全文获取类型
| 收费全文 | 273篇 |
| 免费 | 56篇 |
| 国内免费 | 49篇 |
专业分类
| 测绘学 | 62篇 |
| 大气科学 | 132篇 |
| 地球物理 | 99篇 |
| 地质学 | 14篇 |
| 海洋学 | 10篇 |
| 天文学 | 1篇 |
| 综合类 | 10篇 |
| 自然地理 | 50篇 |
出版年
| 2024年 | 1篇 |
| 2023年 | 3篇 |
| 2022年 | 6篇 |
| 2021年 | 13篇 |
| 2020年 | 11篇 |
| 2019年 | 16篇 |
| 2018年 | 12篇 |
| 2017年 | 16篇 |
| 2016年 | 14篇 |
| 2015年 | 11篇 |
| 2014年 | 19篇 |
| 2013年 | 24篇 |
| 2012年 | 6篇 |
| 2011年 | 12篇 |
| 2010年 | 11篇 |
| 2009年 | 24篇 |
| 2008年 | 40篇 |
| 2007年 | 20篇 |
| 2006年 | 24篇 |
| 2005年 | 18篇 |
| 2004年 | 18篇 |
| 2003年 | 12篇 |
| 2002年 | 5篇 |
| 2001年 | 8篇 |
| 2000年 | 7篇 |
| 1999年 | 6篇 |
| 1998年 | 4篇 |
| 1997年 | 4篇 |
| 1996年 | 3篇 |
| 1995年 | 6篇 |
| 1994年 | 1篇 |
| 1993年 | 1篇 |
| 1992年 | 1篇 |
| 1988年 | 1篇 |
排序方式： 共有378条查询结果，搜索用时 62 毫秒
51.
52.
53.
Kiran Sharma 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(2):137-150
Forest canopy density (FCD) is a major factor in the evaluation of forest status and is an important indicator of possible management interventions. The study uses the FCD model with Landsat TM and Landsat 8 OLI images to assess canopy density in India’s Manipur valley and surrounding hills. Normalized difference built-up index (NDBI) was used to extract built-up areas and population density was retrieved from LandScan data, while elevation and slope were obtained from ASTER DEMs (30 m). Four types of canopy density were delineated with crown cover above 71%, 41–70%, 11–40%, below 10% and areas with no canopy cover, that is 0%. A sharp decline in forest area occurred during 1989–2016 at a rate of loss of 2.9 % year-1 with an average rate of deforestation of 3051 ha year-1. Dense forests exhibited remarkable degradation, especially towards the central valley. The variation in the topographical (elevation and slope) gradient resulted in significant differences in the canopy density over the study area barring some hill slopes. Population pressure and various developmental activities in recent decades led to forest degradation in this fragile yet rich Eastern Himalaya biodiversity hotspot. 相似文献
54.
ABSTRACTWe present a new model extension for the Water balance Simulation Model, WaSiM, which features (i) snow interception and (ii) modified meteorological conditions under coniferous forest canopies, complementing recently developed model extensions for particular mountain hydrological processes. Two study areas in Austria and Germany are considered in this study. To supplement and constrain the modelling experiments with on-site observations, a network of terrestrial time-lapse cameras was set up in one of these catchments. The spatiotemporal patterns of snow depth inside the forest and at the adjacent open field sites were recorded along with snow interception dynamics. Comparison of observed and modelled snow cover and canopy interception indicates that the new version of WaSiM reliably reconstructs the variability of snow accumulation for both the forest and the open field. The Nash-Sutcliffe efficiency computed for selected runoff events in spring increases from ?0.68 to 0.71 and 0.21 to 0.87, respectively. 相似文献
55.
ABSTRACTThroughfall drop size distributions (DSDs) are important for plant–soil interactions. This is the first known study to quantify differences in throughfall DSDs with the presence and absence of foliage. Employing a disdrometer, three parameters solely representing throughfall drip were measured and calculated: maximum drop diameter (DMAX), median volume diameter of drops (D50DR) and relative volume percentage of drops (pDR). Beneath Liriodendron tulipifera L. in Maryland (USA), DMAX, D50DR and pDR were substantially larger when the canopy was unfoliated. In fact, the presence or absence of foliage was one of the primary factors affecting all three throughfall DSDs along with air temperature, according to the boosted regression tree analysis. Experimental results were attributed to differing physical properties of intercepted water between foliated and unfoliated periods and differential water behavior on leaves and bark. Future work should examine the effects of concentrated drip points on the development of throughfall-induced hot spots.
