全文获取类型
收费全文 | 2799篇 |
免费 | 409篇 |
国内免费 | 462篇 |
专业分类
测绘学 | 261篇 |
大气科学 | 207篇 |
地球物理 | 145篇 |
地质学 | 1626篇 |
海洋学 | 251篇 |
天文学 | 3篇 |
综合类 | 286篇 |
自然地理 | 891篇 |
出版年
2024年 | 11篇 |
2023年 | 30篇 |
2022年 | 76篇 |
2021年 | 117篇 |
2020年 | 96篇 |
2019年 | 86篇 |
2018年 | 86篇 |
2017年 | 105篇 |
2016年 | 112篇 |
2015年 | 102篇 |
2014年 | 172篇 |
2013年 | 160篇 |
2012年 | 159篇 |
2011年 | 181篇 |
2010年 | 155篇 |
2009年 | 186篇 |
2008年 | 161篇 |
2007年 | 206篇 |
2006年 | 200篇 |
2005年 | 176篇 |
2004年 | 154篇 |
2003年 | 131篇 |
2002年 | 129篇 |
2001年 | 123篇 |
2000年 | 105篇 |
1999年 | 76篇 |
1998年 | 59篇 |
1997年 | 47篇 |
1996年 | 38篇 |
1995年 | 27篇 |
1994年 | 38篇 |
1993年 | 29篇 |
1992年 | 39篇 |
1991年 | 18篇 |
1990年 | 12篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 5篇 |
1977年 | 9篇 |
1954年 | 1篇 |
排序方式: 共有3670条查询结果,搜索用时 15 毫秒
991.
湿地价值及其有关问题探讨 总被引:9,自引:2,他引:7
当前,全球湿地正处于退化和丧失的威胁之中,其中一个主要原因是对湿地价值认识的不足。通过对湿地价值的来源、基本特征、有关利益群体及不同生态尺度下湿地价值的差异等有关问题的探讨,以期为湿地的保护与管理提供科学依据。 相似文献
992.
数字北京及空间信息资源建设 总被引:1,自引:0,他引:1
首都信息化的战略口号和奋斗目标是建设“数字北京” ,目的是使城市成为有利于人类生存与可持续发展的空间 ,为政府提供更加有效的、科学的管理与决策。本文主要介绍了遥感等空间信息资源在数字北京总体框架中的地位 ,以及整合并开发北京市空间信息资源达到信息共享、服务首都信息化的基本情况 相似文献
993.
994.
Overview of the MITI Nankai Trough Wells: A Milestone in the Evaluation of Methane Hydrate Resources 总被引:4,自引:0,他引:4
Yoshihiro Tsuji Hisashi Ishida Masaru Nakamizu Ryo Matsumoto Satoshi Shimizu 《Resource Geology》2004,54(1):3-10
Abstract. Bottom-simulating reflectors suggestive of the presence of methane hydrates are widely distributed below the ocean floor around Japan. In late 1999, drilling of the MITI Nankai Trough wells was conducted to explore this potential methane hydrate resource and a Tertiary conventional structure. The wells are located in the Northwest Pacific Ocean off Central Japan at a water depth of 945 m. A total of six wells were drilled, including the main well, two pilot wells, and three post survey wells at intervals of 10–100 m. All wells except the first confirmed the occurrence of hydrates based on logging-while-drilling, wire-line logging and/or coring using a pressure and temperature coring system in addition to conventional methods. Based on the various well profiles, four methane hydrate-bearing sand-rich intervals in turbidite fan deposits were recognized. Methane hydrates fill the pore spaces in these deposits, reaching saturation of up to 80 % in some layers. The methane hydrate-bearing turbiditic sand layers are less than 1 m thick, with a total thickness of 12–14 m. The bottom depth of high hydrate concentration correlates well with the depth of the bottom-simulating reflector. Based on these exploration results, the Japanese government inaugurated a 16-year methane hydrate exploitation program in 2001. 相似文献
995.
特种遗产资源研究基本问题--中国世界遗产与国家公园研究形势与任务 总被引:5,自引:3,他引:5
特种遗产是本文对世界遗产与国家公园认定、保护、保存、展示和遗传后代的科学研究所作的理论概括,同时界定了特种遗产与特种遗产资源的概念,讨论了中国的特种遗产———世界遗产与国家公园研究形势,提出了特种遗产资源研究的8个基本问题。 相似文献
996.
997.
998.
In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 105–107 years. Over shorter times, <105 years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 104-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and 14C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic 10Be and 26Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At 104-year timescales, maximum bedrock incision rates are 7 mm/year in the Greater Himalaya and 1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate. 相似文献
999.
Severe problems of fragmented policies and uncoordinated implementation undermine natural resource management in Australia. There have been promising signs of progress through activities such as the National Forest Policy, Council of Australian Government water reforms, National Land and Water Resources Audit, the Murray‐Darling Basin initiative and the National Action Plan for Salinity and Water Quality to integrate resource policies. There have also been some notable successes at State and local level. But fragmentation endures. This paper provides evidence of the enduring problem of fragmentation and presents a framework to analyse Australia's experience in natural resource management. The analysis reveals the multi‐dimensional character of the problems and identifies a diverse set of actions that need to be taken to improve integration in policy and implementation. Particular attention is given to the potential for regional programs to contribute to improved integration and NRM. 相似文献
1000.
Recent changes in the cultural and forest landscapes of the Meseta Purépecha in Michoacán, Mexico as a result of forest degradation underscore the complexity of forest change processes in the tropical highlands of Latin America. Differences in community perception and forest structure and composition between the furniture-making and lumber-producing towns of Pichátaro and Sevina, Purépechan indigenous communities located amidst pine forests on Michoacán's volcanic plateau illustrate the dynamics of this process. We base our comparisons on interviews and field measures of forest structure. Our results show dramatic changes in the forests and cultural landscapes of both communities during the past decade. Following high regional timber exports during the early 1990s, Sevina shifted from a self-sufficient to a timber importation community. By comparison, communal forests and individual parcels in Pichátaro continue to provide wood for approximately 300 wood shops. Field data and forest stand maps confirm the perception of forest degradation in both communities. While Pichátaro has maintained a larger and more diverse forest base to date, stand structure data indicate selective harvesting has led to a shift in dominance toward the less economically desirable pine species and oak. Deforestation and degradation of Sevina and Pichátaro's community forests are symptomatic of both the Meseta Purépecha and Mexico in general. Current forest conditions in both communities justify local, regional, and national concerns regarding declining biodiversity and sustainable economies. 相似文献