bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of biogenic gas bubbles on water flow through poorly decomposed blanket peat
Authors: Beckwith Clive W. , Baird Andrew J.
Published: February 2004

A laboratory investigation was used to determine whether biogenic gas bubbles accumulate and block water‐conducting pores below the water table in poorly decomposed Sphagnum peat. We found that biogenic gas bubbles did accumulate under realistic incubation temperatures. At the end of incubations at 10.5°C, volumetric water contents in two peat samples decreased to between 0.8 and 0.85 (porosity of the samples ranged from 0.96 to 0.97), indicating that the peat was considerably undersaturated with respect to water. Methane was found to be an important constituent of the gas bubbles. The presence of gas bubbles appeared to have a major effect on hydraulic conductivity (K). In control incubations, prior to which the peat had been irradiated and dosed with a biocide, biogenic gas bubbles did not accumulate, and K was 5–8 times higher than at the end of the microbially active incubations. Our results suggest that biogenic gas bubbles have a potentially significant effect on seepage in peat soils.
Historical effects of El Nino and La Nina events on the seasonal evolution of the montane snowpack in the Columbia and Colorado River Basins
Authors: Clark Martyn P. , Serreze Mark C. , McCabe Greg J.
Published: February 2004

Snow‐water equivalent (SWE) data measured at several hundred montane sites in the western United States are used to examine the historic effects of El Nino and La Nina events on seasonal snowpack evolution in the major subbasins in the Columbia and Colorado River systems. Results are used to predict annual runoff. In the Columbia River Basin, there is a general tendency for decreased SWE during El Nino years and increased SWE in La Nina years. However, the SWE anomalies for El Nino years are much less pronounced. This occurs in part because midlatitude circulation anomalies in El Nino years are located 35° east of those in La Nina years. This eastward shift is most evident in midwinter, at which time, SWE anomalies associated with El Nino are actually positive in coastal regions of the Columbia River Basin. In the Colorado River Basin, mean anomalies in SWE and annual runoff during El Nino years depict a transition between drier‐than‐average conditions in the north, and wetter‐than‐average conditions in the southwest. Associations during La Nina years are generally opposite those in El Nino years. SWE anomalies tend to be more pronounced in spring in the Lower Colorado River Basin. Our predictions of runoff reveal modest skill for scenarios using only historic El Nino and La Nina information. Predictions based on the water stored in the seasonal snowpack are, in almost all cases, much higher than those based on El Nino‐Southern Oscillation (ENSO) information alone. However, combining observed midwinter snow conditions with information on seasonal snowpack evolution associated with ENSO improves predictions for basins in which ENSO signals exhibit strong seasonality.
Macrodispersion in a radially diverging flow field with finite Peclet Numbers: 2. Homogenization theory approach
Authors: Attinger S. , Neuweiler I. , Kinzelbach W.
Published: February 2004

We study the transport behavior of a tracer in a radially diverging heterogeneous velocity field. Making use of homogenization theory, we derive effective transport equations. These effective transport equations are very similar to those defined on the local scale. However, the local transport parameters such as local dispersion coefficients are replaced by effective dispersion coefficients. For smoothly varying heterogeneous media, explicit results for effective radial dispersion coefficients are derived. Starting with the purely advective transport behavior (infinite Peclet numbers), we extend our calculations to transport with finite Peclet numbers. We find that the impact of molecular diffusion on the effective dispersivity differs from the impact of local dispersion: Including local dispersion leads to effective dispersivities which are constant and equivalent to the effective dispersivities found in uniform flow configurations. In contrast, effective dispersivities including diffusion are not constant but depend on the radial distance. We compare the results found by homogenization theory with those derived by Neuweiler et al. (this issue) by standard method of moments.
Theoretical basis for the measurement of local transverse dispersion in isotropic porous media
Authors: Cirpka O. A. , Kitanidis P. K.
Published: February 2004

Spreading of a conservative solute in potential flow in a helical porous medium is dominated by the radial velocity distribution within the domain. Higher velocities at the inside of the helix compared to the outside leads to spreading that is balanced by transverse pore‐scale dispersion. The angular moments of the concentration distribution exhibit the typical behavior of Taylor‐Aris dispersion in which the rate of change of the second central moment is inversely proportional to pore‐scale transverse dispersion. We present numerical results for the angular moments over the entire time range and derive analytical expressions for the angular macrodispersion coefficient at the large time limit. We propose a method in which macrodispersion is measured in a helix filled with the porous material of interest in order to infer pore‐scale transverse dispersivity values that are difficult to determine with existing methods.
Stomatal control of transpiration: Examination of the Jarvis‐type representation of canopy resistance in relation to humidity
Authors: Lhomme J.‐P.
Published: February 2004

