15 years of very long baseline interferometry observations of two compact radio sources in Messier 82     

R. J. Beswick  J. D. Riley  I. Marti-Vidal  A. Pedlar  T. W. B. Muxlow  A. R. McDonald  K. A. Wills  D. Fenech  M. K. Argo 《Monthly notices of the Royal Astronomical Society》2006,369(3):1221-1228

  相似文献   

Sub-arcsecond atomic hydrogen absorption in the Seyfert galaxies NGC 7674 and NGC 7469     

R. J. Beswick  A. Pedlar  A. R. McDonald 《Monthly notices of the Royal Astronomical Society》2002,335(4):1091-1099

  相似文献   

Measuring a Low Horizontal Hydraulic Gradient in a High Transmissivity Aquifer     

John P. McDonald 《Ground water》2021,59(5):694-709

High transmissivity aquifers typically have low hydraulic gradients (i.e., a flat water table). Measuring low gradients using water levels can be problematic because measurement error may be greater than the true difference in water levels (i.e., a low signal-to-noise ratio). In this study, the feasibility of measuring a hydraulic gradient in the range of 10⁻⁶ to 10⁻⁵ m/m was demonstrated. The study was performed at a site where the depth to water from land surface ranged from 40.1 to 94.2 m and the aquifer transmissivity was estimated at 41,300 m²/d (hydraulic conductivity of 18,800 m/d). The goals of the study were to reduce measurement error as much as practicable and assess the importance of factors affecting water level measurement accuracy. Well verticality was the largest source of error (0.000 to 0.168 m; median of 0.014 m), and geodetic survey of casing elevations was the next most important source of error (0.002 to 0.013 m; median of 0.005 m). Variability due to barometric pressure fluctuations was not an important factor at the site. Hydraulic heads were measured to an accuracy of ±0.0065 m, and the average hydraulic gradient was estimated to be 8.0 × 10⁻⁶ (±0.9 × 10⁻⁶) m/m. The improvement in accuracy allowed for two reversals in the groundwater flow direction to be identified, after which the gradient averaged 2.5 × 10⁻⁵ (±0.4 × 10⁻⁵) m/m. This study showed it is possible to sufficiently control sources of error to measure hydraulic gradients in the 10⁻⁶ to 10⁻⁵ m/m range.  相似文献   

Chemically stabilized soil organic carbon fractions in a reclaimed minesoil chronosequence: implications for soil carbon sequestration   总被引：1，自引：0，他引：1  

Sriroop Chaudhuri  Louis M. McDonald  Eugenia M. Pena-Yewtukhiw  Jeff Skousen  Mimi Roy 《Environmental Earth Sciences》2013,70(4):1689-1698

With adoption of appropriate reclamation strategies, minesoils can sequester significant amount of soil organic carbon (SOC). The objective of this study was to isolate different SOC fractions and coal-C in a reclaimed minesoil chronosequence and assess effects of increasing time since reclamation on each SOC fraction and selected soil properties. The chronosequence was comprised of four minesoils with time since reclamation ranging between 2 and 22 years. Total SOC (TSOC, summation of all SOC fractions), ranged between 20 and 8 g kg^?1, respectively, at the oldest (Mylan Park) and youngest (WVO1) minesite, indicating increasing SOC sequestration along the chronosequence. The humin fraction accounted for about 43 and 7 % of TSOC, respectively, at Mylan Park and WVO1, indicating increasing humification and biochemical stabilization of SOC with increasing time since reclamation. At WVO1, >60 % of TSOC was apportioned among the acid-hydrolysable (labile) and mineral-bound SOC fractions. Total soil carbon (TSC, TSOC + coal-C) were significantly (p < 0.05) related to the humin fraction in older minesoils, whereas with the acid-hydrolysable (labile) fraction in the younger minesoils indicating that C stabilization mechanisms differed substantially along the chronosequence. Coal-C was unrelated to any SOC fraction at all minesites indicating that SOC sequestration estimations in this chronosequence was unaffected by coal-C. Soil cation exchange capacity and electrical conductivity were significantly (p < 0.05) related to the humin fraction at Mylan Park while to the acid-hydrolysable and mineral-bound SOC fractions at WVO1 indicating that the relative influences of different SOC fractions on soil quality indicators differed substantially along the chronosequence.  相似文献   

Environmental Health Injustice: Exposure to Air Toxics and Children's Respiratory Hospital Admissions in El Paso,Texas     

Sara E. Grineski  Timothy W. Collins  Jayajit Chakraborty  Yolanda J. McDonald 《The Professional geographer》2013,65(1):31-46

