全文获取类型
收费全文 | 732篇 |
免费 | 27篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 71篇 |
地球物理 | 185篇 |
地质学 | 283篇 |
海洋学 | 85篇 |
天文学 | 98篇 |
综合类 | 4篇 |
自然地理 | 30篇 |
出版年
2024年 | 1篇 |
2023年 | 5篇 |
2022年 | 26篇 |
2021年 | 19篇 |
2020年 | 28篇 |
2019年 | 25篇 |
2018年 | 28篇 |
2017年 | 23篇 |
2016年 | 41篇 |
2015年 | 27篇 |
2014年 | 38篇 |
2013年 | 49篇 |
2012年 | 46篇 |
2011年 | 57篇 |
2010年 | 37篇 |
2009年 | 45篇 |
2008年 | 37篇 |
2007年 | 27篇 |
2006年 | 29篇 |
2005年 | 19篇 |
2004年 | 28篇 |
2003年 | 16篇 |
2002年 | 20篇 |
2001年 | 20篇 |
2000年 | 6篇 |
1999年 | 6篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 5篇 |
1995年 | 11篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1978年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1970年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有772条查询结果,搜索用时 437 毫秒
131.
Analysis of aquifer vulnerability and water quality using SINTACS and geographic weighted regression
Jose A. Ramos Leal Felipe O. Tapia Silva Ismael Sandoval Montes 《Environmental Earth Sciences》2012,66(8):2257-2271
Aquifer vulnerability and water quality were assessed in the Central Valleys of Oaxaca (Mexico) using the SINTACS method, based on a geographic information system. SINTACS layers were prepared using data such as climate (rainfall and temperature), water table, hydraulic conductivity, geology, soil type and topographic model. Maps for water quality index (WQI), contamination index and pollution sources index (PSI) were also obtained by this work. Groundwater quality in the Central Valleys may be affected by two factors, those with an anthropogenic origin and those with natural origin. High vulnerability values are located in the valleys of the basin, where granular sediments are exposed. Low vulnerability values are distributed in the basin??s ranges, where metamorphic rocks are found. Given that many of the zones with the highest groundwater vulnerability values correspond to zones with the greatest PSI values, there is great risk of groundwater contamination for the area of study because external (indicated by PSI) and internal (indicated by SINTACS) factors that cause pollution can be frequently observed in the same place. Geographic weighted regression (GWR) is used to test the dependency between WQI as dependent variable and SINTACS, PSI, Urban localities, Agriculture, Pastures and Rivers as predictors. The results indicate the non-stationary behavior of the dependent variable with respect to the predictors. While the obtained GWR models used to model WQI cannot be used in practical situations to predict the behavior of said variable, they can be used to estimate the degree to which the predictors influence the variable of interest. 相似文献
132.
The Biomass Expansion Factor (BEF) and the Root-to-Shoot Ratio (R) are variables used to quantify carbon stock in forests.
They are often considered as constant or species/area specific values in most studies. This study aimed at showing tree size
and age dependence upon BEF and R and proposed equations to improve forest biomass and carbon stock. Data from 70 sample Pinus spp. grown in southern Brazil trees in different diameter classes and ages were used to demonstrate the correlation between
BEF and R, and forest inventory data, such as DBH, tree height and age. Total dry biomass, carbon stock and CO2 equivalent were simulated using the IPCC default values of BEF and R, corresponding average calculated from data used in
this study, as well as the values estimated by regression equations. The mean values of BEF and R calculated in this study
were 1.47 and 0.17, respectively. The relationship between BEF and R and the tree measurement variables were inversely related
with negative exponential behavior. Simulations indicated that use of fixed values of BEF and R, either IPCC default or current
average data, may lead to unreliable estimates of carbon stock inventories and CDM projects. It was concluded that accounting
for the variations in BEF and R and using regression equations to relate them to DBH, tree height and age, is fundamental
in obtaining reliable estimates of forest tree biomass, carbon sink and CO2 equivalent. 相似文献
133.
