全文获取类型
收费全文 | 353篇 |
免费 | 15篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 13篇 |
大气科学 | 65篇 |
地球物理 | 83篇 |
地质学 | 114篇 |
海洋学 | 18篇 |
天文学 | 43篇 |
综合类 | 2篇 |
自然地理 | 31篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 1篇 |
2021年 | 10篇 |
2020年 | 7篇 |
2019年 | 9篇 |
2018年 | 13篇 |
2017年 | 13篇 |
2016年 | 22篇 |
2015年 | 6篇 |
2014年 | 20篇 |
2013年 | 31篇 |
2012年 | 14篇 |
2011年 | 16篇 |
2010年 | 17篇 |
2009年 | 23篇 |
2008年 | 12篇 |
2007年 | 14篇 |
2006年 | 12篇 |
2005年 | 17篇 |
2004年 | 13篇 |
2003年 | 5篇 |
2002年 | 2篇 |
2001年 | 11篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1975年 | 2篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有369条查询结果,搜索用时 0 毫秒
31.
Simulating flow in karst aquifers at laboratory and sub-regional scales using MODFLOW-CFP 总被引:4,自引:1,他引:4
Groundwater flow in a well-developed karst aquifer dominantly occurs through bedding planes, fractures, conduits, and caves created by and/or enlarged by dissolution. Conventional groundwater modeling methods assume that groundwater flow is described by Darcian principles where primary porosity (i.e. matrix porosity) and laminar flow are dominant. However, in well-developed karst aquifers, the assumption of Darcian flow can be questionable. While Darcian flow generally occurs in the matrix portion of the karst aquifer, flow through conduits can be non-laminar where the relation between specific discharge and hydraulic gradient is non-linear. MODFLOW-CFP is a relatively new modeling program that accounts for non-laminar and laminar flow in pipes, like karst caves, within an aquifer. In this study, results from MODFLOW-CFP are compared to those from MODFLOW-2000/2005, a numerical code based on Darcy’s law, to evaluate the accuracy that CFP can achieve when modeling flows in karst aquifers at laboratory and sub-regional (Woodville Karst Plain, Florida, USA) scales. In comparison with laboratory experiments, simulation results by MODFLOW-CFP are more accurate than MODFLOW 2005. At the sub-regional scale, MODFLOW-CFP was more accurate than MODFLOW-2000 for simulating field measurements of peak flow at one spring and total discharges at two springs for an observed storm event. 相似文献
32.
Atle Nesje Denise C. Rüther Jacob C. Yde 《Boreas: An International Journal of Quaternary Research》2023,52(2):194-205
This study presents the sedimentary succession of an outwash plain and an alluvial fan located along the valley Langedalen at the south-eastern side of the Jostedalsbreen ice cap in inner Sogn, western Norway. A newly exposed ~2.8-m-high section along the southern riverbank of Langedøla river shows alternating layers of minerogenic sediments and peat layers with tree logs, identified as Salix sp. The section is situated in the distal part of an alluvial fan built out from the southern slope of Langedalen. Six AMS radiocarbon dates of tree fragments indicate that the accumulation of the fine-grained sediments in the lower part of the section was initiated earlier than the basal radiocarbon date of 914–976 calibrated years CE (1σ age range). These basal, fine-grained sediments are interpreted as proglacial outwash deposited in a floodplain depression or abandoned river channel in a low-energy glaciofluvial environment. Periods of low glacier cover, low river discharge or low-water stands over the floodplain allowed peat formation and the growth of trees and shrubs in the valley. The radiocarbon dates further indicate relatively rapid sediment accretion (~2.7–3 cm a−1) between 190 and 125 cm below the sediment surface, equivalent to approximately 1220 to 1250 cal. a CE (1σ age range). At ~60 cm depth below the surface, dated to approximately 1590 to 1620 cal. a CE (1σ age range), a transition to more coarse-grained, sandy and gravelly sediments indicates increased sediment supply and distal expansion of the alluvial fan. This occurred most likely as a consequence of increased sediment yield from expanding glaciers along the southern valley side of Langedalen as a response to the initial Little Ice Age glacier growth. Based on these results, the accretion and progradation of glacier-fed alluvial fans mainly occur during periods of glacier advance rather than during glacier recession. 相似文献
33.
Carlos M. Duarte Daniel J. Conley Jacob Carstensen María Sánchez-Camacho 《Estuaries and Coasts》2009,32(1):29-36
The implicit assumption of many scientific and regulatory frameworks that ecosystems impacted by human pressures may be reverted
to their original condition by suppressing the pressure was tested using coastal eutrophication. The response to nutrient
abatement of four thoroughly studied coastal ecosystems that received increased nutrient inputs between the 1970s and the
1980s showed that the trajectories of these ecosystems were not directly reversible. All four ecosystems displayed convoluted
trajectories that failed to return to the reference status upon nutrient reduction. This failure is proposed to result from
the broad changes in environmental conditions, all affecting ecosystem dynamics, that occurred over the 30 years spanning
from the onset of eutrophication to the reduction of nutrient levels. Understanding ecosystem response to multiple shifting
baselines is essential to set reliable targets for restoration efforts. 相似文献
34.
