全文获取类型
收费全文 | 860篇 |
免费 | 31篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 55篇 |
大气科学 | 72篇 |
地球物理 | 178篇 |
地质学 | 335篇 |
海洋学 | 30篇 |
天文学 | 133篇 |
综合类 | 11篇 |
自然地理 | 81篇 |
出版年
2023年 | 4篇 |
2022年 | 9篇 |
2021年 | 14篇 |
2020年 | 13篇 |
2019年 | 23篇 |
2018年 | 24篇 |
2017年 | 20篇 |
2016年 | 36篇 |
2015年 | 27篇 |
2014年 | 32篇 |
2013年 | 69篇 |
2012年 | 42篇 |
2011年 | 31篇 |
2010年 | 38篇 |
2009年 | 49篇 |
2008年 | 34篇 |
2007年 | 35篇 |
2006年 | 31篇 |
2005年 | 43篇 |
2004年 | 30篇 |
2003年 | 29篇 |
2002年 | 30篇 |
2001年 | 22篇 |
2000年 | 22篇 |
1999年 | 16篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1984年 | 3篇 |
1982年 | 4篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1957年 | 3篇 |
1956年 | 4篇 |
1953年 | 2篇 |
1951年 | 3篇 |
1948年 | 5篇 |
1928年 | 2篇 |
排序方式: 共有895条查询结果,搜索用时 78 毫秒
791.
Addressing the coral reef crisis in developing countries 总被引:2,自引:0,他引:2
Johann D. Bell Blake D. Ratner Ilona Stobutzki Jamie Oliver 《Ocean & Coastal Management》2006,49(12):976-985
792.
Using analytical approximations we study the effects of different external magnetic configurations on the half-width, mass, and internal magnetic structure of a quiescent solar prominence, modelled as a thin vertical sheet of cool plasma. Firstly, we build up a zeroth-order model and analyse the effects produced by a potential coronal field or a constant- force-free field. This model allows us to obtain the half-width and mass of the prominence for different values of the external field, pressure and shear angle. Secondly, the effects of these external magnetic configurations on a two-dimensional model proposed by Ballester and Priest (1987) are studied. The main effects are a change of the half-width with height, an increase of the mass, a decrease of the magnetic field strength with height and a change in the shape of the magnetic field lines. 相似文献
793.
This is based on a concept worked out by EUROSOLAR, an Association for the Solar Energy Age, of which Hermann Scheer is chairman and Wilfrid Bach is a member of the board. 相似文献
794.
795.
Within the Kyoto Protocol, countries have significant latitude to define a forest. The most important parameter affecting
area designated as forest is the minimum crown density which can be set between 10 and 30 percent. The choice will have implications
for the amount of land available in a country for afforestation and reforestation (A/R) activities within the Clean Development
Mechanism (CDM). In this paper, we used GIS analysis to look at four case studies from countries involved in the EU-funded
ENCOFOR project: Bolivia, Ecuador, Uganda and Kenya. We also looked at the effects of other factors that would exclude land
from eligibility for A/R projects. A low threshold of 10 percent crown cover resulted in exclusion of 70 to 90 percent of
the land area of all countries except Kenya, which differed from the other countries by its large dryland areas. Much less
area was excluded when higher thresholds were used. The spatial analyses showed not only the effects of the choice of the
crown cover criterion, but also where the land was available for CDM activities within each country at different thresholds.
This is the first time that the effect of the crown cover element of the Kyoto forest definition on land availability and
distribution for CDM A/R activities has been quantified. While there are a number of reasons to choose different thresholds,
this analysis suggests that countries that are considering using CDM finance for rural development might want to select the
higher minimum threshold for crown cover in order to maximize their participation and flexibility. 相似文献
796.
Since landfill operations ceased in the early 1980s at the Woodlawn landfill Superfund site in northeastern Maryland, USA,
the carcinogen vinyl chloride monomer (VCM) has persisted in the site ground water at concentrations in places greater than
100 ppb. Investigations of VCM concentrations from monitoring well data from 1981 through 1991, which were published in 2001,
concluded that concentrations were cyclic and were not dissipating. Data from 1991 to 2000 confirm VCM persistence, showing
concentration peaks as late as 1997 in the eastern part of the site. These values are significantly above both drinking water
standards (>MCL of 2 ppb) and lifetime excess cancer risk (E-6 risk level) from exposure since birth (>0.024 ppb). Sources
for VCM are landfill wastes (PVC-sludge) and effluent from an on-site transfer station. In addition to direct release of VCM
from wastes, other chlorinated hydrocarbons, trichloroethene and perchloroethene (TCE and PCE) are also present in the ground
water, and these compounds can degrade into VCM. Persistence of VCM concentrations is the result of leachate generation, mostly
from multi-source wastes in the vadose zone, with subsequent infiltration into the regional aquifer. Increases in VCM concentrations
(new leachate generation) alternated with decreases in VCM concentrations (infiltration without leachate) in a cyclic fashion,
during this 20-year period. 相似文献
797.