Moisture transport and intraseasonal variability in the South America monsoon system 总被引:1,自引:0,他引:1
Leila M. V. Carvalho Ana E. Silva Charles Jones Brant Liebmann Pedro L. Silva Dias Humberto R. Rocha 《Climate Dynamics》2011,36(9-10):1865-1880
This paper examines moisture transport on intraseasonal timescales over the continent and over the South Atlantic convergence zone (SACZ) during the South America (SA) summer monsoon. Combined Empirical Orthogonal Function analysis (EOFc) of Global Precipitation Climatology Project pentad precipitation, specific humidity, air temperature, zonal and meridional winds at 850?hPa (NCEP/NCAR reanalysis) are performed to identify the large-scale variability of the South America monsoon system and the SACZ. The first EOFc was used as a large-scale index for the South American monsoon (LISAM), whereas the second EOFc characterized the SACZ. LISAM (SACZ) index showed spectral variance on 30?C90 (15?C20) days and were both band filtered (10?C100?days). Intraseasonal wet anomalies were defined when LISAM and SACZ anomalies were above the 75th percentile of their respective distribution. LISAM and SACZ wet events were examined independently of each other and when they occur simultaneously. LISAM wet events were observed with the amplification of wave activity in the Northern Hemisphere and the enhancement of northwesterly cross-equatorial moisture transport over tropical continental SA. Enhanced SACZ was observed with moisture transport from the extratropics of the Southern Hemisphere. Simultaneous LISAM and SACZ wet events are associated with cross-equatorial moisture transport along with moisture transport from Subtropical Southwestern Atlantic. 相似文献
134.
Santos RG Martins AS Farias Jda N Horta PA Pinheiro HT Torezani E Baptistotte C Seminoff JA Balazs GH Work TM 《Marine pollution bulletin》2011,62(6):1297-1302
To show the influence of coastal habitat degradation on the availability of food for green turtles (Chelonia mydas), we assessed the dietary preferences and macroalgae community at a feeding area in a highly urbanized region. The area showed low species richness and was classified as degraded. We examined stomach contents of 15 dead stranded turtles (CCL = 44.0 cm (SD 6.7 cm)). The diet was composed primarily of green algae Ulva spp. (83.6%). In contrast, the macroalgae community was dominated by the green alga Caulerpa mexicana. We found a selection for red algae, seagrass and Ulva spp., and avoidance for C. mexicana and brown alga Dictyopteris delicatula. The low diversity of available food items, possibly a result of environmental degradation, likely contributed to the low dietary diversity. The nutritional implications of this restricted diet are unclear. 相似文献
135.
Rodrigo de Freitas Lagoon, located in the Rio de Janeiro City, receives several types of polluted discharges. The knowledge of the sediment microfauna correlated with heavy metal and organic matter concentrations could supply important data about the conditions of the lagoon. The benthic foraminiferal assemblage presented larger diversity and more abundant samples in the lagoon entrance than in the inner area. The Ammonia tepida - Elphidiumexcavatum foraminiferal assemblage is characterized by dwarf, corroded and weak organisms. Agglutinated species were found only near the entrance. Low abundance values and sterility of five samples in the inner area (north/northeast) can be caused by high levels of heavy metals and organic matter. A. tepida shows negative correlation with increasing heavy metals values. PAHs and coprostanol high indexes, and the absence or low presence of microfauna in samples around the lagoon margin confirm illegal flows from gas stations and domestic sewage. 相似文献
136.
Alfredo Alberto Muxel Sonia Maria Nobre Gimenez Flaveli Aparecida de Souza Almeida Rení Ventura da Silva Alfaya Antonio Alberto da Silva Alfaya 《洁净——土壤、空气、水》2011,39(3):289-295
The natural cotton fiber was used to synthesize an anion exchange, containing ZrO2 film on its surface, NCFZC (natural cotton fiber/ZrO2 composite). This anion exchanger was produced by the reaction of the zirconium oxychloride and hydroxyl groups on surface of the natural cotton fiber. The material was used for Cr(VI) ions adsorption studies. Adsorption equilibrium time and optimum pH for Cr(VI) adsorption were found to be 6 h and 4.0, respectively. The Langmuir and Temkin isotherms were used to models adsorption equilibrium data. The adsorption capacity of NCFZC was found to be 1.33 mmol/g. Kinetic studies showed that the rate of adsorption of Cr(VI) on NCFZC obeyed a pseudo‐second‐order kinetic model. 相似文献
137.
Ana Margarida Ribeiro Neiva Paulo Bravo Silva Fernando Corfu João Manuel Farinha Ramos 《Chemie der Erde / Geochemistry》2011,71(3):227-245
Seven distinct phases of Variscan two-mica granite are recognized in the Guarda-Sabugal area. They intruded the Cambrian schist-metagraywacke complex, crystallized in the middle crust, and are syn- to late-D3 (309.2 ± 1.8 Ma), late-D3 (304–300 Ma) and late- to post-D3 (299 ± 3 Ma; ID-TIMS ages on zircon and monazite). Two of the granites, G2 and G5, are close in age and have similar Sr, Nd and O isotope characteristics but contrasting whole rock and mineral features and formed by sequential increasing degree of partial melting of a common metasedimentary protolith. During sequential melting Ti, total Fe, Mg, Ca, Zr, Zn, Sr, Ba and REE contents and (La/Yb)N increase and Si and Rb contents decrease, plagioclase becomes richer in anorthite and biotite and muscovite richer in Ti and Mg. Each of these granites evolved subsequently by fractional crystallization of quartz, K-feldspar, plagioclase, biotite and ilmenite, defining separate series G2–G3–G7 and G5–G6 containing late Sn-bearing differentiates. Two other granites G1 and G4 represent distinct pulses of magma with individual fractionation trends for major and trace elements and distinct (87Sr/86Sr)300, ?Nd300 and δ18O values. 相似文献
138.
