bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal structure of the central sector of the Betic Cordillera (SE Spain)
Authors: Galindo‐Zaldívar J. , Jabaloy A. , González‐Lodeiro F. , Aldaya F.
Published: February 2004

Deep seismic reflection, gravimetric, and magnetometric data allow the main features of the deep structure of the central sector of the Betic Cordillera to be established. The Moho is horizontal or dips slightly toward the S below the mountain chain, and its morphology has no direct relationship with the region's topography. The deep reflectors are not deformed by the Neogene kilometric‐scale folds that produced the main topographic features, probably due to the existence of detachment levels in the crust. The crust is slightly thickened in the Betic Cordillera (almost 35 km) and has an abrupt transition to the thin crust of the Alborán Sea (15 km in the central Alborán Sea) along an E‐W oriented narrow band, subparallel to the coast line, where the Moho dips more than 60°N. The Neogene evolution of the mountain chain caused compressive deformation in the External Zones. In the Internal Zones, pre‐Tortonian low‐angle normal faults developed after the alpine metamorphism, and later high‐ and low‐angle normal faults and strike slip faults occurred up to the Quaternary. Geophysical and field geology data can be used to establish a geological model for the recent evolution of the crustal deformation. In this model, the recent evolution of the region is probably determined by thrusting of the thin continental crust of the Alborán Sea toward the NW over the southern margin of the Iberian Massif. This deformation could be a consequence of the oblique convergent movement between the African and Eurasian plates, which may have uplifted the entire region, causing regressions in the sedimentary basins and the formation of folds and extensional structures in the upper part of the thickened crust.
Relationship between northwestern Tasmania and East Gondwanaland in the Late Cambrian/Early Ordovician: Paleomagnetic evidence
Authors: Li Z. X. , Baillie P. W. , Powell C. M.
Published: February 2004

The 71 samples from 11 sites in the lower Upper Cambrian turbiditic red siltstone and mudstone of northwestern Tasmania have revealed a high‐temperature component which passes a fold test, indicating an age older than Late Devonian. The mean direction of D = 58.3°, I=11.1°, α95 = 10.4°, k = 20.2, is interpreted as of primary or early diagenetic origin, and gives a paleomagnetic pole position at (19.4°S, 28.9°E) with DP = 5.3°, DM = 10.5°. This pole position falls on the Late Cambrian to Early Ordovician part of the East Gondwanaland apparent polar wander path, indicating that if northwestern Tasmania were separated from East Gondwanaland during the Neoproterozoic breakup of the supercontinent Rodinia, it had been accreted to East Gondwanaland by at least the Early Ordovician. Two over‐print poles, one at (50.4°S, 177.4°E) with DP = 6.5°, DM = 6.9°, and the other at (62.0°S, 102.6°E) with DP = 9.9°, DM = 11.3°, suggest that there could have been more than one overprinting event in the region during the breakup between Australia and Antarctica in the late Mesozoic and early Tertiary.
Neogene Patagonian plateau lavas: Continental magmas associated with ridge collision at the Chile Triple Junction
Authors: Gorring Matthew L. , Kay Suzanne M. , Zeitler Peter K. , Ramos Victor A. , Rubiolo Daniel , Fernandez Marisa I. , Panza Jose L.
Published: February 2004

Extensive Neogene Patagonian plateau lavas (46.5° to 49.5°S) southeast of the modern Chile Triple Junction can be related to opening of asthenospheric “slab windows” associated with collisions of Chile Rise segments with the Chile Trench at ≈ 12 Ma and 6 Ma. Support comes from 26 new total‐fusion, whole rock 40Ar/39Ar ages and geochemical data from back arc plateau lavas. In most localities, plateau lava sequences consist of voluminous, tholeiitic main‐plateau flows overlain by less voluminous, 2 to 5 million year younger, alkalic postplateau flows. Northeast of where the ridge collided at ≈12 Ma, most lavas are syncollisional or postcollisional in age, with eruptions of both sequences migrating northeastward at 50 to 70 km/Ma. Plateau lavas have ages from 12 to 7 Ma in the western back arc and from 5 to 2 Ma farther to the northeast. Trace element and isotopic data indicate main‐plateau lavas formed as larger percentage melts of a garnet‐bearing, oceanic island basalt (OIB) ‐like mantle than postplateau lavas. The highest percentage melts erupted in the western and central plateaus. In a migrating slab window model, main‐plateau lavas can be explained as melts that formed as upwelling, subslab asthenosphere which flowed around the trailing edge of the descending Nazca Plate and then interacted with subduction‐altered asthenospheric wedge and continental lithosphere. Alkaline, postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window. Thermal problems of high‐pressure melt generation of anhydrous mantle can be explained by volatiles (H2O and CO2) introduced by the subduction process into slab window source region(s). An OIB‐like, rather than a mid‐ocean ridge basalt (MORB) ‐like source region, and the lack of magmatism northeast of where ridge collision occurred at ≈13 to 14 Ma can be explained by entrainment of “weak” plume(s) or regional variations in an ambient, OIB‐like asthenosphere.
Origin of the Betic‐Rif mountain belt
Authors: Lonergan Lidia , White Nicky
Published: February 2004

