bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactions between clear‐air reflective layers and rain observed with a boundary layer wind profiler
Authors: Cohn S. A. , Rogers R. R. , Jascourt S. , Ecklund W. L. , Carter D. A. , Wilson J. S.
Published: February 2004

We present observations with a 915‐MHz boundary layer profiler of reflective layers in the clear air that persist as light rain falls through them. In the three cases presented a clear‐air echo is initially present over several hours and confined to a horizontal layer a few hundred meters thick. Subsequently, light rain falls through the layer but does not eliminate the spatial pattern of refractive index irregularities which presumably accounts for the clear‐air echo. When the reflectivity of the rain is comparable with that of the clear air, Doppler spectra in the vertical profiler beam have separate modes arising from Rayleigh scattering by the rain and Bragg scattering by the clear air. The modes are distinguishable because of the fall velocity of the drops relative to the air. Using an algorithm to separate the clear air and precipitation components of these bimodal spectra, we can isolate the power and velocity of each mode. This allows us to see interactions of the two backscattering mechanisms. In one puzzling case the clear‐air reflectivity in a layer increases by an order of magnitude as rain falls through it, but in the other cases no such strong effect occurs. We discuss possible causes of the increase in reflectivity but find no convincing explanation.
Geodynamic evolution and thermal history of the central Flin Flon Domain, Trans‐Hudson Orogen: Constraints from structural development, 40Ar/39Ar, and stable isotope geothermometry
Authors: Fedorowich J. S. , Kerrich R. , Stauffer M. R.
Published: February 2004

Shear zones in the Flin Flon area record five episodes of development. These have been studied using structural analysis and 40Ar/39Ar thermochronology. Deformation occurred as a result of collision between the Archean Rae‐Hearne and Archean Superior plates during the Trans‐Hudson Orogeny (1900 to 1700 Ma). The geodynamic evolution encompasses a transition from island arc supracrustal development (∼1900 Ma) to tectonic mountain building under brittleductile conditions (>1860 Ma), two episodes of ductile lineation‐schistosity (L‐S) shear zones on the prograde metamorphic path (1860–1810 Ma), and two episodes of retrograde displacement between 1790 and 1690 Ma, during final suturing of the Archean cratons with Paleoproterozoic assemblages. Metamorphism evolved in three stages. Contact aureoles to granitoid bodies formed at 1860–1840 Ma. Hornblende homfels facies 40Ar/39Ar ages indicate cooling through 500°C at ∼1840 Ma. This was followed by regional heating (burial), which produced regional peak thermal recrystallization at ∼1820–1790 Ma, under lower greenschist to lower amphibolite conditions. Lastly, retrograde metamorphism took place during unroofing between 1790 and 1690 Ma, with a regional cooling rate of 2 °C/m.y.
Late Cretaceous paleomagnetism of the Tombstone district and vicinity: Evidence for a rotational domain boundary in southeastern Arizona
Authors: Hagstrum Jonathan T. , Lipman Peter W. , Moore Richard B.
Published: February 2004

Paleomagnetic data are presented for Late Cretaceous volcanic and intrusive rocks in the Tombstone district and Dos Cabezas Mountains of southeastern Arizona. The resultant paleopole (73°N, 224°E, A95= 8°) is similar to a Late Cretaceous reference pole for North America (82°N, 210°E, A95= 7°), implying that these rocks have not been significantly rotated about vertical or near‐vertical axes relative to the continental interior. Generally, concordant results have also been published for Pennsylvanian terra rosa paleosols within the Black Prince Limestone at localities in the nearby Gunnison Hills, Whetstone Mountains, and Naco Hills. Together, however, these Carboniferous data indicate a 9°±7° clockwise rotation, with respect to North America, similar to that determined for the Colorado Plateau (5°–14°) since the mid‐Cretaceous. Published paleomagnetic data for rocks south and west of the Tombstone vicinity indicate much larger clockwise rotations ranging from ∼25° to ∼60°, implying that a rotational domain boundary runs between the Mustang Mountains and Canelo Hills to the southeast and between the Silver Bell and Tucson Mountains to the northwest. This boundary roughly coincides, in part, with the Sawmill Canyon fault zone and is parallel to other northwest trending strike‐slip faults in the region that were active during the Laramide orogeny. The clockwise rotations in southern Arizona are likely part of a larger pattern of dextral shear along the western margin of North America during the Late Cretaceous and early Tertiary caused by northward oblique subduction of oceanic plates relative to the stable craton.
Cenozoic transtension along the Transantarctic Mountains‐West Antarctic rift boundary, southern Victoria Land, Antarctica
Authors: Wilson Terry J.
Published: February 2004

