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771.
Alan R. Hill 《水文研究》2012,26(20):3135-3146
The effect of preferential flow in soil pipes on nitrate retention in riparian zones is poorly understood. The characteristics of soil pipes and their influence on patterns of groundwater transport and nitrate dynamics were studied along four transects in a 1‐ to >3‐m deep layer of peat and marl overlying an oxic sand aquifer in a riparian zone in southern Ontario, Canada. The peat‐marl deposit, which consisted of several horizontal layers with large differences in bulk density, contained soil pipes that were generally 0.1 to 0.2 m in diameter and often extended vertically for 1 to >2 m. Springs that produced overland flow across the riparian area occurred at some sites where pipes extended to the peat surface. Concentrations of NO3?–N (20–30 mg L?1) and dissolved oxygen (DO) (4–6 mg L?1) observed in peat pipe systems and surface springs were similar to values in the underlying sand aquifer, indicating that preferential flow transported groundwater with limited nitrate depletion. Low NO3?–N concentrations of <5 mg L?1 and enriched δ15N values indicated that denitrification was restricted to small areas of the peat where pipes were absent. Groundwater DO concentrations declined rapidly to <2 mg L?1 in the peat matrix adjacent to pipes, whereas high NO3?–N concentrations of >15 mg L?1 extended over a larger zone. Low dissolved organic carbon values at these locations suggest that supplies of organic carbon were not sufficient to support high rates of denitrification, despite low DO conditions. These data indicate that it is important to develop a greater understanding of pipes in peat deposits, which function as sites where the transport of large fluxes of water with low biogeochemical reaction rates can limit the nitrate removal capacity of riparian zones. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
772.
Evaluation of the MODFLOW-2005 Conduit Flow Process 总被引:1,自引:0,他引:1
The recent development of the Conduit Flow Process (CFP) by the U.S. Geological Survey (USGS) provides hydrogeologic modelers with a new tool that incorporates the non-Darcian, multiporosity components of flow characteristic of karst aquifers. CFP introduces new parameters extending beyond those of traditional Darcian groundwater flow codes. We characterize a karst aquifer to collect data useful for evaluating this new tool at a test site in west-central Florida, where the spatial distribution and cross-sectional area of the conduit network are available. Specifically, we characterize: (1) the potential for Darcian/non-Darcian flow using estimates of specific discharge vs. observed hydraulic gradients, and (2) the temporal variation for the direction and magnitude of fluid exchange between the matrix and conduit network during extreme hydrologic events. We evaluate the performance of CFP Mode 1 using a site-scale dual-porosity model and compare its performance with a comparable laminar equivalent continuum model (ECM) using MODFLOW-2005. Based on our preliminary analyses, hydraulic conductivity coupled with conduit wall conductance improved the match between observed and simulated discharges by 12% to 40% over turbulent flow alone (less than 1%). 相似文献
773.
M. J. Coe A. J. Bird A. B. Hill V. A. McBride M. Schurch J. Galache C. A. Wilson M. Finger D. A. Buckley E. Romero-Colmenero 《Monthly notices of the Royal Astronomical Society》2007,378(4):1427-1433
Multiwavelength observations are reported here of the Be/X-ray binary pulsar system GRO J1008−57. Over ten years worth of data are gathered together to show that the periodic X-ray outbursts are dependant on both the binary motion and the size of the circumstellar disc. In the first instance an accurate orbital solution is determined from pulse periods, and in the second case the strength and shape of the Hα emission line is shown to be a valuable indicator of disc size and its behaviour. Furthermore, the shape of the emission line permits a direct determination of the disc size which is in good agreement with theoretical estimates. A detailed study of the pulse period variations during outbursts determined the binary period to be 247.8 ± 0.4 d, in good agreement with the period determined from the recurrence of the outbursts. 相似文献
774.
Rodney A. Sheets Mary C. Hill Henk M. Haitjema Alden M. Provost John P. Masterson 《Ground water》2015,53(1):151-157
Simulating groundwater flow in a water‐table (unconfined) aquifer can be difficult because the saturated thickness available for flow depends on model‐calculated hydraulic heads. It is often possible to realize substantial time savings and still obtain accurate head and flow solutions by specifying an approximate saturated thickness a priori, thus linearizing this aspect of the model. This specified‐thickness approximation often relies on the use of the “confined” option in numerical models, which has led to confusion and criticism of the method. This article reviews the theoretical basis for the specified‐thickness approximation, derives an error analysis for relatively ideal problems, and illustrates the utility of the approximation with a complex test problem. In the transient version of our complex test problem, the specified‐thickness approximation produced maximum errors in computed drawdown of about 4% of initial aquifer saturated thickness even when maximum drawdowns were nearly 20% of initial saturated thickness. In the final steady‐state version, the approximation produced maximum errors in computed drawdown of about 20% of initial aquifer saturated thickness (mean errors of about 5%) when maximum drawdowns were about 35% of initial saturated thickness. In early phases of model development, such as during initial model calibration efforts, the specified‐thickness approximation can be a very effective tool to facilitate convergence. The reduced execution time and increased stability obtained through the approximation can be especially useful when many model runs are required, such as during inverse model calibration, sensitivity and uncertainty analyses, multimodel analysis, and development of optimal resource management scenarios. 相似文献
775.
