋学   784篇
天文学   171篇
综合类   467篇
自然地理   692篇
  2024年   47篇
  2023年   128篇
  2022年   287篇
  2021年   279篇
  2020年   274篇
  2019年   281篇
  2018年   325篇
  2017年   267篇
  2016年   325篇
  2015年   289篇
  2014年   334篇
  2013年   290篇
  2012年   298篇
  2011年   312篇
  2010年   326篇
  2009年   338篇
  2008年   295篇
  2007年   241篇
  2006年   193篇
  2005年   181篇
  2004年   146篇
  2003年   139篇
  2002年   155篇
  2001年   150篇
  2000年   147篇
  1999年   173篇
  1998年   133篇
  1997年   125篇
  1996年   96篇
  1995年   93篇
  1994年   69篇
  1993年   75篇
  1992年   54篇
  1991年   40篇
  1990年   34篇
  1989年   38篇
  1988年   22篇
  1987年   26篇
  1986年   19篇
  1985年   7篇
  1984年   9篇
  1983年   12篇
  1982年   10篇
  1980年   6篇
  1979年   4篇
  1978年   1篇
  1976年   1篇
  1975年   1篇
  1958年   2篇
  1954年   1篇
排序方式: 共有7099条查询结果,搜索用时 15 毫秒
71.
Barchans of Minqin: Morphometry   总被引:2,自引:0,他引:2  
The barchans on the border of Minqin oasis, China, were surveyed. Our data reconfirmed that the height of slipface is proportional to the width of horns and the baseline can be roughly described by parts of an ellipse and a parabola. We attempted to study dune geomorphology using the theory of differential geometry. The geometrical characteristics of the brink can be quantitatively determined by its curvature and torsion.  相似文献   
72.
空间分析中几种插值方法的比较研究   总被引:2,自引:0,他引:2  
针对常用的空间插值方法在精度分析方面比较的不足,从地质统计学、数学函数和几何方法方面分别选取普通克里金插值法、三次样条函数法和距离权重倒数法进行插值精度的比较与分析。文章分析了各个方法的优缺点和局限性,选取疏密差异较大的均匀和不均匀离散点数据进行实验,利用三次趋势面模型和MATLAB,ArcGIS,GS+Version...  相似文献   
73.
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of  相似文献   
74.
1 INTRODUCTION Scientific management of aquacultural andnatural populations studies on the early life historyof flatfishes in the aspects of their metamorphosisand benthic settlement accompanying pronouncedtransitions in morphology, behavior and habitatecology have received much attention (Youson,1988). Accumulated data showed that temperatureand substrate played a main role in the recruitmentsuccess of flatfish during their early development(Rijnsdorp et al., 1995, Gibson and Robb, 2000…  相似文献   
75.
山东梁邹矿业集团有限公司,位于山东省邹平县境内。由邹平钼业有限公司、邹平铭兴化工有限公司、邹平铭波电源有限公司、邹平铭扬建材有限公司、邹平碑楼铜业有限公司、邹平铭远贸易进出口有限公司组成,是集铜钼采选、精细化工、充电电池、建筑材料、进出口贸易和职工现代服务等为一体的综合性企业集团。其中,钼业公司、化工公司属省二级安全标准化企业,钼业公司是山东省唯一铜钼采选企业,在2000年国家统计局公布的全国最大100家有色金属采选企业中排名第32位。  相似文献   
76.
随着测深技术、GNSS技术、高精密传感器集成化的发展,无人船测量系统为水下地形测量提供了新的思路。结合山东省高密市王吴水库,探讨了无人船水下地形测量的作业流程,通过对比分析,选择最优测线间距获取数据,通过差值分析进行精度评价,数据处理后对成果进行展示。研究区结果表明,无人船测量系统基于单波束测深在中型水库中的应用是十分理想的,测深数据质量较好,能够很好地反映库区的水下地形情况,无人船测量系统在水下地形测量中起着重要作用。  相似文献   
77.
Horizontal starch gel electrophoresis was used to investigate the biochemical genetic structure of Chinese mittencrab Eriocheir sinensis and Hepu mitten crab E. hepuensis. Sixteen putative enzyme-coding loci were examined for forty-nineChinese mitten crabs and thirty-eight Hepu mitten crabs. Nine loci, AAT-1 *, AAT-2 *, G3PDH*, GPI*, IDHP-1 *, IDHP-2 *,MDH-1 *, MDH-2 * and PGM * , are polymorphic in Chinese mitten crab, and seven, AAT-1 *, AAT-2 *, GPI *, IDHP-1 *,MDH-1 *, MDH-2 * and PGM * , are polymorphic in Hepu mitten crab. The proportion of polymorphic loci and the expectedheterozygosity are 0.562 5 and 0.080 3 for Chinese mitten crab, and 0.437 5 and 0.075 4 for Hepu mitten crab. The Nei' s ge-netic distance between two species is 0.002 4.  相似文献   
78.
Fine root is critical to restrain soil erosion and its distribution pattern is of great influence on the restraining effects. This study studied the fine root biomass(Br) distribution of different aged Leucaena leucocephala(5, 10, 15 years) in debris flow source area in Jiangjia Gully by digging downward to the bottom at different distances to stem in three directions on slope. The results showed the Br increased dramatically by 143% from 5 years to 10 years and then rose slowly by 38% from 10 years to 15 years. The Br of 5 years was significantly asymmetric between uphill and alonghill directions, but there was little difference among directions for other ages, and a concentration trend appeared to exist in downhill and alonghill directions. Moreover, fine root(D≤1 mm) was significantly heavier than that of fine root(1mmD≤2 mm), playing a leading role in the vertical distribution of the whole fine root, with a logarithmic or an exponential function. The results presented may shed light on fine root distribution pattern and evaluation of its effect on slope stability in debris flow source area.  相似文献   
79.
董黎明,李向明,冯长春ASTUDYONREGIONALDIFFERENCEOFCHINA'SPAIDURBANLANDUSESYSTEMANDGRADING¥DongLiming;LiXiangming;FengChangchun(PekingUni...  相似文献   
80.
The very low-frequency oscillation (VLFO) on interannual scale in the tropical Pacific is studied by analyzing the main components of the sea surface temperature, sea level pressure, cloudiness, humidity and the equivalent potential temperature over the tropical ocean surface. The nature of two components of VLFO and its propagation features are discussed from the view of air-sea interaction.  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