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   664篇
  免费   56篇
  国内免费   36篇
测绘学   25篇
大气科学   45篇
地球物理   172篇
地质学   375篇
海洋学   31篇
天文学   30篇
综合类   33篇
自然地理   45篇
  2023年   8篇
  2022年   24篇
  2021年   59篇
  2020年   35篇
  2019年   34篇
  2018年   96篇
  2017年   60篇
  2016年   86篇
  2015年   49篇
  2014年   46篇
  2013年   62篇
  2012年   28篇
  2011年   29篇
  2010年   12篇
  2009年   24篇
  2008年   11篇
  2007年   12篇
  2006年   11篇
  2005年   7篇
  2004年   4篇
  2003年   6篇
  2002年   2篇
  2001年   6篇
  2000年   3篇
  1999年   2篇
  1998年   2篇
  1997年   2篇
  1996年   3篇
  1995年   4篇
  1993年   2篇
  1991年   2篇
  1990年   1篇
  1989年   4篇
  1988年   2篇
  1987年   2篇
  1986年   3篇
  1985年   2篇
  1984年   1篇
  1983年   2篇
  1982年   5篇
  1981年   1篇
  1976年   2篇
排序方式: 共有756条查询结果,搜索用时 15 毫秒
41.
Cultural ecology theoretical framework was found suitable to explain unemployment in agriculture in three villages in Bangladesh. Path analysis and multiple regression statistics were used to examine the direct, indirect, and total effects of eight independent human, social, environmental, and technology variables as well as their aggregate contribution on agricultural unemployment. The multiple regression model explains 86.2% of the total variation in unemployment in agriculture; they were followed by environmental constraints and labor saving technology variables.  相似文献   
42.
43.
The assessment of hydrochemical quality of groundwater is very important to explore its nature and usefulness. In this paper, groundwater quality evaluation is carried out in the Balad district, Salah Al-Din Governorate, Iraq. A total of 28 groundwater samples are collected from shallow tube wells and analyzed for various physicochemical parameters. Groundwater suitability for drinking is evaluated based on the World Health Organization (WHO) and Iraqi standards, and suitability of groundwater for irrigation is assessed based on various hydrochemical parameters. The results reveal that the dominant types of groundwater based on piper diagram are mixed CaMgCl and CaCl. Gibbs ratio indicates that the groundwater in the studied area is affected by the evaporation process. The cation-anion exchange reaction in the studied area demonstrates that 54% of the groundwater samples indicate a direct base (cation-anion) exchange reaction, while 46% of the groundwater samples indicating the chloro-alkaline disequilibrium. Furthermore, generally all of the groundwater samples are unsuitable for drinking and irrigation. Cluster analysis reveals two different groups of similarities between the groundwater samples, reflecting different pollution levels in the studied area.  相似文献   
44.
For mineral resource assessment, techniques based on fuzzy logic are attractive because they are capable of incorporating uncertainty associated with measured variables and can also quantify the uncertainty of the estimated grade, tonnage etc. The fuzzy grade estimation model is independent of the distribution of data, avoiding assumptions and constraints made during advanced geostatistical simulation, e.g., the turning bands method. Initially, fuzzy modelling classifies the data using all the component variables in the data set. We adopt a novel approach by taking into account the spatial irregularity of mineralisation patterns using the Gustafson–Kessel classification algorithm. The uncertainty at the point of estimation was derived through antecedent memberships in the input space (i.e., spatial coordinates) and transformed onto the output space (i.e., grades) through consequent membership at the point of estimation. Rather than probabilistic confidence intervals, this uncertainty was expressed in terms of fuzzy memberships, which indicated the occurrence of mixtures of different mineralogical phases at the point of estimation. Data from different sources (other than grades) could also be utilised during estimation. Application of the proposed technique on a real data set gave results that were comparable to those obtained from a turning bands simulation.  相似文献   
45.
Baker's yeast industry has significant effects on environment due to significant water consumption and high strength wastewater production. Effluents from baker's yeast industry are characterized by high concentrations of biological oxygen demand (BOD5), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total dissolved solids (TDS), sulfate, and dark brown color. The aim of this study is to provide a detailed quantitative and qualitative identification and characterization of the wastewater sources from different processes of a baker's yeast industry. It is identified that the most polluted wastewater components of the facility are coming from rotary drum filter and evaporation process. However, the highest pollution loads is identified as tank and equipment cleaning effluent due to very high flow rate from this source. The efforts should be on the way to minimize the water usage for the tank and equipment cleaning process, and to recycle less polluted waste streams back to the process.  相似文献   
46.
47.
The Junipers phoenicea, which covers 70 % of the Jabal Al Akhdar (Green Mountain) in Cyrene on the northeast coast of Libya, has deteriorated over large scales. To deal with this problem, the images of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 8 Operational Land Imager (OLI) in conjunction with the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) were used to map factors controlling the J. phoenicea mortality using a set of automated algorithms and tools. These factors include altitude, slope, aspect, curvature, drainage pattern, seawater intrusion, and land cover. As a first step, changes of J. phoenicea and land cover during the period from the year 2000 to 2015 were mapped. The results showed a sharp decline in J. phoenicea covering an area of 396 km2 (22 %) of the total area. The result also showed that areas at a lower elevation with steep slope and faced to the south and southeast directions have a higher probability of J. phoenicea distribution. The spatial analysis showed a positive correlation between wetness and the intensity of J. phoenicea mortality. The results also show that altitude and slope have the most influencing power on the J. phoenicea morality. This study is of great help for decision makers and agriculture engineers and permits a better understanding of ecological and biomass changes in the Jabal Al Akhdar, Libya, over a regional scale.  相似文献   
48.
Water quality and hydrochemistry of Shariatpur district were evaluated in terms of hydrochemical composition and some important physico-chemical parameters. The groundwater of the study area is good for drinking, domestic as well as for irrigation purposes. Among the major ions, shallow tube well waters give higher concentration of Ca2+ which ranges from 24 to 260 mg/L. The deep tubewell waters show higher concentration of Na+ which varies from 74 to 582 mg/L during dry season. Among the trace elements most of the shallow aquifer samples show higher concentration of Fe2+, Mn2+ and As. Concentration of Fe2+ varies from 0.655 to 18.8 mg/L, and Mn2+ from trace to 0.868 mg/L during dry period. Hydrochemical analyses reveal significant seasonal variation in water quality of shallow aquifer. Both the shallow aquifer and the surface water of the study area are predominantly of Ca–Mg–HCO3 type, while the deep aquifer water is mainly of Na–K–Cl–SO4 type with slight inclination to Ca–Mg–HCO3 type. The study area is suitable for groundwater development if comprehensive and holistic approaches towards water resource management are taken into consideration.  相似文献   
49.
50.
Flash flood assessment and management are necessary for municipal, urban growth planning and emergency action plans. By using the Hydrologic Engineering Centers River Analysis System software, we can model flash flood events and calculate water surface profiles over the length of the modeled stream. After collecting elevation points by using GPS method, the digital elevation model can be calculated for the study area. Najran city has main flood stream passes beside King Abdullah Road based on facts and previous works. A small study area including the mainstream of Wady Najran and King Abdullah Road has chosen as test site. The used methodology has also been proved efficiently for identifying flood inundation maps. Water extent area overlapped by 52–86% for both used methods. At discharge Q = 15 m3/s, the road needs to be protected from the flood.  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