P. Elebert P. J. Callanan A. V. Filippenko P. M. Garnavich G. Mackie J. M. Hill V. Burwitz 《Monthly notices of the Royal Astronomical Society》2008,383(4):1581-1587
We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1−2455. Our R -band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Hα emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km s−1 , placing a lower limit of 0.05 M⊙ on the secondary mass. For a 1.4 M⊙ primary, this implies that the orbital inclination is low, ≲20°. Utilizing the observed relationship between the secondary mass and the orbital period in short-period cataclysmic variables, we estimate the secondary mass to be ∼0.085 M⊙ , which implies an upper limit of ∼2.4 M⊙ for the primary mass. 相似文献
776.
M. E. C. Swanson Max Tegmark rew J. S. Hamilton J. Colin Hill 《Monthly notices of the Royal Astronomical Society》2008,387(4):1391-1402
As galaxy surveys become larger and more complex, keeping track of the completeness, magnitude limit and other survey parameters as a function of direction on the sky becomes an increasingly challenging computational task. For example, typical angular masks of the Sloan Digital Sky Survey contain about N = 300 000 distinct spherical polygons. Managing masks with such large numbers of polygons becomes intractably slow, particularly for tasks that run in time with a naive algorithm, such as finding which polygons overlap each other. Here we present a 'divide-and-conquer' solution to this challenge: we first split the angular mask into pre-defined regions called 'pixels', such that each polygon is in only one pixel, and then perform further computations, such as checking for overlap, on the polygons within each pixel separately. This reduces tasks to , and also reduces the important task of determining in which polygon(s) a point on the sky lies from to , resulting in significant computational speedup. Additionally, we present a method to efficiently convert any angular mask to and from the popular healpix format. This method can be generically applied to convert to and from any desired spherical pixelization. We have implemented these techniques in a new version of the mangle software package, which is freely available at http://space.mit.edu/home/tegmark/mangle/ , along with complete documentation and example applications. These new methods should prove quite useful to the astronomical community, and since mangle is a generic tool for managing angular masks on a sphere, it has the potential to benefit terrestrial mapmaking applications as well. 相似文献
777.
Frank Hill 《Journal of Astrophysics and Astronomy》2008,29(1-2):75-84
A major goal of helioseismology is to understand the mechanism of the solar cycle. In this paper, some results of helioseismic observations relevant to the cycle are briefly reviewed, the current state-of-the-art is discussed, and near-term future directions are sketched out. Topics covered include the internal rotation rate; activity-related parameter variations; the tachocline; far-side imaging; the torsional oscillation; and meridional flows. 相似文献
778.
This paper investigates the instability of Poiseuille flow in a fluid overlying a highly porous material. A two layer approach is adopted, where the Darcy–Brinkman equation is employed to describe the fluid flow in the porous medium, with a tangential stress jump boundary condition at the porous/fluid interface. The basic velocity profiles are continuous due to the interfacial boundary conditions. It is shown that for certain parameter ranges the neutral curves are no longer bimodal, such that the two modes of instability corresponding to the fluid and porous layers, respectively, are not distinct. 相似文献
779.
Bernard Barnier Gurvan Madec Thierry Penduff Jean-Marc Molines Anne-Marie Treguier Julien Le Sommer Aike Beckmann Arne Biastoch Claus Böning Joachim Dengg Corine Derval Edmée Durand Sergei Gulev Elizabeth Remy Claude Talandier Sébastien Theetten Mathew Maltrud Julie McClean Beverly De Cuevas 《Ocean Dynamics》2009,59(3):537-537
780.
Jonathan D. Hill Martin A. Uman Michael Stapleton Douglas M. Jordan Alexander M. Chebaro Christopher J. Biagi 《Journal of Atmospheric and Solar》2010,72(13):913-925
We describe attempts to create ball lightning by directing lightning, triggered from natural thunderclouds using the rocket-and-wire technique, through a variety of materials. Some of the observed phenomena have features in common with natural ball lightning or with laboratory attempts to create it: flame-like luminosity for up to 0.5 s above salt water; constant-luminosity silicon fragments falling for about 1 s under the influence of gravity; a 0.7 m region of stationary luminosity whose bottom was 0.3 m above a stainless steel surface to which arcing had occurred; and a glow for about 0.5 s above pine tree sections. 相似文献