bluuebunny/crossref_metadata_2025 huggingface dataset:
Convergent margin extension associated with arc‐continent collision: The Finsch Deep, Papua New Guinea
Authors: Whitmore G. P. , Johnson D. P. , Crook K. A. W. , Galewsky J. , Silver E. A.
Published: February 2004

The Finsch Deep is an asymmetric rhomboidal basin, with a maximum depth of 5400 m, situated to the north of the Solomon Sea Triple Junction, Papua New Guinea. Anomalous slip vector azimuths, regional plate velocities, earthquake hypocenters, and bathymetric depth relative to surrounding terranes all indicate that the Finsch Deep is not a passive feature developed behind the subduction front. Our interpretation of all available data, including seismological studies, detailed bathymetry, side‐scan character, and seismic sections, suggests that the Finsch Deep has developed due to N‐S extension in the transition zone from continental collision west of the Solomon Sea Triple Junction to oceanic subduction to the east. The key mechanisms thought to drive “convergent margin extension” within the New Guinea Collision are slab pull and oblique subduction. Researchers studying other convergent margins have recognized underplating and tectonic erosion as driving mechanisms for extension; though we have no evidence to either support or discount their presence within the New Guinea Collision.
A shortened intraplate rift system in the Proterozoic Mount Isa terrane, NW Queensland, Australia
Authors: O'Dea Mark G. , Lister Gordon S. , Betts Peter G. , Pound Katherine S.
Published: February 2004

The Leichhardt River Fault Trough of the Mount Isa terrane developed a complex extensional architecture between approximately 1800 and 1600 Ma, forming the underlying template upon which compressional structures were superimposed during the 1590 to 1500 Ma Isan Orogeny. Basin‐fill material accumulated during at least five multiphase periods of rifting and associated postrift subsidence forming a stacked succession of unconformity‐bounded sequences. Initial E‐W extension was associated with a massive magmatic event. Half graben greater than 50 km in width and of alternating asymmetry localized the extrusion of up to 4 km of continental tholeiites. Thereafter a period of N‐S extension resulted in southward tapering north tilted half graben in which synrift basaltic and siliciclastic strata accumulated. N‐S extension was followed by regional postrift subsidence and the deposition of a laterally continuous quartzite‐carbonate package. A multiphase period of E‐W to NW‐SE extension ensued during which time two unconformity‐bounded sequences accumulated. The stratal architectures of these sequences are strongly asymmetric in cross section, exhibiting a pronounced rotational thickening toward the east, consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width. Finally, a period of N‐S extension resulted in the development of E‐W trending F1 drag synclines in the highest level cover rocks. The association of angular unconformities and block‐bounding faults, E‐W trending synclines and E‐W striking faults, and the unique internal fold geometries of fault blocks suggest that many fault‐bounded blocks originated as coherent structural entities during rifting and continued to act as such during subsequent shortening.
Off‐scraping accretionary process under the subduction of young oceanic crust: The Shimanto Belt of Okinawa Island, Ryukyu Arc
Authors: Ujiie Kohtaro
Published: February 2004

The Eocene Kayo Formation exposed at the southwestern end of the Shimanto Belt in the Ryukyu Arc records the progressive development of off‐scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint. The formation consists almost wholly of coherent trench turbidites supplied by axial currents. Detailed structural analysis has revealed that two successive deformation stages are recognized in the Formation. The early deformation (stage 1) occurred by horizontal shortening related to the initial stage of off‐scraping accretion. In this stage, sands were water‐rich and nearly unlithified, and the competence contrast between sandstone and shale was low. The late deformation (stage 2) is marked by the formation of seaward verging imbricate fold‐and‐thrust system within the accretionary wedge. Compared with stage 1, sands in stage 2 were more dewatered and lithified, and the competence contrast between sandstone and shale increased. In accordance with this deformation history, structures caused by high pore fluid pressure indicate increased cohesion of sands during off‐scraping accretion. Studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the Formation became buried and suffered thermal imprint; pressure solution cleavage developed as an axial planar to the stage 2 folds. Thermal imprint on the Formation is characterized by high paleotemperatures at approximately 250°C–300°C in a region of the prehnite‐pumpellyite facies metamorphism. This thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate.
Transpression, displacement partitioning, and exhumation in the eastern Caribbean / South American plate boundary zone
Authors: Avé Lallemant Hans G.
Published: February 2004

The Caribbean/South American plate boundary zone in northeastern Venezuela is a transpressive orogenic belt consisting from north to south of a nascent subduction zone (South Caribbean deformed belt), a volcanic arc (Leeward Antilles arc), a “hinterland” with high‐pressure (P)/low temperature (T) metamorphic rocks (Cordillera de la Costa belt), and a southern nonmetamorphic, foreland fold and thrust belt (Serranía del Interior). The geometry, style, and orientation of mid‐Cretaceous to Tertiary synmetamorphic deformation structures (D1) in the hinterland are compatible with formation in a right‐oblique subduction or collision zone in which displacement partitioning has occurred. Late Oligocene to Recent right‐oblique convergence resulted in the emplacement of the arc and hinterland on the passive South American margin and the formation of the foreland fold and thrust belt (D2); the displacements between the Caribbean and South American plates are partitioned as well. Both D1 and D2 deformations are diachronous: they are older in the west and younger in the east and related to the eastward passage of the Caribbean plate with respect to South America. The ascent, decompression, and exhumation of the high‐P/low‐T metamorphic rocks occurred in two stages: the first in the Cretaceous by arc‐parallel extension (D1) and the second in Neogene time by thrusting (D2) and subsequent erosion.
Do the Current Landscape Evolution Models Show Self‐Organized Criticality?
Authors: Sapozhnikov Victor B. , Foufoula‐Georgiou Efi
Published: February 2004