Brittle fault arrays mapped along the structural boundary between the Transantarctic Mountains and the West Antarctic rift system are oriented obliquely to the axis of the mountains and offshore rift basins. The north to northwest trending regional rift boundary is thus not controlled by continuous rift border faults. Instead, the rift margin trend must be imposed by inherited lithospheric weaknesses along the ancestral East Antarctic craton margin. Fault kinematic solutions indicate that a dextral transtensional regime characterized the rift boundary in the Cenozoic and that dominantly transcurrent motion occurred during the most recent faulting episode. The Transantarctic Mountains are considered to be a rift‐flank uplift, yet no substantial isostatic uplift is expected in a transtensional setting, and the mechanism of large‐magnitude Cenozoic uplift of the mountains remains problematical. Regional deformation patterns in Victoria Land and the Ross Sea can be explained by a transtensional model and are not compatible with large‐magnitude crustal stretching within the West Antarctic rift system in the Cenozoic. The crustal thinning across the rift system more likely took place in the Mesozoic, when major West Antarctic crustal block motions occurred. The Cenozoic intracontinental deformation can be related to plate interaction resulting from the global Eocene plate reorganization, prior to the final separation between Antarctica and a narrow salient of the southeastern Australian margin. Displacement magnitude was probably minor, and thus early Tertiary east–west Antarctic motion is unlikely to account for discrepancies in global plate motion circuits.
Timescales of orogeny: Jurassic construction of the Klamath Mountains
Authors: Hacker Bradley R. , Donato Mary M. , Barnes Calvin G. , McWilliams M. O. , Ernst W. G.
Published: February 2004

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU’s FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 94YCJ2454, Timescales of orogeny: Jurassic construction of the Klamath Mountains, B.R. Hacker, M.M. Donato, C.G. Barnes, M.O. McWilliams, and W.G. Ernst). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.

Classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous. Isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts. Forty‐five new 40Ar/39Ar ages from the Klamath Mountains illuminate the deformation, metamorphism, magmatism, and sedimentation involved in the Jurassic construction of that orogen, leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes. The new geochronologic relationships show that many Jurassic units of the Klamath Mountains had 200 Ma or older volcanoplutonic basement. Subsequent formation of a large ∼170 Ma arc was followed by contractional collapse of the arc. Collision with a spreading ridge may have led to large‐scale NW–SE extension in the central and northern Klamaths from 167 to ∼155 Ma, coincident with the crystallization of voluminous plutonic and volcanic suites. Marked cooling of a large region of the central Klamath Mountains to below ∼350°C at ∼150 Ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels. These data demonstrate that the Klamath Mountains—and perhaps other similar orogens—were constructed during areally and temporally variant episodes of contraction, extension, and magmatism that do not fit classical definitions of orogeny.
Isotopic constraints on the cooling history of the Nanga Parbat‐Haramosh Massif and Kohistan arc, western Himalaya
Authors: George M. , Reddy S. , Harris N.
Published: February 2004

Nineteen 40Ar/39Ar and 22 Rb‐Sr mineral ages are presented for metamorphic rocks and granites collected from the northern part of the Nanga Parbat‐Haramosh Massif (NPHM) and adjacent parts of the Kohistan arc, western Himalaya. The majority of these ages are interpreted as exhumation‐related cooling ages, and they therefore place important constraints on the exhumation history of the region. The 40Ar/39Ar and Rb‐Sr mica cooling ages for areas of the NPHM to the north of the Indus River lie in the range 2–8 Ma and are similar to previously published ages from the Indus gorge itself. Cooling histories for these areas show little systematic variation from east to west, suggesting that the basement may have been exhumed as a relatively rigid block, after the main stages of folding. Mica ages increase dramatically from the NPHM westward into the Kohistan arc, where ages lie in the range 13–30 Ma. The contrast in both cooling ages and the age differential between mineral pairs between the two terranes indicates rapid cooling of the NPHM in the last 10 m.y. compared with the Kohistan arc. The relationship between rates of cooling and exhumation is poorly constrained in regions of rapid uplift, where the characteristics of the evolving geotherm are not well understood. However, the results are consistent with an anomalously rapid exhumation history of the NPHM during the last 10 m.y. compared with the adjacent Kohistan arc. The interpretation of some of the ages documented in this study is problematical. Two biotite samples from contrasting lithologies within the NPHM have yielded higher 87Sr/86Sr ratios than the corresponding whole rocks, but the cause of this isotopic disequilibrium remains unclear. Other biotite samples from the region have consistently yielded younger Rb‐Sr ages than 40Ar/39Ar ages, which may be a reflection of either preferential leaching of 87Sr during fluid infiltration on a regional scale or the presence of excess argon. Alternatively, the relative ages indicate slightly lower closure temperature for strontium diffusion in biotite than for argon diffusion in biotite where samples have undergone rapid cooling.
Tertiary extension in the southern Trento Platform, Southern Alps, Italy
Authors: Zampieri Dario
Published: February 2004