The Jarvis‐type parameterization of canopy resistance is commonly used to model the canopy energy balance in large‐scale applications. In its most complete form it is written as a minimal stomatal resistance multiplied by five stress functions involving solar radiation F1(S), air temperature F2(T), air saturation deficit F3(D), soil or plant water status F4(Ψ), and CO2 concentration of the air F5(C), respectively. One or several functions, however, can be missing according to the experimental conditions or the assumptions made for the modeling process. This general scheme is examined in relation to environmental humidity to determine the exact correspondence between the mathematical representation and the physiological response in terms of canopy resistance and actual transpiration. The strict feedback response, i.e., the response to air or soil humidity through plant water status, is simulated by means of the sole function F4 involving leaf water potential Ψl (without the need for another stress function related to environmental humidity). In this case, canopy resistance increases with increased saturation deficit, and transpiration also increases. When soil water potential decreases, canopy resistance increases and transpiration falls. A feedforward response to air humidity, i.e., a direct response of stomata independent of plant water status, is modeled through the stress function F3(D), with or without the function F4(Ψl), depending on whether the feedforward response is combined with or without a feedback effect. In this latter case, canopy resistance increases with an increased saturation deficit, while the transpiration rate increases up to a threshold and then falls. A strict feedforward response to soil moisture is modeled through function F4 in which soil water potential replaces leaf water potential. When the canopy resistance formulation includes a feedback response, i.e., when the function F4(Ψ) is used with leaf water potential Ψl, Jarvis' scheme is equivalent to the parameterization developed by Monteith (as a function of the rate of transpiration), and the transpiration rate can be rewritten as a simple function of the climatic demand, the soil water availability, and the decoupling factor Ω.
Nonlocal reactive transport in heterogeneous dual‐porosity media with rate‐limited sorption and interregional mass diffusion
Authors: Huang Hai , Hu Bill X.
Published: February 2004

Huang and Hu [2000] developed a nonlocal, first‐order, Eulerian theory for the mean concentration of a conservative tracer in a dual‐porosity medium. Here the results of Huang and Hu are extended to reactive chemical transport under linear nonequilibrium sorption in both mobile and immobile zones. Similar to Huang and Hu [2000], a two‐zone, mobile and immobile, model [van Genuchten and Wierenga, 1976] is adapted to account for the interregional mass transfer. Hydraulic conductivity in the mobile zone, sorption coefficients in both zones, and interregional mass diffusion rate are all assumed to be spatial random variables to account for the inherent spatial variability of physical and chemical properties of a natural medium. The analytical solution for mean concentration in mobile water is given explicitly in Fourier‐Laplace space and numerically converted to real space via a fast Fourier transform method. The results are simplified to conservative transport in a dual‐porosity medium [Huang and Hu, 2000] and reactive transport in a one‐zone model [Hu et al., 1995] under appropriate conditions. The solution provides a tool to investigate the validity of an “effective sorption” method, where various chemical sorption and physical mass transfer processes are treated as a sorption process with effective sorption parameters. This study shows that chemical sorption and interregional mass diffusion may be lumped together as an effective sorption process under specific conditions. Generally speaking, however, such simplification will lead to a significant error in prediction of the plume evolution, especially at late travel time. It is also shown that randomness of the interregional mass transfer process will significantly enhance the plume spreading and lead to a more negatively skewed plume.
Hydraulic conductivity of variably saturated porous media: Film and corner flow in angular pore space
Authors: Tuller Markus , Or Dani
Published: February 2004

Many models for hydraulic conductivity of partially saturated porous media rely on oversimplified representation of the pore space as a bundle of cylindrical capillaries and disregard flow in liquid films. Recent progress in modeling liquid behavior in angular pores of partially saturated porous media offers an alternative framework. We assume that equilibrium liquid‐vapor interfaces provide well‐defined and stable boundaries for slow laminar film and corner flow regimes in pore space comprised of angular pores connected to slit‐shaped spaces. Knowledge of liquid configuration in the assumed geometry facilitates calculation of average liquid velocities in films and corners and enables derivation of pore‐scale hydraulic conductivity as a function of matric potential. The pore‐scale model is statistically upscaled to represent hydraulic conductivity for a sample of porous medium. Model parameters for the analytical sample‐scale expressions are estimated from measured liquid retention data and other measurable medium properties. Model calculations illustrate the important role of film flow, whose contribution dominates capillary flow (in full pores and corners) at relatively high matric potentials (approximately −100 to −300 J kg−1, or −1 to 3 bars). The crossover region between film and capillary flow is marked by a significant change in the slope of the hydraulic conductivity function as often observed in measurements. Model predictions are compared with the widely applied van Genuchten–Mualem model and yield reasonable agreement with measured retention and hydraulic conductivity data over a wide range of soil textural classes.
Large‐eddy simulation over heterogeneous terrain with remotely sensed land surface conditions
Authors: Albertson John D. , Kustas William P. , Scanlon Todd M.
Published: February 2004