In this note we question the relevance of the self‐organized criticality concept as currently applied to landscape evolution modeling. We also express concerns about possible inference problems in testing hypotheses about landscape multifractality using simulated landscapes.
Capture Zone Delineation in Two‐Dimensional Groundwater Flow Models
Authors: Bakker Mark , Strack Otto D. L.
Published: February 2004

Procedures are presented for delineating capture zones of pumping wells. First, an algorithm is presented for determining the envelopes of the capture zones by the use of the locations of stagnation points. Second, it is discussed how the streamlines are determined that separate groundwater originating from different sources (the dividing streamlines). Third, a procedure is presented for constructing the boundary of the capture zone for any given time. The procedures are implemented in an analytic element code and are applied to groundwater flow systems that can be modeled with wells, line sinks, uniform flow, and areal infiltration.
An Intercomparison Study of NEXRAD Precipitation Estimates
Authors: Smith James A. , Seo Dong Jun , Baeck Mary Lynn , Hudlow Michael D.
Published: February 2004

Systematic biases in WSR‐88D (Weather Surveillance Radar–1988 Doppler) hourly precipitation accumulation estimates are characterized from analyses of more than 1 year of WSR‐88D data and rain gage data from the southern plains. Biases are examined in three contexts: (1) biases that arise from the range‐dependent sampling of the WSR‐88D, (2) systematic differences in radar rainfall estimates from two radars observing the same area, and (3) systematic differences between radar and rain gage estimates of rainfall. Range‐dependent biases affect hourly rainfall accumulations products over much of the area covered by the WSR‐88D. Significant underestimation of rainfall occurs within 40 km range of the radar due to bias in reflectivity observations at the higher elevation angles used for rainfall estimation close to the radar. Bright band and anomalous propagation (AP) lead to systematic overestimation of rainfall at intermediate range. Beyond 150 km in spring‐summer and beyond 100 km in winter‐fall, underestimation of precipitation is pronounced due to incomplete beam filling and overshooting of precipitation. Radar‐radar intercomparison studies suggest that radar calibration is a significant problem at some sites. Anomalous propagation during clear‐air conditions, a major problem with previous National Weather Service network radars, has been largely eliminated by the WSR‐88D processing. AP remains a problem for cases in which AP returns are embedded in rain. Radar–rain gage intercomparison analyses indicate systematic underestimation by the WSR‐88D relative to rain gages for paired gage‐radar rainfall estimates. Analyses of spatial coverage of heavy rainfall, however, illustrate fundamental advantages of radar over rain gage networks for rainfall estimation.
Modeling the Large‐Scale Dynamics of Saturated Groundwater Flow Using Spatial‐Filtering Theory: 1. Theoretical Development
Authors: Beckie Roger , Aldama Alvaro A. , Wood Eric F.
Published: February 2004

We develop a model and a framework for understanding the explicitly resolved, large‐scale dynamics of groundwater flow in fully saturated, heterogeneous porous media. The approach is based upon the large eddy simulation (LES) methodology from geophysical fluid dynamics. The idea behind the LES approach is to explicitly resolve the larger scales of flow and provide a closure model for the effect of subgrid scales upon resolved scales. The technical foundation of the method rests on a spatial filtering operation, which allows one to precisely define the scale of the model. We spatially filter Darcy's law and the continuity equation to produce a model for the resolved‐scale dynamics. We develop LES approximations for the resulting flow equations and a closure model for unresolved, subgrid‐scale terms. We model the subgrid terms using resolved‐scale quantities only and thus do not require subgrid‐scale information. The accuracy of these closure models depends upon the scale at which hydraulic conductivity is “measured” by field tests. We characterize the approximations used to model subgrid‐scale effects asymptotically with a small parameter, the dimensionless filter width ϵ, defined as the ratio of the filter width to the dominant length scale of the explicitly resolved variables.
Modeling the Large‐Scale Dynamics of Saturated Groundwater Flow Using Spatial‐Filtering Theory: 2. Numerical Evaluation
Authors: Beckie Roger , Aldama Alvaro A. , Wood Eric F.
Published: February 2004

