Alignment time-scale of the microquasar GRO J1655−40     

Rebecca G. Martin  Christopher A. Tout  J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2008,387(1):188-196

The microquasar GRO J1655−40 has a black hole with spin angular momentum apparently misaligned to the orbital plane of its companion star. We analytically model the system with a steady-state disc warped by Lense–Thirring precession and find the time-scale for the alignment of the black hole with the binary orbit. We make detailed stellar evolution models so as to estimate the accretion rate and the lifetime of the system in this state. The secondary can be evolving at the end of the main sequence or across the Hertzsprung gap. The mass-transfer rate is typically 50 times higher in the latter case but we find that, in both the cases, the lifetime of the mass-transfer state is at most a few times the alignment time-scale. The fact that the black hole has not yet aligned with the orbital plane is therefore consistent with either model. We conclude that the system may or may not have been counter aligned after its supernova kick but that it is most likely to be close to alignment rather than counter alignment now.  相似文献   

In situ characterization of grain‐scale fluvial morphology using Terrestrial Laser Scanning     

Rebecca Hodge  James Brasington  Keith Richards 《地球表面变化过程与地形》2009,34(7):954-968

The grain‐scale morphology of fluvial sediments is an important control on the character and dynamics of river systems; however current understanding of its role is limited by the difficulties of robustly quantifying field surface morphology. Terrestrial Laser Scanning (TLS) offers a new methodology for the rapid acquisition of high‐resolution and high‐precision surface elevation data from in situ sediments. To date, most environmental and fluvial applications of TLS have focused on large‐scale systems, capturing macroscale morphologies. Application of this new technology at scales necessary to characterize the complexity of grain‐scale fluvial sediments therefore requires a robust assessment of the quality and sources of errors in close‐range TLS data. This paper describes both laboratory and field experiments designed to evaluate close‐range TLS for sedimentological applications and to develop protocols for data acquisition. In the former, controlled experiments comprising high‐resolution scans of white, grey and black planes and a sphere were used to quantify the magnitude and source of three‐dimensional (3D) point errors resulting from a combination of surface geometry, reflectivity effects and inherent instrument precision. Subsequently, a methodology for the collection and processing of grain‐scale TLS data is described through an application to a coarse grained gravel system, the River Feshie (D₅₀ 32 to 63 mm). This stepwise strategy incorporates averaging repeat scans and filtering scan artefact and non‐surface points using local 3D search algorithms. The sensitivity of the results to the filter parameter values are assessed by careful internal validation of Digital Terrain Models (DTMs) created from the resulting point cloud data. The transferability of this methodology is assessed through application to a second river, Bury Green Brook, dominated by finer gravel (D₅₀ 18 to 33 mm). The factor limiting the resolution of DTMs created from this second dataset was found to be the relative sizes of the laser footprint and smallest grains. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Changes in sediment and organic carbon accumulation in a highly-disturbed ecosystem: The Sacramento-San Joaquin River Delta (California, USA)     

Elizabeth A. Canuel  Elizabeth J. Lerberg  Rebecca M. Dickhut  Steven A. Kuehl  Thomas S. Bianchi  Stuart G. Wakeham 《Marine pollution bulletin》2009,59(4-7):154

We used the Sacramento-San Joaquin River Delta CA (Delta, hereafter) as a model system for understanding how human activities influence the delivery of sediment and total organic carbon (TOC) over the past 50–60 years. Sediment cores were collected from sites within the Delta representing the Sacramento River (SAC), the San Joaquin River (SJR), and Franks Tract (FT), a flooded agricultural tract. A variety of anthropogenic tracers including ¹³⁷Cs, total DDE (∑DDE) and brominated diphenyl ether (BDE) congeners were used to quantify sediment accumulation rates. This information was combined with total organic carbon (TOC) profiles to quantify rates of TOC accumulation. Across the three sites, sediment and TOC accumulation rates were four to eight-fold higher prior to 1972. Changes in sediment and TOC accumulation were coincident with completion of several large reservoirs and increased agriculture and urbanization in the Delta watershed. Radiocarbon content of TOC indicated that much of the carbon delivered to the Delta is “pre-aged” reflecting processing in the Delta watershed or during transport to the sites rather than an input of predominantly contemporary carbon (e.g., 900–1400 years BP in surface sediments and 2200 yrs BP and 3610 yrs BP at the base of the SJR and FT cores, respectively). Together, these data suggest that human activities have altered the amount and age of TOC accumulating in the Delta since the 1940s.  相似文献   