In recent years, the origin of the Betic‐Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late‐orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic‐Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present‐day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic‐Rif orocline during the early to middle Miocene.
Evaluation of Transient Dissolution of Slightly Water‐Soluble Compounds From a Light Nonaqueous Phase Liquid Pool
Authors: Holman H.‐Y. N. , Javandel I.
Published: February 2004

A transient two‐dimensional mathematical model was developed using anadvection‐dispersion equation to describe the dissolution and transport of slightly watersoluble compounds (SWSCs) from a pool of light nonaqueous phase liquid (LNAPL) into an aquifer. The LNAPL pool is located in a capillary fringe, and the bottom of the pool is in direct contact with the aquifer. In the LNAPL pool, SWSCs are assumed to move by liquid diffusion, and within the aquifer by mass flow under the influence of steady horizontal groundwater flow. The dissolution process along the LNAPL/water interface assumed a local phase equilibrium relation. A formal analytical solution to the mathematical model was first derived in the Laplace transform domain and inverted numerically by means of the Bellman algorithm. This solution was used to calculate thedissolution of a SWSC from an LNAPL pool to groundwater and to identify physicochemical factors that could significantly affect this transient mass transfer process. Results show that within the range of parameters we used in the study, the SWSC flux,which is represented by the modified Damköhler number
, can be related to the Fourier number Fo, the transverse hydrodynamic Peclet number Pet, and the ratio of nonaqueous diffusion Peclet number PeN to PeT(or DT/DN) through a simple power lawrelationship.
Measurement of Evaporation from Bare Soil and its Estimation Using Surface Resistance
Authors: Daamen Carl C. , Simmonds Lester P.
Published: February 2004

Evaporation from soil, Es, is important to land surface energy balance and has been estimated in many studies using a surface resistance approach. We investigate the accuracy of this approach using detailed measurement and simulation. Hourly evaporation rates were measured using microlysimeters and load cells at two semiarid sites with bare soil. A numerical model of water (liquid and vapor) and heat fluxes in a soil profile (the soil water, energy, and transpiration (SWEAT) model) provided an accurate simulation of measured evaporation rates. Using output from SWEAT, relationships between soil resistance rs and soil surface water content θs (0–20 and 0–50 mm) are determined and are then used to estimate Es. These rs‐based models performed well over a period of several days but provided poor estimates of Es on an hourly or even a daily basis. A characteristic divergence between measured Es rates and potential evaporation rates at a time during the early daylight hours was not well simulated by rs‐based models. Anrs(θs) function for a similar soil at a different location underestimated Es by about 60%, Our work suggests that rs calculated from both evaporative demand and near‐surface soil water content θs is likely to be more accurate.
Comparison of the Evaporative Fraction and the Priestley‐Taylor α for Parameterizing Daytime Evaporation
Authors: Crago Richard D.
Published: February 2004

This is a comparison between the evaporative fraction EF, which is the surface latent heat flux divided by the available energy, and the Priestley‐Taylor parameter α. It is shown for the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) that the hypothesis that the midday values of EF and α are the same as their daytime average values must be rejected at the 0.05 level of significance. However, correlations are high between the midday and daytime average values for both variables. Daytime average latent heat fluxes were estimated based on midday values of EF and α along with daytime average available energy and equilibrium evaporation. In addition, the relationships between soil moisture and the midday and daytime average values of EF and α were considered, along with the utility of these relationships in determining daytime total evaporation. The conclusion is that at FIFE formulations based on EF are essentially interchangeable with corresponding formulations based on α.
A fuzzy rule‐based approach to drought assessment
Authors: Pesti Géza , Shrestha Biijaya P. , Duckstein Lucien , Bogárdi István
Published: February 2004

A methodology for predicting regional droughts from atmospheric pressure patterns is presented. Drought characteristics are strongly related to general circulation patterns (CP). CPs are determined from daily atmospheric pressure data. The link between large‐scale CPs and regional scale droughts is modeled using a fuzzy rule‐based approach. A fuzzy rule‐based model operates on an “if” → “then” principle, where “if” corresponds to a vector of fuzzy inputs and “then” corresponds to some fuzzy consequences. The rules are derived from a so‐called training set which includes a daily time series of CP classes and a corresponding monthly sequence of Palmer Drought Severity Indices (PDSI). Split sampling of historical data available for a 35‐year time period is used to derive and then to validate the rules. Then, these fuzzy rules may be applied to predict droughts in terms of atmospheric circulation patterns. The occurrence and persistence of CPs are expected to vary under global climate change. Thus the approach may also be useful in estimating the potential impact of climatic change (e.g., 2 × CO2 scenario) on droughts. The methodology is illustrated using drought index data from New Mexico and atmospheric pressure data over the western United States.
Field Tracer Tests on the Mobility of Natural Organic Matter in a Sandy Aquifer
Authors: McCarthy J. F. , Gu B. , Liang L. , Mas‐Pla J. , Williams T. M. , Yeh T.‐C. J.
Published: February 2004

