bluuebunny/crossref_metadata_2025 huggingface dataset:
Thick‐skinned versus thin‐skinned thrusting: Rheology controlled thrust propagation in the Variscan collisional belt (The southeastern Bohemian Massif, Czech Republic ‐ Austria)
Authors: Fritz Harald , Dallmeyer R. David , Neubauer Franz
Published: February 2004

The Variscan nappe assembly within the southeastern Bohemian Massif includes (1) crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels, (2) nappe assembly of footwall units under very low to low‐grade metamorphic conditions, and (3) frontal, thin‐skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate. Thick‐skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin‐skinned tectonic character of foreland units. 40Ar/39Ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during Late Variscan thrusting (between ∼ 340 Ma and 325 Ma). The 40Ar/39Ar mineral ages together with previously published 40Ar/39Ar and Sm/Nd data record systematically decreasing Variscan mineral ages structurally downward into footwall units. This is explained by rapid exhumation and cooling synchronously with thrusting along crustal‐scale ramps during foreland‐directed nappe propagation. The location of decollement zones within specific structural levels was apparently controlled by crustal‐scale rheological con‐trasts. Late Variscan cooling ages contrast with Cadomian 40Ar/39Ar hornblende and muscovite ages recorded within the structural basement immediately below the major Variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during Variscan thrusting. These record a sequence of Cadomian magmatic and metamorphic events in foreland units including (1) pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions (>610 Ma), (2) greisen formation during a Cadomian collisional event (∼ 600 Ma), and (3) intrusion of calcalkaline granitoid bodies with I‐type affinities (∼ 590 Ma).
Variscan tectonics of central Europe: Postaccretionary intraplate deformation of weak continental lithosphere
Authors: Krohe Alexander
Published: February 2004

The area occupied by the Variscan belt of central Europe forms part of Gondwana‐derived microplates (essentially Avalonia and Armorica) that, according to paleomagnetic data, juxtaposed Baltica and Laurentia from the Upper Ordovician to the Lower Devonian. The actual structure of this area is a puzzle of rather small fault‐bounded crustal blocks, different in P‐T‐t‐histories, which was created by later tectonic processes, at around 360 to 320 Ma. According to a common view and by contrast to paleomagnetic data, the structural record of fault‐bounded blocks is interpreted in terms of Mid‐Devonian to Lower Carboniferous amalgamation of narrow continental plates represented by the fault‐bounded blocks themselves. However, this is not supported in this article. Instead, it will be shown that fault‐systems and fault‐bounded blocks were created by intraplate deformation of a weak domain of Gondwana‐derived continental lithosphere, significantly after its accretion. Deformation was partitioned into newly formed deep‐seated strike‐slip and (kinematically connected) reverse or normal detachment systems. Large translation magnitudes caused juxtaposition of crustal areas of variable thickness. Until they were emplaced along the fault systems, the various categories of fault‐bounded complexes record different tectonic events. Category 1 metamorphic complexes represent deep‐seated parts of thickened crust, some time before unroofing on the detachment systems between about 340 (360) and 320 Ma. However, the record of subduction and extreme crustal thickening during earlier stages of the collision history was erased to a high degree and is only preserved by mineral and structural relics. Strike‐slip and detachment systems propagated across and dismembered tectonic boundaries, formed during these early events. Category 2 metamorphic complexes and the Barrandian Basin recording predominant pre‐Late Devonian metamorphic or stratigraphic events represent the brittle upper crustal parts during continuing metamorphism in the category 1 complexes. Weakly or unmetamorphosed lower Paleozoic category 3 basins represent areas of thinned continental crust and record subsidence before, and to some degree, during activity of detachment systems. The complex overall kinematic pattern of block‐bounding strike‐slip and detachment systems indicates compression and extension acting simultaneously. This probably reflects the effects of continuing oblique plate convergence, rotation, and WSW‐extrusion of fault bounded blocks, in addition to heating, and extensional flow of previously thickened crustal areas.
Miocene detachment in Crete and exhumation P‐T‐t paths of high‐pressure metamorphic rocks
Authors: Jolivet L. , Goffé B. , Monié P. , Truffert‐Luxey C. , Patriat M. , Bonneau M.
Published: February 2004