The southern Trento Platform (Southern Alps, NE Italy) was the locus of abundant mafic volcanism associated with Paleogene extensional tectonics. During the Pal eocene and throughout the Eocene, a NNW–SSE trending graben (the Alpone‐Agno Graben (AAG)) developed in the eastern Lessini Mountains. The graben was filled with basaltic volcaniclastics, calcarenites, and lava flows. The AAG is a half‐graben system probably bounded by rotational listric normal faults. Most faults in the graben dip towards the WSW. Paleostress analysis within the AAG shows that during the Eocene the direction of horizontal extension was ENE–WSW. The volcanic activity continued into the Oligocene; however, the relation between Oligocene tectonics and volcanism remains unclear. Gravimetric and magnetic anomalies in conjunction with the crustal thickness variations suggest a locus of lithospheric thinning slightly off‐axis from the AAG. This can be explained by an overall asymmetric extensional geometry dominated by a WSW dipping detachment fault. The Paleogene geodynamic scenario of the Trento Platform suggests that the AAG developed as a foreland extensional response to the active collisional convergence between the European and Adriatic plates. Neogene compressional tectonics reactivated some of the Paleogene normal faults as strike‐sup faults.
Present‐day vertical displacements in the north‐western Alps and southern Jura Mountains: Data from leveling comparisons
Authors: Jouanne F. , Ménard G. , Darmendrail X.
Published: February 2004

Two high‐precision leveling networks were successively surveyed in France, the NGF, measured during the 1886–1907 period, and the IGN69, measured from 1965 to 1979. The accuracy of these levelings (standard deviation of 1.8mm/√km to 3.8mm/√km) allows us to compute the vertical displacements of the benchmarks between two different eras. The results indicate the occurrence of discrete zones of uplift and subsidence: (1) a regional uplift (up to 1.4 mm/yr) of the Subalpine Massifs; (2) an important uplift of the internal Jura (up to 2 mm/yr); (3) a relative subsidence of the southern part of the Jura (0.8 mm/yr); and (4) a relative subsidence of the Bresse Basin with respect to the external Jura. Comparing the spatial distribution of zones of uplift and their respective vertical displacement rates with a regional structural cross section leads to the conclusion that present‐day uplift of the Belledonne and Bornes Massifs and of the internal parts of the Jura Mountains, can be explained by crustal shortening along a major basement‐involving thrust fault. This fault ramps up under the Bornes Massif from a depth of 12 km to 7 km, turns into a flat under the Molasse Basin, ramps up to the top of the basement at the north‐eastern margin of the internal Jura Mountains, and reaches the surface in the external Jura. The Salève ramp‐anticline is carried by a bifurcation of this thrust. Horizontal displacement rates of 6 mm/yr at the Bornes ramp, 2 mm/yr at the Salève ramp, and 4 mm/yr at the internal Jura ramp have been determined by inversion of profiles of uplift rates. Whether this basement‐involving thrust fault was already active during the Miocene main folding phase of the Jura Mountains or whether it was activated only during Pliocene‐Pleistocene times is subject to debate.
Horizontal aquifer movement in a Theis‐Thiem confined system
Authors: Helm Donald C.
Published: February 2004

The skeletal matrix of an aquifer is known to move laterally toward a discharging well. The mechanisms by which it moves relative to the well have been only partially studied in the past. Examining the relation of drawdown to volume strain for an assumed poroelastic solid has been a common approach. By way of contrast, the present study examines the relation of the velocity of solids (aquifer matrix) to their hydraulic driving forces. The present approach can be considered Darcian and allows the displacement field of the aquifer matrix to be calculated directly. A new formulation of the Darcy‐Gersevanov law of water flowing relative to moving solids is applied to a Theis‐Thiem confined aquifer. The aquifer is found to start with a maximum velocity when the pump is turned on and then to decelerate as time progresses. Dimensionless type curves of horizontal movement of the skeletal matrix are calculated. The resulting equation of aquifer deformation is checked against (1) results from alternatively using the standard poroelastic stress‐strain approach and (2) the fundamental storage equation for transient groundwater flow.
A finite element model for simulating runoff and soil erosion from mechanically treated agricultural lands: 2. Field validation and applications
Authors: Sharda V. N. , Singh Sita Ram , Sastry G. , Dhruvanarayana V. V.
Published: February 2004

The finite element model for simulation of runoff and soil erosion as developed by Sharda and Singh (this issue) is evaluated using data collected from agricultural land treated with major mechanical soil and water conservation measures, namely, contour bunding, graded bunding, bench terracing, and conservation bench terracing. The simulated and experimentally realized hydrographs and soil loss values are in reasonably good agreement for various measures. Probable reasons for discrepancies between the predicted and observed values are discussed. The model has the potential of being used on a single storm or a continuous basis provided the soil, crop, and climatic parameters are precisely known or estimated for a given location and for the period under consideration. The model logically simulates the effects of flow, topographic, soil, and crop parameters such as antecedent moisture level, roughness coefficient, saturated hydraulic conductivity, slope, depth of impoundment, size of outlet, longitudinal slope of the channel, vertical interval, and cropping management factor. The model is found to be quite sensitive to changes in roughness coefficient, rainfall excess rate, and cover management factor, and hence these parameters need to be assessed carefully in the field. The general applicability of the model as a planning tool for soil conservation measures and the scope for future development are also discussed.
Variation of Great Lakes water levels derived from Geosat altimetry
Authors: Morris Charles S. , Gill Stephen K.
Published: February 2004