The field‐scale transport of natural organic matter (NOM) was examined in a two‐well forced gradient injection experiment in a sandy, coastal plain aquifer in Georgetown, South Carolina. Spatial moments described the migration of the center of mass of NOM and conservative tracer. Temporal moments were used to estimate mass loss and retardation of the NOM along a transect of six sampling locations at two depths and at the withdrawal well. Large differences were observed in transport behavior of different subcomponents of NOM. Larger and more strongly binding NOM components in the injection solution are postulated to adsorb and displace weakly binding, low‐molecular weight NOM in groundwater. Conversely, NOM components that were similar to the groundwater NOM were transported almost conservatively, presumably due to “passivation” of the aquifer by previously adsorbed components of the groundwater NOM. NOM may thus exhibit two types of effects on contaminant dynamics in the subsurface. When the equilibria between solution and solid phase NOM is disrupted by introduction of a novel source of NOM, descriptions of the multicomponent transport process are complex and predictive modeling is problematic. Because of the differences in transport behavior of NOM subcomponents, the chemical properties and, more importantly, the functional behavior of NOM with respect to contaminant migration will vary with time and distance along a flow path. However, when groundwater NOM exists at a steady state with respect to adsorption on aquifer surfaces, the migration of NOM, and the contaminant‐NOM complex, may be approximated as the transport of a conservative solute.
Drought and Regional Hydrologic Variation in the United States: Associations with the El Niño‐Southern Oscillation
Authors: Piechota Thomas C. , Dracup John A.
Published: February 2004

Using 94 years of monthly Palmer Drought Severity Index (PDSI) data for 344 climate divisions, this study investigates the hydroclimatic response in the United States to the extreme phases of the Southern Oscillation (El Niño and La Nina). Several regions of coherent response to El Niño‐Southern Oscillation (ENSO) are identified. The strongest relationship between El Niño and extreme drought years is found in the Pacific Northwest. A strong relationship is also noticed in the southern United States, where dry conditions occur consistently during La Niña events. Next, the conditional response in PDSI is evaluated based on the extreme phases of the Southern Oscillation. The PDSI results were compared to similar analyses on 41 years of station temperature, precipitation, and streamflow data. A consistent response is seen in the other hydroclimatic variables, though the most filtered response is seen in PDSI data and streamflow data. The major contribution to the understanding of the ENSO–United States climate relationship is the evaluation of the general form of drought )nd comparison of these results to the fundamental hydrologic processes (precipitation, temperature, and streamflow).
Modeling Bacteria‐Facilitated Transport of DDT
Authors: Saiers James E. , Hornberger George M.
Published: February 2004

Migration of dissolved contaminants through subsurface environments may be enhanced by mobile colloidal particles. Lindqvist and Enfield [1992] have demonstrated that bacteria are capable of adsorbing and accelerating the movement of dichlorodiphenyltrichloroethane (DDT) through columns containing sand. In the work reported here, a mathematical model for describing this bacteria‐facilitated transport is presented. Our results indicate that the experimental data of Lindqvist and Enfield [1992] are most closely described by a model that incorporates a Freundlich isotherm equation for DDT adsorption to the bacteria, a second‐order rate law for bacterial deposition, and a two‐site, kinetic formulation for DDT adsorption by the porous medium.
Simple Disaggregation by Accurate Adjusting Procedures
Authors: Koutsoyiannis Demetris , Manetas Alexandros
Published: February 2004

A multivariate disaggregation method is developed for stochastic simulation of hydrologie series. The method is based on three simple ideas that have been proven effective. First, it starts using directly a typical PAR(1) model and keeps its formalism and parameter set, which is the most parsimonious among linear stochastic models. This model is run for the lower‐level variables without any reference to the known higher‐level variables. Second, it uses accurate adjusting procedures to allocate the error in the additive property, i.e., the departure of the sum of lower‐level variables within a period from the corresponding higher‐level variable. They are accurate in the sense that they preserve explicitly certain statistics or even the complete distribution of lower‐level variables. Three such procedures have been developed and studied in this paper, both theoretically and empirically. Third, it uses repetitive sampling in order to improve the approximations of statistics that are not explicitly preserved by the adjusting procedures. The model, owing to the wide range of probability distributions it can handle (from bell‐shaped to J‐shaped) and to its multivariate framework, is useful for a plethora of hydrologic applications such as disaggregation of annual rainfall or runoff into monthly or weekly amounts, and disaggregation of event rainfall depths into partial amounts of hourly or even less duration. Such real‐world hydrologic applications have been explored in this study to test the model performance, which has proven very satisfactory.
Conditional stochastic analysis of solute transport in heterogeneous, variably saturated soils
Authors: Harter Thomas , Yeh T.‐C. Jim
Published: February 2004