A major problem posed by the geology of Crete is the horizontal contact of an upper unit without Miocene metamorphism onto a metamorphosed lower one with Early Miocene high pressure/low temperature (HP/LT) parageneses. This very sharp contact is roughly parallel to the major Oligo‐Miocene thrust planes which were reactivated as a large‐scale detachment which allowed exhumation of high‐pressure units. We describe the extensional deformation and the metamorphic evolution of the lower plate. Most first‐order deformation features relate to the retrogression from high‐pressure to low‐pressure conditions. A N‐S pervasive stretching is observed everywhere, often associated with a top‐to‐the‐north sense of shear. The extreme variation of thickness of the Phyllite‐Quartzite nappe (upper part of the lower plate) is probably the result of large‐scale boudinage similar to the one seen in large outcrops. The most important observation is the systematic occurrence of fresh carpholite immediately below the base of the Tripolitza nappe except in northwestern Crete where a late extensional shear zone is present. Deeper in the nappe pile carpholite is systematically retrograded. This observation reveals a drastically different PT history for the upper part of the Phyllite‐Quartzite nappe. It also suggests that the late extensional shear zone found along the northern side of Crete cuts inside the metamorphic structure and brings the nonmetamorphosed Tripolitza nappe directly in contact with the deeper parts of the Phyllite‐Quartzite nappe. PT‐t paths suggest a fast temperature decrease in the top of the Phyllite‐Quartzite during retrogression and, hence, during the top‐to‐the‐north shear. The deeper part of the Phyllite‐Quartzite nappe shows a low‐temperature regime throughout, but its PT path includes an isothermal decompression in the first stage. We produce a tentative map of domains having experienced similar PT trajectories during decompression. The overall cool regime is related to the continuous underthrusting of cola continental units during exhumation. Isothermal decompression observed in the core of the Phyllite‐Quartzite Nappe implies fast exhumation during extension and the faster cooling of the upper part is related to a continuous displacement toward the north of a cooler unit during exhumation. Single grain39Ar‐40Ar ages obtained on phengites (15–25 Ma) in various structural sites are in good agreement with these conclusions and with the geological context suggesting that underthrusting of cold units at the front the accretionnary complex occurred contemporaneously with unroofing below a north dipping detachment near the top of the wedge. The age of this detachment is bracketed between the end of the high‐pressure event (20 Ma) and the deposition of the breccia (Early to Middle Miocene) in the Neogene basins.
Parameter Equivalence for the Brooks‐Corey and Van Genuchten Soil Characteristics: Preserving the Effective Capillary Drive
Authors: Morel‐Seytoux Hubert J. , Meyer Philip D. , Nachabe Mahmood , Tourna Jaoudat , van Genuchten M. T. , Lenhard Robert J.
Published: February 2004

This paper provides a simple way to convert Brooks‐Corey (BC) parameters to van Genuchten (vG) parameters and vice versa, for use primarily in situations where saturated conditions are likely to be encountered. Essential in this conversion is the preservation of the maximum value of a physical characteristic, the “effective capillary drive” HcM [Morel‐Seytoux and Khanji, 1974], defined with a good approximation for a soil water and air system as HcM = ∫0∞ krw dhc, where krw is relative permeability (or conductivity) to water and hc is capillary pressure (head), a positive quantity. With this conversion, infiltration calculations are essentially insensitive to the model used to represent the soil hydraulic properties. It is strictly a matter of convenience for the user which expression is used. On the other hand, the paper shows that other equivalences may lead to great variations in predictions of infiltration capacity. Consequently, the choice of the proper equivalence to use in calculations for rainfall‐runoff modeling or for low‐level radioactive waste disposal design is a serious matter.
Evidence for Non‐Gaussian Scaling Behavior in Heterogeneous Sedimentary Formations
Authors: Painter Scott
Published: February 2004

Vertical and horizontal fluctuations in permeability and porosity in sedimentary formations are analyzed and are found to be consistent with scaling models based on Lévy‐stable probability distributions. The approach avoids the assumption of Gaussian behavior and is supported by evidence from horizontal and vertical well logs and from permeability measurements on a sandstone outcrop and segments of core from a heterogeneous formation. The incremental values in these measurement sequences are accurately modeled as having Lévy‐stable distributions. The width of the distribution of increments depends on the spatial scale in a manner consistent with scaling behavior. The width of the distribution is smaller for horizontal increments than for vertical increments, reflecting the reduced variability in the horizontal direction. The scaling parameters are in the range associated with antipersistence and are roughly the same magnitude in the vertical and horizontal directions. The relationships between different physical properties are briefly studied, and it is suggested that they be quantified through off‐diagonal terms in a multivariate Lévy width matrix. Simulations designed to reproduce the observed statistical features are also described. These results have some fundamental implications, as Lévy‐stable distributions require a different set of statistical tools and theoretical methods compared to finite‐variance distributions.
Infiltration of a liquid finger down a fracture into superheated rock
Authors: Phillips O. M.
Published: February 2004

A finger or ribbon of liquid water flowing down a fracture in an unsaturated rock matrix begins to boil when it passes the boiling‐point isotherm. The length of the infiltrating finger increases with time; boiling of the water in the fracture is maintained by heat conduction from the surrounding superheated rock. An intrinsic length scale in this problem is l3 = (ρlQ0h/kmβ)½, where Q0 is the volume flow rate of liquid across the boiling‐point isotherm, h is the enthalpy change on boiling, km is the thermal conductivity of the matrix rock, and β is the ambient temperature gradient. Initially, while (kt)½ <b, the length l(t) of the water finger increases rapidly, proportional to l3 (kt)¼/b½, where k is the thermal diffusivity of the matrix and b is the ribbon width. A corresponding result is given for a cylindrical finger. If water continues to infiltrate beyond the time b2/k, the liquid penetration ultimately stabilizes at a distance l of order l3 below the boiling‐point isotherm, even though the temperature distribution continues to evolve. Numerical calculations show that l depends only weakly on the ribbon width or on the radius of a cylindrical finger.
Dissolution Fingering During the Solubilization of Nonaqueous Phase Liquids in Saturated Porous Media: 1. Model Predictions
Authors: Imhoff Paul T. , Miller Cass T.
Published: February 2004