A framework is developed to explore the coupling between the land and the atmospheric boundary layer using three‐dimensional turbulence simulation over remotely sensed land surface images. The coupled set of equations is integrated with boundary conditions from day 221 of the Monsoon '90 experiment, and analysis is conducted to quantify the transmission of surface heterogeneity information into the atmospheric boundary layer (ABL). The large‐eddy simulation (LES) model incorporates radiant energy availability; spatial fields of remotely observed surface cover, temperature, and moisture; and the ability to account for the separate contributions of soil and vegetation (i.e., two sources) to the mass and energy exchanges. This effort reflects a merging of active lines of research: the use of remotely sensed land surface properties to study water and energy fluxes and the use of LES to study impacts of surface variability on ABL processes. Analysis of the results reveals (1) that the combination of remotely sensed data and LES (in the absence of free parameters) yields regionally averaged land surface fluxes and ranges of spatial variability in the fluxes that compare well to similar measures from a network of flux measuring stations, (2) that the correlation between time‐averaged surface and air temperatures is dependent on the length scale of the surface features, (3) that the horizontal standard deviation of mean air temperature decreased logarithmically with height in the atmospheric surface layer; and (4) that the mean air temperature contains spatial variability induced preferentially from variations in surface temperature occurring at scales >500–1000 m. Hence the feedback strength between the land and the atmosphere is shown to be Scale‐dependent for the range of length scales (i.e., ≤O(10 km)) studied here.
Evolution of bed load grain size distribution with increasing flow strength and the effect of flow duration on the caliber of bed load sediment yield in ephemeral gravel bed rivers
Authors: Powell D. Mark , Reid Ian , Laronne Jonathan B.
Published: February 2004

The grain size distributions of bed load collected during flows that range from the threshold of motion to near bankfull in Nahal Eshtemoa, a coarse‐grained ephemeral stream of southern Israel, give an unequivocal quantification of the change from unequal to near‐equal mobility of grains of all available sizes. This second threshold occurs at ∼4.5 times the shear stress associated with initial motion (4.5тc). The level determined in Eshtemoa is much higher than previously defined by laboratory flume studies. The difference probably reflects the tighter packing and the greater interlock of natural streambeds and carries implications for a number of issues, including the modeling of sediment transport, especially in the context of dryland reservoir sedimentation. The proportion of time that flow exceeds 4.5тc determines the size distribution of cumulative bed load. Using a flow duration/sediment transport approach, in which fractional transport rates by grain size class are predicted with Parker's [1979] bed load function, it can be shown that the size distribution of bed load coarsens as the flow duration curve of individual flash floods incorporates progressively higher flows of variable duration. The method has utility because simple flood parameters such as peak discharge are imperfect predictors of sediment flux, particularly in desert streams where the shape of the flood hydrograph varies considerably from event to event. It is also shown that over a longer period (annual or decadal) the caliber of cumulative bed load depends upon the spectrum of flows in incident floods. The 4‐year hydrograph record in Eshtemoa provides a model‐derived bed load size distribution just finer than the unarmored bed material. The Eshtemoa data set can also be used to show clearly that water discharge in this type of ephemeral stream is “effective” or “dominant” over a wide range of flow stage ranging from 0.4 to 1.1 times bankfull and that bankfull discharge is no more significant than a wide range of other states of flow in transporting bed load.
Stochastic capture zone delineation within the generalized likelihood uncertainty estimation methodology: Conditioning on head observations
Authors: Feyen L. , Beven K. J. , De Smedt F. , Freer J.
Published: February 2004

A stochastic methodology to evaluate the predictive uncertainty in well capture zones in heterogeneous aquifers with uncertain parameters is presented. The approach is based on the generalized likelihood uncertainty estimation methodology. The hydraulic conductivity is modeled as a random space function allowing for the uncertainty that stems from the imperfect knowledge of the parameters of the correlation structure. Parameters are sampled from prior distributions and are used for the generation of a large number of hydraulic conductivity fields, which are subsequently used to solve the groundwater flow equation. A likelihood is calculated for every simulation, based on some goodness‐of‐fit measure between simulated heads and available observations. Using inverse particle tracking, a capture zone is determined which is assigned the likelihood calculated for that particular simulation. Statistical analysis of the ensemble of all simulations enables the predictive uncertainty of the well capture zones to be defined. Results are presented for a hypothetical test case and different likelihood definitions used in the conditioning process. The results show that the delineated capture zones are most sensitive to the mean hydraulic conductivity and the variance, whereas the integral scale of the variogram is the parameter with the smallest influence. For all likelihood measures the prior uncertainty is reduced considerably by introducing the observation heads, but the reduction is most effective for the very selective likelihood definition. The method presented can be used in real applications to quantify the uncertainty in the location and extent of well capture zones when little or no information is available about the hydraulic properties, through the conditioning on head observations.
Trends in Canadian streamflow
Authors: Zhang Xuebin , Harvey K. David , Hogg W. D. , Yuzyk Ted R.
Published: February 2004