A method is developed for the conditional (Monte Carlo) simulation of steady state flow and transient transport from point sources in variably saturated porous media. It combines the geostatistical method, a linearized approximation of the soil water tension perturbation solution, and a finite element numerical model. The method is used to investigate the usefulness of conditional simulation for predicting solute transport under a variety of sampling network designs applied to a number of hypothetical soils. Saturated hydraulic conductivity data yield the largest reduction of conditional uncertainty in relatively wet soils with mild heterogeneities. In highly heterogeneous soils or under dry conditions, soil water tension data by themselves, taken at a sampling density of one to two correlation scales along the expected mean travel path, can greatly reduce prediction uncertainty about solute concentration. Parameter uncertainty about statistical properties of the independent random variables becomes less important as the number of conditioning data increases. However, even with a very high number of sampling data, uncertainty of predicted concentration levels remains significant.
Influence of spatially variable soil hydraulic properties and rainfall intensity on the water budget
Authors: Kim C. P. , Stricker J. N. M.
Published: February 2004

Monte Carlo simulation is employed to investigate the separate and simultaneous effect of horizontal heterogeneity of soil hydraulic properties and rainfall intensity on various statistical properties of the components of the one‐dimensional water budget. One year of micrometeorological time series consisting of half‐hourly values, generated with a one‐dimensional climate model, constitutes the upper boundary conditions. Both a loamy and a sandy soil type are used. Spatial heterogeneity of rainfall is generated by disaggregation of the spatial mean rainfall. Compared with the annual water balance obtained using the scaled average soil hydraulic properties and spatial mean rainfall intensities, spatial heterogeneity of these induces significant changes in the mean values of the annual balance terms. Spatial variation of saturated water content has a negligible effect on the annual water balance and the dynamics of water storage. It appears that a homogeneous “equivalent” soil exists for the sandy soil but not for the loamy soil.
Lagrangian Analysis of Nonreactive Pollutant Dispersion in Porous Media by Means of the Particle Image Velocimetry Technique
Authors: Cenedese Antonio , Viotti Paolo
Published: February 2004

An experimental technique based on image analysis was used to perform a Lagrangian description of passive pollutant particle motion in a three‐dimensional saturated porous medium. To allow for optical access, the experiment was carried out with Pyrex grains as the solid matrix and glycerol as the liquid phase in order to have two phases with the same refractive index. Statistical analysis of the experimental data allowed for estimation of velocity and displacement probability density functions (pdf), velocity component correlation functions, Lagrangian integral scales, and mechanical dispersion coefficient tensor components. The results obtained suggest that the longitudinal velocity component has a log normal pdf while the transversal component has a symmetrical pdf, which is nevertheless not Gaussian for high values of the kurtosis. Furthermore, the velocity components' autocorrelation functions are well represented by exponential laws, and the integral scale is dependent on filtration velocity and grain size. As foreseen in the theory the total displacement pdf shows the tendency to reach normal distribution after many integral scales. The evaluated dispersion coefficient tensor components are dependent on travel time; the components start from zero and reach an asymptotic value after several integral scales. Furthermore, the tensor is anisotropic, with the longitudinal component greater than the transversal one by about 1 order of magnitude. Comparison with other experimental data shows agreement at least for the longitudinal dispersion component. Dagan's linear theory has been used for comparing the analytical longitudinal component of the dispersion tensor with that obtained by means of the experiments.
Predicting the Topographic Limits to a Gully Network Using a Digital Terrain Model and Process Thresholds
Authors: Prosser Ian P. , Abernethy Bruce
Published: February 2004

A digital terrain model is used with process thresholds to predict the extent of a stable gully network in a 5 km2 catchment of the southeastern highlands of Australia. The model, developed by Dietnch et al. [1992, 1993], predicts the topographic controls on channel networks and interprets these in terms of a critical shear stress for channel incision (τc) applied by saturation overland flow. We adapt the model slightly to compare the shear stress applied by Hortonian overland flow to that applied by saturation overland flow. The limits to gully erosion in the catchment are controlled strongly by a topographic threshold that has an inverse relationship between upslope catchment area and local gradient. The topographic threshold for channel incision is reproduced using a simple model of Hortonian overland flow and a τc appropriate for incision into a degraded grass surface (τc = 245 dyn/cm2). This is consistent with historical evidence for the timing of gully erosion. The study confirms a strong topographic control on the extent of the channel network in a catchment significantly different from the western North America catchments where the topographic threshold was first demonstrated. Despite its simplicity, the model for incision by overland flow appears capable of distinguishing the hydrological processes responsible for channel incision when these are reflected in the relationship between channel network and landscape morphology. The model requires relatively simple inputs, suggesting it may be useful for mapping gully erosion hazard in actively eroding catchments.
Implications of Spatial Variability of Soil Physical Properties for Simulation of Evaporation at the Field Scale
Authors: Lewan Elisabet , Jansson Per‐Erik
Published: February 2004