The dissolution of nonaqueous phase liquids (NAPLs) trapped at residual saturation is an important problem at many contaminated groundwater sites. It is well known that NAPL ganglia trapped within the pore space reduce the permeability of the medium to aqueous phase flow. When fluid flow is imposed on such a system, the aqueous phase may interact with the dissolution‐induced permeability changes, leading to fingered patterns. This mechanism is very similar to that of mineral dissolution instabilities, which are a particular example of reactive infiltration instabilities. Extending that literature, we present a nonlinear model describing the dissolution of NAPL ganglia and perform a linear stability analysis of the resultant moving free boundary problem, demonstrating that instabilities may develop from a planar dissolution front. Predicted finger wavelengths are a function of both residual NAPL saturation and the imposed aqueous phase flow rate; they range from centimeters to meters. Experimental observations of dissolution fingering are presented in a companion paper [Imhoff et al., this issue] and are compared with predictions from this model. Dissolution fingering may affect the solubilization of NAPL ganglia in natural environments and in experimental studies of NAPL dissolution intended to quantify mass transfer rates.
Estimating a percent reduction in load
Authors: Millard Steven P.
Published: February 2004

This article extends the work of Cohn et al. [1989] on estimating constituent loads to the problem of estimating a percent reduction in load. Three estimators are considered: the maximum likelihood (MLE), a “bias‐corrected” maximum likelihood (BCMLE), and the minimum variance unbiased (MVUE). In terms of root‐mean‐square error, both the MVUE and BCMLE are superior to the MLE, and for the cases considered here there is no appreciable difference between the MVUE and the BCMLE. The BCMLE is constructed from quantities computed by most regression packages and is therefore simpler to compute than the MVUE (which involves approximating an infinite series). All three estimators are applied to a case study in which an agricultural tax in the Everglades agricultural area is tied to an observed percent reduction in phosphorus load. For typical hydrological data, very large sample sizes (of the order of 100 observations each in the baseline period and after) are required to estimate a percent reduction in load with reasonable precision.
Albedo of dirty snow during conditions of melt
Authors: Conway H. , Gades A. , Raymond C. F.
Published: February 2004

The evolution of spectrally averaged albedo (wavelengths between 0.28 μm and 2.8 μm) of snow surfaces treated with known initial concentrations of particles of submicron‐sized soot and air fall volcanic ash was investigated during conditions of natural melt. Depending on the particle type and concentration, the initial applications reduced the surface albedo to values ranging from 0.18 to 0.41 which were substantially lower than the albedo of the untreated natural snow (about 0.61). Many of the soot particles flushed through the snowpack with the meltwater, and surface concentrations of soot greater than about 5 × 10−7 kg/kg did not persist for more than a few days. The migration of particles to depth caused the snow to brighten after the initial application, thus limiting the amount of albedo reduction and the consequent effects on melting. Nevertheless, the soot remaining near the surface had a substantial, long‐term effect. The residual concentration of 5 × 10−7 kg/kg persisted for several weeks and, compared to the untreated surface, reduced the albedo by about 30% and increased melting by 50%. Particles of volcanic ash with diameters larger than about 5 μm remained at or near the snow surface. Although many of the smaller particles flushed through the snow with the meltwater, the surface albedo was not changed significantly by their removal. The different behaviors of the ash and soot are probably related to the difference in their particle size distributions in relation to the thickness of water films that form the transport paths under conditions of partial saturation that are characteristic of melting snow.
Estimation of Discharge From Three Braided Rivers Using Synthetic Aperture Radar Satellite Imagery: Potential Application to Ungaged Basins
Authors: Smith Laurence C. , Isacks Bryan L. , Bloom Arthur L. , Murray A. Brad
Published: February 2004