This study presents trends computed for the past 30–50 years for 11 hydroclimatic variables obtained from the recently created Canadian Reference Hydrometrie Basin Network database. It was found that annual mean streamflow has generally decreased during the periods, with significant decreases detected in the southern part of the country. Monthly mean streamflow for most months also decreased, with the greatest decreases occurring in August and September. The exceptions are March and April, when significant increases in streamflow were observed. Significant increases were identified in lower percentiles of the daily streamflow frequency distribution over northern British Columbia and the Yukon Territory. In southern Canada, significant decreases were observed in all percentiles of the daily streamflow distribution. Breakup of river ice and the ensuing spring freshet occur significantly earlier, especially in British Columbia. There is also evidence to suggest earlier freeze‐up of rivers, particularly in eastern Canada. The trends observed in hydroclimatic variables are entirely consistent with those identified in climatic variables in other Canadian studies.
The complementary relationship in estimation of regional evapotranspiration: An enhanced advection‐aridity model
Authors: Hobbins Michael T. , Ramírez Jorge A. , Brown Thomas C.
Published: February 2004

Long‐term monthly evapotranspiration estimates from Brutsaert and Strieker's Advection‐Aridity model were compared with independent estimates of evapotranspiration derived from long‐term water balances for 139 undisturbed basins across the conterminous United States. On an average annual basis for the period 1962–1988 the original model, which uses a Penman wind function, underestimated evapotranspiration by 7.9% of precipitation compared with the water balance estimates. Model accuracy increased with basin humidity. An improved formulation of the model is presented in which the wind function and the Priestley‐Taylor coefficient are modified. The wind function was reparameterized on a seasonal, regional basis to replicate independent proxy potential evapotranspiration surfaces. This led to significant differences from the original Penman wind function. The reparameterized wind function, together with a recalibrated Priestley‐Taylor coefficient in the wet environment evapotranspiration formulation, reduced the underestimation of annual average evapotranspiration to only 1.15% of precipitation on an independent set of validation basins. The results offered here lend further support for Bouchet's hypothesis as it applies to large‐scale, long‐term evapotranspiration.
Predicting sorption of groundwater bacteria from size distribution, surface area, and magnetic susceptibility of soil particles
Authors: Bengtsson Göran , Ekere Lina
Published: February 2004

Spatial heterogeneity of hydraulic and sorptive processes may have a strong impact on the mobility of bacteria in porous soils. Whereas heterogeneity at the microscopic solid particl/interstitial pore scale has been conceptually addressed by dividing the porous material into two or more coexisting sorbing fractions, field‐scale spatial variability has been approached by developing stochastic methodologies for coupling advective flow and sorption reactions. This calls for an elucidation of correlations between certain soil parameters and sorption of bacteria, with the purpose of substituting tedious and labourious sorption measurements with indirect methods allowing more rapid screening of field samples. Sorption of two 3H‐labeled strains of groundwater bacteria was examined in test tubes with sandy aquifer material and filter sterilized groundwater from the same aquifer. The aquifer particles were sieved to different size fractions (0–63, 63–106, 106–212, 212–500, 500–1000, and 1000–1500 µm) and mixtures thereof, and their specific surface area, magnetic susceptibility, and mineral composition were determined. Columns filled with the sieved or original aquifer material were saturated with groundwater, and the breakthrough of the labeled bacteria at a flow rate of 0.2 mL min−1 was measured by liquid scintillation. Cell sorption to soil particles in test tube experiments was controlled by magnetic susceptibility, which characterizes the mineral surfaces primarily according to their iron content. In contrast, there was a negative nonlinear regression between the sorption coefficient and the particle size and a positive linear regression with the specific surface area, each with an R2 of ∼0.85 in the column experiments. The work demonstrates that sorption of groundwater bacteria to aquifer material of low organic carbon content can be predicted from the particle size distribution or the specific surface area.
Effects of junction transfer characteristics on transport in fracture networks
Authors: Park Young‐Jin , Lee Kang‐Kun , Berkowitz Brian
Published: February 2004