Editor M.C. Acreman; Associate editor F. Hattermann 相似文献
56.
Robert Andrle 《自然地理学》2013,34(3):270-281
The traditional measure of river-channel sinuosity, P, is shown to vary with scale of measurement, the geometric character of the particular reach, and the starting point of the divider walk. Meander wavelength, L, suffers from non-reproducibility caused by the subjective identification of inflection points, a problem compounded by the irregularity of river meanders. Applicability of the angle measure technique (AMT) to the measurement of meander planform is demonstrated. The AMT considers variations in the complexity of meandering with scale, eliminating the dependence of results on scale of measurement. The AMT also samples the river at random points rather than sequentially, unlike the divider walk used to calculate P, avoiding the dependence of P on the starting position of the divider walk. The AMT also does not require subjective identification of inflection points as wavelength L does. Additionally, the traditional measures P and L are difficult to apply to manifestly underfit rivers. In this study, the AMT is applied to two underfit rivers and measures of meander size and sinuosity are calculated for each set of meanders. [Key words: sinuosity, meanders, wavelength, underfit river, fluvial geomorphology.] 相似文献
57.
Xiong Yan Kanako Muramatsu Masahiro Hirata Kazato Oishi Ichirow Kaihotsu Tamio Takamura 《地球空间信息科学学报》2013,16(2):117-122
We plan to estimate global net primary production (NPP) of vegetation using the Advanced Earth Observing Satellite-II (ADEOS-II) Global Imager (GLI) multi-spectral data. We derive an NPP estimation algorithm from ground measurement data on temperate plants in Japan. By the algorithm, we estimate NPP using a vegetation index based on pattern decomposition (VIPD) for the Mongolian Plateau. The VIPD is derived from Landsat ETM+multi-spectral data, and the resulting NPP estimation is compared with ground data measured in a semi-arid area of Mongolia. The NPP estimation derived from satellite remote sensing data agrees with the ground measurement data within the error range of 15% when all above-ground vegetation NPP is calculated for different vegetation classifications. 相似文献
58.
祁连山南麓退耕地主要植物群落植冠层的截留性能 总被引：12，自引：0，他引：12
退耕地人工群落植冠层的截留能力,是群落生态效益的重要组成部分,这一研究不仅可以掌握退耕还林还草工程作用的机理,还可以更好地指导工程实践,促进工程建设步伐。通过“简易吸水法”,可以快速掌握退耕地植冠层的最大截留潜力。青海省大通县退耕较早、平均年龄约10a以上的青海云杉、华北落叶松等几种人工群落,它们的林冠截留量较高,平均达1．93mm,为当地3种截留量最高、林龄50a之上天然群落的61％。青海云杉、华北落叶松、白桦、青杨、中国沙棘等退耕地人工群落的截留量,基本上显示出与林龄之间呈线性或Cubic、Quadratic正相关关系。在植物群落分层截留、消化吸收降雨的这一空间垂直序列中,植冠层截留量虽然较低,平均仅1．29mm(0．25～2．38mm),但植冠层的主要作用还在于减轻、缓冲雨水直接打击地面,改变降水的侵蚀性危害,而这些作用要远远高于截留量自身的区区数量。 相似文献
59.
60.
中国南方森林冠顶高度Lidar反演—以江西省为例 总被引：1，自引：0，他引：1
激光雷达(Lidar)与光学遥感的有效结合对中国南方区域森林冠顶高度反演意义重大,而国产卫星将为中国森林生态研究提供新的数据源。本文联合利用大脚印激光雷达GLA和国产MERSI数据,在实现GLAS波形数据处理和不同地形条件下森林冠顶高度反演算法基础上,建立了区域尺度不同森林类型林分冠顶高度GLAS+MERSI联合反演关系模型,进行了江西地区森林冠顶高度反演。总体上,GLAS激光雷达森林冠顶高度估算精度较高;且在与MERSI 250 m数据的联合反演模型中,针叶林模型精度较好(R2=0.7325);阔叶林次之(R2=0.6095);混交林较差(R2=0.4068)。分析发现,考虑了光学遥感生物物理参数的GLAS+MERSI联合关系模型在区域森林冠顶高度估算中有较高精度,且在空间分布上与土地覆盖数据分布特征非常一致。 相似文献