全文获取类型
收费全文 | 5749篇 |
免费 | 890篇 |
国内免费 | 1064篇 |
专业分类
测绘学 | 365篇 |
大气科学 | 327篇 |
地球物理 | 1028篇 |
地质学 | 2896篇 |
海洋学 | 1168篇 |
天文学 | 58篇 |
综合类 | 345篇 |
自然地理 | 1516篇 |
出版年
2024年 | 32篇 |
2023年 | 123篇 |
2022年 | 259篇 |
2021年 | 241篇 |
2020年 | 246篇 |
2019年 | 288篇 |
2018年 | 250篇 |
2017年 | 234篇 |
2016年 | 263篇 |
2015年 | 231篇 |
2014年 | 340篇 |
2013年 | 379篇 |
2012年 | 328篇 |
2011年 | 311篇 |
2010年 | 301篇 |
2009年 | 323篇 |
2008年 | 321篇 |
2007年 | 346篇 |
2006年 | 396篇 |
2005年 | 325篇 |
2004年 | 315篇 |
2003年 | 248篇 |
2002年 | 271篇 |
2001年 | 250篇 |
2000年 | 198篇 |
1999年 | 152篇 |
1998年 | 156篇 |
1997年 | 118篇 |
1996年 | 92篇 |
1995年 | 74篇 |
1994年 | 64篇 |
1993年 | 53篇 |
1992年 | 46篇 |
1991年 | 28篇 |
1990年 | 14篇 |
1989年 | 23篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1977年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有7703条查询结果,搜索用时 10 毫秒
111.
During the last decade of the 20th century, extensive conversion in agricultural land use took place in Northeast China. The goal of this study is to ascertain its spatial distribution and regional differentiation, determine its causes, and analyze its environmental impact. Especially we attempt to elucidate how institutional constraints have facilitated the change at a time of agrarian restructuring when newly emerging free market was hybridized with the former planned economy. Information on six categories of land use was mapped from interpretation of Landsat TM images recorded in 1990, 1995 and 2000. Most of land use changes took place during the first half of the decade, coinciding with abrupt and chaotic changes in government directives. Farmland was changed mainly to woodland, water body and built-up areas while woodland and grassland were converted chiefly to farmland. Spatially, the change from farmland to woodland was restricted to the west of the study area. The change from grassland to farmland took place in the grazing and farming interlocked west. These chaotic and occasionally conflicting changes were largely caused by lack of stability and consistency in agricultural land use policies promulgated. They have exerted adverse impacts on the local environment, including land degradation, increased flooding, and modified climate regime. 相似文献
112.
Andrew McGregor 《The Australian geographer》2004,35(2):141-149
This paper presents group research as a potentially powerful research tool for cultural geographers interested in the ways knowledge is produced at the interpersonal level. I argue that groups can create unique research spaces that incorporate 'sociality', the everyday social norms, logics and languages that guide ordinary conversations, into the research agenda. By concentrating on these discursive norms, insights can be obtained into the ways that knowledge is negotiated, produced and constrained in different places. To demonstrate the value of group research, a case study involving Australian environmentalists is analysed, to show how forests are constructed, contested and naturalised within this particular subculture. The article concludes by emphasising the importance of group research to critical geographers interested in social empowerment and change. 相似文献
113.
This paper identifies contrasting environmental discourses on bushfire from the public debate that followed the Victorian fires of 2002–03. Submissions to the inquiry into the 2002–03 Victorian bushfires provide a particularly rich source of documentation. It is argued that environmental events such as bushfire only become political issues, or problems, when they are constituted as such through environmental discourse. Through the analysis, three contrasting discourses on bushfire are identified—here labelled the ‘conservationist’, ‘ruralist’ and ‘wise use’ discourses. In examining how different ‘constructions’ of bushfire have led to conflict, each discourse is shown to consist of a range of actors who draw on shared storylines. Having established a better understanding of the different attitudes, beliefs, interests and values that underpin debates about bushfire, the paper concludes with a brief discussion of the prospects for compromise among the discourses and the potential for improved land and fire management outcomes. 相似文献
114.
