全文获取类型
| 收费全文 | 127篇 |
| 免费 | 2篇 |
专业分类
| 测绘学 | 3篇 |
| 大气科学 | 10篇 |
| 地球物理 | 36篇 |
| 地质学 | 36篇 |
| 海洋学 | 20篇 |
| 天文学 | 7篇 |
| 自然地理 | 17篇 |
出版年
| 2024年 | 1篇 |
| 2020年 | 1篇 |
| 2019年 | 4篇 |
| 2018年 | 2篇 |
| 2017年 | 2篇 |
| 2016年 | 3篇 |
| 2015年 | 4篇 |
| 2014年 | 4篇 |
| 2013年 | 5篇 |
| 2012年 | 4篇 |
| 2011年 | 14篇 |
| 2010年 | 11篇 |
| 2009年 | 13篇 |
| 2008年 | 7篇 |
| 2007年 | 4篇 |
| 2006年 | 7篇 |
| 2005年 | 5篇 |
| 2004年 | 11篇 |
| 2003年 | 2篇 |
| 2002年 | 6篇 |
| 2001年 | 2篇 |
| 2000年 | 1篇 |
| 1999年 | 3篇 |
| 1998年 | 2篇 |
| 1997年 | 3篇 |
| 1996年 | 4篇 |
| 1995年 | 1篇 |
| 1993年 | 2篇 |
| 1969年 | 1篇 |
排序方式： 共有129条查询结果，搜索用时 0 毫秒
31.
Open-framework gravel has permeability, k, above the measurement range of most laboratory constant-head permeameters because the head difference across the length of conventional permeameters is too small to be measured. Here we addressed the challenge of measuring the high k by using a 3 m long permeameter. The head difference over this length was of the order of 10−2 to 10−3 m, which we could measure to the nearest 10−5 m. We collected data over the range of linear, laminar flow to nonlinear, laminar flow to verify that k was measured using data collected within the Darcian regime. We measured k between 4000 and 100,000 Darcies among experiments using different sediments. 相似文献
32.
We demonstrate evidence that past composite based studies centred around the onset of Forbush decrease (FD) events may have improperly isolated the maximal galactic cosmic ray (GCR) decrease associated with the FD events. After an adjustment of the composite to account for such shortcomings we find indications of anomalous cloud cover decreases (of around 3%) located in the upper levels of the troposphere at high southern latitudes. These cloud changes are detectable after latitudinal averaging, suggesting the possibility of a second order relationship between the rate of GCR flux and cloud cover in this region. The maximal cloud change is observed in advance of the maximal GCR decrease; this implies that if the observed cloud changes bear a causal relationship to the rate of GCR flux, then cloud properties may be sensitive to changes in GCR conditions rather than the maximal deviations themselves. 相似文献
33.
Mathew G. Brown T. Andrew Black Zoran Nesic Vanessa N. Foord David L. Spittlehouse Arthur L. Fredeen Rebecca Bowler Nicholas J. Grant Philip J. Burton J. A. Trofymow Dominic Lessard Gesa Meyer 《水文研究》2014,28(8):3326-3340
Over the past decade, British Columbia (BC), has experienced the largest mountain pine beetle (MPB) outbreak on record. This study used the eddy‐covariance (EC) technique to examine the impact of the MPB attack on evapotranspiration (E) and associated canopy characteristics of two lodgepole pine stands with secondary structure (trees, saplings and seedlings surviving the attack) located in central BC. MPB‐06, an 85‐year‐old almost pure stand of pine trees, was first attacked in 2006, and by 2010, ~80% of the trees had been killed. MPB‐03, a 110‐year‐old stand with an overstory consisting of over 90% pine and a developed sub‐canopy, was first attacked in 2003 and by 2007 had > 95% pine canopy mortality. EC measurements began in August 2006 at MPB‐06 and in March 2007 at MPB‐03, and continued for four years. Annual total E ranged from 226 mm to 237 mm at MPB‐06, and from 280 to 297 mm at MPB‐03, showing relatively little year‐to‐year change at both sites over the four years. Increased E from the accelerated growth of the surviving vegetation (secondary structure, shrubs and herbs) compensated for reduction in E due to the death of the overstory. Monthly average daytime canopy conductance, the Priestley–Taylor (α), and the canopy–atmosphere decoupling coefficient (Ω) steadily increased during the growing season reaching approximate maximum values of 5 mm s?1, 0.75 and 0.12, respectively. Potential evapotranspiration was approximated using a vapour pressure deficit‐dependent α obtained at high soil water content. Calculated water deficits indicated some water‐supply limitation to the surviving trees and understory at both sites. Rates of root zone drainage during the growing season were low relative to precipitation. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
34.
The long-term morphodynamic response of estuarine systems to sea level rise and human interference has been studied using a Boolean network approach. The method uses Boolean variables to represent the different elements of the estuarine system, their inter-relationships and the external forcing. The response of a variable to a given change can be deduced from the corresponding Boolean function, which describes the feedbacks between that particular variable and the others in the network. This approach provides qualitative insights into the behaviour of estuary systems without the need for a detailed and quantitative specification of linkages between the various components of the system. 相似文献
35.