The influence of fracture junction, solute transfer characteristics on transport patterns in discrete fracture networks is analyzed. Regular fracture networks with either constant or variable aperture distributions are considered in conjunction with particle tracking methods. Solute transfer probabilities at fracture junctions are determined from analytical considerations. The second spatial moment and the dilution index are used as measures of the spreading and the degree of channelized transport, respectively; these measures also account for varying mixing ratios at fracture junctions under different flow conditions. For fracture networks with variable aperture distributions, mixing conditions at fracture junctions, determined by the local flow conditions and by the junction geometry, are always dominated by complete mixing and streamline routing “end‐member” cases. Moreover, the frequency distribution of the low‐velocity regime that arises from variable aperture distributions strongly affects channelized transport and local flow conditions in fracture networks. Simulations suggest that simplified particle tracking models of solute transport in discrete fracture networks will best represent large‐scale transport by assuming streamline routing, rather than complete mixing, at fracture junctions. Finally, analysis of both the constant and variable aperture distributions indicates that solute spreading may typically be underestimated in forced hydraulic gradient tracer tests owing to changes in local flow conditions at fracture junctions.
Artificial neural network modeling of water table depth fluctuations
Authors: Coulibaly Paulin , Anctil François , Aravena Ramon , Bobée Bernard
Published: February 2004

Three types of functionally different artificial neural network (ANN) models are calibrated using a relatively short length of groundwater level records and related hydrometeorological data to simulate water table fluctuations in the Gondo aquifer, Burkina Faso. Input delay neural network (IDNN) with static memory structure and globally recurrent neural network (RNN) with inherent dynamical memory are proposed for monthly water table fluctuations modeling. The simulation performance of the IDNN and the RNN models is compared with results obtained from two variants of radial basis function (RBF) networks, namely, a generalized RBF model (GRBF) and a probabilistic neural network (PNN). Overall, simulation results suggest that the RNN is the most efficient of the ANN models tested for a calibration period as short as 7 years. The results of the IDNN and the PNN are almost equivalent despite their basically different learning procedures. The GRBF performs very poorly as compared to the other models. Furthermore, the study shows that RNN may offer a robust framework for improving water supply planning in semiarid areas where aquifer information is not available. This study has significant implications for groundwater management in areas with inadequate groundwater monitoring network.
On evaluating the spatial‐temporal variation of soil moisture in the Susquehanna River Basin
Authors: Yu Zhongbo , Carlson T. N. , Barron E. J. , Schwartz F. W.
Published: February 2004

The Hydrologic Model System (HMS), a physically based distributed model, was used to simulate the soil moisture variation during a storm event in the Upper West Branch, a subbasin of the Susquehanna River Basin. The model was calibrated by comparing the simulated temporal daily variation in soil moisture with field data. Data from the Mahantango Watershed within the Susquehanna River Basin were used to drive the HMS for the temporal simulation of soil moisture as a function of soil texture, vegetation, and topography. Spatially distributed data sets with a resolution of 1 km were prepared for the simulations and used to examine the soil moisture variation as caused by the spatial variability in soil texture, vegetation type, and topography. The Pennsylvania State–National Center for Atmospheric Research Mesoscale Meteorological Model (MM5) coupled with the HMS was used to simulate the basin response of hydrologic processes to a storm event. The effect of infiltration on the soil moisture along the flow pathway in the river basin and the overall hydrologic response with observed precipitation compare well with observations. The simulated hydrologic response with the MM5‐simulated precipitation slightly underestimates the actual response.
Flexible scaling model for use in random field simulation of hydraulic conductivity
Authors: Painter Scott
Published: February 2004

A new fractal scaling model is proposed for use in aquifer heterogeneity modeling and simulation. The new model, which is obtained by subordination of fractional Brownian motion, provides a unifying framework for scaling models of heterogeneity in that it includes previous scaling models as special cases and can also be tuned continuously between these models. Choosing the subordinator to be a lognormal distribution results in a non‐Gaussian scaling model with a moderate degree of variability that matches data better than previous fractal scaling models, thus providing more accurate heterogeneity representations for use in stochastic flow and transport predictions. Two new stochastic simulation algorithms are constructed from the new models, one based on sequential simulation algorithms and the other based on probability field simulation. Both algorithms can be made conditional on available measurements.
Seasonally in ENSO‐related precipitation, river discharges, soil moisture, and vegetation index in Colombia
Authors: Poveda Germán , Jaramillo Alvaro , Gil Marta María , Quiceno Natalia , Mantilla Ricardo I.
Published: February 2004

