全文获取类型
收费全文 | 858篇 |
免费 | 27篇 |
国内免费 | 39篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 52篇 |
地球物理 | 217篇 |
地质学 | 345篇 |
海洋学 | 82篇 |
天文学 | 70篇 |
综合类 | 4篇 |
自然地理 | 135篇 |
出版年
2023年 | 3篇 |
2022年 | 5篇 |
2021年 | 12篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 20篇 |
2017年 | 19篇 |
2016年 | 26篇 |
2015年 | 11篇 |
2014年 | 46篇 |
2013年 | 54篇 |
2012年 | 22篇 |
2011年 | 55篇 |
2010年 | 33篇 |
2009年 | 46篇 |
2008年 | 47篇 |
2007年 | 34篇 |
2006年 | 23篇 |
2005年 | 33篇 |
2004年 | 26篇 |
2003年 | 31篇 |
2002年 | 24篇 |
2001年 | 20篇 |
2000年 | 12篇 |
1999年 | 23篇 |
1998年 | 13篇 |
1997年 | 18篇 |
1996年 | 23篇 |
1995年 | 13篇 |
1994年 | 19篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 4篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有924条查询结果,搜索用时 15 毫秒
801.
802.
David Kirkman † David Tytler † Nao Suzuki † Carl Melis Susan Hollywood Kory James Geoffrey So Dan Lubin † Tridivesh Jena Michael L. Norman Pascal Paschos 《Monthly notices of the Royal Astronomical Society》2005,360(4):1373-1380
We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range 2.2 < z < 3.2 . Our measurement of DA excludes absorption from metal lines or the Lyα lines of Lyman-limit systems and damped Lyα systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between 1.6 < z < 2.2 , we find that the redshift evolution of DA is well described over 1.6 < z < 3.2 as A (1 + z )γ , where A = 0.0062 and γ= 2.75 . We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H i absorption than expected at z = 3 , implying that the ultraviolet background is ∼40 per cent higher than expected. Our data appears to be consistent with an H i ionization rate of Γ∼ 1.4 × 10−12 s−1 . 相似文献
803.
804.
The health of red spruce is adversely affected by exposure to acid fog. Epicuticular wax structure over epistomatal chambers, mesophyll membrane calcium and changes in redox state of glutathione are affected in both chamber and field studies and are correlated with leaf exposure to acid fog, but not with soil or total leaf calcium concentrations. Experiments tracking fog deposition under conditions of illumination or darkness, with or without a metabolic inhibitor, support the mechanism of diffusiophoresis linked with photosynthetic gas exchange. Based on deposition pattern and leaf morphology, a stomatal pathway for water and hydrogen ion penetration to leaf interior is proposed. Ecophysiological parameters are discussed in relation to differential sensitivities of red spruce and white pine to ozone and acid fog. 相似文献
805.
Erin Lee Hestir David H. Schoellhamer Jonathan Greenberg Tara Morgan-King Susan L. Ustin 《Estuaries and Coasts》2016,39(4):1100-1112
Submerged aquatic vegetation (SAV) has well-documented effects on water clarity. SAV beds can slow water movement and reduce bed shear stress, promoting sedimentation and reducing suspension. However, estuaries have multiple controls on turbidity that make it difficult to determine the effect of SAV on water clarity. In this study, we investigated the effect of primarily invasive SAV expansion on a concomitant decline in turbidity in the Sacramento-San Joaquin River Delta. The objective of this study was to separate the effects of decreasing sediment supply from the watershed from increasing SAV cover to determine the effect of SAV on the declining turbidity trend. SAV cover was determined by airborne hyperspectral remote sensing and turbidity data from long-term monitoring records. The turbidity trends were corrected for the declining sediment supply using suspended-sediment concentration data from a station immediately upstream of the Delta. We found a significant negative trend in turbidity from 1975 to 2008, and when we removed the sediment supply signal from the trend it was still significant and negative, indicating that a factor other than sediment supply was responsible for part of the turbidity decline. Turbidity monitoring stations with high rates of SAV expansion had steeper and more significant turbidity trends than those with low SAV cover. Our findings suggest that SAV is an important (but not sole) factor in the turbidity decline, and we estimate that 21–70 % of the total declining turbidity trend is due to SAV expansion. 相似文献
806.
