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   4907篇
  免费   963篇
  国内免费   1238篇
测绘学   186篇
大气科学   1174篇
地球物理   1524篇
地质学   2239篇
海洋学   607篇
天文学   228篇
综合类   533篇
自然地理   617篇
  2024年   20篇
  2023年   89篇
  2022年   209篇
  2021年   271篇
  2020年   175篇
  2019年   259篇
  2018年   283篇
  2017年   244篇
  2016年   293篇
  2015年   238篇
  2014年   329篇
  2013年   258篇
  2012年   292篇
  2011年   296篇
  2010年   295篇
  2009年   270篇
  2008年   273篇
  2007年   247篇
  2006年   222篇
  2005年   185篇
  2004年   143篇
  2003年   178篇
  2002年   150篇
  2001年   133篇
  2000年   162篇
  1999年   194篇
  1998年   193篇
  1997年   176篇
  1996年   153篇
  1995年   173篇
  1994年   136篇
  1993年   136篇
  1992年   96篇
  1991年   56篇
  1990年   51篇
  1989年   51篇
  1988年   44篇
  1987年   42篇
  1986年   26篇
  1985年   14篇
  1984年   11篇
  1983年   8篇
  1982年   9篇
  1981年   6篇
  1980年   6篇
  1979年   1篇
  1978年   3篇
  1977年   4篇
  1976年   1篇
  1958年   4篇
排序方式: 共有7108条查询结果,搜索用时 14 毫秒
81.
新疆乌什6.2级地震的中期预测   总被引:2,自引:0,他引:2  
介绍了利用地倾斜资料和中国地壳运动网络工程GPS复测资料对新疆地区震情进行跟踪研究的基本依据、方法和过程,对2005年2月15日发生的乌什Ms6.2地震给出了较好的年度预测意见。  相似文献   
82.
A new record of Platycephalus sp.1 (sensu Nakabo, 2002) was documented based on morphological characters and DNA barcoding. We collected 174 specimens of the genus Platycephalus from Chinese coastal waters of Dongying, Qingdao, Zhoushan, and Beihai. Samples were identified as Platycephalus sp.1 morphologically. The coloration, meristic traits, and morphometric measurements are consistent with previously published records. In brief, it is an orange-brown flathead fish with dark brown spots scattered on head and body, lateral line scales 83 to 99 with one or two spine-bearing anteriormost pored scale, no yellow blotch on the caudal fin. Cytochrome oxidase I subunit (COI) gene fragments were sequenced for phylogenetic analysis. The mean evolutionary distance within the species Platycephalus sp.1 was 0.1%. Net evolutionary distances between Platycephalus sp.1 and other species of Platycephalus ranged from 10.8% to 19.7%, which is much greater than the threshold for species delimitation. The COI sequence analysis strongly supports the validity of Platyceohalus sp.1 at genetic level.  相似文献   
83.
???????6?????л?????????GNSS?????????????????????????????????????????GNSS??????????????о??е???á?  相似文献   
84.
Shi  Ce  Gao  Xiaolong  Liu  Ying  Wang  Chunlin 《中国海洋湖沼学报》2019,37(1):344-349

Self-feeding device is extensively used in aquaculture farms, but for salmonids the individual feeding behavior has seldom been continuously observed. In this article, the individual self-feeding behavior of 10 rainbow trout was continuously monitored with a PIT tag record for 50 days with three replicates. The fish fell into three categories according to their feeding behavior, i.e. high triggering fish (trigger behavior more than 25% of the group, HT), low triggering fish (1%–25%, LT) and zero triggering fish (less than 1%). The results showed that in a group of 10 individual 1–2 HT fish accounted for most of the self-feeding behavior (78.19%–89.14%), which was far more than they could consume. The trigger frequency of the fish was significantly correlated with the initial body weight (P <0.01), however, no significant difference in growth rate among the HT, LT, and ZT fish was observed (P >0.05). Cosinor analysis showed that the two HT fish in the same group had similar acrophase. Though some of the HT fish could be active for 50 d, there were also HT fish decreased triggering behavior around 40 d and the high trigger status was then replaced by other fish, which was first discovered in salimonds. Interestingly, the growth of the group was not affected by the alternation triggering fish. These results provide evidence that in the self-feeding system the HT fish didn’t gain much advantage by their frequent self-feeding behavior, and high trigger status of the HT fish is not only an individual character but also driven by the demand of the group. In the self-feeding system, the critical individual should be closely monitored.