Oliver Gabriel Dagmar Balla Thomas Kalettka Christian Schilling Matthias Zessner 《洁净——土壤、空气、水》2006,34(4):333-345
Research on the sediment‐surface water transition zone in three study site regions, different in hydrological conditions, was conducted to estimate to which extent nitrate in surface water can contribute to riverbed sediment oxidation and thus prevent release of sediment phosphorus to surface water. Consequently, the Du tch Flow Model (DUFLOW) based water quality model “Spreewald” and results from the emission model “ Mo delling N utrient E missions in Ri ver S ystems” (MONERIS) were used to estimate to which extent wastewater treatment plants (WWTPs) could contribute to the NO3– concentration in surface waters if they were operated without denitrification. It is demonstrated that an effective phosphorus retention in sediments by means of surface water NO3– is possible only under specific conditions, i. e. (i) a sufficient supply of surface water to the sediment by advective fluxes, (ii) a sufficient amount of sediment iron to provide phosphorus binding sites in the case of sediment oxidation, (iii) a redox system not leading to a rapid NO3– and FeOOH depletion and to phosphorus release from organic pools by high microbial activities. Model results show that in‐stream denitrification processes counteract a significant increase of NO3– surface water concentrations from WWTPs operated without denitrification during summer, when eutrophication risk through phosphorus is highest. The increase of NO3– surface water concentrations in winter due to decreased denitrification in the surface water is of minor relevance for phosphorus release from sediments. 相似文献
798.
799.
Ute Weckmann Oliver RitterVolker Haak 《Physics of the Earth and Planetary Interiors》2003,138(2):91-112
Magnetotelluric (MT) observations at some sites in the vicinity of the Waterberg Fault/Omaruru Lineament (WF/OL), a major tectono-stratigraphic zone boundary in the Central Zone of the Damara Belt, show evidence for strong three-dimensional (3D) effects. We observe very high skew values, phases over 90°, and a strong correlation of parallel components of the electric and magnetic fields at long periods. Because of the dense site spacing and good spatial coverage, we can positively attribute these effects to local geology and are able to resolve structural detail within the WF/OL. Mapping LaTorraca’s electric characteristic vectors in form of ellipses proved particularly useful in identifying key elements of the conductivity structure for subsequent modelling. 3D and 2D anisotropic modelling can reproduce most of the observed 3D effects. The conductivity anomalies revealed in the area are: (i) a conductive ring structure in the shallow crust along the northern part of the profile; (ii) an anisotropic region in the upper crust with high conductivity parallel to the WF/OL; (iii) anisotropy in the lower crust with a different but undetermined strike direction; and (iv) a shallow elongated conductor sub-normal to the WF/OL. Modelling studies further suggest that the (anisotropic) fault zone is approximately 10 km wide and may reach down to a depth of 14 km or more. 相似文献
800.
The experimental variogram computed in the usual way by the method of moments and the Haar wavelet transform are similar in that they filter data and yield informative summaries that may be interpreted. The variogram filters out constant values; wavelets can filter variation at several spatial scales and thereby provide a richer repertoire for analysis and demand no assumptions other than that of finite variance. This paper compares the two functions, identifying that part of the Haar wavelet transform that gives it its advantages. It goes on to show that the generalized variogram of order k=1, 2, and 3 filters linear, quadratic, and cubic polynomials from the data, respectively, which correspond with more complex wavelets in Daubechies's family. The additional filter coefficients of the latter can reveal features of the data that are not evident in its usual form. Three examples in which data recorded at regular intervals on transects are analyzed illustrate the extended form of the variogram. The apparent periodicity of gilgais in Australia seems to be accentuated as filter coefficients are added, but otherwise the analysis provides no new insight. Analysis of hyerpsectral data with a strong linear trend showed that the wavelet-based variograms filtered it out. Adding filter coefficients in the analysis of the topsoil across the Jurassic scarplands of England changed the upper bound of the variogram; it then resembled the within-class variogram computed by the method of moments. To elucidate these results, we simulated several series of data to represent a random process with values fluctuating about a mean, data with long-range linear trend, data with local trend, and data with stepped transitions. The results suggest that the wavelet variogram can filter out the effects of long-range trend, but not local trend, and of transitions from one class to another, as across boundaries. 相似文献