A technique for using satellite radar altimetry data to estimate the temporal variation of the water level in moderate to large lakes and enclosed seas is described. Great Lakes data from the first 2 years of the U.S. Navy's Geosat Exact Repeat Mission (November 1986 to November 1988), for which there is an improved orbit, are used to demonstrate the technique. The Geosat results are compared to the lake level data collected by the Great Lakes Section, National Ocean Service, National Oceanic and Atmospheric Administration, and are found to reproduce the temporal variations of the five major lakes with root‐mean‐square error (rms) ranging from 9.4 to 13.8 cm and a combined average of 11.1 cm. Geosat data are also analyzed for Lake St. Clair, representing a moderate‐sized lake, with a resulting rms of 17.0 cm. During this study period, the water level in the Great Lakes varied in a typical annual cycle of about 0.2 m (0.5 m for Lake Ontario) superimposed on a general decline of approximately 0.5 m. The altimeter data reproduced the general decline reasonably well for all the lakes, but the annual cycle was obscured in some lakes due to systematic errors in the altimeter data. Current and future altimetry missions will have markedly improved accuracy which will permit many moderate (25 km diameter) or larger lakes or enclosed seas to be routinely monitored.
Spatial variability of small‐basin paleoflood magnitudes for a southeastern Arizona mountain range
Authors: Martínez‐Goytre Juan , House P. Kyle , Baker Victor R.
Published: February 2004

Paleohydrological procedures were used to analyze spatial variations in flood magnitude in drainage basins of the Santa Catalina Mountains (maximum elevation 2791 m), southeastern Arizona. Paleoflood reconstructions in eight watersheds, ranging from 10 to 110 km2, were obtained for the largest floods in stable canyon reaches, yielding discharges ranging from 50 to 270 m3 s−1. Although there is a high correlation between drainage area and flood magnitude, it is also clear that basins located in the southern half of the Santa Catalina Mountains have larger unit discharges (peak discharge divided by drainage area) and that these values decrease toward the northern part of the range. This spatial pattern is probably the result of local hydroclimatological conditions whereby the largest floods are associated with the northward influx of moist, tropical, and subtropical air. For this atmospheric circulation the northern part of the range experiences a rain shadow effect that decreases the unit discharges generated in those basins. The unusually high unit discharge for one of the south facing basins is explained by its basin morphometry. Inverse problem solutions using a rainfall‐runoff model demonstrate that both this basin and another with a comparable drainage area but a much lower estimated peak paleoflood discharge can have their peak flood outputs produced by the same storm intensity. The results demonstrate the potential of paleoflood data collection for understanding spatial variability in small basin flood hydrology.
Transport in three‐dimensionally heterogeneous aquifers: 2. Predictions and observations of concentration fluctuations
Authors: Kapoor Vivek , Gelhar Lynn W.
Published: February 2004

For a multidimensional finite‐size impulse input, analytical solutions to the conservation equation for concentration variance σc2 are presented. Due to the dissipating action of local dispersion, at large times, σc is a decreasing fraction of the mean concentration. The Cape Cod bromide tracer exhibits this decrease. The larger the log conductivity microscale is, the slower is the action of local dispersion, and the slower is the predicted rate of decrease of the ratio of σc and the mean concentration (i.e., the coefficient of variation) with time, at large times, and vice versa. The coefficient of variation increases with distance from the center. A balance between the rates of production and dissipation of σc2 relates it linearly to the squared gradients of the mean concentration field, away from the center of mass. For the zero local dispersion case, σc is an unboundedly growing multiple with time of the mean concentration. The longitudinal spatial second moment and macrodispersivities are insensitive to the inclusion/exclusion of local dispersion and therefore do not differentiate between the concentration fields for the two different cases. In contrast, the spatial‐temporal evolution of σc2 is singularly determined by the dissipating action of local dispersion. Measurements of local dispersivities need to be made along with a characterization of hydraulic conductivity variations to assess contaminant concentrations in aquifers.
Transport in three‐dimensionally heterogeneous aquifers: 1. Dynamics of concentration fluctuations
Authors: Kapoor Vivek , Gelhar Lynn W.
Published: February 2004