Analysis of 41 ERS 1 synthetic aperture radar images and simultaneous ground measurements of discharge for three large braided rivers indicates that the area of active flow on braided river floodplains is primarily a function of discharge. A power law correlation is found between satellite‐derived effective width We and discharge Q, where We is the water surface area within a braided reach divided by the reach length. Synthetic values of We and Q generated from a cellular automata model of stream braiding display a similar power law correlation. Power functions that are fit through plots of We and Q represent satellite‐derived rating curves that can subsequently be used to estimate instantaneous river discharge from space, with errors ranging from tens to hundreds of cubic meters per second. For ungaged rivers, changes in relative discharge can be determined from satellite data alone to determine the shape and timing of annual flows in glacierized basins. Absolute discharge can probably be estimated within a factor of 2. More accurate estimates will require either (1) one or more ground measurements of discharge acquired simultaneously with a satellite image acquisition, or (2) successful parameterization of known morphologic controls such as total sinuosity ∑P, valley slope, bank material and stability, and braid channel hydraulic geometry. Values of total sinuosity ∑P derived from satellite imagery and field measurements from two rivers of braid channel width, depth, velocity, water surface slope, and bed material grain size indicate that while the shape of satellite‐derived We‐Q rating curves may be influenced by all of these variables, the sensitivity of flow area to changing discharge is most dependent upon the degree of braiding. Efforts to monitor river discharge from space will be most successful for intensely braided rivers with high values of total sinuosity. Subsampling of existing daily discharge records from the Iskut River suggests that satellite return times of about 1 week are sufficient for approximating the shape and timing of the seasonal hydrograph in large, glacierized basins. Although errors are large, the presented technique represents the only currently available way to estimate discharge in ungauged braided rivers.
A Model of Groundwater Pollution From an Underground Source
Authors: Pistiner Arieh , Shapiro Michael
Published: February 2004

Saturation distribution within an aquifer of a pollutant discharging from an underground source is modeled by a two‐dimensional, nonlinear diffusion‐convection equation. A closed form self‐similar solution is obtained for the steady saturation distribution in the immiscible zone. The results may be used to rationalize the field data collected for predicting locations of underground leakage sources in aquifers. The solution may be used to check numerical models of the movement of dense liquids in the aquifer.
Turbidity‐Controlled Suspended Sediment Sampling for Runoff‐Event Load Estimation
Authors: Lewis Jack
Published: February 2004

For estimating suspended sediment concentration (SSC) in rivers, turbidity is generally a much better predictor than water discharge. Although it is now possible to collect continuous turbidity data even at remote sites, sediment sampling and load estimation are still conventionally based on discharge. With frequent calibration the relation of turbidity to SSC could be used to estimate suspended loads more efficiently. In the proposed system a programmable data logger signals a pumping sampler to collect SSC specimens at specific turbidity thresholds. Sampling of dense field records of SSC and turbidity is simulated to investigate the feasibility and efficiency of turbidity‐controlled sampling for estimating sediment loads during runoff events. Measurements of SSC and turbidity were collected at 10‐min intervals from five storm events in a small mountainous watershed that exports predominantly fine sediment. In the simulations, samples containing a mean of 4 to 11 specimens, depending on storm magnitude, were selected from each storm's record, and event loads were estimated by predicting SSC from regressions on turbidity. Using simple linear regression, the five loads were estimated with root mean square errors between 1.9 and 7.7%, compared to errors of 8.8 to 23.2% for sediment rating curve estimates based on the same samples. An estimator for the variance of the load estimate is imprecise for small sample sizes and sensitive to violations in regression model assumptions. The sampling method has potential for estimating the load of any water quality constituent that has a better correlate, measurable in situ, than discharge.
Phosphorus Loading to Tropical Rain Forest Streams After Clear‐Felling and Burning in Sabah, Malaysia
Authors: Malmer Anders
Published: February 2004

Most estimates of P export from natural or disturbed humid tropical ecosystems by streams have been based only on export of dissolved P, even though P often is limiting and can be expected to be strongly associated to particles. Therefore loss of ignition (LOI) and particulate P (Ppart) analyses were made on organic and inorganic detritus resulting from surface erosion and on stream‐suspended sediments in an undisturbed rain forest (control), as well as during and after conversion of rain forest into forest plantation. Control forest surface erosion and stream sediments consisted mainly of organics, and dissolved P (Pdiss) dominated over Ppart in stream water. The same relation was found after conversion, with a maximum mean Pdiss/Ppart ratio of up to 10 after burning, compared with 2–2.5 for control forests. This larger difference was assumed to depend on PO4 dissolved from ashes to larger concentrations than could be adsorbed during the short time (<1 hour) to reach peak flow during rainstorms.
Effective Soil Water Characteristics and Ensemble Soil Water Profiles in Heterogeneous Soils
Authors: Smith Roger E. , Diekkrüger Bernd
Published: February 2004