The objective of this paper is to numerically evaluate the spatially filtered flow theory of Beckie et al. [this issue]. We perform two suites of tests to evaluate the theory. In the first suite of tests we examine the accuracy of the spatially filtered Darcy's law approximations. To do this, we use exact filtered conductivities and head gradients in the spatially filtered Darcy's laws to calculate the approximate filtered flux and then compare the approximate flux field to the exact flux field. We determine the exact fields by numerically filtering the primitive fields. In the second suite of tests we solve the model equations to determine the approximate flux solution. We present a simple numerical‐perturbation strategy to solve the model equations. The first suite of tests shows that over a range of filter widths the error in the resolved‐scale flux computed from the second‐order approximate spatially filtered Darcy's law is up to 3 orders of magnitude lower than the zeroth‐order approximation. The second suite of tests, which more closely correspond to the way the theory would be applied in practice, show that the model accuracy and convergence properties are degraded by error from the numerical‐perturbation strategy.
The influence of mass transfer characteristics and porous media heterogeneity on nonaqueous phase dissolution
Authors: Mayer Alex S. , Miller Cass T.
Published: February 2004

A two‐dimensional multiphase flow and species transport model was developed and applied to the case of nonaqueous phase liquid (NAPL) emplacement and dissolution in both homogeneous and heterogeneous porous media systems. Simulations were performed to observe dissolution rate variations and the degree of NAPL‐aqueous phase nonequilibrium as a function of two aqueous phase velocities and five forms of the NAPL‐aqueous phase mass transfer formulation. An integrated form of the Damkohler number was introduced to analyze the degree of NAPL‐aqueous phase nonequilibrium. Mass removal rates for homogeneous media were insensitive to the form of the NAPL‐aqueous phase mass transfer formulation, yielding results similar to a local equilibrium approach for all but one mass transfer formulation. This latter formulation was most sensitive to NAPL saturation and yielded significant nonequilibrium behavior, which was manifested as a decrease in NAPL dissolution rates as the NAPL volume fraction decreased. Variations in mass elution rates between homogeneous and heterogeneous media were observed, with more significant variations found for variances in porous media properties than for horizontal correlation lengths. In heterogeneous media, decreases in dissolution rates were attributed to the existence of relatively immobile regions of NAPL with saturations greater than the residual saturation of the media, so‐called NAPL pools. These results illustrate the importance of the statistical characteristics of heterogeneous porous media on NAPL distribution and dissolution processes.
Aquia Aquifer Dissolved Cl− and 36Cl/Cl: Implications for Flow Velocities
Authors: Purdy Caroline Bascom , Helz George R. , Mignerey Alice C. , Kubik Peter W. , Elmore David , Sharma Pankaj , Hemmick Thomas
Published: February 2004

The Aquia aquifer (southern Maryland) contains a remarkably smooth Cl− profile (0.46–3.23 ppm) along its flow path. This is interpreted as a record of historic changes in the deposition of Cl− in this region. Those changes have been influenced by the rise and fall of sea level, which has altered the distance of the recharge region from the coastline by ∼200 km. The 36Cl concentration along the flow path is not as smooth as the Cl− profile. Historic variations in cosmogenic production, atmospheric transport, precipitation, and evapotranspiration all might have influenced 36Cl concentrations. A general similarity between the 36Cl and Cl− profiles suggests that changes in precipitation and evapotranspiration rates, which influence both tracers similarly, are particularly important. To reconcile 14C, 36Cl, and hydrologic data, we propose a two‐tier model for flow in the Aquia. Shallower portions of the aquifer (<60 m) were subjected to hydraulic gradients and flow rates approximately 5 times larger during the Pleistocene than modern, prepumping rates. At greater depths, flow rates were much slower and less variable; water in this region may be old enough to record some 36Cl decay.
Characterization of the Climatological Variability of Mean Areal Rainfall Through Fractional Coverage
Authors: Seo Dong‐Jun , Smith James A.
Published: February 2004

The role of fractional coverage in climatological variability of mean areal rainfall is investigated. Under second‐order homogeneity assumptions, climatological variability of mean areal rainfall, as measured by its coefficient of variation, is shown to be a function of mean fractional coverage, conditional coefficient of variation of point rainfall, and two correlation scales associated with inner variability and intermittency of point rainfall. To verify the analytical results, empirical analyses are performed using hourly rainfall data from nine Weather Surveillance Radar—1988 Doppier (WSR‐88D) radars in the southern plains, United States. The results indicate that given the local rainfall climatology there exists a catchment scale at which the climatological variability of mean areal rainfall is at its maximum. Existence of such a scale has been hypothesized and demonstrated via an analytical treatment of an idealized situation by Eagleson and Wang [1985], but only in climatological variability of fractional coverage (i.e., variability due only to intermittency). Sensitivity analyses for the correlation‐scale parameters indicate that intermittency plays as important a role as inner variability in shaping the catchment scale‐climatological variability relationship of mean areal rainfall. Decomposition of total variability indicates that over the range of catchment scales that fall within a single WSR‐88D radar umbrella (effective radius of 230 km) the integrated contribution from intermittency is almost as large as that from inner variability.
Water flow paths and residence times in a small headwater catchment at Gårdsjön, Sweden, during steady state storm flow conditions
Authors: Lange Holger , Lischeid Gunnar , Hoch Ralf , Hauhs Michael
Published: February 2004

