全文获取类型
收费全文 | 7169篇 |
免费 | 771篇 |
国内免费 | 835篇 |
专业分类
测绘学 | 1352篇 |
大气科学 | 460篇 |
地球物理 | 886篇 |
地质学 | 2484篇 |
海洋学 | 474篇 |
天文学 | 122篇 |
综合类 | 628篇 |
自然地理 | 2369篇 |
出版年
2024年 | 17篇 |
2023年 | 75篇 |
2022年 | 174篇 |
2021年 | 227篇 |
2020年 | 266篇 |
2019年 | 316篇 |
2018年 | 205篇 |
2017年 | 320篇 |
2016年 | 267篇 |
2015年 | 345篇 |
2014年 | 386篇 |
2013年 | 528篇 |
2012年 | 434篇 |
2011年 | 483篇 |
2010年 | 376篇 |
2009年 | 391篇 |
2008年 | 438篇 |
2007年 | 471篇 |
2006年 | 401篇 |
2005年 | 396篇 |
2004年 | 343篇 |
2003年 | 299篇 |
2002年 | 264篇 |
2001年 | 246篇 |
2000年 | 199篇 |
1999年 | 176篇 |
1998年 | 133篇 |
1997年 | 130篇 |
1996年 | 64篇 |
1995年 | 64篇 |
1994年 | 64篇 |
1993年 | 68篇 |
1992年 | 53篇 |
1991年 | 34篇 |
1990年 | 29篇 |
1989年 | 22篇 |
1988年 | 22篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 4篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有8775条查询结果,搜索用时 15 毫秒
21.
This article describes a unique flood hazard, produced by the dramatic expansion of wetlands in Nelson County, located within
the North American Prairie Pothole Region of North Dakota, USA. There has been an unprecedented increase in the number, average
size, and permanence of prairie wetlands, and a significant increase in the size of a closed lake (Stump Lake) due to a decade-long
wet spell that began in 1993 following a prolonged drying trend. Base-line land cover information from the 1992 USGS National
Land Cover Characterization dataset, and a Landsat TM scene acquired 9 July 2001 are used to assess the growth of the closed
lake and wetland pond surface areas, and to analyze the type and area of various land cover classes inundated between 1992
and 2001. The open water profile in Nelson County changed from one marked by relatively comparable coverage of closed lake
and wetland pond areas in 1992, to one in which wetland open water accounted for the vast majority of total open water in
2001. The bulk of the wetland pond area expansion occurred by displacing existing wetland vegetation and agricultural cropland.
Producers responded to the flood hazard by filing Federal Crop Insurance Corporation (FCIC) claims and enrolling cropland
in the Conservation Reserve Program (CRP), a federal land retirement program. Land taken out of agricultural production has
had an enormous impact upon the agricultural sector that forms the economic base of the rural economy. In 2001 the land taken
out of production due to CRP enrollment and preventive planting claims represented nearly 42% of Nelson County’s 205.2 K ha
base agricultural land. The patterns obtained from this detailed study of Nelson County are likely to be the representative
of the more publicized flood disaster occurring within the Devils Lake Basin of North Dakota. 相似文献
22.
23.
The expansion of offshore activities and the increasing need to meet international and national commitments to biodiversity conservation have led to an enhanced interest in marine spatial planning (MSP) as a tool for sea use management. Several European countries, on their own initiative or driven by European legislation and policy, have taken global leadership in implementing MSP. This article will discuss the Belgian experiences with MSP. It will give a short historical overview based on legal developments and review the implementation process of a ‘Master Plan’ as a spatial management policy for the Belgian Part of the North Sea. Additionally, this article will reflect on the research that has been done in Belgium to apply a land-use planning approach to the marine environment. The MSP process in Belgium shows that a spatial approach to sea use management is possible despite the lack of a legal zoning framework. However, it concludes that a legal basis for MSP, in addition to the current permit system, would provide a more strategic and integrated framework for ecosystem-based, sea use management. 相似文献
24.
25.
26.
The illegal plundering of fishing stocks worldwide is masked behind a maze of corporate structures that makes discovery of the beneficiaries of such activity, difficult, if not impossible to identify. This paper outlines the problem of illegal fishing and then develops a multi-layered response to avert the veil of corporate entity being used as a means of preventing domestic States from answering the principal thematic question—who is the true beneficiary of this illegal behaviour, and who should be held responsible. 相似文献
27.
本文介绍了海水中砷标准物质的研制过程,其中包括溶液的配制、均匀性和稳定性检验,标样的分析方法。并对标准物质的定值数据进行统计处理,两种浓度的定值结果分别为:1.00μg/cm3±0.04μg/cm3和51.0μg/dm3±2.6μg/dm3。两种浓度砷标准物质的稳定期均为一年 相似文献
28.
29.
本文对北部湾三维潮波进行了数值研究,并将所得结果与现有观测资料作了比较。通过比较发现两者相当一致。文中还绘制了M_2和K_1分潮的同潮图,并对其作了讨论。可以断定,北部湾的日潮优势是由日潮共振所引起的。M_2和K_1分潮流的水平分布表明,强流区位于它们各自的蜕化无潮点附近。还讨论了分潮流的铅直结构,可以看出这种结构十分接近实际分潮流场的特征。 相似文献
30.