An analysis of hydrologic variability in Colombia shows different seasonal effects associated with El Niño/Southern Oscillation (ENSO) phenomenon. Spectral and cross‐correlation analyses are developed between climatic indices of the tropical Pacific Ocean and the annual cycle of Colombia's hydrology: precipitation, river flows, soil moisture, and the Normalized Difference Vegetation Index (NDVI). Our findings indicate stronger anomalies during December‐February and weaker during March‐May. The effects of ENSO are stronger for streamflow than for precipitation, owing to concomitant effects on soil moisture and evapotranspiration. We studied time variability of 10‐day average volumetric soil moisture, collected at the tropical Andes of central Colombia at depths of 20 and 40 cm, in coffee growing areas characterized by shading vegetation (“shaded coffee”), forest, and sunlit coffee. The annual and interannual variability of soil moisture are highly intertwined for the period 1997–1999, during strong El Niño and La Niña events. Soil moisture exhibited greater negative anomalies during 1997–1998 El Niño, being strongest during the two dry seasons that normally occur in central Colombia. Soil moisture deficits were more drastic at zones covered by sunlit coffee than at those covered by forest and shaded coffee. Soil moisture responds to wetter than normal precipitation conditions during La Niña 1998–1999, reaching maximum levels throughout that period. The probability density function of soil moisture records is highly skewed and exhibits different kinds of multimodality depending upon land cover type. NDVI exhibits strong negative anomalies throughout the year during El Niños, in particular during September–November (year 0) and June–August (year 0). The strong negative relation between NDVI and El Niño has enormous implications for carbon, water, and energy budgets over the region, including the tropical Andes and Amazon River basin.
A computer model for meandering rivers with multiple bed load sediment sizes: 1. Theory
Authors: Sun Tao , Meakin Paul , Jøssang Torstein
Published: February 2004

A computer model for meandering rivers that couples water flow, bed topography, the sorting of sediments with different grain sizes, and channel migration has been developed using the linear theory of Johannesson and Parker [1989] for the dynamics of meandering rivers and the theory of Parker and Andrews [1985] for bed load sediment transport and sorting in meander bends. The equations for single‐size sediment transport used in the linear theory of Johannesson and Parker were replaced by multiple‐size sediment transport and sorting equations obtained from the theory of Parker and Andrews. This paper is the first of a pair. In this paper, the theoretical framework of the model is presented, and the derivation of the theory from the earlier theories is detailed. It is shown that there are five different regions in the two‐dimensional model parameter space [ϵ(G1 − 3), (π/2)2 G2], where G1 is a measure of the coupling of the longitudinal sediment flow to the longitudinal water flow, ϵ is the rescaled bed friction coefficient, and G2 characterizes the coupling of the transverse (cross channel) sediment flow to the transverse component of the bed inclination. The model is not stable in two of the regions (regions 1 and 2). Alternating bars develop in region 4 but propagate along the channel with damped amplitudes, and no alternating bars develop in region 5. In region 3, which is the boundary between regions 2 and 4, alternating bars develop and propagate undamped throughout the entire channel. Increasing the flöodplain inclination, channel width/depth ratio, average sediment size, or the breadth of the sediment size distribution in the model increases the tendency toward a transition from a stable meandering state to an unstable, presumably braided, state.
A computer model for meandering rivers with multiple bed load sediment sizes: 2. Computer simulations
Authors: Sun Tao , Meakin Paul , Jøssang Torstein
Published: February 2004

The theory for flow and sediment transport in a curved channel that was presented by Sun et al. [this issue], paper 1 of this two‐part study, was used to develop a computer model for meandering rivers with multiple sediment grain sizes. The model, which is based on the linear theory of Johannesson and Parker [1989] for meandering rivers, takes into account the effects of alternate bars. Therefore this is one of the first models that can be used to simulate bar‐bend interactions and their relationships with both the initial growth and the long‐term development of meandering rivers. Unlike the previous theoretical studies of Blondeaux and Seminara [1985] and Johannesson and Parker [1989], the results show that the initial growth of meandering rivers is controlled primarily by the curvature‐related instabilities, and the influence of alternate bars on the initial development of meander loops appears to be negligible. The simulations also show that under certain conditions, meandering rivers do evolve toward a “resonant” state. It appears that finite amplitude perturbations are needed to enhance the initial development of alternate bars in the channel in order for the river to develop toward a bar‐bend resonance state. The model includes the transport and sorting of sediment with a distribution of grain sizes. Heterogeneities in the sedimentary material deposited in the floodplain, as a result of meandering river migration, are realistically simulated.
A Bayesian Joint Probability Approach for flood record augmentation
Authors: Wang Q. J.
Published: February 2004

At‐site flood frequency prediction is often subject to large uncertainties because of the limited length of record available at the gauging station of interest. In many cases, however, one can find a nearby gauging station that has a considerably longer record. In this paper, we introduce a two‐site joint probability approach for the transfer of flood information between two stations. We assume that the annual maximum floods at the two stations follow a bivariate generalized extreme value distribution. We use a Bayesian procedure to estimate the parameters of the distribution by jointly using all the flood data available at the two stations. Efficient transfer of information is achieved by simultaneously carrying out (1) the inference of correlation of floods at the two stations, (2) the transfer of information between the two stations, and (3) the inference of the frequency distributions for the two stations.
Subordinated advection‐dispersion equation for contaminant transport
Authors: Baeumer Boris , Benson David A. , Meerschaert Mark M. , Wheatcraft Stephen W.
Published: February 2004