Since April 1991 the small forested headwater catchment Gl at Gårdsjön (Sweden) has been covered by a roof underneath which natural throughfall is replaced by artificial irrigation with a controlled chemical composition. Here this unique experimental setup was used for a tracer experiment with LiBr. The tracer pulse was applied to a subcatchment of approximately 1000 m2 that was maintained at steady state flow conditions throughout the experiment. Except these steady state flow conditions, the irrigation rates corresponded to a typical storm flow episode. Infiltration of event water was confined to the steep slope of a subcatchment of G1; no water was applied at the boggy valley bottom or close to the weir. An array of groundwater wells, suction lysimeters, and surface water sampling plots was used to document the soil passage of this pulse. Breakthrough in runoff (i.e., streamflow from the weir) occurred in a single peak within about 17 hours when less than 15% of the estimated total soil water in the subcatchment was replaced, indicating a relatively small fraction of mobile water. Tracer concentrations in groundwater wells and surface water in the catchment revealed some shallow, locally confined flow paths through the lower parts of the subcatchment. However, 4 days after application of the tracer the runoff concentration already reached the preevent background level for Br−. Only about 14% of the applied tracer was recovered by this time. Taking this into account, i.e., considering bromide as a nonconservative tracer, a one‐dimensional model application (two‐region convection‐dispersion approach) successfully reproduced breakthrough curves at various places in the catchment. Thus the small portion of mobile water has intensive contact with the resident immobile water along the one‐dimensional flow paths yielding an extremely long tail in the residence time distribution by back diffusion from immobile water. Results of this experiment qualitatively confirm earlier tracer studies under less controlled conditions but are virtually impossible to extrapolate quantitatively to transient flow conditions.
Unsaturated flow in a quasi‐three‐dimensional fractured medium with spatially variable aperture
Authors: Abdel‐Salam Assem , Chrysikopoulos Constantinos V.
Published: February 2004

Transient moisture flow in a variably saturated quasi‐three‐dimensional fracture‐rock matrix system is investigated. The fracture is assumed to possess a spatially variable aperture in its two‐dimensional plane, whereas the rock matrix is treated as a two‐dimensional homogeneous and tight porous medium. The aperture fluctuations in the fracture plane are described stochastically. Moisture exchange between the fracture and the rock matrix is accounted for via an advective coupling term that governs the transfer of moisture at the fracture‐matrix interface and takes into account the effect of a fracture‐surface coating material. Although the variable aperture fracture is two‐dimensional, the coupling term between the fracture and the rock matrix accounts for the three‐dimensional nature of the physical system. The stochastic nonlinear set of partial differential equations is solved numerically by the Galerkin finite element method in conjunction with the Picard iterative scheme and an automatic time step marching. Simulations are performed to investigate phenomena which have been ignored in previous studies. It is demonstrated that, for the case of no moisture exchange with the rock matrix, the moisture follows preferential flow paths within the fracture plane and exhibits pronounced fingering effects. Furthermore, it is shown that the larger the fracture aperture fluctuations the more extended the moisture flow in the fracture. In addition, for the case where there exists moisture exchange with the rock matrix, the movement of the moisture front is considerably reduced, whereas fracture‐surface coatings tend to slow down moisture absorption by the rock matrix.
The hydrology and methylmercury dynamics of a Precambrian shield headwater peatland
Authors: Branfireun Brian A. , Heyes Andrew , Roulet Nigel T.
Published: February 2004

A hydrobiogeochemical investigation of a small headwater peatland located in the Experimental Lakes Area, northwestern Ontario, Canada, examined the surface and subsurface hydrologic pathways and their relation to the movement and spatial variability of methylmercury (CH3Hg+). The hydrology of the peatland controls the mass flux of CH3Hg+ to a downstream pond from the terrestrial ecosystems, and influences the production and/or accumulation of CH3Hg+ in the peatland. Distinct zones of groundwater recharge and discharge were observed within the peatland, and these corresponded, to low and high CH3Hg+ concentrations, respectively, in pore water. The magnitude and flux of CH3Hg+ from the peatland was governed by the area of the peatland and surrounding uplands contributing runoff. There was a threefold increase in CH3Hg+ concentration as stream base flow passed from the origin of the stream at the hillslope‐peatland margin to the stream outlet at the peatland‐lake interface. This relative increase in concentration was maintained during storm flow conditions even though the discharge was up to 10 times greater, indicating that the peatland is a large source of CH3Hg+. Methylmercury‐laden peat pore water found near the surface of the peatland in areas of groundwater discharge moves to the stream as the water table rises to the peat surface. Of a total of 12.6 mg CH3Hg+ leaving the peatland during the 112‐day study period, less than 1% was transported directly by groundwater, 41% was transported in stream base flow, and the remaining 58% was transported by storm flow. However, storm flow conditions occurred only 16% of the time, indicating the significance of episodic CH3Hg+ flux from headwater catchments on the Precambrian Shield.
Intercomparison of Methods for Calculating Potential Evaporation in Regional and Global Water Balance Models
Authors: Federer C. A. , Vörösmarty C. , Fekete B.
Published: February 2004

