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   1669篇
  免费   96篇
  国内免费   98篇
测绘学   73篇
大气科学   69篇
地球物理   331篇
地质学   1053篇
海洋学   105篇
天文学   99篇
综合类   45篇
自然地理   88篇
  2023年   7篇
  2022年   22篇
  2021年   38篇
  2020年   47篇
  2019年   35篇
  2018年   131篇
  2017年   113篇
  2016年   126篇
  2015年   73篇
  2014年   111篇
  2013年   186篇
  2012年   74篇
  2011年   96篇
  2010年   71篇
  2009年   73篇
  2008年   43篇
  2007年   35篇
  2006年   44篇
  2005年   32篇
  2004年   43篇
  2003年   67篇
  2002年   116篇
  2001年   83篇
  2000年   38篇
  1999年   15篇
  1998年   11篇
  1997年   14篇
  1995年   9篇
  1994年   7篇
  1993年   6篇
  1992年   7篇
  1991年   8篇
  1990年   7篇
  1989年   4篇
  1988年   2篇
  1987年   7篇
  1986年   5篇
  1985年   4篇
  1984年   5篇
  1983年   3篇
  1982年   6篇
  1981年   4篇
  1980年   2篇
  1979年   9篇
  1978年   4篇
  1977年   4篇
  1976年   2篇
  1975年   5篇
  1973年   2篇
  1972年   2篇
排序方式: 共有1863条查询结果,搜索用时 62 毫秒
151.
In the last 30 years the climate of the West African Sahel has shown various changes, especially in terms of rainfall, of which inter-annual variabilityis very high. This has significant consequences for the poor-resource farmers, whose incomes depend mainly on rainfed agriculture. The West African Sahel is already known as an area characterized by important interaction between climate variability and key socio-economic sectors such as agriculture and water resources. More than 80% of the 55 million population of West African Sahel is rural, involved in agriculture and stock-farming, the two sectors contributing almost 35% of the countries' GDPs. It is thereforeobvious that climate change seriously affects the economies of these countries. Adding to this situation the high rate of population increase(3%), leading to progressive pressure upon ecosystems, and poorsanitary facilities, one comes to the conclusion that Sahelian countries, Niger amongst them, will be highly vulnerable to climate change.This paper investigates the impact of current climate variability and future climate change on millet production for three major millet-producing regions in Niger. Statistical models have been used to predict the effects of climate change on future production on the basis of thirteen available predictors. Based on the analysis of the past 30-years of rainfall and production data, the most significant predictors of the model are (i) seasurface temperature anomalies, (ii) the amount of rainfall in July, August and September, (iii) the number of rainy days and (iv) the wind erosion factor. In 2025, production of millet is estimated to be about 13% lower as a consequence of climate change, translated into a reduction of the total amount of rainfall for July, August and September, combined with an increase in temperature while maintaining other significant predictors at a constant level. Subsequently,various potential strategies to compensate this loss are evaluated, including those to increase water use efficiency and to cultivate varieties that are adapted to such circumstances.  相似文献   
152.
流域地理景观的GIS数据三维可视化   总被引:17,自引:1,他引:16  
应用以"GIS"为核心的空间信息技术,是开展"数字流域"工程建设自然和最佳的选择。结合"清江流域水文水情与洪水演进仿真系统"的具体研制,分析流域地理数据特点及传统GIS在"数字流域"建设中的优缺点,在应用传统GIS二维方式展示形式管理和预处理先期各类基础流域地理空间数据及其相应属性数据的基础上,为满足仿真系统系列三维可视化要求,建立面向流域空间实体对象的数据模型,定义流域地理空间对象数据结构,将流域地理常规的GIS数据以三维形式展示,在实际的系统开发过程中,取得较好的三维可视化效果,并可满足"数字清江"建设各阶段综合处理、分析、评价、决策以及可视化等方面的需求。  相似文献   
153.
森林火灾遍及全球,损失巨大。火险天气指数(FWI)是森林火灾预警和管理的有效工具。相对湿度(RH)是计算天气火险指数一个非常重要的参数。然而,由插值方法得到的RH在远离气象站点的地方往往存在很大的误差。因此,尝试利用MODIS遥感数据计算基于像元的高分辨率RH。MODIS的MOD07大气廓线产品,可以提供大气可降水量和地表气温数据。基于地面水汽压和大气可降水量的经验关系,利用MODIS数据计算了地表空气的相对湿度,平均绝对误差小于5%。结果表明,利用MODIS数据能简单而有效地计算地表空气的相对湿度,拓展了遥感技术在森林火险管理中的应用。  相似文献   
154.
155.