No author 《Aquatic Sciences - Research Across Boundaries》2005,67(3):392-291
No Abstract. .The following two amendments concern the above mentioned article that appeared electronically Online First on June13, 2005 (this issue, pp. 274–291).1. the first name of the author Cooper is Philip (and not Phillip).2. the correct grant reference number is EVK1-CT-1999-00032 (instead of: EVK1-CT-2002-00121). 相似文献
115.
Delayed Geochemical Hazard (DGH briefly) presents the whole process of a kind of serious ecological and environmental hazard caused by sudden reactivation and sharp release of long-term accumulated pollutant from stable species to active ones in soil or sediment system due to the change of physical-chemical conditions (such as temperature, pH, Eh, moisture, the concentrations of organic matters, etc.) or the decrease of environment capacity. The characteristics of DGH are discussed. The process of a typical DGH can be expressed as a nonlinear polynomial. The points where the derivative functions of the first and second orders of the polynomial reach zero, minimum and maximum are keys for risk assessment and harzard pridication.The process and mechanism of the hazard is due to the transform of pollutant among different species principally. The concepts of "total releasable content of pollutant", TRCP, and "total concentration of active specie", TCAS, are necessarily defined to describe the mechanism of DGH. The possibility of the temporal and spatial propagation is discussed. Case study shows that there exists a transform mechanism of "gradual release" and "chain reaction" among the species of the exchangeable and the bounds to carbonate, iron and manganese oxides and organic matter, thus causing the delayed geochemical hazard. 相似文献
116.
A new methodology for deriving the uplift load–displacement response of long driven piles in cohesionless soils is proposed. This method accounts for the effects of the friction fatigue processes during pile driving and the existence of locked-in residual stresses at the end of pile driving before commencing the pile load test. A hyperbolic formulation is utilized to simulate the nonlinear load transfer curves (the so-called t–z curves). The utility of this technique is demonstrated for a field pullout load test on a driven pile in sand. Predicted and measured load–displacement curves showed good agreement, indicating that this approach yields reasonable results as long as representative input parameters are employed. 相似文献
117.
沿海地区海底地下水排放在北美和欧洲等发达地区受到了越来越多的重视,被认为是一个重要的海岸带陆海相互作用过程。但这一过程在我国尚未引起足够认识,有关研究极少见。海底地下水排放的研究历史不长,只是近十多年才有了快速的发展,有了越来越多的定量研究成果。其研究方法主要有水文计算法、现场实测法和地球化学示踪法,各种方法之间的对比实验是目前的热点问题。沿海地下水排放具有重要的环境意义,它可以是陆地营养物质和污染物质的一个重要排放通道,可以对海岸带环境产生一定影响。我国沿海地区应该加强有关的研究工作,为海岸带环境管理作出贡献。 相似文献
118.
119.
Proof of settlement-time curve appearing “S” shape with ramp load
and its application base on stress-strain relationship 总被引:1,自引:0,他引:1
Based on stress-strain relationship of soil, the proof is made for that s-t curve appears “S” shape when the load is increased linearly, which is similar to construction process. A new model of forecasting the relation of settlement-time is presented. Case history shows the rationality of the method. 相似文献
120.
试论生态环保厕所技术在云南旅游业发展中的可行性 总被引:2,自引:0,他引:2
王静 《云南地理环境研究》2005,17(2):69-72
云南省地处高原,鉴于缺水严重、自然环境脆弱的实际情况,同时,为了突破制约旅游业发展的“旅游厕所”这一瓶颈,云南省现已在走“科技兴旅”的道路,坚持使用高科技、节能环保的生态厕所技术,以确保云南省旅游业持续、快速、健康的发展。由于并非每一种生态环保厕所技术都适用于旅游业,因此,对现今市场上出现的纷繁复杂的生态环保厕所技术进行归类、分析、总结,提出了适应于云南省旅游业发展的生态环保厕所技术类型以及具体的适用场所,进一步指出在选择技术的过程中应考虑的问题。 相似文献