Kinematics of quasars and AGN     

K. I. Kellermann  R. C. Vermeulen  J. A. Zensus  M. H. Cohen  A. West 《New Astronomy Reviews》1999,43(8-10)

The VLBA has been used over a period of four years to study the internal motions within a sample of quasars and AGN. In most sources, features appear to propagate away from the central engine along a well collimated radio jet with apparent transverse velocities between zero and 10c, with some evidence for apparent accelerations and decelerations. The distribution of apparent velocity is not consistent with any simple ballistic model and appears to require either a spread in intrinsic velocity or a difference between the bulk velocity and pattern velocity. The dependence of apparent angular velocity with redshift is consistent with standard Friedmann world models. Further observations of a larger source sample, especially at large redshift may lead to meaningful constraints on world models.  相似文献   

Comments on EPA Guidance for Ground Water Studies     

Stuart Cohen 《Ground Water Monitoring & Remediation》1999,19(2):55-56

"The panel provided good, solid, interdisciplinary advice on a variety of challenging, state-of-the-art issues."  相似文献   

Europeans Test Pesticide Models     

Stuart Cohen 《Ground Water Monitoring & Remediation》1999,19(3):49-50

This column focuses on recent work in Europe. The intent is to present to American readers relevant studies they would not normally see. Much of the following was presented at the 9th International Congress of Pesticide Chemistry last summer in London. The congress was jointly sponsored by the International Union of Pure & Applied Chemistry and the Royal Society of Chemistry.  相似文献   

Comparison of vegetation and stream sediment geochemical patterns in northeastern New South Wales   总被引：1，自引：0，他引：1  

D. R. Cohen  C. M. Silva-Santisteban  N. F. Rutherford  D. L. Garnett  H. M. Waldron 《Journal of Geochemical Exploration》1999,66(3):383

This study compares the geochemical response of stream sediments and adjacent vegetation samples, with variations in drainage catchment lithology and the occurrence of mineralisation, within a 14,000-km² block of the northeastern region of New South Wales, Australia. The area contains a range of lithologies within a Devonian–Permian accretionary complex and Mesozoic sedimentary basin, as well as a wide range of mineral deposits. Sampling was designed to confine each sub-catchment to a single lithological group (mafic and ultramafic rocks, acid intrusives, volcanics, metasedimentary and sedimentary rocks or alluvium). Leaves of over 20 genera, dominated by (Allo-)Casuarina, Eucalyptus, Acacia, Callistemon and Melaleuca, and the <250-μm fraction of the stream sediments were analysed by INAA. The uptake of most trace elements varied between genera, with Callistemon displaying the highest median As and La contents and Eucalyptus the highest Co contents. The stream sediment and vegetation geochemistry reflect both hydromorphic and mechanical dispersion within sub-catchments, with regional patterns dominant over local influences. The vegetation appears to be influenced to a greater extent by hydromorphic dispersion, as indicated by differences in the ratio of leaf to sediment Cr concentrations in sub-catchments draining serpentinites and basalts. Although most known mineral deposits in the region produced anomalies in at least one medium, there was little correlation between the trace element concentrations of the vegetation and stream sediments on a site-by-site basis. A number of Au targets were only detected on the basis of the biogeochemistry and others were only reflected in the stream sediment geochemistry. In general, vegetation displayed more extensive dispersion trains away from mineralisation than did the stream sediments. Differences in the response of the two sampling media suggest their joint use in exploration or environmental surveys to maximise the probability of detecting mineralisation.  相似文献   

湖泊研究的新工具和新视野:第58届IAGLR年会观察     

李刚  Jason D. Stockwell  J. Ellen Marsden  徐耀阳 《湖泊科学》2015,27(4):755-756

<正>以中国为代表的发展中国家城镇化进程正在快速推进,前所未有地改变以欧美发达国家为主体的全球城镇发展格局.与此同时,与城镇化进程相关联的大气、土壤和水体等动植物栖息生境和人类生存环境正发生着深刻变化,并且日益受到社会的普遍关注和广泛重视.湖泊作为一类典型的淡水生境,因具有洪水调蓄、多样性维护、物质循环、环境美化和气候调节等自然生态功能以及供水、水产品生产、休闲娱乐和文化美学等社会经济服务功能,其功能与城镇可持续发展和人类社会文明进步息息相关.为此,本文简要回顾了第  相似文献   

Reducing uncertainty in stochastic streamflow generation and reservoir sizing by combining observed,reconstructed and projected streamflow     

Jason?PatskoskiEmail authorView author&#;s OrcID profile  A.?Sankarasubramanian 《Stochastic Environmental Research and Risk Assessment (SERRA)》2018,32(4):1065-1083

