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   34596篇
  免费   944篇
  国内免费   377篇
测绘学   952篇
大气科学   2815篇
地球物理   7661篇
地质学   12837篇
海洋学   2814篇
天文学   6600篇
综合类   251篇
自然地理   1987篇
  2021年   294篇
  2020年   303篇
  2019年   311篇
  2018年   1146篇
  2017年   1077篇
  2016年   1082篇
  2015年   614篇
  2014年   905篇
  2013年   1671篇
  2012年   1516篇
  2011年   1614篇
  2010年   1161篇
  2009年   1490篇
  2008年   1295篇
  2007年   1280篇
  2006年   1281篇
  2005年   1645篇
  2004年   1679篇
  2003年   1389篇
  2002年   940篇
  2001年   781篇
  2000年   743篇
  1999年   564篇
  1998年   524篇
  1997年   614篇
  1996年   478篇
  1995年   475篇
  1994年   472篇
  1993年   381篇
  1992年   388篇
  1991年   365篇
  1990年   381篇
  1989年   347篇
  1988年   345篇
  1987年   371篇
  1986年   325篇
  1985年   426篇
  1984年   414篇
  1983年   447篇
  1982年   415篇
  1981年   366篇
  1980年   405篇
  1979年   324篇
  1978年   303篇
  1977年   295篇
  1976年   269篇
  1975年   268篇
  1974年   269篇
  1973年   256篇
  1971年   173篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Krainov  S. R.  Ryzhenko  B. N. 《Water Resources》2002,29(1):21-32
The problem of the diversity of the geochemical types of carbon dioxide waters (CDW) in petrografically and mineralogically uniform crystalline rock masses is solved with allowance made for the effect of different boundary conditions (physicochemical parameters) on the geochemical effect of interaction in the rock–water system. The formation of the entire geochemical spectrum of CDW in crystalline rock masses is shown to be explicable on the basis of a model of interaction in granite–water systems at different mass ratios of reacting rock (S) and water (L), different temperatures T, and equilibrium concentrations of dissolved CO2 (P CO2).  相似文献   
102.
The paper presents the first quantitative results of a laboratory study of the velocity field in a two-arm spiral-wave pattern generated in a steady-state fashion by a hydrodynamical instability in a differentially rotating, thin layer of liquid. The liquid layer has a free surface, and the rotational profile includes an interval where the velocity drops abruptly, as in the gaseous disks of spiral galaxies. The properties of anticyclonic vortices observed between the arms of this pattern at the corotation radius are considered.  相似文献   
103.
104.
Contamination of groundwater has become a major concern in recent years. Since testing of water quality of all domestic and irrigation wells within large watersheds is not economically feasible, one frequently used monitoring strategy is to develop contamination potential maps of groundwater, and then prioritize those wells located in the potentially highly contaminated areas for testing of contaminants. However, generation of contamination potential maps based on groundwater sensitivity and vulnerability is not an easy task due inherent uncertainty. Therefore, the overall goal of this research is to improve the methodology for the generation of contamination potential maps by using detailed landuse/pesticide and soil structure information in conjunction with selected parameters from the DRASTIC model. The specific objectives of this study are (i) to incorporate GIS, GPS, remote sensing and the fuzzy rule-based model to generate groundwater sensitivity maps, and (ii) compare the results of our new methodologies with the modified DRASTIC Index (DI) and field water quality data. In this study, three different models were developed (viz. DIfuzz, VIfuzz and VIfuzz_ped) and were compared to the DI. Once the preliminary fuzzy logic-based (DIfuzz) was generated using selected parameters from DI, the methodology was further refined through VIfuzz and VIfuzz_ped models that incorporated landuse/pesticide application and soil structure information, respectively. This study was conducted in Woodruff County of the Mississippi Delta region of Arkansas. Water quality data for 55 wells were used to evaluate the contamination potential maps. The sensitivity map generated by VIfuzz_ped with soil structure showed significantly better coincidence results when compared with the field data.  相似文献   
105.
106.
107.
In a field near M3 photographed in U,B,V from 1963 to 1969 one of 750 compact galaxies shows a brightening of 1 mag in B during 1966/67.  相似文献   
108.
In the Western Lake Baikal, recent Baikal Rift's tectonics control the topography, seismicity, climate, geomorphology, and economy. Scarps, facets, structural terraces, horsts and grabens, and trapezoid valleys can be clearly observed. They have been generated by the recent tectonic movements along the faults and represent a serious geologic hazard. The specific geological conditions predetermine a unique variety of landscapes. Thus, the main type of the economy is tourism. However, unorganized tourism leads to the degradation of the unique landscapes. It increases risk and requires disaster management. Three criteria has been used for risk assessment: (1) degree of geologic hazard; (2) degree of landscape degradation; and (3) degree of the economy's vitality. The high ecological significance and low stability to antropogeneous pressure are typical for landscapes of the Western Lake Baikal. Thus, some special activities of disaster management should be implemented based on our investigation.  相似文献   
109.
The physical-synoptic principles of the automated method of summer precipitation forecasting introduced into operational prognostic practice in 2006 by the decree of the Central Methodical Commission of the Federal Service for Hydrometeorology and Environmental Monitoring (Roshydromet) are presented. Beside that, improved modifications of this method, which are automatically updated, are shown. Validation of those updates based on observational data collected in 2004 has shown improvement of all major criteria of forecasts efficiency. The performed improvement is based on the more detailed quantitative realization of mechanism of various showers formation (including heavy rainfalls).  相似文献   
110.
Authigenic iron and titanium oxides occur in three main textural varieties in the St. Bees Sandstone, a Triassic red bed succession in Cumbria. These are: (a) overgrowths of haematite, titaniferous haematite, anatase, and rutile which generally occur on detrital host grains of similar compositions. The overgrowths may occur as syntaxial rim cements or rhombohedral and prismatic projections and are always in optical continuity, (b) discrete crystals of anatase (including octahedrite) and haematite 10-40 μm in size occur in pore spaces, and (c) fine grained ( < 1 μm) pigmentary haematite occurs as grain coatings around detrital grains and in the interstitial matrix. The euhedral nature and pore-filling habit clearly indicate the authigenic origin of these iron- and titanium oxides. All three textural varieties are believed to have been precipitated from ground waters containing dissolved ferrous and titanium ions. These ions were derived from the intrastratal solution of detrital iron and titanium bearing grains including ferromagnesian silicates and iron-titanium oxides. The precipitation of iron oxides from migrating ground waters in the manner described here goes some way to explaining how thick, uniformly red successions can be formed.  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