Using MODFLOW 2000 to model ET and recharge for shallow ground water problems     

Doble RC  Simmons CT  Walker GR 《Ground water》2009,47(1):129-135

In environments with shallow ground water elevation, small changes in the water table can cause significant variations in recharge and evapotranspiration fluxes. Particularly, where ground water is close to the soil surface, both recharge and evapotranspiration are regulated by a thin unsaturated zone and, for accuracy, must be represented using nonconstant and often nonlinear relationships. The most commonly used ground water flow model today, MODFLOW, was originally designed with a modular structure with independent packages representing recharge and evaporation processes. Systems with shallow ground water, however, may be better represented using either a recharge function that varies with ground water depth or a continuous recharge and evapotranspiration function that is dependent on depth to water table. In situations where the boundaries between recharging and nonrecharging cells change with time, such as near a seepage zone, a continuous ground water flux relationship allows recharge rates to change with depth rather than having to calculate them at each stress period. This research article describes the modification of the MODFLOW 2000 recharge and segmented evapotranspiration packages into a continuous recharge-discharge function that allows ground water flux to be represented as a continuous process, dependent on head. The modifications were then used to model long-term recharge and evapotranspiration processes on a saline, semiarid floodplain in order to understand spatial patterns of salinization, and an overview of this process is given.  相似文献   

Formation and erosion of sediment cover in an experimental bedrock‐alluvial channel          下载免费PDF全文

Rebecca Hodge  Trevor Hoey  Georgios Maniatis  Emilie Leprêtre 《地球表面变化过程与地形》2016,41(10):1409-1420

Sediment grains in a bedrock‐alluvial river will be deposited within or adjacent to a sediment patch, or as isolated grains on the bedrock surface. Previous analysis of grain geometry has demonstrated that these arrangements produce significant differences in grain entrainment shear stress. However, this analysis neglected potential interactions between the sediment patches, local hydraulics and grain entrainment. We present a series of flume experiments that measure the influence of sediment patches on grain entrainment. The flume had a planar bed with roughness that was much smaller than the diameters of the mobile grains. In each experiment sediment was added either as individual grains or as a single sediment pulse. Flow was then increased until the sediment was entrained. Analysis of the experiments demonstrates that: (1) for individual grains, coarse grains are entrained at a higher discharge than fine grains; (2) once sediment patches are present, the different in entrainment discharge between coarse and fine grains is greatly reduced; (3) the sheltering effect of patches also increases the entrainment discharge of isolated grains; (4) entire sediment patches break‐up and are eroded quickly, rather than through progressive grain‐by‐grain erosion; (5) as discharge increases there is some tendency for patches to become more elongate and flow‐aligned, and more randomly distributed across the bed. One implication of this research is that the critical shear stress in bedrock‐alluvial channels will be a function of the extent of the sediment cover. Another is that the influence of sediment patches equalizes critical shear stresses between different grain sizes and grain locations, meaning that these factors may not need to be accounted for. Further research is needed to quantify interactions between sediment patches, grain entrainment and local hydraulics on rougher bedrock surfaces, and under different types of sediment supply. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Evapotranspiration and canopy characteristics of two lodgepole pine stands following mountain pine beetle attack   总被引：1，自引：0，他引：1       下载免费PDF全文

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   

Erratum to: The impact of climate change on California’s ecosystem services     

M. Rebecca Shaw  Linwood Pendleton  D. Richard Cameron  Belinda Morris  Dominique Bachelet  Kirk Klausmeyer  Jason MacKenzie  David R. Conklin  Gregory N. Bratman  James Lenihan  Erik Haunreiter  Christopher Daly  Patrick R. Roehrdanz 《Climatic change》2012,110(3-4):1067-1067

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Carbon losses in soils previously exposed to elevated atmospheric CO₂ in a chaparral ecosystem: Potential implications for a sustained biospheric C sink     

Rebecca J. Trueman  Lina Taneva  Miquel A. Gonzalez-Meler  Walter C. Oechel  Hormoz BassiriRad 《Journal of Geochemical Exploration》2009,102(3):142

