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   1604篇
  免费   122篇
  国内免费   150篇
测绘学   376篇
大气科学   205篇
地球物理   354篇
地质学   326篇
海洋学   114篇
天文学   9篇
综合类   76篇
自然地理   416篇
  2024年   3篇
  2023年   12篇
  2022年   22篇
  2021年   22篇
  2020年   45篇
  2019年   54篇
  2018年   32篇
  2017年   97篇
  2016年   76篇
  2015年   72篇
  2014年   76篇
  2013年   146篇
  2012年   71篇
  2011年   85篇
  2010年   47篇
  2009年   103篇
  2008年   125篇
  2007年   135篇
  2006年   98篇
  2005年   76篇
  2004年   66篇
  2003年   49篇
  2002年   50篇
  2001年   51篇
  2000年   55篇
  1999年   39篇
  1998年   28篇
  1997年   20篇
  1996年   24篇
  1995年   13篇
  1994年   12篇
  1993年   22篇
  1992年   14篇
  1991年   10篇
  1990年   5篇
  1989年   9篇
  1988年   4篇
  1987年   2篇
  1986年   1篇
  1985年   1篇
  1984年   2篇
  1983年   1篇
  1978年   1篇
排序方式: 共有1876条查询结果,搜索用时 15 毫秒
151.
This paper presents the Biogeographic Assessment Framework (BAF), a decision support process for marine spatial planning (MSP), developed through two decades of close collaborations between scientists and marine managers. Spatial planning is a considerable challenge for marine stewardship agencies because of the need to synthesize information on complex socio-ecological patterns across geographically broad spatial scales. This challenge is compounded by relatively short time-frames for implementation and limited financial and technological resources. To address this pragmatically, BAF provides a rapid, flexible and multi-disciplinary approach to integrate geospatial information into formats and visualization tools readily useable for spatial planning. Central to BAF is four sequential components: (1) Planning; (2) Data Evaluation; (3) Ecosystem Characterization; and (4) Management Applications. The framework has been applied to support the development of several marine spatial plans in the United States and Territories. This paper describes the structure of the BAF framework and the associated analytical techniques. Two management applications are provided to demonstrate the utility of BAF in supporting decision making in MSP.  相似文献   
152.
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   
153.
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   
154.
分别对Hinode光学望远镜SOT观测太阳黑子的本影和半影图像以及太阳米粒组织图像进行了空间频谱计算和相关计算,分析了太阳不同结构区域图像的时域和空域特性对哈特曼波前探测精度的影响。计算和分析结果表明,在子孔径波面倾斜小于0.25″情况下,米粒组织图像采样时间在2 min内、本影和半影采样时间在4 min内,对子孔径波面倾斜探测精度的影响极小;并且由太阳目标特性引起的哈特曼波面探测误差随波面畸变量的增大而增大。这些研究结果可为太阳望远镜哈特曼波前探测器研制和应用提供依据和参考。  相似文献   
155.
本文是世界各国日像仪的天线阵性能汇集,包括日、俄、法、中、美等专用日像仪及美国、印度、巴西等兼作日像仪的天线阵,列出了几个典型天线阵列排列及接收系统。附录中列出了等口径及不等口径天线阵列的灵敏度及动态范围计算公式,同时列出了视场、角分辨率等公式。便于读者自行分析,计算,比较各天线阵列性能。  相似文献   
156.
Despite continued uncertainty about the physical realities and political, economic and social implications of peak oil, combined concerns about oil scarcity, climate change and globalisation has spawned an energetic relocalisation movement dedicated to achieving a comprehensive reduction in oil dependency through community-scale initiatives. This paper uses a discourse approach to examine the emergence, geographical spread and practices of the Transition Network, a UK-originated relocalisation movement now involving 186 local initiatives in the UK and other countries. We trace the movement’s drawing upon, and innovation from, discourses and techniques used by other grassroots environmental movements to promote a spatial representation of peak oil as an inevitable and geographically undiscriminating problem, and its use of addiction metaphors and participatory techniques to promote personal and community-scale energy descent initiatives as a viable and necessary alternative to globalisation. We also analyse the spatial representations and techniques used in the Network’s “rhizomic” spread across multiple localities around the world and embedding in communities where relocalisation initiatives are established. We conclude by examining the future challenges these spatial constructions of peak oil pose for the relocalisation movement.  相似文献   
157.
