全文获取类型
收费全文 | 1888篇 |
免费 | 122篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 38篇 |
大气科学 | 165篇 |
地球物理 | 550篇 |
地质学 | 689篇 |
海洋学 | 226篇 |
天文学 | 228篇 |
综合类 | 11篇 |
自然地理 | 112篇 |
出版年
2024年 | 6篇 |
2023年 | 7篇 |
2022年 | 21篇 |
2021年 | 49篇 |
2020年 | 50篇 |
2019年 | 32篇 |
2018年 | 96篇 |
2017年 | 74篇 |
2016年 | 150篇 |
2015年 | 105篇 |
2014年 | 107篇 |
2013年 | 128篇 |
2012年 | 88篇 |
2011年 | 118篇 |
2010年 | 121篇 |
2009年 | 116篇 |
2008年 | 114篇 |
2007年 | 66篇 |
2006年 | 67篇 |
2005年 | 52篇 |
2004年 | 49篇 |
2003年 | 52篇 |
2002年 | 44篇 |
2001年 | 28篇 |
2000年 | 27篇 |
1999年 | 25篇 |
1998年 | 28篇 |
1997年 | 19篇 |
1996年 | 15篇 |
1995年 | 19篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 9篇 |
1991年 | 14篇 |
1990年 | 16篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1978年 | 3篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1957年 | 2篇 |
1956年 | 3篇 |
1954年 | 2篇 |
1950年 | 2篇 |
排序方式: 共有2019条查询结果,搜索用时 31 毫秒
801.
Martinho F Viegas I Dolbeth M Leitão R Cabral HN Pardal MA 《Marine pollution bulletin》2008,56(11):1834-1843
The seasonal variation of five selected multimetric indices for the determination of the Ecological Quality Status (EQS) of transitional waters was evaluated, as well as the indices’ responses to an extreme drought event that occurred in 2005. The database used regards the Mondego River estuary, which was sampled from June 2003 to August 2006 on a monthly basis. Among the selected indices (EBI - Deegan et al. [Deegan, L., Finn, J.T., Ayvazlan, S.G., Ryder-Kieffer, C.A., Buonaccoesi, J., 1997. Development and validation of an Estuarine Biotic Integrity Index. Estuaries 30(3), 601-617], EDI - Borja et al. [Borja, A., Franco, J., Valencia, V., Bald, J., Muxika, I., Belzunce, M.J., Solaun, O., 2004. Implementation of the European Water Framework Directive from the Basque Country (northern Spain): a methodological approach. Marine Pollution Bulletin 48(3-4), 209-218], EFCI - Harrison and Whitfield [Harrison, T.D., Whitfield, A.K., 2004. A multi-metric fish index to assess the environmental condition of estuaries. Journal of Fish Biology 65, 683-710], EBI - Breine et al. [Breine, J.J., Maes, J., Quataert, P., Van den Bergh, E., Simoens, I., Van Thuyne, G., Belpaire, C., 2007. A fish-based assessment tool for the ecological quality of the brackish Scheldt estuary in Flanders (Belgium). Hydrobiologia 575, 141-159] and TFCI - Coates et al. [Coates, S., Waugh, A., Anwar, A., Robson, M., 2007. Efficacy of a multi-metric fish index as an analysis tool for the transitional fish component of the Water Framework Directive. Marine Pollution Bulletin 55, 225-240]), the EBI by Breine et al. (2007) was the only that evidenced clear interannual and seasonal variations. The EQS by the several indices ranged from “Low” to “High”, depending on the index considered, evidencing the high level of mismatch between indices. The results are discussed in the scope of the EU Water Framework Directive, regarding monitoring strategies, application of indices and EQS assessment. 相似文献
802.
Juan-Luis Valera Ana-María Negredo Antonio Villaseñor 《Pure and Applied Geophysics》2008,165(8):1683-1706
Convective removal and mantle delamination are geodynamical mechanisms proposed to explain the presence of extension in the
Alboran Sea within a regional context of compression. Using a new thermo-mechanical algorithm, we present here a quantitative
evaluation and comparison of conceptual models based on these geodynamical mechanisms. In contrast to the in situ convective removal process, the laterally propagating delamination mechanism is shown here to be consistent with first-order
features of the Alboran Sea such as the thinning/thickening distribution, intermediate-depth seismicity and upper mantle structure
imaged by seismic tomography. The lower crust is predicted to reach depths of 100–150 km in some areas, due to mechanically-driven
viscous drag of the downwelling mantle. 相似文献
803.