The present study has been undertaken to determine the nature and the factors controlling the distribution of carbonate content in Kuwait Bay sediments. The carbonate content that may be deposited with Kuwait Bay sediments is attributed to the interplay of the deposits fallout from the dust storms over Kuwaiti territory, the deposits carried in suspension by the currents from Shatt Al-Arab area and to the skeletal parts of microorganisms that may be originated in the sediments. The carbonate content increases in the coarse sediments rather than in the fine sediments. The mineral components of the carbonate present are in the form of calcite, aragonite and some dolomite. Therefore, the texture of the sediments, the rate of deposition in addition to the biological interference may exert a considerable control over carbonate distribution.  相似文献   
156.
A possible suture between an old craton in West Africa and the Arabian—Nubian shield is marked by a persistent zone of Phanerozoic weakness. This zone extends between the Nile and Niger deltas and passes through the Qattara embayment, along the Bahariya fault, through the Kufra basin, between the Tibesti and Jabal al Uwaynat massifs, through the Bahr el Ghazal and Lake Chad areas, and along the Benoue trough.  相似文献   
157.
The Primrose Prospect lies 48 km east of Sable Island on the Scotian Shelf below 80 km of water. The subsurface geology is characterized by the presence of Jurassic salt that has pierced the early Cretaceous sediments and uplifted late Cretaceous and early Tertiary beds to provide localized structural closure, shale compaction, and anomalously high heat gradients in the younger sediments overlying the salt mass.The reservoir rocks interbedded with the compacted shales contain wet gas, condensate, and 31° API oil. The organic matter in these shales, which consists essentially of amorphous material, has undergone thermal alteration and maturation at a very shallow depth, a condition not apparent in coeval beds elsewhere in the area. This localized alteration of the organic matter is considered to have occurred in response to the thermal and pressure effects of the underlying halokinetic structure and could have resulted in the genesis of the mature hydrocarbons.  相似文献   
158.
This article investigates whether the European Union-Mauritania fisheries agreement, which allocates part of the Europe's financial contribution to the conservation of marine ecosystems located within the Banc d’Arguin National Park, can be regarded as a payment for ecosystem service. A framework for qualification as such payment scheme was established based on an extensive literature review. The criteria identified for the qualification as a payment to ecosystem service pertain to: (1) the definition of the ecosystem service(s) involved; (2) the mechanism involved by the payment; and (3) the nature of the transaction. Interviews with local beneficiaries and subsequent data analysis led to the conclusion that this mechanism could be regarded as a payment to ecosystem service and so, through the European Union-Mauritania Fisheries agreement, the European Union were investing to protect local fish resources that could be exploited by its fishing fleet. This agreement, involving the first International Payment to Ecosystem Service of this kind, marks an important step towards better consideration of marine conservation in international public policy and foreign fishing policy in particular. However, this payment is small when compared to revenues generated through the exploitation of developing countries’ fishing grounds by fishing countries. Nevertheless it opens the door for more detailed applications of payment to ecosystem service schemes to other ecosystems contexts, and can provide a useful alternative source of financing of marine biodiversity conservation.  相似文献   
159.
The selection of the potential sites to the creation of an agro-industrial complex requires a large amount of spatial information. Geographical information systems are capable of managing this information, providing the ability to integrate multiple layers of information and deriving new ones. This study incorporates environmental, social, economic, geological and infrastructural data. A multi-criteria analysis is performed to select the potential sites to the creation of an agro-industrial complex in the city of Sfax (the southern coast of Tunisia). The comparison by pair method adopted in this paper, based on a linear combination pondered after having assigned to every factor of decision a coefficient of level-headedness, creates many scenarios among which we selected the sustainable development one. This scenario assigned the highest weight to the economical, environmental and social factors (0.3 for each factor), which shows the presence of nine suitable sectors for the creation of an agro-industrial complex. The ELECTRE method enabled us to make a site ordering for the identification of the most three appropriate sites.  相似文献   
160.
This paper analyses new field and radiometric age data on the timing of extension, sedimentation and magmatism related to the opening of the Red Sea. The first evidence of widespread extension is found in synsedimentary fissures in the fluviatile to shallow marine Tawilah Formation of Cretaceous to early Tertiary age. The first indication of magmatism is the eruption of alkaline basalts (43.5–21.2 Ma) on to marine and non-marine sandstones of lower to middle Tertiary age. The lack of an angular unconformity at the sediment-basalt contact, over a period of 25 Myr, corroborates a diachronous relationship between the Tawilah sandstones and the Yemen Volcanics. These data also indicate that the onset of volcanism in Yemen was in the Eocene, as is the case in Ethiopia and Saudi Arabia. A passive rifting model is supported by early extension and sedimentation followed by syn- and post-tectonic magmatism.  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