全文获取类型
收费全文 | 2762篇 |
免费 | 180篇 |
国内免费 | 24篇 |
专业分类
测绘学 | 44篇 |
大气科学 | 167篇 |
地球物理 | 718篇 |
地质学 | 1329篇 |
海洋学 | 213篇 |
天文学 | 343篇 |
综合类 | 9篇 |
自然地理 | 143篇 |
出版年
2022年 | 22篇 |
2021年 | 48篇 |
2020年 | 42篇 |
2019年 | 61篇 |
2018年 | 99篇 |
2017年 | 103篇 |
2016年 | 119篇 |
2015年 | 95篇 |
2014年 | 121篇 |
2013年 | 200篇 |
2012年 | 109篇 |
2011年 | 175篇 |
2010年 | 151篇 |
2009年 | 173篇 |
2008年 | 142篇 |
2007年 | 89篇 |
2006年 | 114篇 |
2005年 | 113篇 |
2004年 | 91篇 |
2003年 | 91篇 |
2002年 | 78篇 |
2001年 | 53篇 |
2000年 | 47篇 |
1999年 | 39篇 |
1998年 | 33篇 |
1997年 | 29篇 |
1996年 | 28篇 |
1995年 | 28篇 |
1994年 | 35篇 |
1993年 | 17篇 |
1992年 | 27篇 |
1991年 | 34篇 |
1990年 | 19篇 |
1989年 | 25篇 |
1988年 | 13篇 |
1987年 | 23篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 24篇 |
1983年 | 29篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 18篇 |
1979年 | 11篇 |
1978年 | 13篇 |
1977年 | 14篇 |
1976年 | 8篇 |
1975年 | 18篇 |
1974年 | 10篇 |
1973年 | 14篇 |
排序方式: 共有2966条查询结果,搜索用时 296 毫秒
101.
Ran Tao Daniel Strandow Michael Findley Jean‐Claude Thill James Walsh 《Transactions in GIS》2016,20(3):413-425
Territorial control is central to the understanding of violent armed conflicts, yet reliable and valid measures of this concept do not exist. We argue that geospatial analysis provides an important perspective to measure the concept. In particular, measuring territorial control can be seen as an application of calculating service areas around points of control. The modeling challenge is acute for areas with limited road infrastructure, where no complete network is available to perform the analysis, and movements largely occur off road. We present a new geospatial approach that applies network analysis on a hybrid transportation network with both actual road data and hexagon‐fishnet‐based artificial road data representing on‐road and off‐road movements, respectively. Movement speed or restriction can be readily adjusted using various input data. Simulating off‐road movement with hexagon‐fishnet‐based artificial road data has a number of advantages including scalability to small or large study areas and flexibility to allow all‐directional travel. We apply this method to measuring territorial control of armed groups in Sub‐Saharan Africa where inferior transport infrastructure is the norm. Based on the Uppsala Conflict Data Program's (UCDP) Georeferenced Event Data (GED) as well as spatial data on terrain, population locations, and limited transportation networks, we enhance the delineation of the specific areas directly controlled by each warring party during civil wars within a given travel time. 相似文献
102.
103.
Mehdi Adjeroud Carden C. Wallace Pauline Bosserelle Claude Payri Jean‐Louis Menou Michel Pichon 《Marine Ecology》2016,37(3):565-575
Rapa (27°36′ S, 144°20′ W) is a small (~40 km2) volcanic island isolated in the Southern Austral Archipelago, where direct anthropogenic stressors are extremely limited. Here, we present the results of the first quantitative survey of coral community structure across habitats and depths around the island. Despite its geographical isolation in the depauperate South Central Pacific, its small size and unfavourable environmental conditions (competition with macroalgae, low sea surface temperatures, reduced reef accretion), the diversity of scleractinian corals at Rapa is particularly high (112 species from 32 genera, including 37 species of Acropora) in comparison to other French Polynesian islands and subtropical Pacific locations. Our results indicate that the abundance (>100 colonies per 10 m2 recorded at nine of the 17 sampling stations) and cover (>40% at four stations) of corals are relatively high for a marginal reef location. Strong spatial heterogeneity was found, with high variation in diversity, abundance, cover and community composition among stations. Variation in community composition was related to habitat types, with distinct assemblages among fringing reefs within bays, reef formations at bay entrances, and those on the submerged platform surrounding the island. On the platform, a depth gradient was detected, with generic richness, abundance and cover generally greater at deeper stations (18–20 m depth) compared with medium‐depth (10–12 m) and shallow (1–3 m) stations. A gradient was also recorded along bays, with increasing coral diversity and abundance from the bay heads to the bay entrances. The coral community at Rapa was characterized by the presence of several taxa not found in other French Polynesian archipelagos and the rarity of others that are common and abundant in the Society and the Tuamotu islands. Another distinctive feature of reef communities at Rapa is the high cover and dominance of macroalgae, particularly in the shallower parts of the surrounding platform, which probably explains the lower densities of coral colonies recorded there. These characteristics of the diversity and biogeographical composition of coral assemblages at Rapa provide considerable ecological grounds for its conservation. 相似文献
104.
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
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
105.
106.
107.
108.
109.
A simple parameter estimation procedure, designated as integration-based estimation (IBE), was introduced to determine the hydraulic properties of an aquifer using slug test models subjected to certain flow geometries such as radial and spherical flows. The basic idea behind the proposed IBE approach is to link an integration value at pre-defined normalized head levels for field data with that of a theoretical type curve. The IBE method removes the need for the implementation of the classical graphical matching process which would be ineffective to acquire aquifer parameters for non-ideal aquifer conditions. As the second aspect of this study, a new decision tool was suggested to determine the suitable slug test model to be utilized for the site data since diagnosing the flow character properly is of crucial importance for following a convenient analysis procedure. The estimation performance and limitation of the proposed IBE method were tested for several slug test scenarios including radial and spherical flow models with a number of synthetically generated data sets as well as a field application. Results reveal that the IBE together with the identification methodology not only is able to retrieve aquifer parameters as reliable as the existing techniques in the literature but also diagnoses the flow character precisely as demonstrated in this study. 相似文献
110.