Isabel Margarida Horta Ribeiro Antunes Ana Margarida Ribeiro Neiva João Manuel Farinha Ramos Paulo Bravo Silva Maria Manuela Vinha Guerreiro Silva Fernando Corfu 《Chemie der Erde / Geochemistry》2013
In the Segura area, Variscan S-type granites, aplite veins and lepidolite-subtype granitic aplite-pegmatite veins intruded the Cambrian schist-metagraywacke complex. The granites are syn D3. Aplite veins also intruded the granites. Two-mica granite and muscovite granite have similar ages of 311.0 ± 0.5 Ma and 312.9 ± 2.0 Ma but are not genetically related, as indicated by their geochemical characteristics and (87Sr/86Sr)311 values. They correspond to distinct pulses of magma derived by partial melting of heterogeneous metapelitic rocks. Major and trace elements suggest fractionation trends for: (a) muscovite granite and aplite veins; (b) two-mica granite and lepidolite-subtype aplite-pegmatite veins, but with a gap in most of these trends. Least square analysis for major elements, and modeling of trace elements, indicate that the aplite veins were derived from the muscovite granite magma by fractional crystallization of quartz, plagioclase, K-feldspar and ilmenite. This is supported by the similar (87Sr/86Sr)311 and δ18O values and the behavior of P2O5 in K-feldspar and albite. The decrease in (87Sr/86Sr)311 and strong increase (1.6‰) in δ18O from two-mica granite to lepidolite-subtype aplite-pegmatite veins, and the behaviors of Ca, Mn and F of hydroxylapatite indicate that these veins are not related to the two-mica granite. 相似文献
139.
Mário André Trindade Dantas Rodrigo Parisi Dutra Alexander Cherkinsky Daniel Costa Fortier Luciana Hiromi Yoshino Kamino Mario Alberto Cozzuol Adauto de Souza Ribeiro Fabiana Silva Vieira 《Quaternary Research》2013,79(1):61-65
During the Pleistocene a fauna composed of large (biomass > 44 kg) and giant mammals (biomass > 1000 kg) that are usually associated with open environments lived in the Brazilian Intertropical Region. We present here new information concerning the paleoecology and chronology of some species of this megafauna. Carbon isotope analyses were performed for a better understanding of the paleoecology of the species Eremotherium laurillardi (Lund, 1842), Notiomastodon platensis (Ameghino, 1888) and Toxodon platensis (Owen, 1849). The δ13C data allow attributing a generalist diet to these species, which varied according to the kind of habitat in which they lived. In more open habitats all species were grazers; in mixed habitats E. laurillardi and T. platensis were mixed feeders, and N. platensis was grazer; and in more closed habitats all species were mixed feeders. 相似文献
140.
Giovanni Dolif Andre Engelbrecht Alessandro Jatobá Antônio José Dias da Silva José Orlando Gomes Marcos R. S. Borges Carlos A. Nobre Paulo Victor R. de Carvalho 《Natural Hazards》2013,65(3):1831-1847
Natural disasters, particularly those triggered by heavy rainfall, may cause major damage and death. However, if an accurate early warning is issued, the damage can be mitigated. In Latin America and Brazil, characteristics of socioeconomic development often lead to a disorderly growth of cities and, consequently, occupation and irregular construction in risk areas. Therefore, forecasts of heavy rainfall, as well as preventative and mitigatory actions based on meteorological data/alerts, are essential to saving lives and minimizing material loss. An event that would have benefited from such actions is that which occurred in the mountainous region of Rio de Janeiro in January 2011, when over 800 people lost their lives. This work describes the first research initiative on resilience engineering domain in systems to forecast heavy rains in Rio de Janeiro. The results indicate important sources of brittleness in the system that supports the work of meteorologists, mainly related to the technical and organizational framework, and suggests that the main source of resilience in dealing with critical situations is the tacit knowledge of experts. 相似文献