Five methods (Thornthwaite, Hamon, Jensen‐Haise, Turc, and Penman) for estimating potential evaporation for a reference surface (PEr) were compared to four methods (Priestley‐Taylor, McNaughton‐Black, Penman‐Monteith, and Shuttleworth‐Wallace) for estimating surface‐dependent potential evaporation (PEs) using three cover types at each of seven locations from Fairbanks, Alaska, to San Juan, Puerto Rico. For annual PE the PEs methods generally agreed with the PEr methods, but for many locations, differences among methods were hundreds of millimeters per year. No methods were consistently low or high. Three of the PEs methods depend strongly on maximum leaf conductance, for which Körner [1994] provided satisfactory values by cover type. Potential interception ∥PEi∥ can only be estimated appropriately for all cover types by the Shuttleworth‐Wallace method. Use of 5‐day or monthly input data did not greatly degrade results, so use of monthly data to generate PE estimates appears warranted in global water balance models.
Series Solution for Richards Equation Under Concentration Boundary Conditions and Uniform Initial Conditions
Authors: Salvucci Guido Daniel
Published: February 2004

A series solution which expresses the moisture profiles and surface fluxes during infiltration and exfiltration is derived. The solution applies for conditions of fixed surface moisture content and uniform initial moisture content within an unbounded, homogeneous, nonhysteretic soil column. The series derivation is similar to J. R. Philip's but with a transformed time variable. The series converges for infiltration problems involving soils which exhibit steep increase in capillary pressure near saturation. Under these conditions the solution captures the transition from early square root of time behavior to the longtime traveling wave solution without the use of approximate joining techniques. The soil water retention models for which convergence occurs include the widely applied Brooks‐Corey and van Genuchten models, with minor restrictions on the shape parameters of the latter. Other cases examined include infiltration and exfiltration assuming power law, linear, quadratic, and constant dependence of diffusivity and conductivity on moisture content. For these cases the convergence limits were finite. The solution is particularly well suited for infiltration into sandy soils, for which the transition between short‐time and longtime behavior occurs relatively early.
Sensitivity Methods for Time‐Continuous, Spatially Discrete Groundwater Contaminant Transport Models
Authors: Skaggs Todd H. , Barry D. A.
Published: February 2004

Existing sensitivity methods for spatially discrete groundwater contaminant transport models have been developed for time‐stepping numerical algorithms and cannot be readily used with time‐continuous approaches to transport simulation, such as the Laplace transform Galerkin technique. We develop direct and adjoint sensitivity methods in which sensitivity coefficients are computed in the Laplace domain and inverted numerically to the time domain. The methods are computationally efficient when used in conjunction with time‐continuous transport equations. The relative efficiency of the two methods depends on the number of model parameters, number of performance measures, and number of spatial discretization nodes. The adjoint method is favored when the number of performance measures is much smaller than the number of model parameters. The adjoint method is limited in that performance measures are restricted to being linear functions of state variables. A two‐dimensional transport example is developed in detail, and sensitivities with respect to nodal hydraulic conductivities are computed. In the problems analyzed, the direct and adjoint methods are 9 to 156 times faster than the perturbation method, with the computational savings increasing as the size of the problem is increased.
Mixed Layer Convective Turbulence Theory with First International Satellite‐Land Surface Climatology Project Field Experiment Data
Authors: Crago Richard D.
Published: February 2004

The convective turbulence theory (CTT) [Stull, 1994] has several advantages over traditional bulk atmospheric boundary layer similarity formulations for the surface momentum and sensible heat fluxes under free convection. In particular, in CTT surface fluxes at scales of 101–102 km are parameterized directly from surface and mixed layer measurements, without regard to the surface layer profiles or surface roughness. The equations of CTT are tested with the First International Satellite‐Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) data set; the theory is extended so that geostrophic winds may be used in place of mixed layer winds; the formulation is applied to satellite‐derived surface temperature data; and Stull's [1994] mixed convection interpolation formula is tested. The scatter on graphs of the mixed layer transfer coefficients for momentum and sensible heat is greater for the FIFE data than for the Boundary‐Layer Experiment 1983 (BLX83) data used by Stull [1994], and the mean coefficient values are greater, probably because of the use of radiosonde data at FIFE and spatially averaged aircraft data at BLX83. Surface momentum and sensible heat flux estimated on the basis of CTT produced reasonable results but provided little or no improvement over a direct correlation between the reference fluxes and the mixed layer wind speed or the surface‐to‐mixed layer temperature difference. The geostrophic version of CTT provided results comparable to the original CTT. Remotely sensed surface temperatures used with geostrophic CTT allowed reasonable estimates of the surface sensible heat flux.
The behavior of stratified pools in the Wimmera River, Australia
Authors: Western Andrew W. , O'Neill Ian C. , Hughes Roger L. , Nolan Jeremy B.
Published: February 2004