The concentration variance, i.e., mean squared concentration fluctuations, undergoes mean advection, a local dispersive flux, and a macrodispersive flux due to a correlation between squared concentration perturbations and velocity perturbations. The products of the macrodispersion coefficient and the squared gradient of the mean concentration field determine the rate of production of concentration variance. The rate of dissipation of concentration variance is determined by the product of the local dispersion coefficient and the mean squared gradient of the concentration perturbation field. Variance dissipation is represented as a first‐order decay with the decay coefficient equal to twice the sum of the local dispersion coefficient divided by the squared concentration microscale. The concentration microscale, estimated for an advection‐dominated log hydraulic conductivity microscale, is an increasing function of the log conductivity microscale. Thus the larger the log conductivity microscale is, the slower is the rate of dissipation of concentration fluctuations by local dispersion and vice versa. The wave number squared dependence of fluctuation dissipation requires intensive sampling to realistically model the log conductivity spectrum and its microscale, which determines the rate of dissipation of concentration fluctuations by the action of local dispersion. There is no mechanism of destroying concentration fluctuations without the action of local dispersion.
A watershed modeling analysis of fluvial geomorphologic influences on flood peak attenuation
Authors: Woltemade Christopher J. , Potter Kenneth W.
Published: February 2004

Flood peak attenuation caused by storage of flood water on overbank surfaces effectively reduces the magnitude of peak discharges in some, but not all watersheds. Several geomorphic factors that affect the storage and conveyance of flood water were investigated to assess their quantitative influence on downstream peak discharges. The MIKE11 rainfall‐runoff and hydrodynamic models were calibrated for the Grant River watershed, southwestern Wisconsin. Alternative geomorphic conditions were modeled and compared to the original case. Results indicate that channel‐floodplain‐terrace morphology, valley width, stream slope, and hydraulic roughness each influence peak discharges, especially for moderate magnitude (5‐ to 50‐year recurrence interval) floods. Peak discharges varied by as much as 49% between simulations depending on geomorphic conditions. Watersheds that effectively attenuate produce peak discharges that are strongly correlated with total runoff. Watersheds that attenuate little produce peak discharges that exhibit greater variance due to storm intensity and duration.
Multidimensional steady infiltration with prescribed boundary conditions at the soil surface
Authors: Basha H. A.
Published: February 2004

The present work explores a class of analytical solutions of steady moisture movement in unsaturated porous media characterized by an exponential dependence of the hydraulic conductivity. The Green's function method is used to derive a general analytical model which can handle multidimensional steady infiltration problems in a semi‐infinite medium with arbitrary boundary conditions and root uptake forcing functions and for various simple source geometries. The power of the Green's function method is such that a number of new problems were amenable to analysis and solutions to problems already solved were obtained in a form better suited for numerical computation. Of special importance is the capability of the analytical model to handle the difficult problem of mixed boundary conditions in an approximative fashion. The general solution is expressed in integral form from which particular analytical solutions pertaining to cases of surface and subsurface irrigation, root uptake, and isothermal evaporation can be easily deduced. New solutions include the case of surface sources under a prescribed exponential head and flux distribution, an inverse power flux distribution, and periodic root uptake. The inverse power flux distribution allows the approximate modeling of constant potential sources with evaporation at the nonwetted surfaces. It is shown that all two‐ and three‐dimensional infiltration solutions are essentially integrals of two functions: one dependent on the geometry and distribution of the source and the other dependent on the boundary condition at the land surface. Near‐field and far‐field approximations have also been obtained and simple algebraic equations have been derived for the flux and potential beneath the centerline of the source. The proposed model offers the analyst significant flexibility in deriving results and analyzing infiltration phenomena of practical interest in civil and agricultural engineering.
Genetic algorithm solution of groundwater management models
Authors: McKinney Daene C. , Lin Min‐Der
Published: February 2004

Groundwater simulation models have been incorporated into a genetic algorithm to solve three groundwater management problems: maximum pumping from an aquifer; minimum cost water supply development; and minimum cost aquifer remediation. The results show that genetic algorithms can effectively and efficiently be used to obtain globally (or, at least near globally) optimal solutions to these groundwater management problems. The formulation of the method is straightforward and provides solutions which are as good as or better than those obtained by linear and nonlinear programming. Constraints can be incorporated into the formulation and do not require derivatives with respect to decision variables as in nonlinear programming. More complicated problems, such as transient pumping and multiphase remediation, can be formulated and solved using this method. The computational time required for the solution of genetic algorithm groundwater management models increases with the complexity of the problem. The speedup attainable by solving genetic algorithm problems on massively parallel computers is significant for problems where the simulation time required to complete each generation is high.
Atmospheric deposition of nitrogen and phosphorus in the annual nutrient load of Lake Tahoe (California‐Nevada)
Authors: Jassby Alan D. , Reuter John E. , Axler Richard P. , Goldman Charles R. , Hackley Scott H.
Published: February 2004