Francini-Filho RB Moura RL Thompson FL Reis RM Kaufman L Kikuchi RK Leão ZM 《Marine pollution bulletin》2008,56(5):1008-1014
Although reef corals worldwide have sustained epizootics in recent years, no coral diseases have been observed in the southwestern Atlantic Ocean until now. Here we present an overview of the main types of diseases and their incidence in the largest and richest coral reefs in the South Atlantic (Abrolhos Bank, eastern Brazil). Qualitative observations since the 1980s and regular monitoring since 2001 indicate that coral diseases intensified only recently (2005–2007). Based on estimates of disease prevalence and progression rate, as well as on the growth rate of a major reef-building coral species (the Brazilian-endemic Mussismilia braziliensis), we predict that eastern Brazilian reefs will suffer a massive coral cover decline in the next 50 years, and that M. braziliensis will be nearly extinct in less than a century if the current rate of mortality due to disease is not reversed. 相似文献
804.
In November 2002, the sinking of the Prestige cargo ship produced an oil spill of 60,000 tons that affected many areas along the Galician coast (in the northwest of Spain). In a number of rocky shore sites, most organisms (particularly marine mollusks) were nearly extinct at a local scale. We tested whether the local bottleneck/extinction that occurred in affected localities caused any detectable reduction of the genetic diversity in the marine snail Littorina saxatilis, an ovoviviparous rocky shore model species characterized by a low dispersal ability, high population density, and wide distribution range. We compared the level of genetic variation and population differentiation between affected (polluted) and control sites located in seven geographical areas (three sites per area, one impacted and two controls, and two replicates per site) one and a half years after the spill. The analysis included molecular marker variation (microsatellite and AFLP loci) and quantitative trait genetic variation for shell variables in embryos extracted from pregnant females. Our results indicate that the affected populations did not show a significant overall reduction in genetic diversity when compared to the controls, suggesting that the species is highly resistant to losing genetic variability as a consequence of a local short-term pollution process in spite of its low dispersal ability and direct development. However, some genetic effects were detected in the polluted populations, particularly for quantitative shell traits and AFLPs, consistent with local adaptations resulting from the fuel contamination. 相似文献
805.
806.
Elisabete S. Braga Vitor C. Chiozzini Gláucia B.B. Berbel João C.C. Maluf Valquíria M.C. Aguiar Marcela Charo Daniel Molina Sílvia I. Romero Beatriz B. Eichler 《Continental Shelf Research》2008
Nutrient distributions observed at some depths along the continental shelf from 27°05′S (Brazil) to 39°31′S (Argentina) in winter, 2003 and summer, 2004 related to salinity and dissolved oxygen (mL L−1) and saturation (%) data showed remarkable influences of fresh water discharge over the coastal region and in front of the La Plata estuary. In the southern portion of the study area different processes were verified. Upwelling processes caused by ocean dynamics typical of shelf break areas, eddies related to surface dynamics and regeneration processes confirmed by the increase of nutrients and the decrease of dissolved and saturation oxygen data were verified. High silicate concentrations in the surface waters were identified related to low salinities (minimum of 21.22 in winter and 21.96 in summer), confirming the importance of freshwater inputs in this region, especially in winter. Silicate concentration range showed values between 0.00 and 83.52 μM during winter and from 0.00 to 41.16 μM during summer. Phosphate concentrations worked as a secondary trace of terrestrial input and their values varied from 0.00 to 3.30 μM in winter and from 0.03 to 2.26 μM in summer; however, in shallow waters, phosphate indicated more clearly the fresh water influence. The most important information given by nitrate concentrations was the presence of water from SACW upwelling that represents a new source of nutrients for marine primary production. Nitrate maximum values reached 41.96 μM in winter and 33.10 μM in summer. At a depth ∼800 m, high nitrate, phosphate and silicate concentrations were related to Malvinas Current Waters, Subantarctic Shallow Waters and Antarctic Atlantic Intermediate Waters (AAIW). Dissolved oxygen varied from 3.41 to 7.06 mL L−1 in winter and from 2.65 to 6.85 mL L−1 in summer. The percentage of dissolved oxygen saturation in the waters showed values between 48% and 113% in winter and from 46% to135% in summer. The most important primary production was verified in the summer, and situations of undersaturation were mainly observed below 50 m depth and at some points near the coast. The anti-correlation between nutrients and dissolved oxygen which showed evident undersaturation also revealed important potential sites of remineralization processes. The nutrient behaviours showed some aspects of the processes that occur over the Southwestern South Atlantic continental shelf and in their land–sea interfaces between Mar del Plata and Itajaí. 相似文献
807.