Numerous inland Australian streams contain density‐stratified or saline pools, which are usually located on channel bends. Saline pools consist of a layer of saline water underlying a layer of fresh water. Saline pools generally form as a result of saline groundwater seeping into the stream and collecting in scour depressions during periods of low flow. Inflows of saline river water can also collect in scour depressions. Field and laboratory investigations of saline pool mixing by overflowing fresh water reveal that mixing depends on a balance between interfacial shear and buoyancy forces acting on a thin dense layer flowing up the downstream slope of the scour depression, and on the bend sharpness. Convection associated with surface cooling also causes mixing. A model for saline pools formed by groundwater inflows and mixed by fresh overflows is proposed and applied to several saline pools in the Wimmera River.
An Intercomparison of Techniques to Determine the Area‐Averaged Latent Heat Flux from Individual in Situ Observations: A remote Sensing Approach Using the European Field Experiment in a Desertification‐Threatened Area Data
Authors: Pelgrum H. , Bastiaanssen W. G. M.
Published: February 2004

A knowledge of the area‐averaged latent heat flux 〈λE〉 is necessary to validate large‐scale model predictions of heat fluxes over heterogeneous land surfaces. This paper describes different procedures to obtain 〈λE〉 as a weighted average of ground‐based observations. The weighting coefficients are obtained from remote sensing measurements. The remote sensing data used in this study consist of a Landsat thematic mapper image of the European Field Experiment in a Desertification‐Threatened Area (EFEDA) grid box in central Spain, acquired on June 12, 1991. A newly developed remote sensing algorithm, the surface energy balance for land algorithm (SEBAL), solves the energy budget on a pixel‐by‐pixel basis. From the resulting frequency distribution of the latent heat flux, the area‐averaged latent heat flux was calculated as 〈λE〉 = 164 W m−2. This method was validated with field measurements of latent heat flux, sensible heat flux, and soil moisture. In general, the SEBAL‐derived output compared well with field measurements. Two other methods for retrieval of weighting coefficients were tested against SEBAL. The second method combines satellite images of surface temperature, surface albedo, and normalized difference vegetation index (NDVI) into an index on a pixel‐by‐pixel basis. After inclusion of ground‐based measurements of the latent heat flux, a linear relationship between the index and the latent heat flux was established. This relationship was used to map the latent heat flux on a pixel‐by‐pixel basis, resulting in 〈λE〉 = 194 W m−2. The third method makes use of a supervised classification of the thematic mapper image into eight land use classes. An average latent heat flux was assigned to each class by using field measurements of the latent heat flux. According to the percentage of occurrence of each class in the image, 〈λE〉 was calculated as 110 W m−2. A weighting scheme was produced to make an estimation of 〈λE〉 possible from in situ observations. The weighting scheme contained a multiplication factor for each measurement site in order to compensate for the relative contribution of that site to 〈λE〉. It was shown that 〈λE〉 derived as the arithmetic mean of 13 individual in situ observations leads to a difference of 34% (〈λE〉 = 104 W m−2), which emphasizes the need for improved weighting procedures.
Nonergodic Solute Transport in Three‐Dimensional Heterogeneous Isotropic Aquifers
Authors: Zhang You‐Kuan , Zhang Dongxiao , Lin Jie
Published: February 2004

The general expressions for the time‐dependent ensemble averages of the second spatial moments 〈A〉 and the effective dispersivities γ, defined as (½μ)(d;〈A〉/dt) where μ is the magnitude of the mean flow velocity μ, are evaluated in order to study the effect of initial plume size on 〈A〉 and γ in three‐dimensional heterogeneous isotropic aquifers under the first‐order approximation to the particle displacement. The results confirm previous findings that 〈A〉 and γ generally approach their respective ergodic limits X and α as the size of a source increases, where X and α are the single particle displacement covariance and the associated dispersivity, and that the transverse lengths of a source are more important than the longitudinal length for the ergodic condition to be met. The longitudinal dispersion of a nonergodic plume becomes Fickian or the effective asymptotic longitudinal dispersivity is constant at late time as long as one of the initial lateral lengths of the plume is nonzero, while the transverse dispersion is always non‐Fickian and the effective asymptotic transverse dispersivities are always zero regardless of the initial plume size. The most important and interesting findings are, when the longitudinal length l1 of an initial plume is larger than the lateral lengths l2 and l3, both effective longitudinal and transverse dispersivities γii (i= 1, 2, 3) increase to their respective peaks at early time, then γ11 decreases toward an asymptotic constant, whose value depends on the values of l2 and l3 (γ11 → 0 if l2 = l3 = 0), whereas γ22 and γ33 decrease to below zero (i.e., become negative), increase again, and finally approach zero independent of the lateral lengths of the source. Comparison of the current study with a numerical simulation shows good agreement between the calculated and simulated longitudinal second spatial moments.
Estimation and validation of contemporaneous PARMA Models for streamflow simulation
Authors: Rasmussen Peter F. , Salas Jose D. , Fagherazzi Laura , Rassam Jean‐Claude , Bobée Bernard
Published: February 2004

Seasonal streamflow series generally exhibit periodicity in the autocovariance structure. Such periodicity can be represented by PARMA models, i.e., autoregressive moving average (ARMA) models with parameters that vary with the seasons. Statistical properties of low‐order models such as the PARMA(2,2) model are examined. The periodic moment equations are derived; they can be used to compute the periodic covariance structure of a given model. Simulation of streamflow at several sites can be done using the contemporaneous PARMA model. The main problem in using such models is to determine the covariance matrices of innovations. Traditionally, this has been done by the method of maximum likelihood. However, this method generally leads to significant underestimation of the cross correlation of flows. A moment estimator is developed herein for the contemporaneous PARMA(2,2) model along with three approximate moment‐based estimators for those cases where a feasible moment solution cannot be obtained. The applicability of the proposed methods is illustrated by fitting PARMA models to weekly flow data for two catchments in the Ottawa River basin.
Groundwater age dating with chlorofluorocarbons
Authors: Oster H. , Sonntag C. , Münnich K. O.
Published: February 2004