Soil water retention curves in eight soil profiles on a sandy soil were determined by field measurements of soil water content and soil water matric potential made during 19 days of continuous drying using time‐domain‐reflectometry and tensiometers. Evaporation from the different profiles was simulated using the retention curves and estimated unsaturated conductivity functions, meteorological data as driving variables, and measured soil water matric potentials at 40 cm depths as the lower boundary condition. Good agreement was obtained between simulated and measured evaporation from different locations in the field. Areal mean evaporation obtained as the arithmetic mean of individual simulations was 2% smaller than the evaporation obtained using mean physical properties in one simulation. This difference was small compared to the variation in simulated evaporation between individual locations (62–142% of the meanevaporation).
The moving blocks bootstrap versus parametric time series models
Authors: Vogel Richard M. , Shallcross Amy L.
Published: February 2004

The application of a parametric time series model to a water resources problem involves selecting a model and estimating its parameters, both steps adding uncertainty to the analysis. The moving blocks bootstrap is a simple resampling algorithm which can replace parametric time series models, avoiding model selection and only requiring an estimate of the moving block length. The moving blocks bootstrap resamples the observed time series using approximately independent moving blocks. A Monte Carlo experiment is performed involving the use of a time series model to estimate the storage capacity S of a surface water reservoir. Our results document that the bootstrap always produced storage estimates with lower root‐mean‐square‐error than a parametric alternative, even when no model error is introduced into the parametric scheme. These results suggest that the moving blocks bootstrap can provide a simple and attractive alternative to more complex multivariate ARMA models.
Sand Erosion by Mobile Predaceous Stream Insects: Implications for Ecology and Hydrology
Authors: Statzner Bernhard , Fuchs Ulrike , Higler Lambertus W. G.
Published: February 2004

Despite increasing knowledge of the ability of keystone animal species (“ecosystem engineers”) to change their physical environment, there is little and inconsistent evidence that benthic invertebrates affect the erosion of bottom material in streams. Therefore we designed field stream experiments and observations to investigate the effect of mobile predaceous stonefly (Dinocras cephalotes) larvae on sand erosion. When short of prey, the stoneflies erode sand from stream riffles thereby deepening the interstices between cobbles. On the basis of reasonable assumptions, we speculate that Dinocras has an erosion potential of about 200–400 kg sand m−2 yr−1 at natural population densities under favorable flow conditions. We consider the possible implications of the bioturbation potential of stream invertebrates for ecology (habitat quality and disturbance) and hydrology (sand transport and stability of coarse stream bottoms), which call for joint research on this novel role invertebrates play in the functioning of stream ecosystems.
Reaction‐Transport Models for Radiocarbon in Groundwater: The Effects of Longitudinal Dispersion and the use of Sr Isotope Ratios to Correct for Water‐Rock Interaction
Authors: Johnson T. M. , DePaolo D. J.
Published: February 2004

We expand a model for isotope ratios in groundwater‐rock systems to consider the ratio of 14C to dissolved inorganic carbon, or “percent modern carbon.” The model couples one‐dimensional advection and dispersion with water‐rock interaction and radioactive decay. We use a Fickian formulation to model longitudinal macrodispersion and present a formula for the groundwater velocity error introduced when dispersion is ignored, as a function of dispersivity and decrease in percent modern carbon. For example, if the longitudinal dispersivity were one tenth the length of observation and percent modern carbon decreased tenfold along a flow path, the error would be 23%. We also present a method for inversion of 87Sr/86Sr and 14C data to obtain carbonate dissolution rates and dissolution‐corrected water velocities and apply this technique using published data from the Lincolnshire aquifer, England. The results are similar to those obtained using the δ13C method.
Influence of the Depth Hoar Layer of the Seasonal Snow Cover on the Ground Thermal Regime
Authors: Zhang T. , Osterkamp T. E. , Stamnes K.
Published: February 2004

Snow cover is a good insulator since it has low thermal conductivity. The structure of the seasonal snow cover usually consists of higher‐density layers at the top with coarse, lower‐density depth hoar layers at the base. Because of its much smaller thermal conductivity, changes in depth hoar fraction can have a significant impact on the insulating effect of the seasonal snow cover. A one‐dimensional finite difference conductive heat transfer model with phase change was applied to investigate the effect of variations in the depth hoar fraction of the seasonal snow cover on the ground thermal regime. The snow cover was treated as a single layer with the effective thermal properties determined with considerations of the effect of the wind slab and depth hoar layer. The model was applied to investigate the thermal regime of the active layer and permafrost from October 5, 1986, through July 13, 1991, at West Dock near Prudhoe Bay, Alaska. Results indicate that the calculated temperatures on the ground surface, in the active layer, and in deep permafrost are in excellent agreement with the measured temperatures. A sensitivity study shows that in permafrost regions, variations in the depth hoar fraction from 0.0 to 0.6 can increase the daily ground surface temperature 12.8°C and the mean annual ground surface temperature by 5.5°C and delay the active layer freeze‐up by several months. In nonpermafrost regions an increase in the depth hoar fraction from 0.0 to 0.6 can increase the daily ground surface temperature by 8.4°C and the mean annual ground surface temperature by up to 2.4°C and reduce the seasonal freezing depth by up to 80%.
Multifractal Properties of Daily Rainfall in Two Different Climates
Authors: Svensson C. , Olsson J. , Berndtsson R.
Published: February 2004

