The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia   总被引：1，自引：0，他引：1  

G. O. Walker  J. H. K. Ma  E. Golton 《Annales Geophysicae》1994,12(2-3):195-209

Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9○N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.  相似文献   

Nonaqueous Phase Hydrocarbon in a Fine-Grained Sandstone: 1. Comparison Between Measured and Predicted Saturations and Mobility     

David Huntley  Robert N. Hawk  Helen P. Corley 《Ground water》1994,32(4):626-634

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Salt accumulation in semi-arid floodplain soils with implications for forest health     

Ian D. Jolly  Glen R. Walker  Peter J. Thorburn 《Journal of Hydrology》1993,150(2-4):589-614

Dieback of native Eucalyptus largiflorens forests is an increasing problem on the floodplains of the lower River Murray, southern Australia. Salinisation of floodplain soils, as a result of the changed hydrological management of the River Murray, appears to be a primary cause of the dieback. Regulation of the River Murray has reduced the frequency of large flood events by a factor of approximately three and caused groundwater levels beneath floodplains to rise. The higher water tables have resulted in increased discharge of the naturally saline groundwater in the floodplains by evapotranspiration, and the decreased incidence of large floods has reduced floodwater recharge and hence leaching of salt from floodplain soils. Use of soil physical properties for a range of floodplain soils, combined with measurements of groundwater discharge from bare and vegetated sites, suggests that the time-scale for complete soil salinisation can, at worst, be less than 20 years. Moreover, salt accumulation at most sites will continue to occur as the present flooding regime (of which there is limited scope for improvement) appears incapable of providing the leaching required to counteract accumulation. The analyses carried out here suggest that the ‘critical’ water table depth (below which groundwater discharge is balanced or exceeded by floodwater recharge) needs to be increased by 14–55% (the more clayey the soil, the larger the increase) to prevent salt accumulation. Failure to implement schemes which lower the water tables beneath the floodplain may, in the long term, cause serious damage to these important riparian forests.  相似文献   

GIS-based multicriteria evaluation for earthquake response: a case study of expert opinion in Vancouver,Canada     

Walker  Blake Byron  Schuurman  Nadine  Swanlund  David  Clague  John J. 《Natural Hazards》2021,105(2):2075-2091

Natural Hazards - GIS-based multicriteria evaluation (MCE) provides a framework for analysing complex decision problems by quantifying variables of interest to score potential locations according...  相似文献   

Laboratory Experiments and Grain Based Discrete Element Numerical Simulations Investigating the Thermo-Mechanical Behaviour of Sandstone     

Woodman  James  Ougier-Simonin  Audrey  Stavrou  Anastasios  Vazaios  Ioannis  Murphy  William  Thomas  Mark E.  Reeves  Helen J. 《Geotechnical and Geological Engineering》2021,39(7):4795-4815

Geotechnical and Geological Engineering - Thermo-mechanical loading can occur in numerous engineering geological environments, from both natural and anthropogenic sources. Different minerals and...  相似文献   

A method for quantitative pyrite abundance in mine rock piles by powder X-ray diffraction and Rietveld refinement     

Erik J. Oerter  George H Brimhall Jr.  Jennifer Redmond  Bruce Walker 《Applied Geochemistry》2007,22(12):2907

The abundance of pyrite and other sulfide minerals in mine rock piles is a potentially significant if not a determinative factor in terms of the geochemical and geomechanical evolution of the dumps as oxidation produces acid solutions that drive hydrolysis reactions. A technique is presented here that supports the quantitative determination of pyrite abundance in mine rock dumps by heavy liquid mineral separation to concentrate pyrite for powder X-ray diffraction and then Rietveld method refinement of the diffraction data on a large number of samples using commonly available laboratory equipment. In order to improve and constrain the accuracy of XRD results, binary (pyrite-quartz) and 6-part mineral mixtures (pyrite and rock-forming andesite minerals) spanning a wide range of pyrite concentrations were prepared gravimetrically and run as standards. These standards were then used to minimize errors in pyrite abundance data by constraining key input parameters in the Rietveld refinement. A new polynomial relationship was derived between diffracting crystallite size and the Brindley microabsorption correction input size. This method provides a means to determine uncertainties in pyrite abundance, whereas conventional Rietveld refinement techniques done without the use of standards yield only statistical measures of the least-squares fit, rather than absolute uncertainties in mineral constituent weight percentages. The technique was applied to a number of mine rock pile samples and the uncertainty in the results determined by applying the relationship derived from the 6-part gravimetric standards to the results of the Brindley corrected Rietveld refinements. Uncertainties determined by this method are found to be on the order of ±10% for samples with pyrite content greater than 10 wt% and ±30% for samples with pyrite content less than 10 wt%. In order to evaluate the technique’s improvement upon traditional visual mineral abundance estimation the quantitative results are compared to manual volumetric estimates.  相似文献   

The role of vegetation in the retention of fine sediment and associated metal contaminants in London's rivers          下载免费PDF全文

Helen M. Gibbs  Angela M. Gurnell  Catherine M. Heppell  Kate L. Spencer 《地球表面变化过程与地形》2014,39(8):1115-1127