A mathematical method called subordination broadens the applicability of the classical advection‐dispersion equation for contaminant transport. In this method the time variable is randomized to represent the operational time experienced by different particles. In a highly heterogeneous aquifer the operational time captures the fractal properties of the medium. This leads to a simple, parsimonious model of contaminant transport that exhibits many of the features (heavy tails, skewness, and non‐Fickian growth rate) typically seen in real aquifers. We employ a stable subordinator that derives from physical models of anomalous diffusion involving fractional derivatives. Applied to a one‐dimensional approximation of the MADE‐2 data set, the model shows excellent agreement.
Interannual variability of accumulated snow in the Columbia Basin, British Columbia
Authors: Hsieh William W. , Tang Benyang
Published: February 2004

Snow water equivalent anomalies (SWEA) measured around April 1 by stations in the Columbia basin area in British Columbia, Canada, were studied for their interannual variability during the period 1950–1999, particularly in relation to El Niño/La Niña events and to high and low Pacific–North American (PNA) atmospheric circulation patterns. Composites of the SWEA showed that SWEA were negative during El Niño years, positive during La Niña years, negative during high PNA years, and positive during low PNA years. High PNA appeared to have the most impact on the SWEA, followed by La Niña, El Niño, and low PNA. In the Columbia basin area, La Niña effects (relative to El Niño effects) on SWEA decrease northward and eastward but strengthen with elevation. Composites of the Pacific sea surface temperature anomalies (SSTA) during the 10 lowest SWEA years revealed weak signals, with El Niño warm SSTA present only during spring and early summer in the preceding year and the SSTA pattern consistent with a high PNA present by fall and winter. In contrast, composites of the SSTA during the 10 highest SWEA years showed strong La Niña cool SSTA starting around May in the preceding year and lasting onto winter.
Deep lithospheric dynamics beneath the Sierra Nevada during the Mesozoic and Cenozoic as inferred from xenolith petrology
Authors: Lee Cin‐Ty , Rudnick Roberta L. , Brimhall George H.
Published: February 2004

Peridotite xenoliths erupted in late Miocene basalts (∼8 Ma) in the central Sierra Nevada sample a lithosphere that is vertically stratified in terms of age and thermal history. The deeper portions (∼45–100 km) have asthenospheric osmium isotopic compositons and possess textural and chemical evidence for cooling from >1100° to 700–820°C. The shallower portions (<60 km) have unradiogenic Os isotopic compositions, which yield Proterozoic model ages, and contain orthopyroxenes that record temperatures as low as 670°C in their cores and heating up to 900°C on their rims. These observations suggest that the deeper xenoliths represent fragments of hot asthenosphere that upwelled to intrude and/or underplate the overlying Proterozoic lithosphere represented by the shallower xenoliths. The contrasting thermal histories between the shallow and deep xenoliths suggest that hot asthenosphere and cold lithosphere were suddenly juxtaposed, a feature consistent with the aftermath of rapid lithospheric removal or sudden intrusion of asthenospheric mantle into the lithosphere rather than passive extension. On the basis of regional tectonics and various time constraints, it is possible that this lithospheric removal event was associated with the generation of the Sierra Nevada granitic batholith during Mesozoic subduction of the Farallon plate beneath North America. Pleistocene basalt‐hosted xenoliths record a different chapter in the geodynamic history of the Sierras. These xenoliths are relatively fertile, come from depths shallower than 45–60 km, are characterized by asthenospheric Os isotopic compositions, record hot equilibration temperatures (1000°–1100°C), and show no evidence for cooling. The strong contrast in composition and thermal history between the Pleistocene and late Miocene suites indicate that the post‐Mesozoic lithospheric mantle, as represented by the latter, was entirely replaced by the former. The hot Pleistocene peridotites may thus represent new lithospheric additions associated with a post‐Miocene lithospheric removal event or extension. High elevations, low sub‐Moho seismic velocities, and the presence of fast velocity anomalies at 200 km depth may be manifestations of this event. If lithospheric removal occurred in the Mesozoic and Cenozoic, the observations presented here place constraints on the styles of lithospheric removal. In the Mesozoic, the lithospheric mantle was only partially removed, whereas in the Pliocene, the entire lithospheric mantle and probably the mafic lower crust were removed.
Separation of high field strength elements (Nb, Ta, Zr, Hf) and Lu from rock samples for MC‐ICPMS measurements
Authors: Münker Carsten , Weyer Stefan , Scherer Erik , Mezger Klaus
Published: February 2004

