Comments on “Two-dimensional concentration distribution for mixing-controlled bioreactive transport in steady-state” by O.A. Cirpka and A.J. Valocchi     

Haibing Shao  Florian Centler  Cecilia De Biase  Martin Thullner  Olaf Kolditz 《Advances in water resources》2009

In this comment we present a re-analysis of the analytical solution presented by Cirpka and Valocchi for steady-state concentrations of dissolved bioreactive compounds and bacterial biomass in porous media. We discuss the validity range of the analytical solution. In particular, the criterion used to determine the sustainability of biomass is revisited. This re-analysis shows that the ω$\omega$ criterion used by Cirpka and Valocchi is only a necessary but not a sufficient criterion to determine the bioreactive zones. As a consequence, the analytical solution does not provide the exact distribution of compounds throughout the domain, but can serve as upper or lower boundaries for species concentrations at a given location. These conclusions are supported by the simulation results obtained from an established reactive transport model.  相似文献   

Phosphorus sources and demand during summer in a eutrophic lake     

Amy M. Kamarainen  Rachel M. Penczykowski  Matthew C. Van de Bogert  Paul C. Hanson  Stephen R. Carpenter 《Aquatic Sciences - Research Across Boundaries》2009,71(2):214-227

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Melting of enriched mantle beneath Pitcairn seamounts: Unusual U–Th–Ra systematics provide insights into melt extraction processes     

Bernard Bourdon  James A. Van Orman 《Earth and Planetary Science Letters》2009,277(3-4):474-481

U-series systematics as well as Sr isotopes were measured on young seamount lavas from the Pitcairn hotspot collected during the Polynaut cruise. The combined U-series and Sr isotope data reveal typical mixing relationships between two endmembers. One typical ‘plume’ endmember with radiogenic ⁸⁷Sr/⁸⁶Sr and relatively low ²³⁰Th/²³⁸U and a ‘lithosphere’ endmember with less radiogenic ⁸⁷Sr/⁸⁶Sr and relatively larger ²³⁰Th/²³⁸U. Remarkably, all the lavas, except for a few arguably older samples, are characterized by ²²⁶Ra deficits relative to ²³⁰Th. On the basis of water content and trace element systematics, we argue that this is due to melting in the presence of phlogopite, which is only stable at lithosphere temperatures. A melting model including the diffusive exchange of elements among phlogopite, garnet and melt is used to constrain melting conditions of the lithosphere. These unusual ²²⁶Ra–²³⁰Th signatures can be explained by relatively slow melting rates at low matrix porosity. Our model also demonstrates that the effective partitioning behavior is dependent on the melting rate. A simple thermal model for lithosphere heating and melting is in good agreement with predicted melting rates.  相似文献   

Wave train model for knickpoint migration     

Nicolas Loget  Jean Van Den Driessche 《Geomorphology》2009,106(3-4):376-382

Rivers respond to a drop in their base level by incising the topography. The upstream propagation of an incision, as usually depicted by a knickpoint migration, is thought to depend on several parameters such as the drainage area, lithology, and the amplitude of the base level drop. We first investigate the case of the Messinian Salinity Crisis that was characterized by the extreme base level fall (1500 m) of the Mediterranean Sea at the end of the Miocene. The response of drainage areas of three orders of magnitude (10³ to 10⁶ km²) highlights the dominant role of the drainage area (with a square root relationship) in controlling the knickpoint migration after a base level fall. A compilation of mean rates of knickpoint propagation for time durations ranging from 10² to 10⁷ years displays a similar relationship indicating that successive wave trains of knickpoint can migrate in a river: first, wave trains linked to the release of the alluvial cover and then, wave trains related to the bedrock incision, which correspond to the real time response of rivers. Wave trains with very low retreat rates (long lived knickpoints > 1 My) rather correspond to the response time of regional landscape.  相似文献   

多代理人系统在商业街消费者行为模拟中的应用——以上海南京东路为例   总被引：4，自引：1，他引：3  

朱玮  王德 《地理学报》2009,64(4)

用模型方法研究消费者行为经历了从集合模型到个体模型的过程.随着计算机技术的进步,近十余年来多代理人技术得到了长足的发展.当前,将消费者个体行为模型和多代理人系统相结合来模拟商业街消费者活动,已经成为相关研究和实践领域的前沿方法.其主要优点在于能够使相关人员以单个消费者为基础,整合复杂的相关因素,预测整体或局部商业空间的动态运行效果,作为规划开发的依据.本文介绍由笔者基于NetLogo开发的一个模拟商业街消费者行为的多代理人模拟平台,包括系统的建构和模拟的流程.该模拟系统的核心由四个基于有限理性原理的消费者决策模型构成:回家决策、方向选择决策、休息决策、商店光顾决策.然后应用这个平台于研究上海南京东路消费者行为的实例.该数据收集于2007年,现场记录了消费者在南京东路的活动过程,并被用于拟合行为模型.将模拟的代理人个体行为集合后,于三方面同观察到的消费者实际行为作比较,包括从事不同活动的消费者数量随时间的变化、在各街道段中活动的消费者数量随时间的变化,以及单个商店中的消费者人次和延时;同时分析了模拟中的问题.结果显示该模拟平台总体上能够较好地预测消费者集合行为.  相似文献   