Reservoir sizing is one of the most important aspects of water resources engineering as the storage in a reservoir must be sufficient to supply water during extended droughts. Typically, observed streamflow is used to stochastically generate multiple realizations of streamflow to estimate the required storage based on the Sequent Peak Algorithm (SQP). The main limitation in this approach is that the parameters of the stochastic model are purely derived from the observed record (limited to less than 80 years of data) which does not have information related to prehistoric droughts. Further, reservoir sizing is typically estimated to meet future increase in water demand, and there is no guarantee that future streamflow over the planning period will be representative of past streamflow records. In this context, reconstructed streamflow records, usually estimated based on tree ring chronologies, provide better estimates of prehistoric droughts, and future streamflow records over the planning period could be obtained from general circulation models (GCMs) which provide 30 year near-term climate change projections. In this study, we developed paleo streamflow records and future streamflow records for 30 years are obtained by forcing the projected precipitation and temperature from the GCMs over a lumped watershed model. We propose combining observed, reconstructed and projected streamflows to generate synthetic streamflow records using a Bayesian framework that provides the posterior distribution of reservoir storage estimates. The performance of the Bayesian framework is compared to a traditional stochastic streamflow generation approach. Findings based on the split-sample validation show that the Bayesian approach yielded generated streamflow traces more representative of future streamflow conditions than the traditional stochastic approach thereby, reducing uncertainty on storage estimates corresponding to higher reliabilities. Potential strategies for improving future streamflow projections and its utility in reservoir sizing and capacity expansion projects are also discussed.  相似文献   

Experimental validation of seismic retrofit solutions for URM chimneys     

Marta Giaretton  Jason M. Ingham  Dmytro Dizhur 《Bulletin of Earthquake Engineering》2018,16(1):295-313

Clay-brick unreinforced masonry (URM) chimneys are a common architectural feature of vintage domestic and commercial URM and timber-framed buildings widely encountered worldwide. URM chimneys include a cantilevered appendage above the roofline that during earthquakes-induced shaking is susceptible to significant damage and collapse, which can result in fatalities, injuries, and property damage. To mitigate this risk, many vintage URM chimneys are removed, braced, or eventually replaced with modern alternatives. The earthquake performance of 15,400 URM chimneys damaged during the 2010/2011 Canterbury earthquake sequence was investigated using a photographic database and insurance claims data. The widespread damage and failure of a large proportion of retrofitted chimneys was the impetus for an experimental shake-table testing campaign that was undertaken to assess the performance of five prototypical URM chimneys in their as-built and retrofitted conditions. Cost-effective retrofit techniques with low aesthetic impact, including the use of near surface mounted carbon-fibre reinforced polymer strips and post-tensioning, were investigated. Tested chimneys were constructed to replicate the most commonly encountered geometric characteristics and construction details collected during an in-field pilot survey of 351 undamaged vintage URM chimneys.  相似文献   

The likelihood of having a household emergency plan: understanding factors in the US context     

Rivera  Jason D. 《Natural Hazards》2020,104(2):1331-1343

Natural Hazards - Individual household emergency planning is the most fundamental and can be the least expensive way to prepare for natural disasters. However, despite government and nonprofit...  相似文献   

Thresholds for post-wildfire debris flows: Insights from the Pinal Fire,Arizona, USA     

Carissa A. Raymond  Luke A. McGuire  Ann M. Youberg  Dennis M. Staley  Jason W. Kean 《地球表面变化过程与地形》2020,45(6):1349-1360

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post-wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h⁻¹ lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils. © 2019 John Wiley & Sons, Ltd.  相似文献   

The role of antecedent groundwater heads in controlling transient aquifer storage and flood peak attenuation in karst watersheds     

Patricia Spellman  Jason Gulley  Jonathan B. Martin  Jeremy Loucks 《地球表面变化过程与地形》2019,44(1):77-87

Transient storage of floodwaters in aquifers is known to attenuate peak flows in rivers and drive subsurface dissolution. Transient aquifer storage could be enhanced in watersheds overlying karst aquifers where caves facilitate surface and groundwater exchange. Few studies, however, have examined controls on, or magnitudes of, transient aquifer storage or flood peak attenuation in karstic watersheds. Here we evaluate flood peak attenuation with multiple linear regression analyses of 10 years of river and groundwater data from the Suwannee River, which flows over the karstic upper Floridan aquifer in north-central Florida and experiences frequent flooding. Regressions show antecedent river stage exerts the dominant control on magnitudes of transient aquifer storage, with recharge and time to peak having secondary controls. Specifically, low antecedent stages result in larger magnitudes of transient aquifer storage and thus greater flood attenuation than conditions of elevated antecedent stage. These findings suggest subsurface weathering, including cave formation and enlargement, caused by transient aquifer storage could occur on a more frequent basis in aquifers where groundwater table elevation is lowered due to anthropogenic or climatic influences. Our work also shows that measures of groundwater table elevation prior to an event could be used to improve predictive flood models. © 2018 John Wiley & Sons, Ltd.  相似文献