The application of multiple collector inductively coupled plasma source mass spectrometry (MC‐ICPMS) to 176Lu‐176Hf and 92Nb‐92Zr chronometry has been hampered by complex Zr‐Hf purification procedures that involve multiple ion exchange column steps. This study presents a single‐column separation procedure for purification of Hf and Lu by ion exchange using Eichrom® Ln‐Spec resin. The sample is loaded in pure HCl, and element yields are not dependent on the sample matrix. For 92Nb‐92Zr chronometry, a one‐column procedure for purification of Zr using Biorad® AG‐1‐× 8 resin is described. Titanium and Mo are completely removed from the Zr, thus enabling accurate 92Zr measurements. Zirconium and Nb are quantitatively separated from rock samples using Eichrom Ln‐Spec resin, allowing measurements of Zr/Nb with a precision of better than ±5% (2σ). The Ln‐Spec and anion resin procedures may be combined into a three‐column method for separation of Zr‐Nb, Hf, Ta, and Lu from rock samples. For the first time, this procedure permits combined isotope dilution measurements of Nb/Ta, Zr/Hf, and Lu/Hf using a mixed 94Zr‐176Lu‐180Hf‐180Ta tracer. Analytical protocols for Zr and Hf isotope measurements using the Micromass Isoprobe, a second generation, single‐focusing MC‐ICPMS, are reported. Using the Isoprobe at Münster, 2σ external precisions of ±0.5ɛ units for Hf and Zr isotope measurements are achieved using as little as 5 ng (Hf) to 10 ng (Zr) of the element. The 176Hf/177Hf and Lu/Hf for rock reference materials agree well with other published MC‐ICPMS and thermal ionization mass spectrometry (TIMS) data.
Improvements in noble gas separation methodology: A nude cryogenic trap
Authors: Lott Dempsey E.
Published: February 2004

A new noble gas cryogenic trap design employing polished stainless steel as the active trapping surface is described and tested. This trap offers significant advantages over the charcoal trap systems now commonly used for the preparation of noble gas samples for mass spectrometric analysis. Aliquots of Ne, Ar, Kr, and Xe are quantitatively trapped at less than 9 K and selectively released (≥99% at 20 K, 85 K, 110 K, 135 K, respectively). These release temperatures are significantly lower than obtained with conventional charcoal traps. The sorption‐desorption characteristics of the trap for the individual gases and neon‐argon mixtures are examined in detail as a function of trap temperature. The low release temperatures and improved separation for these noble gases will decrease processing times, reduce blanks and memory, and enhance the purity of noble gas sample preparation as compared to charcoal traps.
Using ocean margin density to constrain ocean circulation and surface wind strength in the past
Authors: Lynch‐Stieglitz Jean
Published: February 2004

The density structure along the ocean margins carries an integrated imprint of ocean circulation. The difference in pressure between the eastern and western margins of the ocean reflects the net meridional geostrophic transport across the ocean basin. If one assumes that the wind driven circulation is closed in the upper ocean, the wind driven component of the net geostrophic transport is equal in magnitude and opposite in direction to the Ekman transport. The remainder of the net meridional geostrophic transport is the upper branch of the thermohaline meridional overturning circulation. The meridional geostrophic transports calculated using the ocean margin density data collected during World Ocean Circulation Experiment (WOCE) reflect both the wind driven and thermohaline overturning circulations. Density along the ocean margins for times in the past can be estimated from the oxygen isotopic composition of benthic foraminifera. These ocean margin density estimates can then be used to help constrain the shear in the meridional overturning circulation, the strength of zonal surface winds over the oceans, and the resulting wind driven circulation. These data will also provide an important complement to existing paleoceanographic reconstructions of surface conditions and deep water tracer fields which are currently used to constrain models of past ocean circulation.
Measurements and simulation of stratospheric NO3 at mid and high latitudes in the northern hemisphere
Authors: Renard J.‐B. , Taupin F. G. , Rivière E. D. , Pirre M. , Huret N. , Berthet G. , Robert C. , Chartier M. , Pepe F. , George M.
Published: February 2004

Simultaneous measurements of NO3, along with those of O3, NO2, and aerosol extinction coefficient, have been performed during the night by the AMON instrument since 1992 at high and midlatitudes and by the SALOMON instrument since 1998 at midlatitude. Observations are conducted using the stellar and lunar occultation methods, respectively. Vertical profiles of NO3 are obtained after inversion of the optical depth spectra recorded from 650 to 670 nm, including the 662‐nm absorption line. Five profiles at midlatitude and two profiles at high latitude are presented. Comparisons with a box model constrained with measured ozone and temperatures (and NO2 at high latitude) have been performed, taking into account the uncertainties in the rate constants of the reactions leading to NO3 equilibrium. The modeling results can reproduce part of the observations, taking into account possible errors in the rate constants, temperature, or NO3 absorption cross sections. Some disagreements nevertheless remain between observations and modeling outputs. In the middle stratosphere they could result from gradients of temperature. Below 30 km, other phenomena could be invoked to explain the disagreements. At high latitude the presence of solid polar aerosols induces an artifact in the data reduction. At midlatitude, large increases observed in the NO3 concentration profiles obtained between 1992 and 1994 are real. A speculative hypothesis involving volcanic aerosols is proposed.