全文获取类型
收费全文 | 154篇 |
免费 | 9篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 13篇 |
地球物理 | 60篇 |
地质学 | 55篇 |
海洋学 | 17篇 |
天文学 | 11篇 |
综合类 | 2篇 |
自然地理 | 3篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 5篇 |
2018年 | 3篇 |
2016年 | 2篇 |
2015年 | 2篇 |
2012年 | 4篇 |
2011年 | 3篇 |
2010年 | 2篇 |
2009年 | 2篇 |
2008年 | 7篇 |
2007年 | 7篇 |
2006年 | 9篇 |
2005年 | 1篇 |
2004年 | 10篇 |
2003年 | 4篇 |
2002年 | 9篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 14篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 4篇 |
1975年 | 2篇 |
排序方式: 共有177条查询结果,搜索用时 15 毫秒
41.
Kenya's rate of natural population increase exceeds 4.0%/year. At this rate, Kenya's population of 23.5 million will expand to 35 million by the year 2000. Rural migrants are being forced out of the highlands into marginal arid and semiarid regions to the east and south in the eastern ecological gradient including Meru, Kitui, Machakos, and Kajiado districts. The people have become victims of marginalization by which the productivity of a unit of land declines relative to the demands of its occupants. The concept of carrying capacity means the number of people a given area can sustain over the long term. In Maasailand, 3.5 standard stock units (450 kg each) are required per adult equivalent for full subsistence, about 7 cows/person. For the Maasai pastoralists, carrying capacities were examined at 2 levels of subsistence: 100% from the herds and 80% from the herds; 2 technological levels; and population-growth rates of 2%, 2.5%, and 3%/annum. Using the median, 3.5%/year, population-growth scenario these districts will have almost 5 million inhabitants in the year 2000. Poverty at technology level I for 40% of them, or for 2 million people, is implausible. Technology level II implies that current rural-development programs will succeed with technological innovations for farm households, access to credit, and markets for their produce. Level II is likely to prevail toward the end of the century for the majority of farmers. Level III necessitates best agricultural and livestock technology as well as the best management. At most, 25% of the households of the eastern ecological gradient could enter this realm by the year 2000. Current strategies of voluntary family planning, rural development emerging from an antiquated extension system, inability to address inequity in land distribution, and laissez-faire resource management are inadequate to deal with the pace of change. 相似文献
42.
Kelly-Gerreyn BA Hydes DJ Hartman MC Siddorn J Hyder P Holt MW 《Marine pollution bulletin》2007,54(7):850-862
This study evaluates the ship of opportunity (Ferrybox) concept for both sustained monitoring of UK shelf sea waters and numerical model validation. Release of phosphate from the wreck of a chemical tanker (MV Ece) in the western English Channel (49.73 degrees N, 3.25 degrees W) in March 2006 is used to demonstrate the importance of sustained observations in decision support systems and policy development. The Ferrybox system continuously collects sea surface (5m) data from a suite of autonomous electronic sensors installed on a passenger ferry operating year-round between Portsmouth (UK) and Bilbao (Spain). The detection of anomalously high concentrations of phosphate (1.54mmolm(-3), four times the usual level) and onset of phytoplankton growth close to the wreck site in March 2006 was placed in the context of multiple years of measurements (phosphate, nitrate, silicate and chlorophyll) collected from the Ferrybox system (2003-2006) and the long-term time series station E1 (50.03 degrees N, 4.65 degrees W, 1930-1987) in the English Channel. With regard to decision support, release of phosphate from the tanker is unlikely to pose a threat as phytoplankton growth at the end of winter is not unusual in this region and dissolved inorganic nitrogen rather than phosphate (DIN:DIP=10-18) is likely to ultimately limit algal growth in spring 2006. With regard to policy development, the Oslo and Paris (OSPAR) commissions recommendation of sampling every three years in "non-problem areas" is likely to provide statistically inadequate data, given the interannual and decadal variability identified in the Ferrybox and E1 data: the Ferrybox data show that oceanic winter nutrient concentrations varied by 35-50% between 2003/2004 and 2005/2006 due to deeper mixing of water off-shelf in early 2005/2006 and comparisons between the Ferrybox and E1 years show that the western English Channel is currently experiencing a low in phosphate concentrations similar to those in the 1960s. The importance of Ferrybox data in evaluating the reliability of predictive operational models needed in decision support is also demonstrated, by highlighting both strengths and weaknesses in a state-of-the-art ecosystem model designed for UK shelf waters. 相似文献
43.
44.
Beaumont NJ Austen MC Atkins JP Burdon D Degraer S Dentinho TP Derous S Holm P Horton T van Ierland E Marboe AH Starkey DJ Townsend M Zarzycki T 《Marine pollution bulletin》2007,54(3):253-265
This paper identifies and defines ecosystem goods and services provided by marine biodiversity. Case studies have been used to provide an insight into the practical issues associated with the assessment of marine ecosystem goods and services at specific locations. The aim of this research was to validate the definitions of goods and services, and to identify knowledge gaps and likely difficulties of quantifying the goods and services. A validated theoretical framework for the assessment of goods and services is detailed, and examples of the goods and services at a variety of case study areas are documented. These results will enable future assessments of marine ecosystem goods and services. It is concluded that the utilisation of this goods and services approach has the capacity to play a fundamental role in the Ecosystem Approach, by enabling the pressures and demands of society, the economy and the environment to be integrated into environmental management. 相似文献
45.
46.
47.
48.
50.