The multifractal properties of daily rainfall were investigated in two contrasting climates: an east Asian monsoon climate (China) with an extreme rainfall variability and a temperate climate (Sweden) with a moderate rainfall variability. First, daily time series were studied. The results showed that daily rainfall in both climates can be viewed as the result of a multiplicative cascade process for the range 1–32 days. The temporal data exhibited scaling for moments of orders up to 2.5 in the monsoon area and up to 4.0 in the temperate area and showed clear multifractal properties in both climates. Second, daily spatial rainfall distributions were pooled into different rainfall‐generating mechanism groups, and each group was analyzed separately. The spatial data for all rainfall mechanisms in the two climates exhibited scaling for moments of orders up to 4.0. The scaling regime was 15–180 km (225–32,400 km2) in the monsoon climate and 7.5–90 km (55–8100 km2) in the temperate climate. A multifractal framework seemed well suited for description of convective‐type rainfall in both climates, but its suitability for frontal rainfall in the two regions was less clear. Although the frontal rainfall exhibited scaling, the almost linear τ(q) functions suggested monofractality.
Role of Organic Solutes in the Chemistry Of Acid‐Impacted Bog Waters of the Western Czech Republic
Authors: Hruška Jakub , Johnson Chris E. , Krám Pavel
Published: February 2004

In many regions, naturally occurring organic acid anions can effectively buffer mineral acid inputs from atmospheric deposition, moderating their effect on surface water pH. We studied the effect of chronically high inputs of acid rain on the chemistry of three brown‐water streams in the western Czech Republic. The dissolved organic acids in the streams were similar in character to those of other systems in Europe and North America. The site densities (the carboxyl group content per mass of C) were similar to values reported from Fenno‐Scandia, and the relationship between the apparent pKa and pH conformed to those from two North American studies. Sulfate and organic acid anions (OA−) were the dominant anions in all three streams, yet despite high dissolved organic carbon and total organic acid concentrations, OA − comprised only 21–32% of total anion charge. This pattern was due to very high sulfate concentrations and, in two of the streams, a low degree of dissociation of the organic acids, probably the results of high long‐term inputs of strong acids. Stream water pH was highly correlated to sulfate concentration, but uncorrelated with OA−, suggesting that free acidity is controlled by strong mineral acids rather than organic acids. Thus future reductions in strong acid inputs should result in increased pH and a return to organic control over acid‐base chemistry.
Computed tomography imaging of air sparging in porous media
Authors: Chen May‐Ru , Hinkley Richard E. , Killough John E.
Published: February 2004

This paper describes the use of an X ray computerized tomography scanner for noninvasive, three‐dimensional, high‐resolution imaging of air distribution patterns during air sparging in water‐saturated sandpacks. Experiments were performed in Plexiglas cylindrical cells using well‐defined Ottawa sands. The reconstructed images provide detailed porosity and air saturation distributions previously unavailable. Two classes of behavior were observed. In high‐permeability (106 darcy) uniform sandpacks a relatively few, discrete, and tortuous air pathways were formed. Steady state was rapidly attained, and the intrachannel air saturations were low (0.10). In contrast, low‐permeability (3 darcy) sandpacks increased the breadth of air contact, delayed attainment of steady state, and displayed high air saturations (0.50). The concept of a hydrodynamic stagnation saturation and standard one‐dimensional fractional flow theory proved useful in explaining the experiments. Numerical simulation confirms the explanations, even when capillary pressure and compressibility effects are included.
Evaluation of Spatially Distributed Ground‐Based and Remotely Sensed Data to Estimate Spatially Distributed Sensible Heat Fluxes
Authors: Qualls Russell J. , Brutsaert Wilfried
Published: February 2004

The First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) was initiated partly to improve our ability to model spatial distributions of surface‐atmospheric fluxes over hilly prairie. Monin‐Obukhov similarity was used to calculate sensible heat fluxes (Hc) at an array of ten FIFE flux measurement sites for comparison with measured sensible heat fluxes (Hm). Data were collected within the dynamic sublayer on a clear morning when there was strong solar heating of the surface, winds in excess of 5 m s−1, and uniformly wet soil conditions. The sensitivity of correlations between Hc and Hm to spatial variability of air (Ta) and aircraft‐based (thermal infrared multispectral scanner (TIMS)) remotely sensed surface (Tr) temperatures, wind speed )u), and an atmospheric stability parameterization (ψh) was examined. Hc was found to depend on the spatial variability of Tr and u but not on Ta and ψh. Furthermore, approximately half the discrepancy between Hc and Hm may be attributed to uncertainty in Hm.
Estimating Lithologic and Transport Properties in Three Dimensions Using Seismic and Tracer Data: The Kesterson aquifer
Authors: Hyndman David W. , Gorelick Steven M.
Published: February 2004

