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排序方式: 共有1273条查询结果,搜索用时 15 毫秒
121.
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The effect of drawdown on suspended solids and phosphorus export from Columbia Lake,Waterloo, Canada
Mike Shantz Elizabeth Dowsett Emma Canham Guillaume Tavernier Mike Stone Jonathan Price 《水文研究》2004,18(5):865-878
This study examines the effect of drawdown on the timing and magnitude of suspended solids and associated phosphorus export from a 12 ha reservoir located in an urbanized watershed in southern Ontario, Canada. Water level in Columbia Lake was lowered by 1·15 m over a 2‐week period in November 2001. The total phosphorus (TP) concentrations ranged from 63 to 486 µg L?1 in Columbia Lake and 71 to 373 µg L?1 at its outflow. All samples exceeded the Provincial Water Quality Objective of 30 µg TP L?1. Outflow concentrations of suspended solids and TP increased significantly with decreasing lake level and were attributed to the resuspension of cohesive bottom sediments that occurred at a critical threshold lake level (0·65 m below summer level). Suspended solids at the outflow consisted of flocculated cohesive materials with a median diameter (D50) of c. 5 µm. Particulate organic carbon accounted for 8·5% of the suspended solids export by mass. A total mass of 18·5 t of suspended solids and 62·6 kg TP was exported from Columbia Lake, which represents a significant pulse of sediment‐associated P to downstream environments each autumn during drawdown. The downstream impacts of this release can be minimized if the water level in Columbia Lake is lowered no more than 0·5 m below summer levels. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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Alexandre Faure Jonathan Tennyson 《Monthly notices of the Royal Astronomical Society》2001,325(1):443-448
Molecular R -matrix calculations are performed to give rotational excitation rates for electron collisions with linear molecular ions. Results are presented for CO+ , HCO+ , NO+ and H2 + up to electron temperatures of 10 000 K. De-excitation rates and critical electron densities are also given. It is shown that the widely used Coulomb–Born approximation is valid for Δ j =1 transitions when the molecular ion has a dipole greater than about 2D, but otherwise is not reliable for studying electron-impact rotational excitation. In particular, transitions with Δ j >1 are found to have appreciable rates and are found to be entirely dominated by short-range effects. 相似文献
124.
Graham Jonathan D. López Ariste Arturo Socas-Navarro Hector Tomczyk Steven 《Solar physics》2002,208(2):211-232
We investigate the diagnostic potential of polarimetric measurements with filtergraph instruments. Numerical simulations are used to explore the possibility of inferring the magnetic field vector, its filling factor, and the thermodynamics of model atmospheres when only a few wavelength measurements are available. These simulations assume the magnetic Sun to be represented by Milne–Eddington atmospheres. The results indicate that two wavelength measurements are insufficient to reliably determine the magnetic parameters, regardless of whether magnetograph techniques or least-squares fitting inversions are used. However, as few as four measurements analyzed with the inversion technique provide enough information to retrieve the intrinsic magnetic field with an accuracy better than 10% in most cases. 相似文献
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Galateia Terti Isabelle Ruin Sandrine Anquetin Jonathan J. Gourley 《Natural Hazards》2015,79(3):1481-1497
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Decadal‐Scale Changes of the Ödenwinkelkees,Central Austria,Suggest Increasing Control of Topography and Evolution Towards Steady State
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Jonathan L. Carrivick Katie Berry Martin Geilhausen William H.M. James Christopher Williams Lee E. Brown David M. Rippin Steve J. Carver 《Geografiska Annaler: Series A, Physical Geography》2015,97(3):543-562
Small mountain glaciers have short mass balance response times to climate change and are consequently very important for short‐term contributions to sea level. However, a distinct research and knowledge gap exists between (1) wider regional studies that produce overview patterns and trends in glacier changes, and (2) in situ local scale studies that emphasise spatial heterogeneity and complexity in glacier responses to climate. This study of a small glacier in central Austria presents a spatiotemporally detailed analysis of changes in glacier geometry and changes in glaciological behaviour. It integrates geomorphological surveys, historical maps, aerial photographs, airborne LiDAR data, ground‐based differential global positioning surveys and Ground Penetrating Radar surveys to produce three‐dimensional glacier geometry at 13 time increments spanning from 1850 to 2013. Glacier length, area and volume parameters all generally showed reductions with time. The glacier equilibrium line altitude increased by 90 m between 1850 and 2008. Calculations of the mean bed shear stress rapidly approaching less than 100 kPA, of the volume–area ratio fast approaching 1.458, and comparison of the geometric reconstructions with a 1D theoretical model could together be interpreted to suggest evolution of the glacier geometry towards steady state. If the present linear trend in declining ice volume continues, then the Ödenwinkelkees will disappear by the year 2040, but we conceptualise that non‐linear effects of bed overdeepenings on ice dynamics, of supraglacial debris cover on the surface energy balance, and of local topographically driven controls, namely wind‐redistributed snow deposition, avalanching and solar shading, will become proportionally more important factors in the glacier net balance. 相似文献
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129.
Julien?LeutholdEmail author Jonathan?D.?Blundy Richard?A.?Brooker 《Contributions to Mineralogy and Petrology》2015,169(2):12
The 176Lu–176Hf and 147Sm–143Nd decay systems are routinely used to determine garnet (Grt)–whole-rock (WR) ages; however, the 176Lu–176Hf age of garnet is typically older than the 147Sm–143Nd age determined from the same aliquots. Here we present experimental data for Lu3+ and Hf4+ diffusion in garnet as functions of temperature, pressure and oxygen fugacity and show that the diffusivity of Hf4+ in almandine/spessartine garnet is significantly slower than that of Lu3+. The diffusive closure temperature (T C) of Hf4+ is significantly higher than that of Nd3+, and although this property is partly responsible for the observed 176Lu–176Hf and 147Sm–143Nd Grt–WR age discrepancies, the difference between the T C-s of Lu3+ and Hf4+ could lead to apparent Grt–WR 176Lu–176Hf ages that are skewed from the age of Hf4+ closure in garnet. In addition, the slow diffusivity of Hf4+ indicates that the bulk of metamorphic garnets retain a substantial fraction of prograde radiogenic 176Hf throughout peak metamorphic conditions, a phenomenon that further complicates the interpretation of 176Lu–176Hf garnet ages and invalidates the use of analytical T C expressions. We argue that the diffusion of trivalent rare earth elements in garnet becomes much faster when their concentration level falls below a few hundred ppm, as in the experiments of Tirone et al. (Geochim Cosmochim Acta 69: 2385–2398, 2005), and further argue that this low-concentration mechanism is appropriate for modeling the susceptibility of 147Sm–143Nd garnet ages to diffusive resetting. 相似文献
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