  相似文献   
85.
Adjusting and optimizing land use structure is one of the essential approaches to solve the conflict between land supply and demand. In this study,an uncertain interval multi-objective linear programming model was established and applied to analyzing the suitability of land use structure in Pi County of Sichuan Province. An adjustment scheme for optimizing land use structure was proposed on the basis of development planning drawn up by the local government. The results are summarized as follows: 1) the opti...  相似文献   
86.
We used data from bottom trawl surveys to study the factors influencing the abundance of small yellow croaker, Larimichthys polyactis, in the southern Yellow Sea (SYS) and the East China Sea (ECS). The resource density index (RDI) was generally higher in summer and autumn than in spring and winter. RDIs were also significantly greater in the SYS than in the ECS in summer and autumn. The bottom water salinity and depth of spatial distribution of small yellow croaker was similar between the two areas in summer, but different in other seasons. Regression analysis suggested that environmental factors such as bottom water temperature, salinity, and depth influenced the RDIs in summer in these areas. Growth condition factor (GCF) in the two areas varied monthly and the croaker in the SYS grew more slowly than those in the ECS. This was likely due to the low bottom temperature of the Yellow Sea Cold Water Mass in summer and autumn or to higher human fishing pressure in the ECS. To ensure sustainable utilization of the croaker stocks in these regions, we recommend reducing the fishing intensity, increasing the cod-end mesh size, and improving the protection of juveniles.  相似文献   
87.
针对卫星激光数据处理算法这一难题,分析了COMPASS系统GEO卫星激光观测数据的质量,讨论了反求工程和计算机视觉中的曲线识别与激光观测数据预处理的异同,提出了基于点云曲线辨识的卫星激光数据预处理算法。该算法将卫星激光观测数据预处理视为有序带噪声的空间曲线重建问题,利用COMPASS系统多圈GEO卫星实测数据进行验证。结果表明:该方法与常用算法一致率在85%以上,能够实现激光观测数据的自动处理与满足导航系统的需要。  相似文献   
88.
???й?????????IGS????,??????FES2004????????NAO99b?????????????????????????????????λ?????????????????????????????????λ????????cm??????????????U????????????????????????3~4????????????????????????????????mm????????????????????????С????????????????GPS????????????????10-8??????????????????????????????????С??mm?????????????????????????????1 cm??  相似文献   
89.
??????GPS/??????????????λ????????????????Helmert????????????????????GPS/????????????????????????????????????÷???????????????????????????  相似文献   
90.
The nitrogen(N) and phosphorus(P) budget and the ecological efficiency of a polyculture system of sea cucumber(Apostichopus japonicus), jellyfish(Rhopilema esculenta) and shrimp(Fenneropenaeus chinensis) were studied in a cofferdam, 120.2 ha in size. The nutrients were supplied by spring tide inflow. In total, 139600 kg N yr-1 and 9730 kg P yr-1 input to the system; while 118900 kg N yr-1 and 2840 kg P yr-1 outflowed from the system concurrently, thus the outflow was 85.7%(N) and 29.2%(P) of inflow. The production of N and P was 889.5 kg yr-1 and 49.28 kg yr-1(sea cucumber) and 204 kg yr-1 and 18.03 kg yr-1(jellyfish and shrimp), respectively. The utilization rate of N and P by polycultured animals was 7.8‰ and 6.9‰, respectively, 21.9% and 38% higher than that of monocultured sea cucumber. Our results indicated that the polyculture system was an efficient culture system of animals and a remediation system of coastal environment as well; it scavenged 14.3% and 70.8% of N and P, respectively. Such an ecological efficiency may be improved further by increasing either the stocking density or the size of sea cucumber or both.  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