Changing atmospheric conditions often result in a data distribution shift in remote sensing images for different dates and locations making it difficult to discriminate between various classes of interest. To alleviate this data shift issue, we introduce a novel supervised classification framework, called Classify-Normalize-Classify (CNC). The proposed scheme uses a two classifier approach where the first classifier performs a rough segmentation of the class of interest (COI) in the input image. Then, the median signal of the estimated COI regions is subtracted from all image pixels values to normalize them. Finally, the second classifier is applied to the normalized image to produce the refined COI segmentation. The proposed methodology was tested to detect deforestation using bitemporal Landsat 8 OLI images over the Amazon rainforest. The CNC framework compared favorably to benchmark masks of the PRODES program and state-of-the-art classifiers run on surface reflectance images provided by USGS. 相似文献
35.
Jacob M. Bendle Neil F. Glasser 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(1):130-145
The cirques of Snowdonia, North Wales were last occupied by glacier ice during the Younger Dryas Chronozone (YDC), c. 12.9–11.7 ka. New mapping presented here indicates 38 small YDC cirque glaciers formed in Snowdonia, covering a total area of 20.74 km2. Equilibrium line altitudes (ELAs) for these glaciers, calculated using an area–altitude balance ratio (AABR) approach, ranged from 380 to 837 m asl. A northeastwards rise in YDC ELAs across Snowdonia is consistent with southwesterly snow-bearing winds. Regional palaeoclimate reconstructions indicate that the YDC in North Wales was colder and drier than at present. Palaeotemperature and annual temperature range estimates, derived from published palaeoecological datasets, were used to reconstruct values of annual accumulation and ‘winter balance plus summer precipitation’ using a degree-day model (DDM) and non-linear regression function, respectively. The DDM acted as the best-estimate for stadial precipitation and yielded values between 2073 and 2687 mm a?1 (lapse rate: 0.006 °C m?1) and 1782–2470 mm a?1 (lapse rate: 0.007 °C m?1). Accounting for the potential input of windblown and avalanched snow onto former glacier surfaces, accumulation values dropped to between 1791 and 2616 mm a?1 (lapse rate: 0.006 °C m?1) and 1473–2390 mm a?1 (lapse rate: 0.007 °C m?1). The spatial pattern of stadial accumulation suggests a steep precipitation gradient and provides verification of the northeastwards rise in ELAs. Glaciers nearer the coast of North Wales were most responsive to fluctuations in climate during the YDC, responding to sea-ice enforced continentality during the coldest phases of the stadial and to abrupt warming at the end of the stadial. 相似文献
36.
DNA tracers with information capacity and high detection sensitivity tested in groundwater studies 总被引:1,自引:0,他引:1
In order to investigate the usefulness of unique synthetic DNA tracers in groundwater, a field experiment was conducted in
Norway. DNA tracers and a sodium-chloride tracer were injected into an aquifer. The transport of DNA molecules was interpreted
by comparing with the plume of chloride ions under forced-gradient steady-state flow conditions. Spatial concentration moments
described the migration of conservative tracers. Mobility and migration of DNA in groundwater demonstrate that DNA tracers
can be detected by using the polymerase chain reaction (PCR) and DNA sequence analysis. The results indicate that DNA tracers
can be valuable tools as tracers in groundwater investigations.
Received, June 1997 / Revised, July 1998, December 1998 / Accepted, January 1999 相似文献
37.
Pankaj Khanna Heath H. Hopson André W. Droxler Dylan A. Droxler Daniel Lehrmann Brian Kubik Jacob Proctor Pulkit Singh Paul M. Harris 《Sedimentology》2020,67(2):1135-1160
Although Late Cambrian microbial build-ups were recognized in the Point Peak Member of the Wilberns Formation in Central Texas (USA) nearly 70 years ago, only a few studies focused specifically on the build-ups themselves. This study focuses on the interpretation of the regional (15 measured sections described in literature representing an area of 8000 km2) and local (field and drone photogrammetry studies in a 25 km2 area from within south Mason County) microbial build-up occurrence, describes their growth phases and details their interactions with the surrounding inter-build-up sediments. The study establishes the occurrence of microbial build-ups in the lower and upper Point Peak members (the Point Peak Member is informally broken up into the lower Point Peak and the upper Point Peak members separated by Plectotrophia zone). The lower Point Peak Member consists of three <1 m thick microbial bioherms and biostrome units, in addition to heterolithic and skeletal/ooid grainstone and packstone beds. One, up to 14 m thick, microbial unit associated with inter-build-up skeletal and ooid grainstone and packstone beds, intercalated with mixed siliciclastic–carbonate silt beds, characterizes the upper Point Peak member. The microbial unit in the upper Point Peak member displays a three-phase growth evolution, from an initial colonization phase on flat based, rip-up clast lenses, to a second aggradation and lateral expansion phase, into a third well-defined capping phase. The ultimate demise of the microbial build-ups is interpreted to have been triggered by an increase of water turbidity caused by a sudden influx of fine siliciclastics. The lower Point Peak member represents inner ramp shallow subtidal and intertidal facies and the upper Point Peak member corresponds to mid-outer ramp subtidal facies. Understanding the morphological architecture and depositional context of these features is of importance for identifying signatures of early life on Earth. 相似文献
38.