One‐dimensional vertical flows through spatially heterogeneous areas are studied with numerical solutions for a variety of soil types, to ascertain the behavior of the ensemble. Soil heterogeneity is treated using the distributions of parameters describing the characteristic relations of conductivity, water content, and pressure head. Values of the critical parameters, taken from a modified Brooks‐Corey or van Genuchten model, have previously 7lsqb;Smith and Diekkrüger, 1992] been shown in two extensive data sets to exhibit little interdependence. The static ensemble characteristics, except for saturated and residual water content parameters, are shown to have regular bias compared to a soil with the mean characteristic parameters. The measured data also are shown not to support the popular similar media heterogeneity model. Using both analytic approximate flow models and an ensemble solution of Richards' equation, the effective dynamic ensemble characteristics in three example soils are studied. As suggested in previous studies, such effective characteristics are conditional to flow conditions and are here shown to be quite different from ensemble static characteristics. This indicates that irrespective of the sample size, effective soil dynamic characteristics are not measurable for samples possessing significant heterogeneity.
Stochastic Analysis of Adsorbing Solute Transport in Two‐Dimensional Unsaturated Soils
Authors: Yang Jinzhong , Zhang Renduo , Wu Jinquan , Allen Myron B.
Published: February 2004

Adsorbing solute transport in two‐dimensional heterogeneous unsaturated soil was studied by means of stochastic numerical simulations. Heterogeneities in the soil's hydraulic properties and in the adsorption isotherm were simulated using random fields having specified statistical structures. Macrodispersion was analyzed using the spatial moments of numerically generated solute plumes. Among different realizations of the heterogeneous soil, the discrepancies between second‐order moments and macrodispersion coefficients were large. Macrodispersivities of unsaturated soils increased with decreasing water content. Also, heterogeneous adsorption of solute enhanced the solute spreading. When the adsorption coefficient was negatively correlated with the saturated hydraulic conductivity, solute spreading was greater than when adsorption was uncorrelated or positively correlated with the conductivity.
Boundary Conditions for Convergent Radial Tracer Tests and Effect of Well Bore Mixing Volume
Authors: Zlotnik Vitaly A. , Logan J. David
Published: February 2004

Convergent radial flow tracer tests have a complex spatial nonaxial transport structure caused by the flow in the vicinity of the injection well and its finite mixing volume. The formulation of the boundary value problem, and especially the treatment of the boundary conditions at the injection well, is nontrivial. Hodgkinson and Lever [1983],Moench [1989, 1991], and Welty and Gelhar [1994] have developed different models and methods for the analysis of breakthrough curves in the extraction well. To extend interpretation techniques to breakthrough curves in the zone between injection and extraction wells, an analysis of conventional transport models is given, and improved boundary conditions are formulated for a convergent radial tracer test problem. The formulation of the boundary conditions is based upon a more detailed analysis of the kinematic flow structure and tracer mass balance in the neighborhood of the injection well. Two practical applications of revised boundary conditions for field data analysis are given. First, the note explains anomalous high well bore mixing volumes of injection wells found by Cady et al. [1993] and allows one to establish the role of mixing versus other processes (retardation, matrix diffusion, etc.). Second, it is shown that the improper use of Moench's [1989] model can produce bias in the characteristics of breakthrough curves in the extraction well under conditions that involve a significant mixing factor in the injection well. A numerical example indicates an error in peak concentrations on a breakthrough curve by as much as 70% and in peak arrival time by 10% for Peclet numbers Pe=102. The effect becomes slightly less significant for Pe=1.
Cost Implications in Achieving Alternative Water Quality Targets
Authors: Schleich Joachim , White David , Stephenson Kurt
Published: February 2004

Excessive nutrient loading poses significant water quality problems in many water bodies across the country. An important question that must be addressed when nutrient reduction policies are devised is where nutrient reduction targets will be applied within the watershed. This paper examines the cost implications of establishing three possible nutrient reduction targets in different locations along the Fox‐Wolf River basin in northeast Wisconsin. A linear programming model calculates the total cost of achieving a 50% phosphorus load reduction target established in various locations throughout the basin. Two strategies establish phosphorus reduction targets for each of the 41 subwatersheds, and the third approach establishes a single 50% target reduction at Green Bay for the entire watershed. The results indicate that achieving target phosphorus reductions at the subwatershed level is over 4 times more expensive than achieving the same percentage phosphorus reduction for the watershed as a whole.
Estimating Local Bed Shear Stress from Velocity Observations
Authors: Wilcock Peter R.
Published: February 2004

Replicate velocity observations using conventional equipment under typical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measurements. The bed shear velocity u* can be estimated within 3% using the depth‐averaged velocity in the vertically averaged logarithmic velocity profile. To be accurate, this method is limited to relatively simple flow geometries which may be expected to have the appropriate velocity structure. Estimates of u* made using a single near‐bed velocity observation are less precise by a factor of 3 because of the larger uncertainty associated with a single observation. Accuracy of this method requires appropriate flow conditions only near the bed, so it may be applied in a wider range of flow conditions, including spatially variable flow. Estimates of u* from the slope of the near‐bed velocity profile are the least precise and require the most restrictive flow conditions for accuracy but offer the advantage that they may be made without independent knowledge of the bed roughness.
On Hack's Law
Authors: Rigon Riccardo , Rodriguez‐Iturbe Ignacio , Maritan Amos , Giacometti Achille , Tarboton David G. , Rinaldo Andrea
Published: February 2004