808.
Summary Rainfall anomaly patterns are obtained for the city of Barcelona from a statistical and a spectral point of view. The time
series consists of monthly rainfall amounts recorded over 128 years without interruption. Monthly positive and negative anomalies,
obtained as the difference between monthly amounts and monthly threshold values, are used for both types of analyses. The
threshold levels are derived form the deciles of theoretical monthly rainfall distributions, which have been previously modelled
by the gamma distribution. Positive and negative anomalies of the monthly rain amounts are investigated for these threshold
levels. The statistical analysis is applied to each decile considered, yielding empirical exponential laws that can be used
to forecast the cumulative number of episodes of consecutive months with either positive or negative anomalies equalling or
exceeding a fixed length. A set of linear laws, relating the expected rainfall amount cumulated during an episode of a fixed
length, is also deduced. It is worthy of mention that, independently of the decile considered, all the exponential and linear
laws have satisfactory regression coefficients. At the same time, it has also been possible to establish the evolution of
the coefficients of these laws with respect to the different deciles considered. The exponential laws for episodes of positive
and negative anomalies are the starting point, together with two hypotheses, to model probabilities of repeated long episodes
over an arbitrary number of years and their return periods in terms of the Poisson distribution model. Moreover, power spectra
are derived for anomalies relative to the 50% decile at monthly and seasonal scale. The spectral estimates obtained are then
compared with theoretical spectra deduced from possible Markovian or random behaviour of the time series of anomalies. Finally,
the significant spectral peaks are discussed and compared with other significant spectral components deduced for some areas
of the Mediterranean domain.
Received November 11, 1999 Revised February 28, 2000 相似文献
809.
A Matrix Approach Coupled with Monte Carlo Techniques for Solving the Net Radiative Balance of the Urban Block 总被引:2,自引:0,他引:2
Marta J. N. Oliveira Panão Helder J. P. Gonçalves Paulo M. C. Ferrão 《Boundary-Layer Meteorology》2007,122(1):217-241
A new method is developed for solving the shortwave and longwave net radiative balance of a three-dimensional urban structure,
represented by parallelepiped blocks uniformly distributed in each direction. The method is based on a novel approach to determine
the shape factors among surfaces, which are estimated by Monte Carlo techniques due to the complex geometry associated with
the three-dimensional urban structure. Then, a set of linear equations is solved to quantify the radiative balance, in order
to obtain their exact solution, considering all the inter-reflections among surfaces. The comparison between the new and the
ray-tracing tracking methods resulted in a Pearson correlation coefficient of 0.996. However, by integrating the linear equations’
exact solution with Monte Carlo techniques, the new method reduces by a factor of 36 the central processing unit (CPU) time
used to perform the calculations of the ray-tracing tracking method. The use of the model for a sensitivity study allows us
to verify the effective absorptance and emittance increases with the canyon aspect ratio of the urban layout. An urban structure
formed by square cross-sectional blocks absorbs more solar radiation than an urban structure formed by rectangular cross-sectional
blocks. The approximation of a specific geometry for an equivalent bi-dimensional infinite street can be applied for rectangular
cross-sectional blocks, where the width is 11 times or more greater than the depth dimension. 相似文献
810.
Summary ?One of the most important features in analysing the climatology of any region is to study the precipitation and its periodicity
of different harmonics in order to study the behavior of the observed data. In this study the amplitude of frequencies, phase
angle and basic statistical parameters are calculated in order to depict spatial characteristics of precipitation over Jordan.
Precipitation records of 17 stations were chosen according to climatic regions of Jordan. The first and second harmonic analyses
explain more than 90% of the precipitation variation in Jordan effectively. The amplitudes of the first and second harmonic
were calculated in order to describe the climatic regions in the country. The maximum amplitudes were found in the northern
mountainous region. The phase angle representing the time of maximum rainfall is also used in the form of a contour chart.
It is found that Jordan has its main rainfall season in winter with maximum around January. The coefficient of variation shows
the high variability of rainfall of the country.
Received February 4, 2002; revised August 1, 2002; accepted August 6, 2002 相似文献