全文获取类型
收费全文 | 2075篇 |
免费 | 125篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 43篇 |
大气科学 | 169篇 |
地球物理 | 522篇 |
地质学 | 891篇 |
海洋学 | 234篇 |
天文学 | 242篇 |
综合类 | 6篇 |
自然地理 | 105篇 |
出版年
2024年 | 4篇 |
2023年 | 9篇 |
2022年 | 28篇 |
2021年 | 42篇 |
2020年 | 43篇 |
2019年 | 43篇 |
2018年 | 101篇 |
2017年 | 92篇 |
2016年 | 119篇 |
2015年 | 96篇 |
2014年 | 110篇 |
2013年 | 179篇 |
2012年 | 94篇 |
2011年 | 140篇 |
2010年 | 139篇 |
2009年 | 140篇 |
2008年 | 127篇 |
2007年 | 84篇 |
2006年 | 71篇 |
2005年 | 51篇 |
2004年 | 54篇 |
2003年 | 53篇 |
2002年 | 45篇 |
2001年 | 31篇 |
2000年 | 29篇 |
1999年 | 18篇 |
1998年 | 19篇 |
1997年 | 19篇 |
1996年 | 20篇 |
1995年 | 20篇 |
1994年 | 21篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1991年 | 15篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 5篇 |
排序方式: 共有2212条查询结果,搜索用时 419 毫秒
71.
72.
73.
74.
Yan Gao Adrian Ghilardi Jaime Paneque-Galvez Margaret Skutsch Jean François Mas 《国际地球制图》2016,31(9):1019-1031
This study assesses whether MODIS Vegetation Continuous Fields percent tree cover (PTC) data can detect deforestation and forest degradation. To assess the usefulness of PTC for detecting deforestation, we used a data set consisting of eight forest and seven non-forest categories. To evaluate forest degradation, we used data from two temperate forest types in three conservation states: primary (dense), secondary (moderately degraded) and open (heavily degraded) forest. Our results show that PTC can differentiate temperate forest from non-forest categories (p = 0.05) and thus suggests PTC can adequately detect deforestation in temperate forests. In contrast, single-date PTC data does not appear to be adequate to detect forest degradation in temperate forests. As for tropical forest, PTC can partially discriminate between forest and non-forest categories. 相似文献
75.
76.
77.
78.
Ontogenic variation and effect of collection procedure on leaf biomechanical properties of Mediterranean seagrass Posidonia oceanica (L.) Delile
下载免费PDF全文
![点击此处可从《Marine Ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Carmen B. de los Santos Bárbara Vicencio‐Rammsy Gilles Lepoint François Remy Tjeerd J. Bouma Sylvie Gobert 《Marine Ecology》2016,37(4):750-759
Leaf mechanical traits are important to understand how aquatic plants fracture and deform when subjected to abiotic (currents or waves) or biotic (herbivory attack) mechanical forces. The likely occurrence of variation during leaf ontogeny in these traits may thus have implications for hydrodynamic performance and vulnerability to herbivory damage, and may be associated with changes in morphologic and chemical traits. Seagrasses, marine flowering plants, consist of shoot bundles holding several leaves with different developmental stages, in which outer older leaves protect inner younger leaves. In this study we examined the long‐lived seagrass Posidonia oceanica to determine ontogenic variation in mechanical traits across leaf position within a shoot, representing different developmental stages. Moreover, we investigated whether or not the collection procedure (classical uprooted shoot versus non‐destructive shoot method: cutting the shoot without a portion of rhizome) and time span after collection influence mechanical measurements. Neither collection procedure nor time elapsed within 48 h of collection affected measurements of leaf biomechanical traits when seagrass shoots were kept moist in dark cool conditions. Ontogenic variation in mechanical traits in P. oceanica leaves over intermediate and adult developmental stages was observed: leaves weakened and lost stiffness with aging, while mid‐aged leaves (the longest and thickest ones) were able to withstand higher breaking forces. In addition, younger leaves had higher nitrogen content and lower fiber content than older leaves. The observed patterns may explain fine‐scale within‐shoot ecological processes of leaves at different developmental stages, such as leaf shedding and herbivory consumption in P. oceanica. 相似文献
79.
Juan Pérez Quezadas Victor M. Heilweil Alejandra Cortés Silva Luis Araguas María del Rocío Salas Ortega 《Hydrogeology Journal》2016,24(8):1953-1966
Geochemistry and environmental tracers were used to understand groundwater resources, recharge processes, and potential sources of contamination in the Rio Actopan Basin, Veracruz State, Mexico. Total dissolved solids are lower in wells and springs located in the basin uplands compared with those closer to the coast, likely associated with rock/water interaction. Geochemical results also indicate some saltwater intrusion near the coast and increased nitrate near urban centers. Stable isotopes show that precipitation is the source of recharge to the groundwater system. Interestingly, some high-elevation springs are more isotopically enriched than average annual precipitation at higher elevations, indicating preferential recharge during the drier but cooler winter months when evapotranspiration is reduced. In contrast, groundwater below 1,200 m elevation is more isotopically depleted than average precipitation, indicating recharge occurring at much higher elevation than the sampling site. Relatively cool recharge temperatures, derived from noble gas measurements at four sites (11–20 °C), also suggest higher elevation recharge. Environmental tracers indicate that groundwater residence time in the basin ranges from 12,000 years to modern. While this large range shows varying groundwater flowpaths and travel times, ages using different tracer methods (14C, 3H/3He, CFCs) were generally consistent. Comparing multiple tracers such as CFC-12 with CFC-113 indicates piston-flow to some discharge points, yet binary mixing of young and older groundwater at other points. In summary, groundwater within the Rio Actopan Basin watershed is relatively young (Holocene) and the majority of recharge occurs in the basin uplands and moves towards the coast. 相似文献
80.