Atmospheric deposition provides most of the dissolved inorganic nitrogen (DIN) and total nitrogen in the annual nutrient load of Lake Tahoe. Deposition also contributes significant amounts of soluble reactive phosphorus (SRP) and total phosphorus loading but plays less of a role than in the case of nitrogen. Most of the DIN probably originates outside of the drainage basin in urban and agricultural areas to the south and west. Spatial patterns of SRP deposition differ from those of DIN and suggest a within‐basin terrestrial source, such as leachate from windblown dust or other particles. Because of atmospheric N deposition, the N∶P (molar) ratio in combined loading is well above the Redfield ratio of 16 and consistent with an observed shift from colimitation by N and P to persistent P limitation in the lake phytoplankton.
Aquifer remediation: A method for estimating mass transfer rate coefficients and an evaluation of pulsed pumping
Authors: Harvey Charles F. , Haggerty Roy , Gorelick Steven M.
Published: February 2004

When pumping contaminated water from an aquifer, contaminant concentrations often decline rapidly, only to rebound when the pump is shut off. One reason for this behavior is that contaminants in the mobile phase are readily removed, but mass transfer from the immobile to the mobile phase is rate limited. Pulsed pumping, in which the pump is periodically turned off, has been suggested as a means to enhance remediation. We conducted a comprehensive comparison of pulsed pumping and continuous pumping for the case of a Gaussian plume, subject to first‐order mass transfer during transport, in an aquifer with no regional gradient and constant mass transfer rate coefficients. We developed a Laplace domain Green's function solution for concentrations during pumping periods and coupled it with an analytic solution for concentrations during resting periods. First, we used this model to provide a simple type curve that can be used to estimate the mobile‐immobile phase mass transfer coefficients from field data. Second, the mass and concentration removal histories were determined during pulsed pumping and during continuous pumping. The continuous pumping rate removed the same volume of water over the duration of remediation. We investigated the effects of physical parameters such as the mass transfer rate coefficients, and engineering design parameters such as the length of resting periods. Under the conditions considered, our evaluation shows that (1) for equal volumes of water removed, pulsed pumping does not remove more contaminant mass than pumping continuously at an average rate; (2) if the duration of the resting period is too large, then pulsed pumping removes much less mass than continuous pumping at the average rate, and (3) if the pulsed and continuous pumping rates are the same, pulsed pumping will take longer than continuous pumping to clean up the aquifer, but will require significantly less time during which the pump operates.
An exact nonlinear correction to transverse macrodispersivity for transport in heterogeneous formations
Authors: Dagan Gedeon
Published: February 2004

The random, two‐dimensional, and steady in time Eulerian velocity field is assumed to be given. It pertains to flow through heterogeneous porous media of spatially variable permeability. Advective transport of a passive scalar is modeled by the Lagrangian approach by seeking the statistical moments of the trajectory of a tagged particle. The trajectories' variances and the associated dispersion coefficients are evaluated by a perturbation expansion in the velocity variances and by Corrsin's conjecture. First‐order results obtained in the past show that for flow through porous formations of log‐permeability, two‐point covariance of finite integral scale, the transverse dispersion coefficient tends asymptotically to zero. In contrast, Corrsin's conjecture leads to a finite dispersion coefficient, quadratic in the velocity variance. To investigate this effect, an exact nonlinear correction to the transverse dispersion coefficient is derived for a normal velocity field. It is shown that this term tends to zero asymptotically, in contrast with the result based on Corrsin's conjecture. Furthermore, an illustrative computation shows the transport‐related nonlinear correction to be small at any time.
Water movement in glass bead porous media: 2. Experiments of infiltration and finger flow
Authors: Lu T. X. , Biggar J. W. , Nielsen D. R.
Published: February 2004

This paper presents experimental observations of infiltration and finger flow in glass beads. In paper 1 (Lu et al., this issue), we showed that the total surface tensile force is much greater in initially wet profiles than in initially dry profiles. During capillary rise in glass beads, the “jump” process takes place for an initially dry condition, whereas in an initially wet profile not only a jump process but a film thickening associated with film flow characterizes capillary rise. In this paper, infiltration experiments into initially dry glass beads show that the wetting front is relatively saturated and flat compared with the unsaturated and irregular wetting front into an initially wet profile. In the experiments of finger flow, photographs show that the tip of the finger is completely water saturated and that no partially saturated zones exist around the saturated tip. The fingers initiated in a dry zone disappear when they reach an initially wet lower zone even when the packing conditions of the glass beads are identical. Hence the criterion for instability when water is applied at a rate less than the value of the saturated hydraulic conductivity does not apply to an initially wet condition. When a fine layer of glass beads lies on a coarse layer that is initially dry, fingering will take place during infiltration and flow is unstable. If the coarse lower layer is initially wet, finger flow does not develop, and the flow remains stable. More investigations are required to ascertain threshold values of the initial water content causing instability of water movement in porous media.
Modeling of reactive groundwater transport governed by biodegradation
Authors: Zysset Andreas , Stauffer Fritz , Dracos Themistocles
Published: February 2004

