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   167篇
  免费   19篇
  国内免费   20篇
林业   42篇
农学   6篇
基础科学   18篇
  40篇
综合类   52篇
农作物   9篇
水产渔业   3篇
畜牧兽医   10篇
园艺   12篇
植物保护   14篇
  2024年   1篇
  2023年   5篇
  2022年   9篇
  2021年   15篇
  2020年   15篇
  2019年   11篇
  2018年   9篇
  2017年   16篇
  2016年   13篇
  2015年   8篇
  2014年   9篇
  2013年   8篇
  2012年   17篇
  2011年   8篇
  2010年   11篇
  2009年   13篇
  2008年   6篇
  2007年   5篇
  2006年   9篇
  2005年   5篇
  2004年   4篇
  2003年   2篇
  2002年   1篇
  2001年   2篇
  1997年   1篇
  1994年   1篇
  1987年   2篇
排序方式: 共有206条查询结果,搜索用时 15 毫秒
1.
1990—2019年阿拉尔垦区植被覆盖时空变化特征分析   总被引:2,自引:0,他引:2  
宋奇  高琪  马自强  王楠  王明玥  彭杰 《草地学报》2021,29(5):1014-1024
监测植被覆盖变化对区域生态环境评价和可持续发展具有重要意义。本文基于Landsat遥感影像和相关统计数据,运用像元二分模型和重心转移模型等方法分析阿拉尔垦区1990—2019年近30年的植被覆盖时空变化特征。结果表明:1990—2019年,阿拉尔垦区平均植被覆盖面积呈增加趋势,高和极高植被覆盖区的面积呈增加趋势,差异显著,分别增加了674 km2和1 147 km2;阿拉尔垦区植被覆盖变化存在时段性和区域性差异,时段上,2005—2010年垦区西北部开始出现植被,以中覆盖类型为主,区域上,垦区西北部植被覆盖的增加趋势最显著;近30年,垦区植被覆盖重心整体向东北方向转移;气候对阿拉尔垦区植被覆盖变化有一定的影响,但人类活动影响最直接。在未来发展中应注重保护自然植被,有计划地开垦土地,合理调整垦区农业结构,科学分配水权,保证垦区生态用水,实现可持续发展。  相似文献   
2.
【目的】探究Landsat8 OLI数据和KNN算法在森林蓄积量估测中的潜力。【方法】以湖南省湘潭县为研究区,采用Landsat8 OLI数据和同时期的二类调查数据,通过距离相关系数筛选特征,分别采用线性回归模型(MLR)、K-近邻模型(KNN)、距离加权KNN模型(DW-KNN)和优化欧式KNN模型(FW-KNN)对森林蓄积量进行估测。使用十折交叉方法进行精度检验,对检验结果进行对比分析。【结果】3种KNN模型的估测结果均高于传统的线性模型,并且在3种KNN模型中,FW-KNN算法效果最好,决定系数达到0.69,为3种模型中最高;3种KNN模型中,本研究优化欧氏距离KNN模型的估测精度最高,其均方根误差为30.3%,相比于传统KNN模型的均方根误差降低了5.1%,相比于DW-KNN模型降低了3.3%。【结论】采用DW-KNN蓄积量估测结果明显优于其他两种模型,说明通过特征与蓄积量的相关性优化样本间的距离是一种可行的KNN优化方法。  相似文献   
3.
基于Landsat和MODIS数据融合的农牧区NPP模拟   总被引:2,自引:0,他引:2  
天山北坡是中国重要的农牧业发展基地,利用遥感数据准确获取植被净初级生产力(Net primary productivity,NPP)的时空信息,对于合理分配农牧业草地资源具有重要意义。由于受到天气影响及卫星传感器受到时间分辨率和空间分辨率的限制,获取既具有中空间分辨率、又具有高时间分辨率的遥感数据比较困难。本文基于中空间分辨率Landsat 8 OLI数据与高时间分辨率MODIS数据,采用遥感数据时空融合STARFM算法,获取中空间分辨率和高时间分辨率序列的遥感数据,以天山北坡中段区域为实验区,结合CASA模型,对区域内植被NPP进行模拟。结果表明,2016年内8个时期,融合后的NDVI数据与对应时刻的Landsat 8 OLI NDVI数据的相关系数不小于0.759,偏差在0.006 2~0.009 4之间,均方根误差在0.074~0.135之间;利用融合数据与CASA模型协同模拟的NPP具有良好的空间细节信息,NPP模拟值与野外实测值决定系数R~2为0.860 1,表明两者具有较好的相关性。本研究为多源遥感影像融合技术与光能利用率模型协同模拟NPP提供了新的思路。  相似文献   
4.
