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   4787篇
  免费   148篇
  国内免费   61篇
测绘学   166篇
大气科学   427篇
地球物理   1042篇
地质学   1769篇
海洋学   435篇
天文学   852篇
综合类   11篇
自然地理   294篇
  2023年   14篇
  2022年   44篇
  2021年   62篇
  2020年   67篇
  2019年   61篇
  2018年   149篇
  2017年   144篇
  2016年   189篇
  2015年   118篇
  2014年   163篇
  2013年   253篇
  2012年   197篇
  2011年   285篇
  2010年   214篇
  2009年   302篇
  2008年   242篇
  2007年   204篇
  2006年   233篇
  2005年   222篇
  2004年   301篇
  2003年   231篇
  2002年   152篇
  2001年   108篇
  2000年   104篇
  1999年   84篇
  1998年   95篇
  1997年   67篇
  1996年   40篇
  1995年   45篇
  1994年   40篇
  1993年   35篇
  1992年   41篇
  1991年   27篇
  1990年   38篇
  1989年   17篇
  1988年   17篇
  1987年   23篇
  1986年   19篇
  1985年   16篇
  1984年   22篇
  1983年   23篇
  1982年   14篇
  1981年   24篇
  1978年   22篇
  1977年   20篇
  1976年   16篇
  1975年   18篇
  1974年   16篇
  1973年   15篇
  1971年   12篇
排序方式: 共有4996条查询结果,搜索用时 15 毫秒
81.
Planning for source water protection in Ontario   总被引:2,自引:0,他引:2  
Ontario began developing a drinking water source protection planning process in 2000, after a rural community's water supply was contaminated by pathogens from agricultural runoff. An expert committee report has recommended legislating a new source protection “sphere of jurisdiction” for municipalities; however, no specific new municipal tools or authorities have been proposed as yet. This paper reports findings from an evaluation of the extent to which existing institutional arrangements for land use planning and water management facilitate or constrain source protection by municipalities. A case study of the Regional Municipality of Waterloo suggests there is the potential to link land use planning more strongly to water management through creative use of existing tools, forgoing the need for major institutional changes.  相似文献   
82.
83.
84.
85.
Mössbauer spectra (MS) of blue, green and yellow beryl (ideally Be3Al2Si6O18) containing approximately 1% of iron were obtained at 295 and 500 K. Room temperature (RT) spectra of both blue and green samples showed the presence of an asymmetric Fe2+ doublet (ΔE Q~2.7 mm/s, δ~1.1 mm/s), with a very broad low-velocity peak. There is no clear evidence for the presence of a ferric component. The MS of the yellow sample at RT consists of an intense central absorption with parameters typical for Fe3+E Q~0.4 mm/s, δ~0.29 mm/s), plus an apparently symmetrical Fe2+ doublet. This sample acquires a light-blue shade upon heating in air at about 620 K. Thermal treatments at high temperatures caused no significant changes in the MS, but the green and yellow beryl acquire a blue colour. All these results are interpreted in relation to the existence of channel water and the distribution of iron among the available crystallographic sites.  相似文献   
86.
87.
88.
Using entropy theory, a methodology was developed for the evaluation and redesign of groundwater quality monitoring wells in the Gaza Strip in Palestine. Essential to the methodology is the development of a Transinformation Model (TM) which yields the amount of information transfer and the dependency between the wells as a function of distance. The TM parameters, such as the minimum transinformation and the range, were employed for evaluating the network which revealed that most of the distances between wells were less than the range. It also indicated that a high percentage of redundant information existed in the network. Therefore, the network was reduced by superimposing a square pattern over the monitored area and selecting one well per square block in a stratified pattern. The methodology was tested using the chloride data collected from 1972–2000 from 417 groundwater quality monitoring wells in the Gaza Strip. The number of the groundwater quality monitoring wells in the Gaza Strip was reduced by 53%, while there was 26% redundant information based on the minimum existing distance between wells. This methodology is meant to help monitor the groundwater quality (salinity) in the Gaza Strip.  相似文献   
89.
A generic network design in close range photogrammetry is one where optimal multi-ray intersection geometry is obtained with as few camera stations as practicable. Hyper redundancy is a concept whereby, once the generic network is in place, many additional images are recorded, with the beneficial impact upon object point precision being equivalent to the presence of multiple exposures at each camera position within the generic network. The effective number of images per station within a hyper redundant network might well be in the range of 10 to 20 or more. As is apparent when it is considered that a hyper redundant network may comprise hundreds of images, the concept is only applicable in practice to fully automatic vision metrology systems, where it proves to be a very effective means of enhancing measurement accuracy at the cost of minimal additional work in the image recording phase. This paper briefly reviews the network design and accuracy aspects of hyper redundancy and illustrates the technique by way of the photogrammetric measurement of surface deformation of a radio telescope of 26 m diameter. This project required an object point measurement accuracy of σ  = 0·065 mm, or 1/400 000 of the diameter of the reflector.  相似文献   
90.
Atmospheric dimethyl sulfide (DMS) and sulfur dioxide (SO2) concentrations were measured at Baring Head, New Zealandduring February and March 2000. Anti-correlated DMS and SO2 diurnalcycles, consistent with the photochemical production of SO2 from DMS, were observed in clean southerly air off the ocean. The data is used to infer a yield of SO2 from DMS oxidation. The estimated yields are highly dependent on assumptions about the DMS oxidation rate. Fitting the measured data in a photochemical box model using model-generated OH levels and the Hynes et al. (1986) DMS + OH rate constant suggests that theSO2 yield is 50–100%, similar to current estimates for the tropical Pacific.However, the observed amplitude of the DMS diurnal cycle suggests that the oxidation rate is higher than that used by the model, and therefore, that theSO2 yield is lower in the range of 20–40%.  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