The identification of aquifer heterogeneities, particularly flow paths and barriers, has become a critical research topic in hydrology. Cross‐well seismic tomography may provide the needed resolution when used in conjunction with hydraulic head and tracer concentration measurements. We demonstrate a field application and sensitivity analysis of the split inversion method (SIM), which combines seismic, hydraulic, and tracer data to estimate the three‐dimensional zonation of aquifer properties along with the hydraulic properties for these zones. For the Kesterson aquifer in the San Joaquin Valley, California, we first invert seismic travel times measured between six well pairs to obtain seismic slowness (1/seismic velocity) cross sections, or tomograms. We then use conditional simulation to provide three‐dimensional seismic slowness realizations. Next, the SIM is used to split several realizations into three lithologic zones and assign hydraulic properties to the zones to best match six tracer concentration histories.
Effect of Local Dispersion on Solute Transport in Randomly Heterogeneous Media
Authors: Zhang Dongxiao , Neuman Shlomo P.
Published: February 2004

Recently, an exact Eulerian‐Lagrangian theory of advective transport in space‐time random velocity fields was developed by one of us. We present a formal extension of this theory that accounts for anisotropic local dispersion. The resultant (conditional) mean transport equation is generally nonlocal in space‐time. To assess the impact of local dispersion on the prediction of transport under uncertainty, we adopt a first‐order pseudo‐Fickian approximation for this equation. We then solve it numerically by Galerkin finite elements for two‐dimensional transport from an instantaneous square source in a uniform (unconditional) mean flow field subject to isotropic local dispersion. We use a higher‐order approximation to compute explicitly the standard deviation and coefficient of variation of the predicted concentrations. Our theory shows (in an exact manner), and our numerical results demonstrate (under the above closure approximations), that the effect of local dispersion on first and second concentration moments varies monotonically with the magnitude of the local dispersion coefficient. When this coefficient is small relative to macrodispersion, its effect on the prediction of nonreactive transport under uncertainty can, for all practical purposes, be disregarded. This is contrary to some recent assertions in the literature that local dispersion must always be taken into account, no matter how small.
Dual‐Porosity Fractal Models for Transient Flow Analysis in Fissured Rocks
Authors: Hamm Se‐Yeong , Bidaux Pascal
Published: February 2004

A dual‐porosity fractal model is derived that considers transient flow from block to fissure, skin between the fissure and the block, and storage capacity and skin effect on the pumping well. Type curves for different flow dimensions are provided. They concern the pumping well and observation wells located either in the fissure system or in a matrix block. The interpretation of a field case using this model shows good agreement between the theoretical drawdown and the observed data, with consistent results from the pumping well and the observation well. A flow dimension of 1.45 is obtained.
Modeling Hydraulic Conductivity of a Complex Confining Layer at Various Spatial Scales
Authors: Bierkens Marc F. P.
Published: February 2004

The spatial variation of the hydraulic conductivity of a complex confining layer in the Netherlands is modeled at four different scales: the scale of sediment cores (10−1–100 m), the scale of the numerical model blocks (101–102 m), the local scale (102–103 m), and the regional scale (103–105 m). Two stochastic methods are combined: (1) conditional (multiple indicator) simulation of texture classes to obtain three‐dimensional images of the lithology of the confining layer from borehole descriptions and (2) the statistical upscaling procedure of Indelman and Dagan [1993a, b] to derive the first‐ and second‐order statistics of hydraulic conductivities at the scale of the numerical model blocks. When the upscaling of the core‐scale conductivities is ignored, the local‐ and regional‐scale groundwater flow through the confining layer is predicted wrongly.
A comparison of surface sampling methods for coarse fluvial sediments
Authors: Wohl Ellen E. , Anthony Deborah J. , Madsen Susan W. , Thompson Douglas M.
Published: February 2004

In order to characterize variability associated with sampling coarse fluvial sediment, surface grain‐size distributions were characterized at eight sample sites within gravel‐ to boulder‐bed channels. Four methods were used: (1) a random walk, (2) a sampling grid spaced at an interval equal to the intermediate diameter of the largest clast in the sampling area, (3) a sampling grid spaced at one half the interval of method 2, and (4) a randomly chosen subsection within which a patch count of most clasts at least partially exposed at the surface was conducted. At each site, six replicates of each method were performed, with each replicate consisting of 100 clasts. Sampling was performed by a single operator at six of the sites and by multiple operators at two of the same sites and at two additional sites. The research objectives were to characterize variability among replicates of a method, among methods, and among operators. Sample sites were divided into relatively well sorted (inclusive graphic standard deviation <1.4) and poorly sorted (≥1.5). The first three methods produce statistically indistinguishable values of D50 and D84 for both well‐sorted and poorly sorted sites when performed by a single operator. Multiple operators may produce statistically different population measures (D50, D84, sample distributions, and variance) using any of the four methods at either type of site. Because of operator‐induced variability, we caution against direct comparisons of grain‐size distributions between channels or within a single channel through time if more than one operator performs the sampling.
Geochemical Estimates of Paleorecharge in the Pasco Basin: Evaluation of the Chloride Mass Balance Technique
Authors: Murphy Ellyn M. , Ginn Timothy R. , Phillips Jerry L.
Published: February 2004