Along the mid- and north Atlantic coasts of the USA, over 90 % of salt marshes have been ditched. Ditching was largely abandoned by the mid-twentieth century; however, techniques that create permanent shallow water pools for mosquito control and bird habitat are increasingly being applied to marshes of the USA and elsewhere. Salt marshes in Plum Island Sound, Massachusetts, and Barnegat Bay, New Jersey, were used to examine differences between areas that have been ditched and those altered to increase the density of shallow pools in water table dynamics, salinity, soil and porewater chemistry, as well as short-term sedimentation, accretion, and elevation change rates. We found that the area with plugged ditches, berms, and pools in Plum Island had less drainage, higher salinity and porewater sulfide and ammonium concentrations, and higher soil organic matter than the adjacent ditched area. Despite averaging 8 cm lower in elevation, the Plum Island ditched area had less sediment deposition and was composed of higher elevation plant species than the area with plugged ditches, berms, and shallow pools. Elevation increased in the ditched area at a rate of 3.2 ± 0.5 mm/year, but elevation change was variable in the area with pools. In Barnegat Bay, the marsh area with pools and ditches had less sediment deposition and surface accretion than the ditch-only area, associated, in part, with the higher elevation. An average elevation difference of 4.5 cm was associated with a sixfold difference in mineral sediment deposition. Temporal sediment deposition and surface accretion was important in the ditch-only area but was absent or muted in the area with numerous pools. Elevation increased in both marsh areas at an average rate of 1.8 ± 0.8 mm/year, less than half the long-term average local rate of sea-level rise. Our results illustrate how physical manipulations including changes to tidal hydrology and surface topography interact with elevation to influence short-term biophysical feedbacks. 相似文献
807.
808.
Kathleen R. Laird Astrid Michels Chloë T.L. Stuart Susan E. Wilson William M. Last Brian F. Cumming 《Quaternary Science Reviews》2007,26(25-28):3328-3343
Paleoclimatic records from the climatically sensitive Canadian prairies are relatively rare due to the scarcity of study sites with continuous Holocene stratigraphic sequences. Oro Lake, a meromictic lake in the dry grasslands of Saskatchewan (Canada), contains a continuous Holocene diatom record spanning the last 10,000 years. Here we present analyses at three different time scales and resolution: (1) 1–3 yr resolution of the past 80 years, (2) century-scale analysis of the Holocene, and (3) decadal-scale analysis of the past 7000 years. Recent changes in the diatom assemblages and their respective salinity inferences were significantly related to measured effective moisture (precipitation minus evaporation, P−ET). The droughts of the 1930s, and a wet period during the 1950s are clearly evident in the diatom record, suggesting the Oro Lake record contains a sensitive archive of past climatic conditions. Century-scale analysis of the diatom record during the Holocene is consistent with a cool and moist climate in the early Holocene (prior to ca 9700 cal yr BP, 8600 14C yr BP). An abrupt increase in diatom-inferred salinity at 9600 cal yr BP (8500 14C yr BP) indicates the onset of an arid climate, with continuing arid conditions throughout the mid-Holocene. Decadal-scale analysis of the past 7000 years suggests that the mid-Holocene was more complex, with extended periods of increased variability in precipitation, particularly between ca 5800–3600 cal yr BP (5000–3200 14C yr BP) which is characterized by intervals of increased effective moisture. The past 2000 years is characterized by reduced salinities and generally wetter conditions in comparison to the mid-Holocene. The combination of the different scales of analyses in this study provides a detailed account of the dynamic nature of climate from sub-decadal to millennial scale in the Oro Lake region within the Palliser Triangle. Climate model predictions suggest that the Canadian prairie region may see a higher frequency of extreme droughts under projected global warming, potentially similar to the most arid periods seen during the mid-Holocene when many lake basins completely dried out. 相似文献
809.
Catherine Christenson David J. Hart Michael Cardiff Susan Richmond Dante Fratta 《Ground water》2022,60(3):426-433
Communication of hydrologic data to the public can be improved by connecting data to the places they represent. In our example of data communication, we coupled hydrologic data with simultaneously collected video as both a scientific and public engagement tool. This note presents a method for collecting spatially and temporally dense datasets of water-quality and geophysical data on small streams and lakes, and for displaying the data in a user-friendly format using commercially available software. With this method, multiple instruments are mounted on a canoe and a controlled survey float is carried out to collect data. The data stream is georeferenced and logged using an Arduino microcontroller to provide detailed information about spatial variability. We employed these continuous data-collection methods at small streams and lakes across Wisconsin, USA. Comparison of stream-float sensor data to lab reported data, data collected by alternative sensors, and previously collected data in our study areas indicates that the low-cost temperature, electrical conductivity, pH, and dissolved oxygen sensors performed well. GoPro cameras recorded video throughout the duration of data collection. Our established water-quality and geophysical data collection methods are inexpensive, fast, and reliable, which qualify them as excellent tools for fine-scale spatial understanding of stream and lake habitats' health. Data-rich video connects point measurements of water properties to the appearance of the native environment. This method helps improve our understanding of groundwater and surface water interactions in complex hydrogeologic systems, enhance communication amongst stakeholders, and provide context when monitoring and managing sensitive habitats. 相似文献
810.
Spring flux as an indicator of source geomorphology,substrata, and temperature conditions in springs
Hydrogeology Journal - Spring flux (m/s), defined as spring flow (m3/s) divided by the area over which groundwater emerges (m2) at a spring, provides a meaningful way to distinguish between... 相似文献