全文获取类型
收费全文 | 267篇 |
免费 | 24篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 35篇 |
地球物理 | 67篇 |
地质学 | 93篇 |
海洋学 | 23篇 |
天文学 | 37篇 |
综合类 | 3篇 |
自然地理 | 32篇 |
出版年
2022年 | 4篇 |
2021年 | 3篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 16篇 |
2017年 | 7篇 |
2016年 | 12篇 |
2015年 | 15篇 |
2014年 | 11篇 |
2013年 | 31篇 |
2012年 | 19篇 |
2011年 | 18篇 |
2010年 | 17篇 |
2009年 | 15篇 |
2008年 | 11篇 |
2007年 | 12篇 |
2006年 | 7篇 |
2005年 | 4篇 |
2004年 | 6篇 |
2003年 | 7篇 |
2002年 | 11篇 |
2001年 | 1篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1987年 | 2篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1966年 | 2篇 |
排序方式: 共有293条查询结果,搜索用时 31 毫秒
111.
Latent and sensible heat flux predictions from a uniform pine forest using surface renewal and flux variance methods 总被引:1,自引:0,他引:1
Gabriel Katul Cheng-I Hsieh Ram Oren David Ellsworth Nathan Phillips 《Boundary-Layer Meteorology》1996,80(3):249-282
A surface renewal model that links organized eddy motion to the latent and sensible heat fluxes is tested with eddy correlation measurements carried out in a 13m tall uniform Loblolly pine plantation in Duke Forest, Durham, North Carolina. The surface renewal model is based on the occurance of ramp-like patterns in the scalar concentration measurements. To extract such ramp-like patterns from Eulerian scalar concentration measurements, a newly proposed time-frequency filtering scheme is developed and tested. The time-domain filtering is carried out using compactly-supported orthonormal wavelets in conjunction with the Universal Wavelet Thresholding approach of Donoho and Johnstone, while the frequency filtering is carried out by a band-pass sine filter centered around the ramp-occurrence frequency as proposed by other studies. The method was separately tested for heat and water vapour with good agreement between eddy correlation flux measurements and model predictions. The usefulness of the flux-variance method to predict sensible and latent heat fluxes is also considered. Our measurements suggest that the simple flux-variance method reproduces the measured heat and momentum fluxes despite the fact that the variances were measured within the roughness sublayer and not in the surface layer. Central to the predictions of water vapour fluxes using the flux-variance approach is the similarity between heat and water vapour transport by the turbulent air flow. This assumption is also investigated for this uniform forest terrain. 相似文献
112.
Fábio Roland Luciana O. Vidal Felipe S. Pacheco Nathan O. Barros Arcilan Assireu Jean P. H. B. Ometto André C. P. Cimbleris Jonathan J. Cole 《Aquatic Sciences - Research Across Boundaries》2010,72(3):283-293
Hydroelectric reservoirs generate energy without significant combustion of fossil fuels. However, these systems can, potentially,
emit greenhouse gases (GHG’s) at a rate which may be significant at the global scale, and, possible, co-equal, per kilowatt-hour,
to that from conventional coal or oil-fired systems. Although much of the new construction of hydroelectric reservoirs is
in the tropics, most of the data on GHG emissions comes from temperate regions. Further, much of the existing data on reservoir
gas emissions comes from single sites, usually near the terminal dams. Large tropical reservoirs often involve the impoundments
of river systems with complex morphology which in turn can cause spatial heterogeneity in gas flux. We evaluated spatial and
seasonal variability in CO2 concentrations and gas flux for five large (50–1,400 km2) reservoirs in the Cerrado region of Brazil. Most of data set (87% of all measurements) showed CO2 supersaturation and net efflux to the atmosphere. There was as much or more variation in pCO2 over space and among seasons. The large studied reservoirs showed different zones in terms of CO2 emission because those fluxes are dependent on flooded biomass, watershed input of organic matter and dam operation regime.
Here we demonstrate that the reservoirs in the Brazilian Cerrado have low rates of CO2 emissions compared to existing global comparisons. Our results suggest that ignoring the spatial variability can lead to
more than 25% error in total system gas flux. 相似文献
113.
The exchage equilibrium
has been used to measure activity-composition relations along the olivine join FeSi0.5O2−MgSi0.5O2 at 1400 K and 1 atm pressure. Equilibrium Fe−Mg partitioning between the two phases was determined by reversing the compositions
of olivine coexisting with oxide and matallic iron over the composition range Fo23 to Fo92. A detailed study of the thermodynamic properties of the oxide phase has recently been made by Srečec et al. and we have
confirmed their results in the composition range of interest. Application of the oxide data to the exchange equilibrium enables
the properties of olivine to be determined. Within experimental uncertainly (Fe, Mg)Si0.5O2 olivine can, at 1400 K, be treated as a symmetric solution with W
Fe-Mg
o1
of 3.7±0.8 kJ/mol. The data permit the presence of only very slight asymmetry in the series. The data do not support recent
assertions that olivine is highly non-ideal (W≈10 kJ/mol) under these conditions. 相似文献
114.