Hack's law is reviewed, emphasizing its implications for the elongation of river basins as well as its connections with their fractal characteristics. The relation between Hack's law and the internal structure of river basins is investigated experimentally through digital elevation models. It is found that Hack's exponent, elongation, and some relevant fractal characters are closely related. The self‐affine character of basin boundaries is shown to be connected to the power law decay of the probability of total contributing areas at any link and to Hack's law. An explanation for Hack's law is derived from scaling arguments. From the results we suggest that a statistical framework referring to the scaling invariance of the entire basin structure should be used in the interpretation of Hack's law.
A Note on Nonergodic Transport of a Passive Solute in Partially Saturated Anisotropic Heterogeneous Porous Formations
Authors: Russo David
Published: February 2004

First‐order analysis, based on a stochastic continuum presentation of the Eulerian velocity in a partially saturated porous formation and a general Lagrangian description of the transport, was used to investigate the effects of a few length scales, characteristics of the heterogeneous transport domain, on nonergodic transport of passive solutes and its tendency to ergodic transport in steady state, gravity‐dominated, unsaturated flow. Results of the analyses show that, as in ergodic transport, for given finite travel time, t, and fixed length‐scale of the planar input zone, ℓ, the principal components of the scaled macrodispersion tensor, D′ij (i, j = 1, 2, 3), are controlled by the length‐scale ratios p = Iyh/Iyυ and η = λ/Iyυ, where Iyυ and Iyh are the length scales of the formation heterogeneity in the vertical and the horizontal directions, respectively, and λ is the macroscopic capillary length‐scale. The results, that for fixed Iyυ and given p, η and t the principal components of D′ij are controlled by the ratio ℓ′ = ℓ/Iyh and that they may reach their ergodic limits if ℓ′ is sufficiently large, are in agreement with results of previous studies of nonergodic transport in aquifers. New findings obtained in the present investigation suggest that when the mean unsaturated flow is vertical and the typical travel distance is small compared with the travel distance required for the plume to reach its asymptotic, Fickian behavior, the tendency of the principal components of the preasymptotic D′ij to their ergodic limits is enhanced by increasing p (larger stratification) and decreasing η (coarser‐textured soil material). The effects of both p and η on the tendency of the transport to its ergodic limit, however, decrease with increasing travel time and vanishes at the large time limit, in which the transport approaches Fickian behavior.
Streamflow simulation: A nonparametric approach
Authors: Sharma Ashish , Tarboton David G. , Lall Upmanu
Published: February 2004

In this paper kernel estimates of the joint and conditional probability density functions are used to generate synthetic streamflow sequences. Streamflow is assumed to be a Markov process with time dependence characterized by a multivariate probability density function. Kernel methods are used to estimate this multivariate density function. Simulation proceeds by sequentially resampling from the conditional density function derived from the kernel estimate of the underlying multivariate probability density function. This is a nonparametric method for the synthesis of streamflow that is data‐driven and avoids prior assumptions as to the form of dependence (e.g., linear or nonlinear) and the form of the probability density functions (e.g., Gaussian). We show, using synthetic examples with known underlying models, that the nonparametric method presented is more flexible than the conventional models used in stochastic hydrology and is capable of reproducing both linear and nonlinear dependence. The effectiveness of this model is illustrated through its application to simulation of monthly streamflow from the Beaver River in Utah.
Soil and moisture independent estimation of stage‐two evaporation from potential evaporation and albedo or surface temperature
Authors: Salvucci Guido Daniel
Published: February 2004

The relation between stage‐one (atmosphere limited) and stage‐two (soil limited) evaporation is analyzed under the assumption that moisture profiles approximately preserve similarity during simultaneous drying and draining. The assumption is shown to be reasonable (through comparison with a numerical solution of Richards equation for homogeneous soils) under a wide range of conditions. The similarity assumption is used to derive a simple approximate expression for maximum soil evaporation (the exfiltration capacity) under the influence of gravity. The time compression approximation is used to relate stage‐one and stage‐two drying. It is demonstrated in many observed cases that the time at which stage‐two evaporation is reached is beyond the convergence limit (tgrav) of the often used one‐ or two‐term Philip type exfiltration capacity and consequently that stage‐two drying rates are better approximated by the gravity‐dominated asymptotic limit of the exfiltration solution. Combining the asymptotic limit of the similarity solution with the time compression approximation yields a simple diagnostic equation for estimating stage‐two drying: f=[8/π2]ēptd/t. The equation does not require in situ estimates of soil hydraulic properties and moisture state. It depends only on the average stage‐one evaporation ēp and the time between wetting (irrigation or rainfall) and the onset of stage‐two drying td. As has been demonstrated in the past, this time td may be estimated from the abrupt change that occurs at the transition from stage‐one to stage‐two drying in the ratio of daily averaged surface to air temperature or in the short wave albedo of bare soils. In essence, the approach here uses the readily obtainable (possibly via remote sensing) time td in place of information on vadose zone hydraulic properties and initial soil moisture conditions. The method is tested with considerable success against published evaporation data from laboratory soil columns, weighing bare soil lysimeters, agricultural bare soil plots, and a natural grassland.
A comparative modeling study of soil water dynamics in a desert ecosystem
Authors: Kemp Paul R. , Reynolds James F. , Pachepsky Yakov , Chen Jia‐Lin
Published: February 2004

