全文获取类型
收费全文 | 77398篇 |
免费 | 13420篇 |
国内免费 | 17455篇 |
专业分类
测绘学 | 7745篇 |
大气科学 | 10909篇 |
地球物理 | 18349篇 |
地质学 | 39169篇 |
海洋学 | 11275篇 |
天文学 | 3596篇 |
综合类 | 5373篇 |
自然地理 | 11857篇 |
出版年
2024年 | 266篇 |
2023年 | 800篇 |
2022年 | 2074篇 |
2021年 | 2539篇 |
2020年 | 2761篇 |
2019年 | 3483篇 |
2018年 | 2609篇 |
2017年 | 3100篇 |
2016年 | 3221篇 |
2015年 | 3523篇 |
2014年 | 4378篇 |
2013年 | 4759篇 |
2012年 | 4664篇 |
2011年 | 5017篇 |
2010年 | 4144篇 |
2009年 | 5160篇 |
2008年 | 5101篇 |
2007年 | 5630篇 |
2006年 | 5365篇 |
2005年 | 4559篇 |
2004年 | 4308篇 |
2003年 | 3929篇 |
2002年 | 3399篇 |
2001年 | 2980篇 |
2000年 | 2792篇 |
1999年 | 2589篇 |
1998年 | 2321篇 |
1997年 | 2067篇 |
1996年 | 1882篇 |
1995年 | 1594篇 |
1994年 | 1542篇 |
1993年 | 1330篇 |
1992年 | 1038篇 |
1991年 | 742篇 |
1990年 | 622篇 |
1989年 | 517篇 |
1988年 | 389篇 |
1987年 | 245篇 |
1986年 | 180篇 |
1985年 | 141篇 |
1984年 | 66篇 |
1983年 | 48篇 |
1982年 | 55篇 |
1981年 | 39篇 |
1980年 | 45篇 |
1979年 | 46篇 |
1978年 | 49篇 |
1977年 | 44篇 |
1976年 | 27篇 |
1954年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 781 毫秒
971.
Frederick T. Short Evamaria W. Koch Joel C. Creed Karine M. Magalhães Eric Fernandez & Jeffrey L. Gaeckle 《Marine Ecology》2006,27(4):277-289
Seagrasses are an important coastal habitat worldwide and are indicative of environmental health at the critical land–sea interface. In many parts of the world, seagrasses are not well known, although they provide crucial functions and values to the world's oceans and to human populations dwelling along the coast. Established in 2001, SeagrassNet, a monitoring program for seagrasses worldwide, uses a standardized protocol for detecting change in seagrass habitat to capture both seagrass parameters and environmental variables. SeagrassNet is designed to statistically detect change over a relatively short time frame (1–2 years) through quarterly monitoring of permanent plots. Currently, SeagrassNet operates in 18 countries at 48 sites; at each site, a permanent transect is established and a team of people from the area collects data which is sent to the SeagrassNet database for analysis. We present five case studies based on SeagrassNet data from across the Americas (two sites in the USA, one in Belize, and two in Brazil) which have a common theme of seagrass decline; the study represents a first latitudinal comparison across a hemisphere using a common methodology. In two cases, rapid loss of seagrass was related to eutrophication, in two cases losses related to climate change, and in one case, the loss is attributed to a complex trophic interaction resulting from the presence of a marine protected area. SeagrassNet results provide documentation of seagrass change over time and allow us to make scientifically supported statements about the status of seagrass habitat and the extent of need for management action. 相似文献
972.
Wind and waves are major forces affecting the geomorphology and biota in coastal areas. We present a generally applicable method for measuring and calculating fetch length, fetch direction and wave exposure. Fetch length and direction, measured by geographic information system-based methods, are used along with wind direction and wind speed data to estimate wave height and period by applying forecasting curves. The apparent power of waves approaching the shore, used as a proxy for wave exposure, is then calculated by a linear wave model. We demonstrate our method by calculating fetch lengths and wave exposure indices for five areas with varying exposure levels and types of meteorological conditions in the Finnish Archipelago Sea, situated in the northern Baltic Sea. This method is a rapid and accurate means of estimating exposure, and is especially applicable in areas with geomorphologically varying and complicated shorelines. We expect that our method will be useful in several fields, such as basic biogeographical and biodiversity research, as well as coastal land-use planning and management. 相似文献
973.
根据海洋调查历史资料 ,对台湾东北海域冷水块的时空分布、结构、特点进行分析 ,主要结果如下 :(1 )首次利用卫星跟踪漂流浮标的轨迹 ,佐证冬季该冷水块的存在。 (2 )鉴于该冷水的重要性 ,建议对它给予称谓 ,暂定名为“彭花棉”冷水块。 (3 )该冷水块是黑潮次表层水涌升的结果 ,是黑潮对我国近海海洋环境影响的典型实例之一。 (4)该冷水块具有低温、高盐、高密、低氧、高磷、高硅和高氮等特性 相似文献
974.
975.
976.
In this paper, the theoretical analysis and experimental studies are employed to investigate the reflection characteris-tics of partial standing waves caused by wave overtopping and sloping top of structures. Based on the principle of conser-vation of wave energy flux, the third-order Stokes wave theory is used to formulate the reflection coefficient at wave overtopping; the calculation results are regressed into an applied expression. A series of experiments of wave reflection for a vertical-wall structure with chamfered and overhanging upper sections are carried out to investigate the influence of top slope on wave reflection. The regularity of variation of wave reflection in this case is analysed based on the experimental results. 相似文献
977.
978.
INTRODUCTIONTheDenglouCape ,inthesouthwestoftheLeizhouPeninsula ,isatthenorthmarginoftropicalzone .SeveralresearchesandcartographiesoftheregionalgeomorphologyandQuater narygeology ,whichwerecarriedoutinthepast,allincludethisarea (MGL ,SCSIO ,CAS ,1 978;GPCSGRCZT… 相似文献
979.
980.
结合国内外海岸线分形研究进展 ,以江苏省海岸线为例 ,系统地探讨了不同比例尺地图上海岸线长度与分维的变化及其规律。指出海岸线长度与地图比例尺之间存在有 lg L =algk b的基本关系 ;其次 ,对于同一条海岸线来说 ,使用量规法和网格法计算所得分维结果是不同的 ,使用量规法的计算结果大于使用网格法的计算结果 ;再者 ,在不同比例尺地图上 ,同一海岸线的分维是不同的 ,随着地图比例尺的逐渐变小 ,海岸线分维相应地逐渐变小 ,且在不同比例尺间使用量规法计算所得海岸线分维结果的差值大于使用网格法计算所得海岸线分维结果的差值 相似文献