Organic and inorganic compounds infiltrating into groundwater are subject to redox reactions catalyzed by bacteria. These bacteria tend to form conglomerates and to adsorb to the solid matrix. Assuming a biofilm concept, the mass transport in and near such a biofilm is a controlling factor for different processes affecting the microbial activity, such as the supply of the substrates to the bacteria or the detachment of biofilm parts by shear forces. We present here a macroscopic model for the transport of dissolved substances in groundwater‐biofilrn systems. This model conceptualizes the diffusion dominated microscopic transport processes within the biofilm by using a logistic approach based on a dififusional limitation. As biological processes, the growth, the maintenance, and the decay of the bacterial population are explicitly accounted for. The model is formulated for a one‐dimensional flow and transport field and is numerically solved with an operator splitting technique as described by Zysset et al. (1994). It is able to reproduce the transport of nitrate and sulfate in two laboratory column experiments in which these substances served as dissolved electron acceptors in bacterially mediated redox reactions.
The Southern Oscillation index as a predictor of the probability of low streamflows in New Zealand
Authors: Moss M. E. , Pearson C. P. , McKerchar A. I.
Published: February 2004

The Southern Oscillation index (SOI) can yield information about subsequent streamflows on the South Island of New Zealand; however, the relationship between streamflow and SOI may be nonlinear and heteroscedastic. To deal with such difficulties, the conditional probability of streamflow's being below a critical magnitude, given a prior observation of SOI, can be considered a random variable, with its probability density function (pdf) estimated by Bayesian analysis of existing observations of streamflow and SOI. The conditional pdf can yield a probabilistic forecast of critically low streamflows given a precursor value of SOI. In the Clutha River basin of New Zealand, average austral summer inflow to the headwater lakes has an unconditional probability of nonexceedance of 360 m3/s of approximately 17%. However, during a moderate La Niña, average austral spring SOI = 12, the conditional probability of nonexceedance has an expected value of 55%. For an El Niño with a spring SOI of −12, the expected probability of nonexceedance is 18%; for a neutral SOI of 0, the expected probability is 10%. Thus, the probability of nonexceedance of seasonal streamflow can be seen to vary by a factor of more than 5 as a function of the SOI.
Simulation of surfactant‐enhanced aquifer remediation
Authors: Brown Chris L. , Pope Gary A. , Abriola Linda M. , Sepehrnoori Kamy
Published: February 2004

Surfactant‐enhanced aquifer remediation (SEAR) is currently under active investigation as one of the most promising alternatives to conventional pump‐and‐treat remediation for aquifers contaminated by dense nonaqueous phase organic liquids. An existing three‐dimensional finite‐difference enhanced oil recovery simulator is adapted to model the SEAR process. This simulator incorporates the complex chemistry and multiphase transport behavior of surfactant/water/organic mixtures in permeable media. Model governing equations and parameter requirements are discussed, and simulations are employed to illustrate some important issues potentially affecting SEAR performance at the field scale. Simulations suggest that the total time for remediation could be reduced by more than an order of magnitude over conventional remediation approaches by employing SEAR. The assumptions, approximations, and conditions required to achieve such a favorable result are identified, and the importance of modeling as a quantitative tool for the assessment of SEAR is highlighted.
Self‐organized river basin landscapes: Fractal and multifractal characteristics
Authors: Rodriguez‐Iturbe Ignacio , Marani Marco , Rigon Riccardo , Rinaldo Andrea
Published: February 2004

In recent years a new theory of the evolution of drainage networks and associated landscapes has emerged, mainly in connection with the development of fractal geometry and of self‐organized criticality (SOC) concepts. This theory has much improved our understanding of the mechanisms which determine the structure of natural landscapes and their dynamics of evolution. In the first part of this paper the main ideas in the theory of landscape self‐organization are outlined, and some remarkable features of the resulting structures are presented. In the second part we apply theoretical tools developed in the context of multifractal fields to the study of the scaling properties of the field of elevations of SOC landscapes. We observe that such landscapes appear to be more complex than simple self‐affine fractals, although in some cases a simple fractal framework may be adequate for their description. We also show that multiple scaling may emerge as a result of heterogeneity in the field properties reflecting climate and geology.
Learning disaggregation technique for the operation of long‐term hydroelectric power systems
Authors: Saad M. , Turgeon A. , Bigras P. , Duquette R.
Published: February 2004

This paper describes a nonlinear disaggregation technique for the operation of multireservoir systems. The disaggregation is done by training a neural network to give, for an aggregated storage level, the storage level of each reservoir of the system. The training set is obtained by solving the deterministic operating problem of a large number of equally likely flow sequences. The training is achieved using the back propagation method, and the minimization of the quadratic error is computed by a variable step gradient method. The aggregated storage level can be determined by stochastic dynamic programming in which all hydroelectric installations are aggregated to form one equivalent reservoir. The results of applying the learning disaggregation technique to Quebec's La Grande river are reported, and a comparison with the principal component analysis disaggregation technique is given.
Hydrodynamic description for advective sediment transport processes and rill initiation
Authors: Loewenherz‐Lawrence D. S.
Published: February 2004

