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   5981篇
  免费   1057篇
  国内免费   1217篇
测绘学   584篇
大气科学   761篇
地球物理   1061篇
地质学   3617篇
海洋学   859篇
天文学   137篇
综合类   540篇
自然地理   696篇
  2024年   52篇
  2023年   156篇
  2022年   198篇
  2021年   214篇
  2020年   172篇
  2019年   233篇
  2018年   220篇
  2017年   132篇
  2016年   183篇
  2015年   219篇
  2014年   306篇
  2013年   234篇
  2012年   283篇
  2011年   299篇
  2010年   271篇
  2009年   260篇
  2008年   242篇
  2007年   257篇
  2006年   266篇
  2005年   271篇
  2004年   250篇
  2003年   261篇
  2002年   211篇
  2001年   229篇
  2000年   207篇
  1999年   201篇
  1998年   237篇
  1997年   235篇
  1996年   239篇
  1995年   237篇
  1994年   210篇
  1993年   173篇
  1992年   200篇
  1991年   177篇
  1990年   156篇
  1989年   112篇
  1988年   60篇
  1987年   41篇
  1986年   37篇
  1985年   49篇
  1984年   43篇
  1983年   45篇
  1982年   33篇
  1981年   30篇
  1980年   22篇
  1979年   16篇
  1978年   7篇
  1959年   8篇
  1958年   10篇
  1957年   6篇
排序方式: 共有8255条查询结果,搜索用时 325 毫秒
811.
812.
813.
树木葱茂,绿草如茵,座座房屋整齐排列,条条道路笔直洁净,村民们或三三两两结伴散步,或围坐家门话桑麻,孩子们互相追逐、嬉戏,尽享童年的快乐,一片欢乐和谐的景象,这就是镇平县城郊乡大奋庄村今日的写照。可就在几年前,这里还是另一幅情景:废弃的老宅有的倒塌、破败,生活垃圾、动物粪便到处乱堆,别说往里走,就是  相似文献   
814.
地质工作如何"更加紧密地与国民经济与社会发展相结合,更加主动地为经济与社会发展服务",是我们面对的时代课题。在国家加强地质工作的大好形势下,强化"大地质"意识则是  相似文献   
815.
介绍了公路收费系统中所需的实时数据通信功能的设计,并通过组件方式,将其制作成具有一定安全性的通用组件,提供给应用系统调用,从而提高开发效率.  相似文献   
816.
蒙特卡罗方法也称统计试验法.它是根据统计抽样理论,通过对随机变量函数的概率模拟、统计试验来近似求解的方法. 矿产资源无论是作为地质过程的产物还是作为地质观测的结果,它都具有随机的性质.对资源量的计算,必然受概率法则的支配,因而是一定概率意义下的估计.由于蒙特卡罗方法能够正确地模拟随机变量的分布,再现它的取值规律,因而在矿产资源总量预测中被广泛用作资源估计的方法.  相似文献   
817.
818.
Fouling diatoms are a main component of biofilm,and play an important role in marine biofouling formation. We investigated seasonal variations in fouling diatom communities that developed on glass slides immersed in seawater,on the Yantai coast,northern Yellow Sea,China,using microscopy and molecular techniques. Studies were conducted during 2012 and 2013 over 3,7,14,and 21 days in each season. The abundance of attached diatoms and extracellular polymeric substances increased with exposure time of the slides to seawater. The lowest diatom density appeared in winter and the highest species richness and diversity were found in summer and autumn. Seasonal variation was observed in the structure of fouling diatom communities. Pennate diatoms Cylindrotheca,Nitzschia,Navicula,Amphora,Gomphonema,and Licmophora were the main fouling groups. Cylindrotheca sp. dominated in the spring. Under laboratory culture conditions,we found that Cylindrotheca grew very fast,which might account for the highest density of this diatom in spring. The lower densities in summer and autumn might result from the emergence of fouling animals and environmental factors. The Cylindrotheca sp. was identified as Cylindrotheca closterium using18 S rDNA sequencing. The colonization process of fouling diatoms and significant seasonal variation in this study depended on environmental and biological factors. Understanding the basis of fouling diatoms is essential and important for developing new antifouling techniques.  相似文献   
819.
Apostichopus japonicus(Holothuroidea,Echinodermata) is an ecological and economic species in East Asia.Conventional biometric monitoring method includes diving for samples and weighing above water,with highly variable in weight measurement due to variation in the quantity of water in the respiratory tree and intestinal content of this species.Recently,video survey method has been applied widely in biometric detection on underwater benthos.However,because of the high flexibility of A.japonicus body,video survey method of monitoring is less used in sea cucumber.In this study,we designed a model to evaluate the wet weight of A.japonicus,using machine vision technology combined with a support vector machine(SVM) that can be used infield surveys on the A.japonicus population.Continuous dorsal images of free-moving A.japonicus individuals in seawater were captured,which also allows for the development of images of the core body edge as well as thorn segmentation.Parameters that include body length,body breadth,perimeter and area,were extracted from the core body edge images and used in SVM regression,to predict the weight of A.japonicus and for comparison with a power model.Results indicate that the use of SVM for predicting the weight of 33 A.japonicus individuals is accurate(R~2=0.99) and compatible with the power model(R~2=0.96).The image-based analysis and size-weight regression models in this study may be useful in body weight evaluation of A.japonicus in lab and field study.  相似文献   
820.
Spatio-seasonal patterns of fish diversity,Haizhou Bay,China   总被引:3,自引:0,他引:3  
Spatial-seasonal patterns in fish diversity in Haizhou Bay were studied based on stratified random surveys conducted in 2011.Principal component analysis was conducted to distinguish different diversity components,and the relationships among 11 diversity indices were explored.Generalized additive models were constructed to examine the environmental effects on diversity indices.Eleven diversity indices were grouped into four components:(1) species numbers and richness,(2) heterogeneous indices,(3) evenness,and(4) taxonomic relatedness.The results show that diversity indices among different components are complementary.Spatial patterns show that fish diversity was higher in coastal areas,which was affected by complex bottom topography and spatial variations of water mass and currents.Seasonal trends could be best explained by the seasonal migration of dominant fish species.Fish diversity generally declined with increasing depth except for taxonomic distinctness,which increased with latitude.In addition,bottom temperature had a significant effect on diversity index of richness.These results indicate that substrate complexity and environmental gradients had important influences on fish diversity patterns,and these factors should be considered in fishery resource management and conservation.Furthermore,diversity in two functional groups(demersal/pelagic fishes) was influenced by different environmental factors.Therefore,the distribution of individual species or new indicators in diversity should be applied to examine spatio-seasonal variations in fish diversity.  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