We compared three different soil water models to evaluate the extent to which variation in plant growth form and cover and soil texture along a topographic gradient interact to affect relative rates of evaporation and transpiration under semiarid conditions. The models all incorporated one‐dimensional distribution of water in the soil and had separate functions for loss of water through transpiration and soil evaporation but differed in the degree of mechanism and emphasis. PALS‐SW (Patch Arid Lands Simulator‐Soil Water) is a mechanistic model that includes soil water fluxes and emphasizes the physiological control of water loss by different plant life forms along the gradient. 2DSOIL is a mechanistic model that emphasizes the physical aspects of soil water fluxes. SWB (Soil Water Budget) is a simple water budget model that has no soil water redistribution and includes simplified schemes for soil evaporation and transpiration by different life forms. The model predictions were compared to observed soil water distributions at five positions along the gradient. All models predicted soil water distributions reasonably well and, for the most part, predicted similar trends along the transect in the fractions of water lost as soil evaporation versus transpiration. Transpiration was lowest (about 40% of total evapotranspiration (ET)) for the creosote bush community, which had the lowest plant cover (30% peak cover). The fraction of ET as transpiration increased with increasing plant cover, with 2DSOIL predicting the highest transpiration (60% of total ET) for the mixed vegetation community (60% peak cover) on relatively fine textured soil and PALSr SW predicting highest transpiration (69% of total ET) for the mixed vegetation community (70% peak cover) on relatively coarse textured soil. The community type had an effect on the amount of water lost as transpiration primarily via depth and distribution of roots. In this respect, PALS‐SW predicted greatest differences among stations as related to differences in plant community types. However, since PALS‐SW did not provide as good of fit with the soil moisture data as did 2DSOIL, the differences in the morphology and physiology of the life‐forms may be secondary to the overall control of water loss by the primary factors accounted for in 2DSOIL: vertical distribution of soil moisture, degree of canopy cover, and evaporative energy budget of the canopy. Soil texture interacted with the amount and type of plant cover to affect evaporation and transpiration, but the effect was relatively minor.
Numerical simulation of a heavy rainfall event during the PRE‐STORM Experiment
Authors: Zhao Wenjie , Smith James A. , Bradley A. Allen
Published: February 2004

Numerical simulation of a storm that occurred along the Kansas‐Oklahoma border June 9–10, 1985, during the Preliminary Regional Experiment for STORM‐Central (PRE‐STORM) was carried out to examine the effect of atmospheric moisture on extreme rainstorms. This storm developed in a weak synoptic forcing environment and produced more than 200 mm of rainfall in a few hours. Heavy rainfall was confined to a small area and storm structure has been described as that of a chaotic convective system. Rainfall events like the June 9–10 storm which produce large rainfall amounts in small areas over short time periods and which have weak synoptic scale forcing are a particular challenge for numerical modeling. These events are also of particular importance for problems of engineering hydrometeorology, which motivates the analyses presented in this paper. The model used in this study is the nonhydrostatic version of the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model (MM5). Three nested domains were used for modeling the June 9–10 storm. The grid spacing is 90 km for the outer, coarse grid, which covers most of the continental United States; 30 km for the intermediate grid (covering the central and southern plains); and 10 km for the local grid covering the PRE‐STORM region. Using operationaland PRE‐STORM surface and upper air observations, the model accurately reproduces storm total rainfall and storm location. To examine the role of atmospheric water vapor for heavy rainfall events, a series of model runs are carried out in which initial moisture is varied from 75% to 125% of the observed value. Storm total rainfall, storm structure, and storm timing, as well as all elements of the atmospheric water budget, exhibit a striking sensitivity to changes in atmospheric water vapor. These results have significant implications for engineering hydrometeorology procedures, especially procedures used for probable maximum precipitation (PMP) analyses.
Energetics of sediment‐laden streamflows
Authors: Mendoza Cesar , Zhou Donghuo
Published: February 2004

The present approach, which considers a diluted sediment suspension as a perturbation to a clear‐water flow, permits a direct comparison of the energetics of turbulent sediment‐laden and clear‐water open‐channel flows of constant slope and depth. On the basis of the Boussinesq approximation to the momentum equations for the flow with sediment in suspension and the Navier‐Stokes equations for the clear‐water flow, a depth‐averaged equation for the change in turbulence energy dissipation rate and an equation for the flow energy budget are obtained. From the analysis of these equations it becomes clear that the presence of the suspension affects the distribution of energy in the flow; also, the turbulence kinetic energy can increase, decrease, or remain the same depending on the sediment loading conditions, and its rate of dissipation can increase, decrease, or remain the same depending on the magnitude of the flow hydraulic parameters relative to that of the loading conditions. These findings provide additional insight into the energetics of these complex flows, agree qualitatively with experimental data, and complement available models.
Hydrogeological modeling of the saline hot springs at the Sea of Galilee, Israel
Authors: Gvirtzman Haim , Garven Grant , Gvirtzman Gdaliahu
Published: February 2004