http://www.sciencedirect.com/science/article/pii/S1674987110000113   总被引:3,自引:0,他引:3  
<正>Against the current background of global climate change,the study of variations in the soil carbon pool and its controlling factors may aid in the evaluation of soil's role in the mitigation or enhancement of greenhouse gas.This paper studies spatial and temporal variation in the soil carbon pool and their controlling factors in the southern Song-nen Plain in Heilongjiang Province,using soil data collected over two distinct periods by the Multi-purpose Regional Geochemical Survey in 2005—2007, and another soil survey conducted in 1982—1990.The study area is a carbon source of 1479 t/km~2 and in the past 20 years,from the 1980s until 2005.the practical carbon emission from the soil was 0.12 Gt.Temperature,which has been found to be linearly correlated to soil organic carbon,is the dominant climatologic factor controlling soil organic carbon contents.Our study shows that in the relevant area and time period the potential loss of soil organic carbon caused by rising temperatures was 0.10 Gt,the potential soil carbon emission resulting from land-use change was 0.09 Gt,and the combined potential loss of soil carbon(0.19 Gt) caused by warming and land-use change is comparable to that of fossil fuel combustion(0.21 Gt).Due to the time delay in soil carbon pool variation,there is still 0.07 Gt in the potential emission caused by warming and land-use change that will be gradually released in the future.  相似文献   
158.
European Union expansion over the last decade has generated many studies, surpassed in number only by governmental comment and media headlines following this general theme. Issues and knowledge explored and expressed have been many, one being a hard to evidence crime-migration nexus leading to constructive empirical studies focussing on the accession to the EU of predominantly East European nations and ensuing migratory patterns. Appertaining to this study is geographic focus on crime committed by inter-EU migrants in England, resulting from holistic research of a contemporary EU statute concerning inter-state sharing of bio-informatics, seeking contextual knowledge to the benefit of policy makers. The geography of inter-EU crime in England identifies spatial dispersion and regional areas of activity on a meso scale. The activity of individual nationalities is examined in a spatial manner displaying distinct differences in movements on a regional (meso) scale. Results improve on current knowledge of the criminality of inter-EU migrants but also identify and call for further research in a conceptual development as localised criminal activity draws an international investigatory perspective.  相似文献   
159.
The importance of urban growth processes and their spatial characteristics has led to a growing interest in monitoring these phenomena. Spatial metrics are widely employed for this purpose, appearing in an increasing number of studies where they are used to characterise growth patterns and their evolution over time. This paper presents an analysis of urban growth patterns using spatial metrics in the Algarve (southern Portugal), an area of considerable urban dynamics associated with tourism. Two datasets were used (CORINE 1:100,000 maps and COS 1:25,000 maps) and two time periods (1990 and 2006–2007) in order to compare the different urban land use patterns detected and their evolution over time. The results show differences in urban land use patterns and associated processes at each scale, with stable land use patterns predominating at the 1:100,000 scale whereas the 1:25,000 scale showed a move towards more dispersed patterns. These results have enabled an assessment of the principal differences in urban growth patterns observed at both scales, and the implications for planning these entail.  相似文献   
160.
Successful implementation of a forest based climate change mitigation mechanism such as REDD + depends on robust and available methods for measurement and estimation of forest degradation. Currently available methods are for application in single-hit degradation incidents in high density humid forests. However, it has been suggested that gradual degradation, especially in dry forests, is more widespread and that methods are needed for measuring and estimating associated emissions. We assess the applicability of an indirect remote sensing approach for monitoring forest degradation: infrastructure and other indicators of human activities are mapped and used for spatial prediction of degradation activities. For proxy variables we tested distance to forest edge, distance to roads, and population pressure calculated as the sum of inhabitants per pixel in the Landscan 2010 population raster dataset multiplied by an inverse power distance decay function. Wood extraction incidents were counted in 160 plots in two dry forests in Tanzania with infrastructural entry from one side only. We analyzed the spatial pattern of forest degradation as a function of the chosen proxy variables using zero inflated count models which allows for an excess of zero counts. A jack-knife bootstrap using 10,000 runs was applied to optimize the population distance decay function. We found that the impact of forest degradation is highest near high population concentration, above 1000 individuals. Furthermore, distance to nearest forest edge or road was a significant proxy for estimation of the number of wood extraction incidents (p < 0.001), where degradation incidents decreased with increasing distance to forest edge or road. At 3000 m from the forest edge towards the forest core the probability of wood extraction is 20% and dropping. The population distance decay function was found to have a steep decline indicating a relative small impact on forest degradation. Further, and perhaps larger, studies are needed to be able to recommend a distance decay function for general application in Tanzania. However, the results are useful for understanding spatial patterns of wood harvesting as a function of distance to nearest forest edge or road in dry Miombo woodland areas with average population pressure at 1685 ± 101 persons within a radius of 4000 m from the wood extraction sites.  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