5.
针对精确获取大尺度空间范围内农业大棚的分布情况并进行长时间的序列动态监测存在数据量大、计算效率低、精度不高等问题,利用Google Earth Engine(GEE)云平台能够实现快速存取、实时处理海量卫星数据,基于多时相Landsat影像进行农业大棚时序光谱特征和纹理特征的自动提取,采用随机森林算法实现山东省农业大棚的遥感分类,从而生成了山东省近30年农业大棚的空间分布和时空动态变化图。结果表明,本文分类流程具有较高的分类精度,其平均总体精度达到91.63%,Kappa系数均值为0.8642。经分析,山东省农业大棚从1990年的6.67 km^2增加到2018年的9919.40 km^2,增长速度为354.03 km^2/a。  相似文献   
6.
  1. Coastal and estuarine waters are important ecosystems with high primary and secondary productivity, but they are prone to the impacts of habitat loss caused by anthropogenic activities. For species exclusively inhabiting coastal and estuarine waters, such as the Indo‐Pacific humpback dolphin, Sousa chinensis , irreversible habitat loss can have dramatic implications for population viability.
  2. A Landsat image database was used to determine the extent of coastal changes along the northern Beibu Gulf, where a large humpback dolphin population is found. The results were compared with the standardized sighting gradient (SPUF) determined from a questionnaire survey of fishermen and likely core habitats identified by application of a global digital elevation model.
  3. Both SPUF and likely core habitat results indicated a continuous distribution of the humpback dolphin along the northern Beibu Gulf. Landsat images revealed that 129.6 km2 of coastal waters were permanently lost in the past 40 years, 60 km2 within the likely core habitats. Although this may be considered small, the impact of such habitat loss could be substantial in some local habitats.
  4. The humpback dolphin population in the northern Beibu Gulf should be regarded as one management unit, with two or more social subunits. Immediate systematic surveys are needed to fill information gaps on true distribution range and habitat‐use patterns.
  5. Habitat protection actions for dolphins in the northern Beibu Gulf should include both core and linking habitats, including enacting protected areas in core habitats, mitigating anthropogenic impacts in likely habitats, restoring both coastal waters and surrounding landscape quality, effective treatment of industrial sewage discharge, and comprehensive environmental impact assessments for the planning of coastal development projects.
  相似文献   
7.
模拟多光谱卫星传感器数据的冬小麦白粉病遥感监测   总被引:1,自引:0,他引:1  
为了解利用遥感技术快速大范围监测小麦白粉病病害情况的可行性,以Landsat5TM波段响应函数为基础,将地面实测冠层高光谱数据模拟为TM多光谱数据,从而分析卫星传感器多光谱波段对病害的响应情况,并构建多光谱指数(PMSI)估测白粉病严重度。在此基础上,采用2010年星-地配套数据对PMSI估测精度进行验证。结果表明,PMSI能够较准确地反映冬小麦白粉病发生的程度,获得较理想的病情严重度反演精度(r2=0.475,RMSE=0.129)。因此采用多光谱卫星遥感影像在小麦大面积种植区域进行病害监测具有应用潜力。  相似文献   
8.