A physically based model for advective sediment transport during rill initiation is developed based on (1) a short‐wavelength rescaling of the momentum balance equations for the surface water flux; (2) a deforming coordinate system which characterizes local morphological changes of the surface during erosion; and (3) a generalized transport function for the surface sediment flux which distinguishes the contributions of diffusive and advective transport. A linear stability analysis of the coupled system indicates that an intrinsic minimum transverse length scale for rill initiation is forced by the cross‐slope pressure gradient in the surface water. The wavelength of this initial rill spacing is dependent upon the relative significance of the advective component of sediment transport and the hydraulic efficiency of the surface water flux and is independent of the total length of the hillslope. The hydrodynamic model presented can also be used to evaluate the nonlinear development of rills and rill networks, subsequent to initiation.
On determining resistance to interrill overland flow
Authors: Parsons Anthony J. , Abrahams Athol D. , Wainwright John
Published: February 2004

Simulated rainfall was applied to runoff plots of four sizes at three sites to generate overland flow. The hydraulic resistance to the flow was assessed using the Darcy‐Weisbach friction factor which was calculated both from direct measurement of discharge and depth of flow and by fitting model hydrographs based on the kinematic wave equations to the rising limbs of the runoff hydrographs. Results show that there is no consistent variation in the friction factor as a function either of plot size or of the method used in its calculation. However, the direct measurement of depth and discharge appears to give more reliable results. Values for the Darcy‐Weisbach friction factor observed here are an order of magnitude greater than those obtained by Abrahams et al. (1994) in experiments on nearby plots but where runoff was generated by flow onto the upper end of the plots from a trickle pipe. It is concluded that determination of friction factors to be used in modeling runoff and soil erosion must be made by direct measurement of flow hydraulics of rain‐induced overland flow rather than from the runoff hydrograph or by using trickle‐induced flow.
Transport of Atmospheric Trace Gases to the Water Table: Implications for Groundwater Dating with Chlorofluorocarbons and Krypton 85
Authors: Cook P. G. , Solomon D. K.
Published: February 2004

Chlorofluorocarbons and krypton 85 are trace gases whose atmospheric concentrations have been increasing over the past few decades. As they are soluble in water, they have been used as groundwater age indicators over timescales ranging from a few years to a few decades. In this paper we show that the time lag for transport of these atmospheric trace gases through the unsaturated zone is an important consideration when dating groundwaters that are recharged through thick unsaturated zones. The apparent time lag is largely dependent on the gas solubility, the gas diffusion coefficient, and the soil water content. In coarse‐grained soils the lag time will typically range between 1 and 2 years for a water table depth of 10 m to between 8 and 15 years for a water table depth of 30 m. For the shallower water tables (<10 m) the effect is not significant, as it is within the error of the dating methods. However, for deeper water tables (>10 m), if this effect is not considered, the use of these gaseous tracers will overestimate the age of the groundwater. In very fine‐grained soils where the soil water content in the unsaturated zone may be close to saturation, the effect will be much more pronounced.
Eulerian‐Lagrangian Analysis of Transport Conditioned on Hydraulic Data: 4. Uncertain Initial Plume State and Non‐Gaussian Velocities
Authors: Zhang Dongxiao , Neuman Shlomo P.
Published: February 2004

In previous papers of this series we described an analytical‐numerical method to predict deterministically solute transport under uncertainty based on a new Eulerian‐Lagrangian theory. We examined the effect that conditioning on hydraulic data has on predicted velocities and concentrations due to instantaneous point and nonpoint sources and discussed the same effect on spatial plume moments, total mass flow rate across a “compliance surface,” cumulative mass release across this surface, and the corresponding travel time distribution. Our analysis to date assumed that the initial state of the plume (the source term or initial concentration) is known with certainty and that the groundwater velocity field is Gaussian. In reality, the initial state of the plume is almost never known with certainty, especially when this state is inferred by sampling a plume at some arbitrary time t0 following introduction of the solute into the subsurface (as is the case at many contaminated sites). Likewise, Monte Carlo simulations have shown that in Gaussian log hydraulic conductivity or log transmissivity fields, of the kind often indicated by in situ hydraulic test data, the longitudinal velocity becomes rapidly lognormal as the variance of the heterogeneities increases. In this paper we show how to handle both of these complications by means of our analytical‐numerical method of computation. By revisiting our previous examples, we explore the effects that uncertainty in plume initial state and non‐Gaussian velocities may have on predicted plume concentrations and uncertainty due to instantaneous point and nonpoint sources with and without conditioning on measurements of log transmissivity or hydraulic head in two dimensions. One of our findings is that the more common Gaussian models are nonconservative (in a regulatory sense) when applied during early dimensionless times to plumes that are initially short in the direction of mean flow.