基于SD法的城市感知研究——以浙江台州地区为例   总被引：2，自引：0，他引：2  

王德  张昀  崔昆仑 《地理研究》2009,28(6):1528-1536

借助SD法即语义差别法获取人们对台州地区9个城市的整体感知,并研究这些感知与代表城市特征的客观指标之间的关系,为营造宜人的城市环境提供参考。结果表明,台州城市群总体特点较为中庸,但各城市特点各异,基本能分为三组;城市的感知和城市的客观指标之间存在着一定关系;与客观指标预期相关的感知和实际相关的感知存在一定出入;各种指标对感知的影响力由大到小依次为经济指标、规模指标、区位指标和建设指标;经济总量对感知的作用大于人均经济量的作用;受到客观指标影响程度由大到小的感知依次是:城市现代化的感知、城市规模和繁华度的感知、城市有序度和城市心理距离的感知、城市尺度的感知;通过进一步的回归分析,最后得到了特定感知量级下对应的城市客观指标值。  相似文献   

Discovery of magnetic fields in three He variable Bp stars with He and Si spots     

M. Briquet  S. Hubrig  M. Schller  P. De Cat 《Astronomische Nachrichten》2007,328(1):41-45

It is essential for the understanding of stellar structure models of high mass stars to explain why constant stars, nonpulsating chemically peculiar hot Bp stars and pulsating stars co‐exist in the slowly pulsating B stars and β Cephei instability strips. We have conducted a search for magnetic fields in the four Bp stars HD55522, HD105382, HD131120, and HD138769 which previously have been wrongly identified as slowly pulsating B stars. A recent study of these stars using the Doppler Imaging technique revealed that the elements He and Si are inhomogeneously distributed on the stellar surface, causing the periodic variability. Using FORS 1 in spectropolarimetric mode at the VLT, we have acquired circular polarisation spectra to test the presence of a magnetic field in these stars. A variable magnetic field is clearly detected in HD55522 and HD105382, but no evidence for the existence of a magnetic field was found in HD131120. The presence of a magnetic field in HD138769 is suggested by one measurement at 3σ level. We discuss the occurrence of magnetic B stars among the confirmed pulsating B stars and find strong magnetic fields of order kG and oscillations to be mutually exclusive. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Variability of the Fe K line relativistic component in a sample of Seyfert 1 galaxies     

B. De Marco  M. Cappi  M. Dadina  G. G. C. Palumbo 《Astronomische Nachrichten》2006,327(10):1028-1031

We present the analysis of X‐ray spectral variability made on a sample of 7 Seyfert 1 bright galaxies, using XMM‐Newton data. From the “XMM‐Newton Science Archive” we selected those bright Seyfert 1 showing one or more prominent flares in their 2–10 keV light curves. For each of them we extracted spectra in 3 different time intervals: before, during and after the flare. We fitted them with a simple power law and then shifted a narrow emission and absorption line template across the 2.5–10 keV data, in order to investigate the presence of line‐like features with a confidence level greater than 99%. Some highly significant features were detected in 3 out of 7 sources studied. In particular, the 3 sources, namely PG 1211+143, NGC 4051 and NGC 3783, showed the presence of a variable emission feature in the 4.5–5.8 keV band, characterized by an increase of its intensity after the flare peak. Because of the observed variability pattern, this feature seems to be ascribable to a reverbered redshifted relativistic component of the Fe K line. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Identifying causes of ground-penetrating radar reflections using time-domain reflectometry and sedimentological analyses   总被引：5，自引：0，他引：5  

Remke L. Van Dam  & Wolfgang Schlager 《Sedimentology》2000,47(2):435-449

Ground-penetrating radar (GPR) is a geophysical technique widely used to study the shallow subsurface and identify various sediment features that reflect electromagnetic waves. However, little is known about the exact cause of GPR reflections because few studies have coupled wave theory to petrophysical data. In this study, a 100- and 200-MHz GPR survey was conducted on aeolian deposits in a quarry. Time-domain reflectometry (TDR) was used to obtain detailed information on the product of relative permittivity (ɛ_r) and relative magnetic permeability (μ_r), which mainly controls the GPR contrast parameter in the subsurface. Combining TDR data and lacquer peels from the quarry wall allowed the identification of various relationships between sediment characteristics and ɛ_rμ_r. Synthetic radar traces, constructed using the TDR logs and sedimentological data from the lacquer peels, were compared with the actual GPR sections. Numerous peaks in ɛ_rμ_r, which are superimposed on a baseline value of 4 for dry sand, are caused by potential GPR reflectors. These increases in ɛ_rμ_r coincide with the presence of either organic material, having a higher water content and relative permittivity than the surrounding sediment, or iron oxide bands, enhancing relative magnetic permeability and causing water to stagnate on top of them. Sedimentary structures, as reflected in textural change, only result in possible GPR reflections when the volumetric water content exceeds 0·055. The synthetic radar traces provide an improved insight into the behaviour of radar waves and show that GPR results may be ambiguous because of multiples and interference.  相似文献   

Minimal Detectable Biases of Cross-Correlated GPS Observations     

Kees De Jong 《GPS Solutions》2000,3(3):12-18

In this article, analytic expressions are given for the minimal detectable biases (MDBs) of dual-frequency cross-correlated global positioning system (GPS) single difference code and carrier observations. They are given for three different short baseline models. (Short here implies that orbital and atmospheric uncertainties are assumed absent.) A comparison is made with the expressions for the MDBs of non-cross-correlating receivers. Finally, in order to get a better understanding of them, MDBs are visualized as a function of the parameters on which they depend, such as the number ob tracked satellites and the number of observation epochs. ? 2000 John Wiley & Sons, Inc.  相似文献