CFCs are useful tracers for age dating young water. Two critical assumptions are typically invoked: (1) the initial concentration needs to be known, and (2) the tracer must be stable. A series of 8000 CFC air data from four sites on the west European continent (from high‐alpine clean air down to urban sites) show site‐specific CFC excesses relative to the global background ranging from 125% at an urban site (Heidelberg) to only 30% at a rural site (Wachenheim). In both cases we find regular diurnal, weekly, and annual variations but also episodic variations due to changing air mass and sporadic local emission as well as a decrease of the CFC excess due to decreasing emissions in Europe. However, in soil air above the groundwater table the CFC excess variation is low‐pass filtered, allowing reconstruction of the site‐specific CFC input anomaly in relation to the global background. Thus local CFC soil air measurements can provide a valuable local correction factor for CFC dating and extend the applicability of CFC studies in specific environments. To investigate the chemical stability assumption, five field sites and two laboratory experiments show CFC 11 and CFC 12 degradation under anoxic conditions with degradation rates ranging from 0.05 up to 3×105 pmol L−1 yr−1. However, the CFC 12/CFC 11 degradation ratio is found to be constant over six orders of magnitude, with CFC 12 always being less reactive by a factor of about 10.
Asymptotic analysis of nonlinear equilibrium solute transport in porous media
Authors: Jaekel Uwe , Georgescu Adelina , Vereecken Harry
Published: February 2004

We discuss the asymptotic behavior of a solute plume undergoing reversible sorption governed by a Freundlich isotherm after single‐pulse injection. Our analysis predicts that the concentration at a fixed position decays asymptotically like a power law with an exponent α = 1/(1 − n) where n is the Freundlich exponent. Correspondingly, the shape of tail is time invariant. The results are checked by comparison with numerical solutions for one‐dimensional transport in a homogeneous medium. Some further asymptotic results for this case are derived. The power law behavior provides an alternative way to derive the Freundlich n parameter from breakthrough curves in comparison to the use of inverse estimation methods. This is especially the case when evaluating breakthrough curves obtained in two‐ or three‐dimensional flow domains, for which indirect estimation of parameters becomes very difficult.
Modeling water retention curves of sandy soils using neural networks
Authors: Schaap Marcel G. , Bouten Willem
Published: February 2004

We used neural networks (NNs) to model the drying water retention curve (WRC) of 204 sandy soil samples from particle‐size distribution (PSD), soil organic matter content (SOM), and bulk density (BD). Neural networks can relate multiple model input data to multiple model output data without the need of an a priori model concept. In this way a high performance black‐box model is created, which is very useful in a data exploration effort to assess the maximum obtainable prediction accuracy. We used a series of NN models with an increasing parametrization of input and output variables to get a better interpretability of model results. In the first two models we used the nine PSD fractions, BD, and SOM as input, while we predicted the nine points of the water retention curve. These NNs had 12 input and 9 output variables, predicting WRCs with an average root‐mean‐square residual (RMSR) water content of 0.020 cm3 cm−3. After a few intermediary models with increasing parametrization of PSD and WRC using (adapted) van Genuchten [1980] equations we arrived at a final NN model that used six input variables to predict three van Genuchten [1980] parameters resulting in a RMSR of 0.024 cm3 cm−3. We found saturated and residual water contents to be unrelated to the PSD, BD, or SOM, therefore the saturated water content was considered to be an independent input variable, while the residual water content was set to zero. Sensitivity analyses showed that the PSD had a major influence on the shape of the WRC, while BD and SOM were less important. On the basis of these sensitivity analyses we established more explicit equations that demonstrated similarity relations between PSD and WRC and incorporated effects of SOM and BD in an empirical way. Despite the fact that we considered a large number of linear and nonlinear variants these equations had a weaker performance (RMSR: 0.029 cm3 cm−3) than the NN models, proving the modeling power of that technique.
Forcing, intermittency, and land surface hydrologic partitioning
Authors: Marani Marco , Grossi Giovanna , Napolitano Francesco , Wallace Michael , Entekhabi Dara
Published: February 2004