The linear and weakly nonlinear dynamics of long, low frequency, spatially growing baroclinic waves embedded in slowly varying zonal flow on a β-plane channel are examined in a continuous model of the atmosphere. For a basic state jet flow possessing a locally unstable region, the nonlinear solution yields a maximum amplitude that is located near the region of maximum baroclinicity and substantially upstream of the maximum amplitude obtained from linear theory. The difference between the linear and nonlinear solutions is due to the time-averaged wave fluxes becoming large enough in the nonlinear problem to stabilize the flow prior to reaching the location (jet center) where the basic state baroclinicity and locally computed linear growth rate are maximized. 相似文献
115.
The direct precipitation of apatite in seawater is inhibited by Mg ions. The action of various factors on this precipitation is studied: addition of Ca, addition of F, reduction of pH and simultaneous effect of reduction of pH and addition either of F or of Ca. It is established that the reduction of the pH of seawater to 7 allows the precipitation of the apatite with a Ca/Mg ratio in the water of 1.2. The favourable environment for the settlement of natural apatite must then be either neutral or acid. This result is corroborated by the re-study of Gulbrandsen's equation. Such an acid environment with a high phosphorus content may be the result of oxidation of large quantities of organic matter due to previous intense biological activity. The acidity is probably brought about by bubbling of CO2. 相似文献
116.
Methods for detecting regime shifts in large marine ecosystems: a review with approaches applied to North Pacific data 总被引:1,自引:0,他引:1
This note provides a brief review of five analytical methods previously used or promoted for diagnosing regime shifts in marine ecosystems. The methods discussed are: (i) principal components analysis, (ii) compositing average standard deviates, (iii) autoregressive moving average and intervention analysis modeling, (iv) vector autoregressive process modeling, and (v) Fisher information. Assessments of the relative strengths and weaknesses for the different analytical approaches are also offered. Some of these methods are applied to a collection of fishery oceanographic time series for the N. Pacific to illustrate aspects of their relative utility and limitations for diagnosing regime shift behavior. One recommendation for future studies is to analyze biotic and abiotic time series separately in order to identify ecosystem state variables of interest and to better isolate ecosystem behaviors from other influences like environmental change. Methods that allow for quantitative assessments of the statistical significance of hypothesized regime shifts should be favored over those that do not. Analyses of especially large collections of time series may benefit from first using a data compression technique and then applying one of the methods that are more appropriate for just one or a small number of time series. Because of the difficulties in observing and adequately documenting many aspects of marine ecosystem variability, it is crucial that future research attempt to combine empirical studies of large marine ecosystems with theoretical and modeling studies of other systems for which the dynamics and predictability are better understood. With such a comparative approach it should be possible to refine conceptual and simulation models, while also identifying crucial gaps in existing observations. 相似文献
117.
Nathan Hawley 《Geo-Marine Letters》1981,1(1):7-10
Flume observations of the consolidation of mud beds deposited from an unsteady flow show that 10-mm layer undergoes considerable
consolidation during the time of deposition and within the first hour after deposition ceases, but little additional consolidation
during the next 24 hr. A certain minimum thickness, between 3 and 10 mm, is required for any consolidation to occur. The results
support the hypothesis that thick mud layers may form in nearshore subtidal areas as the result of accumulation of thinner
layers deposited during successive tidal cycles, but only if each individual layer is able to withstand erosion during the
subsequent cycle. 相似文献
118.
We used a numerical model forced with three different scenarios to analyze Baffin Bay circulation sensitivity to runoff around Baffin Bay, especially the Greenland runoff, for the past (1970–2010) and future (2010–2099). We observed an overall decrease in transport from the Arctic to the North Atlantic for the volume, heat, and freshwater over the time period as well as an augmentation of the freshwater and heat in Baffin Bay. In the early 1990s, the increase in heat in Baffin Bay was consistent with an increase in the West Greenland Irminger Water (WGIW) inflow at Davis Strait while later West Greenland Shelf Water played an important role in the heat import, sustaining the idea that the West Greenland Current might have an impact on the melt of West Greenland tidewater glaciers. The increase in freshwater and later in heat in Baffin Bay leads to changes in the steric height inside Baffin Bay, which leads to changes in the circulation. After 1978, the WGIW reaches the North Water polynya and recirculates into the Baffin Bay gyre where it accumulates over time. In the future experiment, the dynamic changes in Baffin Bay are mainly related to the accumulation of heat inside the gyre. 相似文献
119.
Nathan Frey 《Urban geography》2017,38(10):1534-1549
This study examines equity issues in access to urban environmental amenities using Washington, D.C. as a case study. The amenities that are the focus of this study, urban trees and urban parks, have a range of social and health benefits for city dwellers, and Washington, D.C. is a major city with a large, diverse population and long-standing geographic divisions by race and class. Using spatial regression techniques, the study examines disparities in access to these amenities by race, ethnicity, and socioeconomic status at citywide and sub-city scales. Mixed evidence is found for disparities in access across amenities and across scales. The results emphasize the need for studies of access to urban environmental amenities to pay closer attention to the differences between amenities and to issues of scale. 相似文献
120.