Meteoric fresh groundwater from shallow aquifers and hot brines from deep aquifers mix while emerging from several springs along the western coast of the Sea of Galilee, a freshwater lake located within the Dead Sea Rift Valley, Israel. After the rainy season, when elevations of the groundwater table rise in the regional aquifers and discharge rates of springs increase, solute concentrations decrease at Tabha springs but, surprisingly, increase at Fulya springs, apparently suggesting two different salinization mechanisms. Two detailed geologic cross sections were constructed, one across the rift valley at Tabha and a second at Fulya, each about 6 km deep and 70 km long. The hydrodynamics in these cross sections were analyzed using a two‐dimensional finite element code that solves the coupled variable‐density groundwater flow and heat transfer equations. Numerical simulations indicate that a topography‐driven flow model explains both spring systems, and the opposite salinity behavior results from the different hydrogeological configurations of the two subsurface drainage basins. At Fulya, both aquifers, the shallow one and the deeper one, are partially phreatic, whereas at Tabha, the deeper aquifer is totally confined. The response of springs to changes in elevation of groundwater table were simulated, reproducing field observations. This analysis has implications for the management scheme for the lake and its surrounding aquifers.
Resolution and uncertainty in hydrologic characterization
Authors: Vasco D. W. , Datta‐Gupta Akhil , Long Jane C. S.
Published: February 2004

Hydrologists have applied inverse techniques to obtain estimates of subsurface permeability and porosity variations and their associated uncertainties. Although inverse methods are now well established in hydrology, important aspects of inverse theory, the analysis of resolution, and the trade‐off between model parameter resolution and model parameter uncertainty have not been utilized. In this paper the concept of model parameter resolution is incorporated into the analysis of hydrological experiments. Model parameter resolution is a measure of the spatial averaging implicit in estimates of a distributed hydrological property such as permeability. There are two important uses of resolution and uncertainty estimates in hydrology. The first use is to plan a hydrologic testing program. Resolution matrices can be developed for proposed well tests in a variety of synthetic media. Then the effectiveness of the test design can be evaluated in terms of model parameter resolution and uncertainty. Secondly, when real data are available and used in an inversion determining the distribution of hydrologic parameters, estimates of model parameter resolution and uncertainty analysis can indicate the reliability of the solution. For synthetic tests in which the hydraulic conductivity varies and porosity does not, it is found that tracer data can provide better spatial resolution of subsurface hydraulic conductivity variations than transient pressure data. Pressure data are most sensitive to hydraulic conductivity variations immediately surrounding the well. Both pressure and tracer data better determine barriers to flow rather than channels to flow. The methodology is applied to a set of transient pressure data gathered at the Grimsel Rock Laboratory of the Swiss National Cooperative for the Storage of Radioactive Waste. In the fracture under study a low hydraulic conductivity region appears to partition the fracture plane into two distinct zones.
Decision analysis of shoreline protection under climate change uncertainty
Authors: Chao Philip T. , Hobbs Benjamin F.
Published: February 2004

If global warming occurs, it could significantly affect water resource distribution and availability. Yet it is unclear whether the prospect of such change is relevant to water resources management decisions being made today. We model a shoreline protection decision problem with a stochastic dynamic program (SDP) to determine whether consideration of the possibility of climate change would alter the decision. Three questions are addressed with the SDP: (l) How important is climate change compared to other uncertainties?, (2) What is the economic loss if climate change uncertainty is ignored?, and (3) How does belief in climate change affect the timing of the decision? In the case study, sensitivity analysis shows that uncertainty in real discount rates has a stronger effect upon the decision than belief in climate change. Nevertheless, a strong belief in climate change makes the shoreline protection project less attractive and often alters the decision to build it.
Synthesizing shallow seasonal snow covers
Authors: Shook K. , Gray D. M.
Published: February 2004

A method of synthesizing a snowfield from point field measurements, which is based on the fractal structure of the water equivalent, is presented. Two steps are involved in the synthesis. First, the method of fractal sum of pulses is used to generate the spatial distribution of the snow water equivalent in an array. Second, the synthetic snowfield is adjusted to take on the statistical properties of the natural snow cover. The two‐parameter lognormal probability density function fitted to field data is used for this purpose. Examples of the application of the lognormal distribution for describing the water equivalent of snow covers on various landscapes encountered in a prairie environment are presented. The geometrical properties (fractal dimension(s)) of snow patches that result from melting a synthetic snow cover agree closely with those of patches of an ablated natural snow cover.