Because of sampling problems, the precipitation estimates from satellite remote sensing are aggregated over time (typically monthly) and over space. Since land surface hydrologic processes have threshold and nonlinear dependencies on precipitation, coarse‐resolution precipitation observations may not be directly used in hydrologic models. Differences in the character of intermittency of precipitation, the averaged values remaining the same, can in fact yield large differences in the hydrologic partitioning and therefore in the resulting climate. In this paper an equilibrium hydrologic model is used to study the influence of intermittency on the way precipitation is partitioned into different hydrological quantities. The parameters defining intensity and duration of storms are varied (keeping total precipitation volume constant), and the resulting effects on the partitioning into runoff, evaporation, recharge, and soil moisture storage are determined. It is found that the character of intermittency in storm arrivals has a large impact on the hydrologic partitioning. Furthermore, investigations on the sensitivity of hydrologic partitioning on soil type and water table depth show that rainfall intermittency plays a major role irrespective of these other factors. Runoff generation, evaporation, and groundwater recharge are the three main components of the water balance. Depending on the soil type and climate, the main competition is between different combinations of these three loss mechanisms. It is concluded that the temporal structure of storms has a strong influence on the long‐term equilibrium state of the hydrological system.
Competitive Sorption and Displacement of Hydrophobic Organic Contaminants in Saturated Subsurface Soil Systems
Authors: McGinley P. M. , Katz L. E. , Weber W. J.
Published: February 2004

The sorption of tetrachloroethylene and trichlorobenzene by a subsurface soil that exhibits moderately nonlinear sorption isotherms for each solute was found to be strongly competitive, while that by a soil for which both solutes exhibit more linear sorption isotherms was essentially additive. The isotherm nonlinearity and competitive sorption observed in the former case suggest that a greater fraction of the sorption by that soil occurred on or within tightly ordered or condensed organic matrices rather than within more amorphous soil organic matter. Ideal adsorbed solution theory was used in conjunction with a Freundlich isotherm in a dual‐domain model to characterize mathematically the sorption behavior of these two soils in column‐type reactor systems. Model parameters obtained independently from breakthrough curves for single‐solute systems were used to predict breakthrough curves for bisolute systems. The results demonstrate that (1) even moderate isotherm nonlinearity can affect breakthrough curve behavior in single‐solute systems, and (2) the effects of isotherm nonlinearity on solute breakthrough are significantly increased by the additional manifestation of competitive effects in bisolute systems.
Concentration‐discharge relationships in runoff from a steep, unchanneled catchment
Authors: Anderson Suzanne Prestrud , Dietrich William E. , Torres Raymond , Montgomery David R. , Loague Keith
Published: February 2004

The observation that “old” water dominates storm runoff suggests that release of low‐solute water from soils rather than rainwater must cause storm runoff dilution. This inference is supported by sprinkling experiments in an 860‐m2 catchment in the Oregon Coast Range, in which >200 mm of both high and low ionic strength precipitation produced similar concentration‐discharge trends. Rainwater chemistry was buffered as it traveled through catchment soils: the amount of sprinkling‐derived water in the runoff increased during long periods of steady discharge but was not accompanied by a change in runoff solute concentrations. Stored water plays a role in runoff dilution as well. Nearly all runoff from the catchment passes through underlying weathered bedrock rather than perching and discharging only through soil. Bedrock water composition appears to vary through storm events, as the average contact time of water with rock declines with increasing discharge, a behavior at odds with the concept of stable end‐members.
An analytical model for nonlinear adsorptive transport through layered soils
Authors: Wu Y. S. , Kool J. B. , Huyakorn P. S. , Saleem Z. A.
Published: February 2004

Solute transport with nonlinear adsorption occurs in many situations involving inorganic chemical and metal contamination in soil and groundwater systems. The resulting isotherms can be highly nonlinear, and numerical solutions of the transport equation can encounter severe convergence difficulties. An analytical solution is developed for simulating one‐dimensional transport in the layered soil column with nonlinear adsorption effects. The analytical solution is an exact solution for nondispersive transport and becomes an approximate one when dispersion effects are included. The accuracy of the solution has been examined using a numerical model under advection‐dominated conditions. The derived analytical solution can be used for any form of nonlinear sorption isotherms given as closed‐form functions, or tabulated data, with a general time‐dependent source boundary condition. The heterogeneity of soils is handled by treating the soil column as a layered system. The resulting analytical model is suitable for estimating the leaching potential of metals or other solutes in the subsurface and for determining travel times of contaminants. Our analysis shows that the shape and nonlinearity of the sorption isotherm may have a controlling influence on transport behavior. Evaluation and application examples are provided to demonstrate applicability of the solution to solute and metal transport with both functional and tabular nonlinear isotherms.
Streamflow simulation for continental‐scale river basins
Authors: Nijssen Bart , Lettenmaier Dennis P. , Liang Xu , Wetzel Suzanne W. , Wood Eric F.
Published: February 2004

A grid network version of the two‐layer variable infiltration capacity (VIC‐2L) macroscale hydrologic model is described. VIC‐2L is a hydrologically based soil‐ vegetation‐atmosphere transfer scheme designed to represent the land surface in numerical weather prediction and climate models. The grid network scheme allows streamflow to be predicted for large continental rivers. Off‐line (observed and estimated surface meteorological and radiative forcings) applications of the model to the Columbia River (1° latitude‐longitude spatial resolution) and Delaware River (0.5° resolution) are described. The model performed quite well in both applications, reproducing the seasonal hydrograph and annual flow volumes to within a few percent. Difficulties in reproducing observed streamflow in the arid portion of the Snake River basin are attributed to groundwater‐surface water interactions, which are not modeled by VIC‐2L.