The Pasco Basin in southeastern Washington State provides a unique hydrogeologic setting for evaluating the chloride mass balance technique for estimating recharge. This basin was affected by late Pleistocene catastrophic floods when glacial dams in western Montana and northern Idaho were breached. It is estimated that multiple Missoula floods occurred between ∼13,000 and 15,000 years B.P. and reached a high water elevation of ∼350 m. These floods removed accumulated chloride from the sediment profile, effectively resetting the chloride mass balance clock at the beginning of the Holocene. The rate of chloride accumulation qCl in the sediments was determined by two methods and compared. The first method measured qCl by dividing the calculated natural fallout of 36Cl by a measured ratio of 36Cl/Cl in the pore water, while the second method used the total mass of chloride in the profile divided by the length of time that atmospheric chloride had accumulated since the last flood. Although the two methods are based on different approaches, they showed close agreement. In laboratory studies the sediment to water ratio for chloride extraction was sensitive to the grain size of the sediments; low extraction ratios in silt loam sediments led to significant underestimation of pore water chloride concentration. Br/Cl ratios were useful for distinguishing nonatmospheric (e.g., rock) sources of chloride. Field studies showed little spatial variability in estimated recharge at a given site within the basin but showed significant topographic control on recharge rates in this semiarid environment. An extension of the conventional chloride mass balance model was used to evaluate chloride profiles under transient, time‐varying annual precipitation conditions. This model was inverted to determine the paleorecharge history for a given soil chloride profile, and the parameters of the root extraction model required to estimate paleoprecipitation
Observations of Flow and Sediment Entrainment on a Large Gravel‐Bed River
Authors: Wilcock Peter R. , Barta Alan F. , Shea Conor C. , Kondolf G. Mathias , Matthews W. V. Graham , Pitlick John
Published: February 2004

Constant‐discharge reservoir releases on the Trinity River, California, provide an unusual opportunity to unambiguously relate flow and gravel entrainment on a large gravel‐bed river. Bed shear stress т0 was estimated using local observations of depth‐averaged velocity. Gravel entrainment was measured using large tracer gravel installations. Lateral variability of т0 is large, even for straight channels with simple, trough‐like geometry. No simple relation exists between local and cross‐section mean values of т0 . Fine grains (less than 8 mm; 20–30% of the bed material) are transported at lower discharges than coarse grains. Scour to the base of the bed surface layer occurs at a dimensionless shear stress тg* ≈ 0.035, for тg* formed using local т0 and the median grain size of the gravel portion of the bed. The dimensionless reference transport rate W* = 0.002, often used as a surrogate for the threshold of grain motion, occurs at nearly the same тg*. At smaller тg*, entrainment and transport rates decrease rapidly, becoming vanishingly small at тg* ≈ 0.031. Even at very small gravel transport rates, all sizes are transported, although the coarsest sizes are in a state of partial transport in which only a portion of the exposed grains are entrained. Both entrainment and cumulative transport observations suggest that maximum scour depth for plane‐bed transport is slightly less than twice the surface layer thickness.
Effective Dispersion of a Solute Cloud in a Chemically Heterogeneous Porous Medium: Comparison of Two Ensemble‐Averaging Procedures
Authors: Metzger D. , Kinzelbach H. , Kinzelbach W.
Published: February 2004

We consider solute transport in a porous medium for which we assume the retardation factor R(x), resulting from linear chemical adsorption, to be stochastically varying in space. For large times, the evolution of a solute plume developing from a pointlike, instantaneous solute injection is described by its effective velocity and dispersion. We calculate such quantities using perturbation theory and two different averaging procedures. The first and correct procedure calculates the central moments of the cloud for a given aquifer realization and averages over the ensemble afterward. The second method, which is mathematically less demanding, obtains large‐scale transport coefficients from the moments of the ensemble‐averaged concentration distribution. This last approach is often used to replace the correct procedure, tacitly assuming that the two averaging methods lead to the same effective quantities. We show that the results actually differ in one dimension, whereas the difference vanishes in higher dimensions. The effective retardation factor is found to be the ensemble average of the corresponding small‐scale quantity. The effective dispersion coefficient, on the other hand, differs from the retarded small‐scale dispersion coefficient. It is significantly enhanced by the inhomogeneous fluctuations of the disordered medium.