Timely and accurate mapping of anthropogenic and natural disturbance patterns can be used to better understand the nature of wildlife habitats, distributions and movements. One common approach to map forest disturbance is by using high spatial resolution satellite imagery, such as Landsat 5 Thematic Mapper (TM) or Landsat 7 Enhanced Thematic Mapper plus (ETM+) imagery acquired at a 30 m spatial resolution. However, the low revisit times of these sensors acts to limit the capability to accurately determine dates for a sequence of disturbance events, especially in regions where cloud contamination is a frequent occurrence. As wildlife habitat use can vary significantly seasonally, annual patterns of disturbance are often insufficient in assessing relationships between disturbance and foraging behaviour or movement patterns.The Spatial Temporal Adaptive Algorithm for mapping Reflectance Change (STAARCH) allows the generation of high-spatial (30 m) and -temporal (weekly or bi-weekly) resolution disturbance sequences using fusion of Landsat TM or ETM+ and Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. The STAARCH algorithm is applied here to generate a disturbance sequence representing stand-replacing events (disturbances over 1 ha in area) for the period 2001-2008, over almost 6 million ha of grizzly bear habitat along the eastern slopes of the Rocky Mountains in Alberta. The STAARCH algorithm incorporates pairs of Landsat images to detect the spatial extent of disturbances; information from the bi-weekly MODIS composites is used in this study to assign a date of disturbance (DoD) to each detected disturbed area. Dates of estimated disturbances with areas over 5 ha are validated by comparison with a yearly Landsat-based change sequence, with producer's accuracies ranging between 15 and 85% (average overall accuracy 62%, kappa statistic of 0.54) depending on the size of the disturbance event. The spatial and temporal patterns of disturbances within the entire region and in smaller subsets, representative of the size of a grizzly bear annual home range, are then explored. Disturbance levels are shown to increase later in the growing season, with most disturbances occurring in late August and September. Individual events are generally small in area (<10 ha) except in the case of wildfires, with, on average, 0.4% of the total area disturbed each year. The application of STAARCH provides unique high temporal and spatial resolution disturbance information over an extensive area, with significant potential for improving understanding of wildlife habitat use.  相似文献   
9.
遥感信息普遍存在着尺度效应,合适的空间分辨率可以反映特定目标的空间结构特性。基于地理学第一规律,选择了目前主要采用的2种方法——局部变异和变异函数对最优尺度的选择进行研究,并针对传统方法的局限性提出了改进方案。通过同一地区的遥感卫星Landsat 7,Spot-5/HRG和QuickBird遥感影像,对不同的景观区域采用不同的方法进行了比较研究。根据实验,得出了局部变异适合微观、变异函数则更适用于宏观问题的结论,并得到了不同数据源在不同景观类型下的最优尺度。最后,根据最优尺度选择的结果,讨论了不同数据源的适用性。图4表4参19  相似文献   
10.
Monitoring the dynamics of soil salinization is of great importance for agricultural production. This study selected Yucheng County, a typical county on the Huang-Huai-Hai Plain (HHHP) of China, as the study area and evaluated the spatial and temporal variation of soil salinization. Three methods, consisting of principal component analysis (PCA) transformation, tasseled cap (TC) transformation, and optimal band combination (OBC), were used to extract information from an early Landsat multispectral scanner (MSS) image from 1984, and their advantages were compared. In addition, OBC was used on a thematic mapper (TM) image from 2009. An iteratively self-organizing data analysis algorithm was used together with prior knowledge of likely classifications to interpret the MSS and TM images for data classification. Finally, a transfer matrix method was used to assess the spatial and temporal variability of soil salinization and analyze the driving factors of soil salinization. Compared to PCA transformation and OBC, TC transformation was a more effective method for extracting soil salinization information from the MSS sensor. The results indicate that a soil area of approximately 298 km2 was affected by salinity in 1984 in Yucheng County, of which 5.40%, 11.96%, and 12.75% were classified as being subject to slight, moderate, and severe salinization, respectively. In 2009, the saline area was reduced to only 146 km2, of which 10.70% and 3.75% were characterized by slight to moderate salinization and no severe salinization, respectively. The saline land decreased at an average rate of 6 km2 per year. This decrease was probably a result of lower groundwater depth, increased organic fertilizer or crop straw in soil, changed land use type, and increased vegetation coverage.  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