Many urban rivers receive significant inputs of metal‐contaminated sediments from their catchments. Restoration of urban rivers often creates increased slack water areas and in‐channel vegetation growth where these metal‐contaminated sediments may accumulate. Quantifying the accumulation and retention of these sediments by in‐channel vegetation in urban rivers is of importance in terms of the planning and management of urban river restoration schemes and compliance with the Water Framework Directive. This paper investigates sediment properties at four sites across three rivers within Greater London to assess the degree to which contaminated sediments are being retained. Within paired restored and unrestored reaches at each site, four different bed sediment patch types (exposed unvegetated gravel, sand, and silt/clay (termed ‘fine’) sediments, and in‐channel vegetated sediments) were sampled and analysed for a range of metals and sediment characteristics. Many samples were found to exceed Environment Agency guidelines for copper (Cu), lead (Pb) and zinc (Zn) and Dutch Intervention Values for Cu and Zn. At all sites, sediments accumulating around in‐channel vegetation were similar in calibre and composition to exposed unvegetated fine sediments. Both bed sediment types contained high concentrations of pseudo‐total and acetic acid extractable metal concentrations, potentially due to elevated organic matter and silt/clay content, as these are important sorbtion phases for metals. This implies that the changed sediment supply and hydraulic conditions associated with river restoration may lead to enhanced retention of contaminated fine sediments, particularly around emergent plants, frequently leading to the development of submerged and emergent landforms and potential river channel adjustments. High pseudo‐total metal concentrations were also found in gravel bed sediments, probably associated with iron (Fe) and manganese (Mn) oxyhydroxides and discrete anthropogenic metal‐rich particles. These results highlight the importance of understanding the potential effects of urban river restoration upon sediment availability and channel hydraulics and consequent impacts upon sediment contaminant dynamics and storage. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Isotopic investigation of runoff generation in a glacierized catchment in northern Sweden   总被引：2，自引：0，他引：2       下载免费PDF全文

Helen E. Dahlke  Steve W. Lyon  Peter Jansson  Torbjörn Karlin  Gunhild Rosqvist 《水文研究》2014,28(3):1383-1398

In this study, summer rainfall contributions to streamflow were quantified in the sub‐arctic, 30% glacierized Tarfala (21.7 km²) catchment in northern Sweden for two non‐consecutive summer sampling seasons (2004 and 2011). We used two‐component hydrograph separation along with isotope ratios (δ¹⁸O and δD) of rainwater and daily streamwater samplings to estimate relative fraction and uncertainties (because of laboratory instrumentation, temporal variability and spatial gradients) of source water contributions. We hypothesized that the glacier influence on how rainfall becomes runoff is temporally variable and largely dependent on a combination of the timing of decreasing snow cover on glaciers and the relative moisture storage condition within the catchment. The results indicate that the majority of storm runoff was dominated by pre‐event water. However, the average event water contribution during storm events differed slightly between both years with 11% reached in 2004 and 22% in 2011. Event water contributions to runoff generally increased over 2011 the sampling season in both the main stream of Tarfala catchment and in the two pro‐glacial streams that drain Storglaciären (the largest glacier in Tarfala catchment covering 2.9 km²). We credit both the inter‐annual and intra‐annual differences in event water contributions to large rainfall events late in the summer melt season, low glacier snow cover and elevated soil moisture due to large antecedent precipitation. Together amplification of these two mechanisms under a warming climate might influence the timing and magnitude of floods, the sediment budget and nutrient cycling in glacierized catchments. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Evaluation of Zerovalent Zinc for Treatment of 1,2,3‐Trichloropropane‐Contaminated Groundwater: Laboratory and Field Assessment     

Alexandra J. Salter‐Blanc  Eric J. Suchomel  John H. Fortuna  James T. Nurmi  Chris Walker  Tom Krug  Suzanne O'Hara  Nancy Ruiz  Theresa Morley  Paul G. Tratnyek 《Ground Water Monitoring & Remediation》2012,32(4):42-52

The efficacy and feasibility of using zerovalent zinc (ZVZ) to treat 1,2,3‐trichloropropane (TCP)‐contaminated groundwater was assessed in laboratory and field experiments. In the first portion of the study, the reactivity of commercially available granular ZVZ toward TCP was measured in bench‐scale batch‐reactor and column experiments. These results were used to design columns for on‐site pilot‐scale treatment of contaminated groundwater at a site in Southern California. Two of the ZVZ materials tested were found to produce relatively high rates of TCP degradation as well as predictable behavior when scaling from bench‐scale to field testing. In addition, there was little decrease in the rates of TCP degradation over the duration of field testing. Finally, no secondary impacts to water quality were identified. The results suggest that ZVZ may be an effective and feasible material for use in engineered treatment systems, perhaps including permeable reactive barriers.  相似文献   

A risk-mitigation approach to the management of induced seismicity   总被引：2，自引：0，他引：2  

Julian J. Bommer  Helen Crowley  Rui Pinho 《Journal of Seismology》2015,19(2):623-646

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