Aeolian erosion of flat, arid landscapes is induced (and sustained) by the aerodynamic surface stress imposed by flow in the atmospheric surface layer. Conceptual models typically indicate that sediment mass flux, Q (via saltation or drift), scales with imposed aerodynamic stress raised to some exponent, n, where \(n > 1\). This scaling demonstrates the importance of turbulent fluctuations in driving aeolian processes. In order to illustrate the importance of surface-stress intermittency in aeolian processes, and to elucidate the role of turbulence, conditional averaging predicated on aerodynamic surface stress has been used within large-eddy simulation of atmospheric boundary-layer flow over an arid, flat landscape. The conditional-sampling thresholds are defined based on probability distribution functions of surface stress. The simulations have been performed for a computational domain with \(\approx 25 H\) streamwise extent, where H is the prescribed depth of the neutrally-stratified boundary layer. Thus, the full hierarchy of spatial scales are captured, from surface-layer turbulence to large- and very-large-scale outer-layer coherent motions. Spectrograms are used to support this argument, and also to illustrate how turbulent energy is distributed across wavelengths with elevation. Conditional averaging provides an ensemble-mean visualization of flow structures responsible for erosion ‘events’. Results indicate that surface-stress peaks are associated with the passage of inclined, high-momentum regions flanked by adjacent low-momentum regions. Fluid in the interfacial shear layers between these adjacent quasi-uniform momentum regions exhibits high streamwise and vertical vorticity. 相似文献
39.
A. F. Carril C. G. Menéndez A. R. C. Remedio F. Robledo A. S?rensson B. Tencer J.-P. Boulanger M. de Castro D. Jacob H. Le Treut L. Z. X. Li O. Penalba S. Pfeifer M. Rusticucci P. Salio P. Samuelsson E. Sanchez P. Zaninelli 《Climate Dynamics》2012,39(12):2747-2768
The ability of four regional climate models to reproduce the present-day South American climate is examined with emphasis on La Plata Basin. Models were integrated for the period 1991–2000 with initial and lateral boundary conditions from ERA-40 Reanalysis. The ensemble sea level pressure, maximum and minimum temperatures and precipitation are evaluated in terms of seasonal means and extreme indices based on a percentile approach. Dispersion among the individual models and uncertainties when comparing the ensemble mean with different climatologies are also discussed. The ensemble mean is warmer than the observations in South Eastern South America (SESA), especially for minimum winter temperatures with errors increasing in magnitude towards the tails of the distributions. The ensemble mean reproduces the broad spatial pattern of precipitation, but overestimates the convective precipitation in the tropics and the orographic precipitation along the Andes and over the Brazilian Highlands, and underestimates the precipitation near the monsoon core region. The models overestimate the number of wet days and underestimate the daily intensity of rainfall for both seasons suggesting a premature triggering of convection. The skill of models to simulate the intensity of convective precipitation in summer in SESA and the variability associated with heavy precipitation events (the upper quartile daily precipitation) is far from satisfactory. Owing to the sparseness of the observing network, ensemble and observations uncertainties in seasonal means are comparable for some regions and seasons. 相似文献
40.
Jacob Garty Lior Weissman Tal Levin Rachel Garty-Spitz Haya Lehr 《Journal of Atmospheric Chemistry》2004,49(1-3):251-266
To assess the impact of UV-B radiation combined with NaHSO3 in solution under laboratory conditions, we compared the amounts of stress-ethylene produced by two lichen species of the same genus. Ramalina lacera, an epiphytic Mediterranean lichen, was found to be rather sensitive to UV-B combined with NaHSO3, relative to the epilithic desert lichen R. maciformis. The impact of high temperatures with FeCl2 in solution, measured by amounts of stress ethylene accentuated the adaptability of R. maciformis to desert conditions, whereas R. lacera appeared to be rather sensitive to extreme temperatures especially in the case of chemical contamination. The adaptability of R. maciformis to UV-B appears to be related to the photoprotective capabilities of lichen substances. 相似文献