Mario A. Aurelio Jocelyn B. Galapon Violet T. Hizon Dominic B. Sadsad 《Island Arc》2009,18(1):144-154
The structural signature in the area between the Baguio mineral district and Ansagan, Tuba, Benguet in the South Central Cordillera, northern Luzon, Philippines, is dominated by northeast- to ENE-trending faults, contained within a NNW–SSE-trending transtensional strip. This 50-km-long, 25-km-wide elongated tectonic zone is bounded to the west by the Pugo Fault and to the east by the Tebbo Fault, both being branches of the Philippine Fault System. Detailed structural geological (particularly microtectonic) analysis of fracture and mineral vein systems indicates strong conformity with the regional structural direction. Computed extensional stress axis σ3 directions are oriented N150° on average, sub-parallel to the strike of the bounding faults. The existence of known mineral deposits and prospects within the tectonic strip implies an intimate relationship between transtension and mineral occurrence. 相似文献
36.
Factal interrelationships between topography and structure are investigated in two areas of the North American central Appalachian Mountains: one in the intensely deformed Valley and Ridge province and the other in the relatively undeformed foreland area of the Appalachian Plateau province. In the Valley and Ridge province the fractal dimensions of topographic and structural relief vary systematically along the strike of major folds following a second-order polynominal trend. Cross-correlation of the fractal dimensions of topography to structure indicates that there is a significant positive correlation between the two. Fractal analysis of topography in the relatively undeformed foreland area of the Appalachian Plateau revealed no significant variation in the fractal characteristics of topography across the study area, consistent with the lack of near-surface structure. However, fractal analysis of deeper structures beneath the Plateau area undertaken using reflection seismic data revealed step-wise increases in fractal dimension from the deeply buried Precambrian basement to the near-surface. These vertical changes in fractal dimension can be related to the tectonic history of the area. Taken together, these studies indicate that fractal analysis provides a means to quantify and compare the influence of near-surface structure on topographic development and lateral and vertical structural variability. Fractal analysis provides a means to characterize the systematic changes in the complex patterns formed by topography and structure and the interrelationships between them. Similarity in their fractal characteristics implies similarity in the relative amplitude and abundance of different wavelength features in the topographic or structural profile. © 1998 John Wiley & Sons, Ltd. 相似文献
37.
Permeability, k, and porosity, φ, were measured in mixtures of fine, medium, and coarse sand, where the volume fraction of each of the three components was systematically varied. The k was modeled well by the Kozeny‐Carman equation for three‐component mixtures by using a representative grain size parameter, d, computed by averaging the grain diameters of components recursively, with averaging methods based on whether finer components exist in sufficient volume to fill the pores within coarser components. The φ was modeled well by using linear interpolation with piecewise‐planar models. We explored the use of differing numbers of piecewise‐planar elements in the model, and illustrate the trade‐off between the increased accuracy and the increased data requirements that both come from adding more elements. The k model is a function of both d and φ, but more sensitive to d. The k model gave results consistent with measured values when computed using either measured φ values, or values from any of the φ models. 相似文献
38.
39.
Food-insecure households in many countries depend on international aid to alleviate acute shocks and chronic shortages. Some food security programmes (including Ethiopia’s Productive Safety Net Program–PSNP – which provides a case study for this article) have integrated aid in exchange for labour on public works to reduce long-term dependence by investing in the productive capacity and resilience of communities. Using this approach, Ethiopia has embarked upon an ambitious national programme of land restoration and sustainable land management. Although the intent was to reduce poverty, here we show that an unintended co-benefit is the climate-change mitigation from reduced greenhouse gas (GHG) emissions and increased landscape carbon stocks. The article first shows that the total reduction in net GHG emissions from PSNP’s land management at the national scale is estimated at 3.4 million?Mg?CO2e?y?1 – approximately 1.5% of the emissions reductions in Ethiopia’s Nationally Determined Contribution for the Paris Agreement. The article then explores some of the opportunities and constraints to scaling up of this impact.Key policy insights
Food security programmes (FSPs) can contribute to climate change mitigation by creating a vehicle for investment in land and ecosystem restoration.
Maximizing mitigation, while enhancing but not compromising food security, requires that climate projections, and mitigation and adaptation responses should be mainstreamed into planning and implementation of FSPs at all levels.
Cross-cutting oversight is required to integrate land restoration, climate policy, food security and disaster risk management into a coherent policy framework.
Institutional barriers to optimal implementation should be addressed, such as incentive mechanisms that reward effort rather than results, and lack of centralized monitoring and evaluation of impacts on the physical environment.
Project implementation can often be improved by adopting best management practices, such as using productive living livestock barriers where possible, and increasing the integration of agroforestry and non-timber forest products into landscape regeneration.
40.