全文获取类型
收费全文 | 975篇 |
免费 | 19篇 |
国内免费 | 57篇 |
专业分类
林业 | 124篇 |
农学 | 30篇 |
基础科学 | 94篇 |
172篇 | |
综合类 | 461篇 |
农作物 | 42篇 |
水产渔业 | 9篇 |
畜牧兽医 | 46篇 |
园艺 | 55篇 |
植物保护 | 18篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 30篇 |
2021年 | 21篇 |
2020年 | 17篇 |
2019年 | 25篇 |
2018年 | 8篇 |
2017年 | 22篇 |
2016年 | 41篇 |
2015年 | 60篇 |
2014年 | 56篇 |
2013年 | 47篇 |
2012年 | 79篇 |
2011年 | 90篇 |
2010年 | 85篇 |
2009年 | 89篇 |
2008年 | 86篇 |
2007年 | 78篇 |
2006年 | 49篇 |
2005年 | 42篇 |
2004年 | 20篇 |
2003年 | 21篇 |
2002年 | 15篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 2篇 |
排序方式: 共有1051条查询结果,搜索用时 312 毫秒
61.
Timely and accurate information on crop conditions obtained during the growing season is of vital importance for crop management.
High spatial resolution satellite imagery has the potential for mapping crop growth variability and identifying problem areas
within fields. The objectives of this study were to use QuickBird satellite imagery for mapping plant growth and yield patterns
within grain sorghum fields as compared with airborne multispectral image data. A QuickBird 2.8-m four-band image covering
a cropping area in south Texas, USA was acquired in the 2003 growing season. Airborne three-band imagery with submeter resolution
was also collected from two grain sorghum fields within the satellite scene. Yield monitor data collected from the two fields
were resampled to match the resolutions of the airborne imagery and the satellite imagery. The airborne imagery was related
to yield at original submeter, 2.8 and 8.4 m resolutions and the QuickBird imagery was related to yield at 2.8 and 8.4 m resolutions.
The extracted QuickBird images for the two fields were then classified into multiple zones using unsupervised classification
and mean yields among the zones were compared. Results showed that grain yield was significantly related to both types of
image data and that the QuickBird imagery had similar correlations with grain yield as compared with the airborne imagery
at the 2.8 and 8.4 m resolutions. Moreover, the unsupervised classification maps effectively differentiated grain production
levels among the zones. These results indicate that high spatial resolution satellite imagery can be a useful data source
for determining plant growth and yield patterns for within-field crop management. 相似文献
62.
Within-field spatial variability is related to multiple factors that can be time-independent or time-dependent. In this study,
our working hypothesis is that a multi-time scale analysis of the dynamics of spatial patterns can help establish a diagnosis
of crop condition. To test this hypothesis, we analyzed the within-field variability of a sugarcane crop at seasonal and annual
time scales, and tried to link this variability to environmental (climate, topography, and soil depth) and cropping (harvest
date) factors. The analysis was based on a sugarcane field vegetation index (NDVI) time series of fifteen SPOT images acquired
in the French West Indies (Guadeloupe) in 2002 and 2003, and on an original classification method that enabled us to focus
on crop spatial variability independently of crop growth stages. We showed that at the seasonal scale, the within-field growth
pattern depended on the phenological stage of the crop and on cropping operations. At the annual scale, NDVI maps revealed
a stable pattern for the two consecutive years at peak vegetation, despite very different rainfall amounts, but with inverse
NDVI values. This inversion is linked with the topography and consequently to the plant water status. We conclude that (1)
it is necessary to know the crop growing cycle to correctly interpret the spatial pattern, (2) single-date images may be insufficient
for the diagnosis of crop condition or for prediction, and (3) the pattern of vigour occurrence within fields can help diagnose
growth anomalies.
相似文献
Pierre TodoroffEmail: |
63.
华东师范大学牧草选育研究课题组培育的普通小麦(Triticum aestivum)与天蓝偃麦草(Agropyron glaucum)远缘杂交的高度不育的后代再与通小麦回交,经七年选育现已获得育性恢复的一年生粮草兼用型饲革新品系,进行细胞学分裂时期的行为观察表明:在花粉母细胞减数分裂各个时期中,可见到染色体呈正常的分裂活动的分裂相。对根尖细胞有丝分裂活动观察,证明该品系为稳定的整倍体。 相似文献
64.
65.
66.
连云港城市扩展及其驱动力分析 总被引:1,自引:0,他引:1
利用多时相遥感影像、社会经济统计数据以及其他辅助资料,在基于遥感影像专题信息提取和GIS空间叠置分析技术的基础上对连云港市1978~2005年的城市扩展及其侵占周围土地的情况进行了系统的分析,并对连云港市扩展的驱动力进行了初步的探讨.结果表明,连云港的城市扩展很快,2005年的城市面积是1978年的4倍多,而且在不同的空间位置城市扩展的速度和规模也有较大的差异,城市扩展主要是侵占周边的优质农田.连云港市临海的地理位置和快速的经济发展决定着城市扩展的空间方位和速度,是城市扩展的主要驱动力. 相似文献
67.
68.
A long-term (1983–2000) global dataset of Earth’s surface daily-integrated UV exposure was developed from a combination of ISCCP-D1 3 h reflectance measurements (in order to resolve the diurnal variation of cloud conditions) and TOMS total ozone amount. The inversion algorithm developed in our previous study was employed with modifications addressing the conversion of visible reflectance to UV albedo and narrowband UV albedo to broadband albedo over the range of 280–400 nm. Validation of the product was carried out using ground-based measurements at six stations. In general, good agreements between the estimated and measured daily UV exposure are found at most stations; the relative mean and root mean square (RMS) difference varies from 3 to 14% and from 20 to 45%, respectively. Among the stations, San Diego has a fairly low mean difference (9%) and the lowest RMS difference (33%), owing to the prevailing clear sky or uniform cloud cover condition. The RMS difference increases with cloud amount, which is largely caused by mismatch between satellite and ground-based measurements. The effect of diurnal variation in atmospheric opacity associated primarily with cloud on the estimation of daily erythemal UV doses is investigated with both ground-based measurements and ISCCP-D1 data. It is found that daily erythemal UV doses estimated from only a noontime satellite “snapshot” may incur errors larger than 20%, which may be reduced for long-term averaging. 相似文献
69.
水分亏缺指数(WDI)是建立在作物水分胁迫指数理论基础上,并假设陆地表面温度是冠层温度与土壤表面温度线性加权及土壤与植被冠层之间不存在感热交换的情况下,结合陆气温差与植被指数得到的区域干旱评价指标。本文利用MODISTerra陆地表面温度和植被指数数据产品,采用NDVITs空间法分别计算了2000年4月上旬和5月中旬气温空间分布的基础上,结合陆气温差和植被指数计算全国范围WDI,并与表层土壤含水量进行对比。结果表明:WDI能够比较合理地用来监测区域干旱,不仅适用于裸地条件,还能够有效地应用于完全植被或部分植被覆盖条件下的干旱监测,克服了CWSI只能应用于郁闭植被冠层的限制。 相似文献
70.