全文获取类型
收费全文 | 898篇 |
免费 | 38篇 |
国内免费 | 19篇 |
专业分类
测绘学 | 17篇 |
大气科学 | 44篇 |
地球物理 | 203篇 |
地质学 | 532篇 |
海洋学 | 44篇 |
天文学 | 70篇 |
综合类 | 11篇 |
自然地理 | 34篇 |
出版年
2022年 | 12篇 |
2021年 | 15篇 |
2020年 | 18篇 |
2019年 | 16篇 |
2018年 | 63篇 |
2017年 | 72篇 |
2016年 | 68篇 |
2015年 | 39篇 |
2014年 | 57篇 |
2013年 | 72篇 |
2012年 | 54篇 |
2011年 | 50篇 |
2010年 | 30篇 |
2009年 | 48篇 |
2008年 | 53篇 |
2007年 | 23篇 |
2006年 | 28篇 |
2005年 | 18篇 |
2004年 | 28篇 |
2003年 | 24篇 |
2002年 | 14篇 |
2001年 | 7篇 |
2000年 | 11篇 |
1999年 | 3篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 9篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1966年 | 3篇 |
1960年 | 1篇 |
排序方式: 共有955条查询结果,搜索用时 406 毫秒
211.
Groundwater of Egypt: “an environmental overview” 总被引:1,自引:0,他引:1
Although Egypt has the great Nile River, which is the main supply of water, Egypt’s water is limited to 55.5 billion m3 per annum. Owing to the rapid growth of the population and the increasing consumption of water in agriculture, industry,
domestic use, etc., it is expected that Egypt will rely to some extent on groundwater to develop the new projects such as
Tushka in Upper Egypt and East Oweinat. Issues related to groundwater in Egypt are identified with the common geological features
associated with formation of the aquifers and demonstrating the location of the main resources of groundwater, followed by
the main objective of this paper, which is addressing the environmental issues related to groundwater in Egypt. Several studies
have been reviewed and personal communication made with the authorities to introduce this work and provide an overview of
the groundwater quality problems in Egypt with examples from different parts of the country. 相似文献
212.
Mohamed El Wartiti Amina Malaki Mohamed Zahraoui Abdelilah El Ghannouchi Felice Di Gregorio 《Environmental Geology》2008,55(2):415-422
Across the northwestern Tabular Middle Atlas of Morocco there are many examples of landscapes, rocks and fossils that provide
key evidence of a particular moment or period in Earth history. Such Earth heritage sites are important for educating the
general public in environmental matters. They also serve as tools for demonstrating sustainable development and for illustrating
methods of site conservation as well as remembering that rocks, minerals, fossils, soils, landforms form an integral part
of the natural world. The significance of certain sites for aesthetic or tourism reasons is obvious. There are numerous geosites,
which could contribute to effective exploitation of geotourism, often in conjunction with ecotourism. The strategy employed
to such sites involves close consultation with all communities in the vicinity of the respective geosite and is not only aimed
at tourism and education, but also at sustainable improvement of the infrastructure of the people of this area. Geological
heritage sites, properly managed, can generate employment and new economic activities, especially in regions in need of new
or additional sources of income. 相似文献
213.
Geophysical boundary-layer flows feature complex dynamics that often evolve with time; however, most current knowledge centres on the steady-state problem. In these atmospheric and oceanic boundary layers, the pressure gradient, buoyancy, Coriolis, and frictional forces interact to determine the statistical moments of the flow. The resulting equations for the non-stationary mean variables, even when succinctly closed, remain challenging to handle mathematically. Here, we derive a simpler physical model that reduces these governing unsteady Reynolds-averaged Navier–Stokes partial differential equations into a single first-order ordinary differential equation with non-constant coefficients. The reduced model is straightforward to solve under arbitrary forcing, even when the statistical moments are non-stationary and the viscosity varies in time and space. The model is successfully validated against large-eddy simulation for, (1) time-variable pressure gradients, and (2) linearly time-variable buoyancy. The new model is shown to have a superior performance compared to the classic Blackadar solutions (and later improvements on these solutions), and it covers a much wider range of conditions. 相似文献
214.
The effects of dry summer ambient temperature on some physiological and productive responses were studied in an exotic European breed (Holstein–Friesian) and a local zebu breed (Butana ecotype) of cattle in Sudan. Rectal temperature and respiration rate were positively correlated, and dry matter intake and milk yield were negatively correlated with variation in ambient temperature. However, significant adverse effects on high ambient temperature were only observed in the exotic breed. 相似文献
215.
Electron microprobe analyses of small chlorite grains (10–20 μm width) in diagenetically altered rocks from three deep wells
and a series of outcrop samples ranging in maximum age from 4 to 1300 Ma show an increasingly smaller range in grain-to-grain
composition as a function of the average age of the specimens in a series. The scatter of composition in a sample in each
series changes little with depth despite present day, or estimated maximum temperature differences of 70° C to 100° C from
top to bottom. Comparison of these chlorite compositions with those published for geothermal and metamorphic rocks formed
under conditions of 300–450° C indicates that the homogeneity of chlorite composition is a function of both the temperature
of equilibration and its duration. Apparently total homogenization of Fe/(Fe+Mg), Al/(Al+Mg+Fe) content will occur after 1010 years at low temperatures. Simple calculations indicate that a slow process such as solid state diffusion could be responsible
for the eventual homogenization of mineral grain composition to reach chemical equilibrium after phase equilibrium has been
reached. 相似文献
216.
217.
Arid to semi-arid regions are characterized by low levels of surface water and low annual precipitation (generally <350 mm/year). In such areas, groundwater must be used to meet all the needs of the population for water. As a consequence, careful management is required to ensure the sustainability of this scarce resource in response to the demands of urban centers, industry, agriculture, and tourism. The concept of the aquifer recharge rate is particularly useful in the quantification of these groundwater resources and can be used to form the basis of a decision support system. This study determined the potential recharge rate in the Haouz aquifer using a multi-criteria analysis that included both the major and minor factors influencing the rate of infiltration of water into the aquifer. The analysis was based on the use of a geographical information system supported by remote sensing techniques to develop thematic data layers. These layers were then used to describe the spatial variation of the factors influencing the recharge rate of the aquifer and were subsequently integrated and analyzed to derive the spatial distribution of the potential recharge. This approach was used to classify the Haouz Plain (Morocco) into three different zones with respect to the recharge rate, with recharge rates ranging from 3.5 to 18.2 %. 相似文献
218.
219.
Twelve calcareous nannofossil biozones of Late Oligocene-Late Miocene in Northern Egypt were defined and correlated with their corresponding biozones in Egypt and other parts of the world. These are arranged from the top to base as Zone NN12, Zone NN11, Zone NN10, Zone NN8, Zone NN7, Zone NN6 Zone NN5, NN4, Zone NN3, Zone NN2 Zone NP25 and Zone NP24. In the present study (Boughaz-1 Well), the Late Miocene unconformably overlies the Middle Miocene. This unconformity surface is recognized by the missing of calcareous nannofossil zones NN7 to NN9. While, in North Sinai (Malha-1 Well), the Early/Middle Miocene boundary cannot be recognized, where the Middle Miocene unconformably overlies the topmost Oligocene, and it is defined by the missing calcareous nannofossil zones NN1 to NN4. 相似文献
220.