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   1058篇
  免费   52篇
  国内免费   46篇
测绘学   229篇
大气科学   149篇
地球物理   156篇
地质学   210篇
海洋学   136篇
天文学   5篇
综合类   65篇
自然地理   206篇
  2024年   2篇
  2023年   13篇
  2022年   13篇
  2021年   20篇
  2020年   42篇
  2019年   22篇
  2018年   11篇
  2017年   62篇
  2016年   36篇
  2015年   43篇
  2014年   77篇
  2013年   67篇
  2012年   38篇
  2011年   62篇
  2010年   45篇
  2009年   59篇
  2008年   55篇
  2007年   62篇
  2006年   50篇
  2005年   49篇
  2004年   32篇
  2003年   54篇
  2002年   44篇
  2001年   27篇
  2000年   30篇
  1999年   21篇
  1998年   18篇
  1997年   14篇
  1996年   9篇
  1995年   9篇
  1994年   10篇
  1993年   12篇
  1992年   16篇
  1991年   5篇
  1990年   11篇
  1989年   2篇
  1988年   1篇
  1987年   2篇
  1986年   3篇
  1985年   2篇
  1984年   1篇
  1983年   2篇
  1982年   2篇
  1980年   1篇
排序方式: 共有1156条查询结果,搜索用时 15 毫秒
121.
This paper draws upon research in Victoria's Goulburn Broken Catchment exploring landholder responses to dryland salinity. It has been assumed that part of the explanation for limited adoption of recommended practices was that landholders were unaware of the extent of dryland salinity. Socio-economic data from a mail survey was combined in a Geographic Information System (GIS) with other layers, including the location of discharge sites and depth to saline ground water. Comparisons of expert maps and landholder identified salinity sites suggested that landholders in the upper catchment had excellent knowledge of the current extent of salinity on their property. At the same time, the expert maps failed to predict half of the saline-affected sites identified by landholders. It seems that the extensive community education effort undertaken in this region had been successful in raising salinity awareness. Our research also highlighted that most landholders were not concerned about the impacts of dryland salinity and appear to believe they can 'live with salt'. This is an important issue because the small, diffused amounts of salt exported from these properties in the upper catchment are/will have a substantial impact downstream. The research methodology and findings have important policy and management implications, and these are discussed.  相似文献   
122.
The spatial structure of fishery resources influences stock dynamics and finally the fishery. Therefore, this aspect should be included as a key topic in the assessment and management of fisheries. The fishery of the sea urchin Paracentrotus lividus in Galicia has been used as case study to demonstrate how the mismatch between biological, fishery and management scales causes failures in the governance, giving rise to over-exploitation. P. lividus is spatially distributed in nested biological units: patches, micro-stocks, local populations and metapopulations. Fishing operations are local exploiting micro-stocks; however management units in Galician comprise usually more than one local population. This pattern allows the depletion of several micro-stocks without any short-term signals in the exploitation rates over the complete managed territory. Management units should be redefined according to the boundaries of the local populations. In addition, the implementation of reserve networks or a rotation system could allow to effectively managing the resource at a fine-scale. Any of both regulations could also compensate the inverse density dependence that regulates recruitment and fecundity in this species.  相似文献   
123.
Historic land use can exert strong land-use legacies, i.e., long-lasting effects on ecosystems, but the importance of land-use legacies, alongside other factors, for subsequent forest-cover change is unclear. If past land use affects rates of forest disturbance and afforestation then this may constrain land use planning and land management options, and legacies of current land management may constrain future land use. Our goal was to assess if and how much land-use legacies affect contemporary forest disturbance, and the abundance of different forest types in the Carpathian region in Eastern Europe (265,000 km2, encompassing parts of Poland, Slovakia, Ukraine, Romania, Hungary, and Czech Republic). We modeled contemporary forest disturbance (based on satellite image analysis from 1985 to 2010) as a function of historic land use (based on digitized topographic maps from 1860 and 1960). Contemporary forest disturbance was strongly related to historic land use even when controlling for environmental, accessibility and socio-political variation. Across the Carpathian region, the odds of forest disturbance were about 50% higher in areas that were not forested in 1860 (new forests) compared to areas that were forested then (old forests). The forest disturbance in new forests was particularly high in Poland (88% higher odds), Slovakia (69%) and Romania (67%) and persisted across the entire range of environmental, accessibility and socio-political variation. Reasons for the observed legacy effects may include extensive plantations outside forest ranges, predominantly spruce, poplar, and black locust, which are prone to natural disturbances. Furthermore, as plantations reach harvestable age of about 70 years for pulp and 120 year for saw-timber production, these are likely to be clear-cut, producing the observed legacy effects. Across the Carpathians, forest types shifted towards less coniferous cover in 2010 compared to the 1860s and 1960s likely due to extensive historic conifer harvest, and to recent natural disturbance events and clear-cuts of forest plantations. Our results underscore the importance of land-use legacies, and show that past land uses can greatly affect subsequent forest disturbance for centuries. Given rapid land use changes worldwide, it is important to understand how past legacies affect current management and what the impact of current land management decisions may be for future land use.  相似文献   
124.