Between 1996 and 2001 an experimental set up in a chaparral community near San Diego, CA, examined various plant and ecosystem responses to CO₂ concentrations ranging from 250 to 750 μl l^− 1. These experiments indicated a significant increase in soil C sequestration as CO₂ rose above the ambient levels. In 2003, two years after the cessation of the CO₂ treatments, we returned to this site to examine soil C dynamics with a particular emphasis on stability of specific pools of C. We found that in as little as two years, C content in the surface soils (0–15 cm) of previously CO₂ enriched plots had dropped to levels below those of the ambient and pretreatment soils. In contrast, C retained in response to CO₂ enrichment was more durable in the deeper soil layers (> 25 cm deep) where both organic and inorganic C were on average 26% and 55% greater, respectively, than C content of ambient plots. Using stable isotope tracers, we found that treatment C represented 25% of total soil C and contributed to 55% of soil CO₂ efflux, suggesting that most of treatment C is readily accessible to decomposers. We also found that, C present before CO₂ fumigation was decomposed at a faster rate in the plots that were exposed to elevated CO₂ than in those exposed to ambient CO₂ levels. To our knowledge, this is the first report that allows for a detail accounting of soil C after ceasing CO₂ treatments. Our study provides a unique insight to how stable the accrued soil C is as CO₂ increases in the atmosphere.  相似文献   

Early Ipswichian (last interglacial) sea level rise in the channel region: Stone Point Site of Special Scientific Interest,Hampshire, England     

Rebecca M. Briant  Martin R. Bates  Steve Boreham  Nigel G. Cameron  G. Russell Coope  Michael H. Field  B. Marcus Hatch  Jonathan A. Holmes  David H. Keen  Aiobhean A. Kilfeather  Kirsty E.H. Penkman  Rianne M.J. Simons  Jean-Luc Schwenninger  Francis F. Wenban-Smith  Nicola J. Whitehouse  John E. Whittaker 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(1):1-26

Constraining the speed of sea level rise at the start of an interglacial is important to understanding the size of the ‘window of opportunity’ available for hominin migration. This is particularly important during the last interglacial when there is no evidence for significant hominin occupation anywhere in Britain. There are very few finer grained fossiliferous sequences in the Channel region that can be used to constrain sea level rise and they are preserved only to the north of the Channel, in England. Of these, the sequence at Stone Point SSSI is by far the most complete. Data from this sequence has been previously reported, and discussed at a Quaternary Research Association Field Meeting, where a number of further questions were raised that necessitated further data generation. In this paper, we report new data from this sequence – thin section analysis, isotopic determinations on ostracod shells, new Optical Stimulated Luminescence ages and Amino Acid Recem analyses. These show early sea level rise in this sequence, starting during the pre-temperate vegetation zone IpI, but no early warming. The implications of this almost certainly last interglacial sequence for the human colonisation of Britain and our understanding of the stratigraphic relationship of interglacial estuarine deposits with their related fluvial terrace sequences is explored.  相似文献   

Review: Current and emerging methods for catchment-scale modelling of recharge and evapotranspiration from shallow groundwater     

Rebecca C. Doble  Russell S. Crosbie 《Hydrogeology Journal》2017,25(1):3-23

A review is provided of the current and emerging methods for modelling catchment-scale recharge and evapotranspiration (ET) in shallow groundwater systems. With increasing availability of data, such as remotely sensed reflectance and land-surface temperature data, it is now possible to model groundwater recharge and ET with more physically realistic complexity and greater levels of confidence. The conceptual representation of recharge and ET in groundwater models is critical in areas with shallow groundwater. The depth dependence of recharge and vegetation water-use feedback requires additional calibration to fluxes as well as heads. Explicit definition of gross recharge vs. net recharge, and groundwater ET vs. unsaturated zone ET, in preparing model inputs and reporting model results is necessary to avoid double accounting in the water balance. Methods for modelling recharge and ET include (1) use of simple surface boundary conditions for groundwater flow models, (2) coupling saturated groundwater models with one-dimensional unsaturated-zone models, and (3) more complex fully-coupled surface-unsaturated-saturated conceptualisations. Model emulation provides a means for including complex model behaviours with lower computational effort. A precise ET surface input is essential for accurate model outputs, and the model conceptualisation depends on the spatial and temporal scales under investigation. Using remote sensing information for recharge and ET inputs in model calibration or in model–data fusion is an area for future research development. Improved use of uncertainty analysis to provide probability bounds for groundwater model outputs, understanding model sensitivity and parameter dependence, and guidance for further field-data acquisition are also areas for future research.  相似文献