Although much environmental justice research tacitly assumes that unequal environmental exposures produce geographic disparities in adverse health outcomes, very few empirical environmental justice studies have tested that assumption. This article does so by using estimates of exposure to air toxics disaggregated by emission source (point and mobile) to predict children's hospitalization rates for both asthma and respiratory infections in El Paso, Texas. Air toxics emissions from most source categories were found to be significant predictors of children's respiratory infection hospitalization rates, but not asthma hospitalization rates, at the census tract level. Findings suggest that sociospatial disparities in respiratory infection rates might be linked to environmental inequalities.  相似文献   

Modelling the impact of agroforestry on hydrology of Mara River Basin in East Africa   总被引：1，自引：0，他引：1       下载免费PDF全文

Hosea M. Mwangi  Stefan Julich  Sopan D. Patil  Morag A. McDonald  Karl‐Heinz Feger 《水文研究》2016,30(18):3139-3155

Land‐use change is one of the main drivers of watershed hydrology change. The effect of forestry related land‐use changes (e.g. afforestation, deforestation, agroforestry) on water fluxes depends on climate, watershed characteristics and spatial scale. The Soil and Water Assessment Tool (SWAT) model was calibrated, validated and used to simulate the impact of agroforestry on the water balance in the Mara River Basin (MRB) in East Africa. Model performance was assessed by Nash–Sutcliffe Efficiency (NSE) and Kling–Gupta Efficiency (KGE). The NSE (and KGE) values for calibration and validation were: 0.77 (0.88) and 0.74 (0.85) for the Nyangores sub‐watershed, and 0.78 (0.89) and 0.79 (0.63) for the entire MRB. It was found that agroforestry in the watershed would generally reduce surface runoff, mainly because of enhanced infiltration. However, it would also increase evapotranspiration and consequently reduce baseflow and overall water yield, which was attributed to increased water use by trees. Spatial scale was found to have a significant effect on water balance; the impact of agroforestry was higher at the smaller headwater catchment (Nyangores) than for the larger watershed (entire MRB). However, the rate of change in water yield with an increase in area under agroforestry was different for the two and could be attributed to the spatial variability of climate within the MRB. Our results suggest that direct extrapolation of the findings from a small sub‐catchment to a larger watershed may not always be accurate. These findings could guide watershed managers on the level of trade‐offs that might occur between reduced water yields and other benefits (e.g. soil erosion control, improved soil productivity) offered by agroforestry. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Marine biosecurity: The importance of awareness, support and cooperation in managing a successful incursion response     

Piola RF  McDonald JI 《Marine pollution bulletin》2012,64(9):1766-1773

Shipping is almost certainly the most prevalent human-mediated transport vector for non-indigenous species (NIS) within the marine environment. The Royal Australian Navy (RAN) has long acknowledged the importance of sound environmental management and in recent years has taken a proactive approach to addressing risks associated with marine biosecurity. primarily as a result of biofouling on Navy vessel returning from overseas operations. This paper describes two case studies that highlight the effectiveness of the RAN marine biosecurity management framework in identifying an unwanted marine species on Navy vessels, and the successful biosecurity management program that ensued. In particular, the early detection and identification of a suspect NIS, the quick response to the discovery and the collaborative approach adopted between the RAN and the Government regulatory agency (Western Australian Department of Fisheries) charged with coordinating the incursion response serves as a model for how future incursion responses should be reported and managed.  相似文献   

Bulge–disc decompositions and structural bimodality of Ursa Major cluster spiral galaxies     

Michael McDonald †  Stéphane Courteau  R. Brent Tully 《Monthly notices of the Royal Astronomical Society》2009,393(2):628-640

We present bulge and disc (B/D) decompositions of existing   K '  surface brightness profiles for 65 Ursa Major (UMa) cluster spiral galaxies. This improves upon the disc-only fits of Tully et al. The 1996 disc fits were used by Tully & Verheijen for their discovery of the bimodality of structural parameters in the UMa cluster galaxies. It is shown that our new one-dimensional B/D decompositions yield disc structural parameters that differ only slightly from the basic fits of Tully et al. and evidence for structural bimodality of UMa galaxies is maintained. Our B/D software for the decomposition of one-dimensional surface brightness profiles of galaxies uses a non-linear minimization scheme to recover the best-fitting Sérsic bulge and the exponential disc while accounting for the possible presence of a compact nucleus and spiral arms and for the effects of seeing and disc truncations. In agreement with Tully & Verheijen, we find that the distribution of near-infrared disc central surface brightnesses is bimodal with an F-test confidence of 80 per cent. There is also strong evidence for a local minimum in the luminosity function at     . A connection between the brightness bimodality and a dynamical bimodality, based on new H  i linewidths, is identified. The B/D parameters are presented in Table 1 .