The Himalayan Yew (Taxus baccata subsp, wallichiana) is an endangered native high value medicinal plant of the Himalayan Region. The several medicinal properties of the bark and leaves of this species have increased its risk of extinction due to pressures for utilization. It is also subjected to harvest for fuelwood. The species does not regenerate well from seed and that is another risk factor. The objective of this research was to investigate the population ecology of the species as a foundation for its conservation. Six forest communities in the Khokhan Wildlife Sanctuary where the species is present were sampled. The abundance of the species, impacts of harvesting and its current regeneration patterns indicate that it may soon be extirpated from the Sanctuary. A plan for conserving the remaining sub-populations is presented. It could provide a template for conservation in other locations where the species is at risk.  相似文献   
125.
A fossil geothermal area is hosted by the Carboniferous, Permian and Bunter sandstones of the Offenburg intramontane trough in the central Black Forest. The hydrothermal alteration is identified on the basis of newly formed sericites, which appear as pseudomorphs after feldspar and filling of pore spaces. According to K–Ar dating of sericite, serititization occurred about 145 Ma ago (Jurassic). On the basis of 18O analyses of sericite, sericite composition and vitrinite reflectance, the hydrothermal fluids had temperatures of 150–210 °C. Because their electrolyte content was low, these fluids are assumed to have derived from meteoric water. A second pulse of electrolyte-rich hydrothermal fluids resulted in quartz overgrowths. Fluid mobilization seems to be linked to the disintegration of Pangaea and to reactivated fault systems extending from the crystalline basement into the intramontane sediments.  相似文献   
126.
本文从实际出发,阐述“三点并举,拓展设计市场;两极并重,促进企业发展”的发展思路及其在促进企业发展中所起到的作用,探讨勘察设计单位自身发展的历程和模式。  相似文献   
127.
The identification of fossil conifer stomata was conducted from a lake sedimentary sequence on the Liupan Mountains, southwestern Loess Plateau, in order to reconstruct detailed forest history and assess the potential of using stomata as a supplement to pollen analysis to determine the local presence of conifers over the last 3200 years. The pollen has already been analyzed in our previous study. Reference conifer stomata were prepared for the fossil stomata identification and demonstrate that the conifer stomata could be identified to at least genus level. Our stomata and pollen results reveal three clear stages of the forest recession during the late Holocene: mixed conifer-deciduous forest, steppe-forest and steppe. Combined stomata and pollen results confirm the local presence of conifer trees (Abies and Pinus) in the vicinity of the Tianchi Lake. Abundant Abies trees existed during 3200-2200 cal yr BP. Then it decreased and finally demised at ca.210 cal yr BP. Pinus trees have been continuously present with very low abundance throughout the late Holocene. Application of comprehensive studies comprising both stomata and pollen analyses allowed a detailed determination of the stages of conifer trees presence in the vicinity of the Tianchi Lake. Our study suggests that the analysis of fossil stomata is a valuable methodological tool for the provision of unambiguous evidence of the past local presence of the coniferous taxa in this region.  相似文献   
128.
近年来,遥感技术的飞速发展使得影像数据的使用也越来越广泛和深入,而影像分辨率的增加,使得数据量急剧增长,对数据的存储和管理提出了很高的要求。针对传统遥感影像数据组织存储的不足,本文提出了一种大文件金字塔数据存储结构,用来高效存储管理影像数据;同时针对多时相遥感影像数据,提出了基于版本机制的影像数据管理模式。实验表明这种方法具有更高的存储检索效率。  相似文献   
129.
学校图书馆是图书资料的供应与服务基地 ,其藏书量与藏书类别的多少 ,以及图书馆各专业情报资料源的数量与管理和服务水平 ,是办学条件的重要标志之一 ,因而 ,测绘图书情报资料的信息源建设、测绘科技文献的开发与利用和测绘科技情报资料为师生的服务 ,在测绘教育中有着举足轻重的地位与作用。  相似文献   
130.
近年来湖南省在地勘行业管理上开展了法规法制建设、规划计划编制、勘查秩序维护、勘查成果管理、勘查外部环境改善等方面的工作,“八五”后3年继续开展法制建设、勘查登记管理、勘查成果管理、建立行业统计系统、做好服务协调工作,推广改革经验等。  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