全文获取类型
收费全文 | 3420篇 |
免费 | 543篇 |
国内免费 | 385篇 |
专业分类
测绘学 | 161篇 |
大气科学 | 57篇 |
地球物理 | 1406篇 |
地质学 | 1168篇 |
海洋学 | 420篇 |
天文学 | 17篇 |
综合类 | 149篇 |
自然地理 | 970篇 |
出版年
2024年 | 16篇 |
2023年 | 30篇 |
2022年 | 63篇 |
2021年 | 125篇 |
2020年 | 156篇 |
2019年 | 154篇 |
2018年 | 128篇 |
2017年 | 145篇 |
2016年 | 135篇 |
2015年 | 141篇 |
2014年 | 149篇 |
2013年 | 236篇 |
2012年 | 141篇 |
2011年 | 183篇 |
2010年 | 149篇 |
2009年 | 206篇 |
2008年 | 199篇 |
2007年 | 210篇 |
2006年 | 229篇 |
2005年 | 188篇 |
2004年 | 167篇 |
2003年 | 165篇 |
2002年 | 133篇 |
2001年 | 143篇 |
2000年 | 111篇 |
1999年 | 105篇 |
1998年 | 88篇 |
1997年 | 82篇 |
1996年 | 61篇 |
1995年 | 48篇 |
1994年 | 61篇 |
1993年 | 34篇 |
1992年 | 25篇 |
1991年 | 27篇 |
1990年 | 14篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 19篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 3篇 |
排序方式: 共有4348条查询结果,搜索用时 859 毫秒
51.
52.
53.
我国低山丘陵区水土流失生态环境背景分析 总被引:6,自引:0,他引:6
我国是一个以山地为主的国家,山地多为大江大河的源头,而低山丘陵为这些大江大河支流的发源地,这些区域的水土流失必然会带来环境问题。本文以TM影象为数据源,以GIS为技术支撑,从DEM中获取低山丘陵层,以此为MASK层,提取水土流失及相应的生态环境背景各层,包括土地利用、坡度、年降水、积温、植被等生态背景层,这些背景层是多年数据的平均,然后把水土流失层与环境背景各层分别叠加,分析不同环境背景下水土流失状况,从而为治理水土流失提出科学依据。 相似文献
54.
55.
1 Introduction Soil is not only the major natural resource on which human being depends for the production of food, feed, fiber, renewable energy and raw materials, but also plays a key role in maintaining the complex terrestrial ecosystems and climate systems of this planet. Recent rapid increase in the human population is placing a great strain on the worlds soil resources. Only about 11% of the global land surface covered by the soils are being used to raise crops and livestock, in other … 相似文献
56.
Rebecca J. Kent 《The Geographical journal》2002,168(1):48-56
The soil conservation campaigns that have been a prominent feature of Jamaican agricultural policy since the 1950s are frequently presented as having failed to ameliorate the problem of soil erosion in hillside agriculture. A case study of a small farming community in the Blue Mountains explores the development of the soil conservation practices currently employed by farmers. The use of trash barriers in carrot farming is described and the origins of this technology and its subsequent adaptation are considered. The study concludes that extension interventions and government policy have influenced the development of current soil conservation practices; practices which have their roots in indigenous techniques. Other factors such as the adaptation of trash barriers to suit local conditions and their importance in soil fertility management also play a role in their widespread use in the study area. The study demonstrates that to understand the process of technical change in farming communities, it is necessary to consider a range of factors, external and internal, technical and social, that have over time influenced farmers' decision making. 相似文献
57.
近、现代黄河三角洲地貌形态反演 总被引:5,自引:0,他引:5
1855年黄河改夺大清河河道后于山东利津县东北入海。至今黄河尾癌河道历经10次大的改道变迁,塑造了近、现代黄河三角洲,形成了7个主要的亚三洲堆积体。该文利用黄河尾癌河道改道资料,近、现代河口流路变迁图,近、现代黄河三角洲体系图以及1985-1986年的水文工程地质钻孔资料,在研究黄河三角洲沉积模式的基础上,对工程地质钻孔数据进行沉积层序分析,将今论古,反推近、现代黄河三角洲沉积地貌,研究结果近似展示了近、现代黄河三角洲二维平面的发育演变和其时其地的地貌形态。 相似文献
58.
59.
60.
Exploring some relationships between biological soil crusts, soil aggregation and wind erosion 总被引:5,自引:0,他引:5
A portable wind tunnel was used to test the contribution of biological and physical elements to overall soil aggregation on a soil dominated by biological soil crusts in south-eastern Australia. After moderate disturbance and simulated wind erosion, 90% of surface aggregates on the loamy soil and 76% on the sandy soil were dominated by biological elements (cryptogams). Lower levels of biological bonding were observed on the severely disturbed treatment. Linear regression indicated a significant positive relationship (r2=0·72) between biological soil crust cover and dry aggregation levels greater than 0·85mm. To maintain sediment transport below an erosion control target of 5gm−1s−1 for a 65kmh−1 wind at 10m height, a crust cover of approximately 20% is required. When a multiple regression model which sequentially fitted biological crust cover and dry aggregation greater than 0·85mm was applied to the data, dry aggregation accounted for more of the variation in sediment transport rate than biological crust cover. These data were used to develop a conceptual model which integrates crust cover and dry aggregation, and provides a useful framework within which to predict the likely impacts of changes in soil crust cover and aggregation. 相似文献