bluuebunny/crossref_metadata_2025 huggingface dataset:
Contemporary strain rates in the northern Basin and Range province from GPS data
Authors: Bennett R. A. , Wernicke B. P. , Niemi N. A. , Friedrich A. M. , Davis J. L.
Published: May 2003

We investigate the distribution of active deformation in the northern Basin and Range province using data from continuous GPS (CGPS) networks, supplemented by additional campaign data from the Death Valley, northern Basin and Range, and Sierra Nevada–Great Valley regions. To understand the contemporary strain rate field in the context of the greater Pacific (P)–North America (NA) plate boundary zone, we use GPS velocities to estimate the average relative motions of the Colorado Plateau (CP), the Sierra Nevada–Great Valley (SNGV) microplate, and a narrow north‐south elongate region in the central Great Basin (CGB) occupying the longitude band 114–117°W. We find that the SNGV microplate translates with respect to the CP at a rate of 11.4 ± 0.3 mm yr−1oriented N47 ± 1°W and with respect to NA at a rate of ∼12.4 mm yr−1also oriented N47°W, slower than most previous geodetic estimates of SNGV‐NA relative motion, and nearly 7° counterclockwise from the direction of P‐NA relative plate motion. We estimate CGB‐CP relative motion of 2.8 ± 0.2 mm yr−1oriented N84 ± 5°W, consistent with roughly east‐west extension within the eastern Great Basin (EGB). Velocity estimates from the EGB reveal diffuse extension across this region, with more rapid extension of 20 ± 1 nstr yr−1concentrated in the eastern half of the region, which includes the Wasatch fault zone. We estimate SNGV‐CGB relative motion of 9.3 ± 0.2 mm yr−1oriented N37 ± 2°W, essentially parallel to P‐NA relative plate motion. This rate is significantly slower than most previous geodetic estimates of deformation across the western Great Basin (WGB) but is generally consistent with paleoseismological inferences. The WGB region accommodates N37°W directed right lateral shear at rates of (1) 57 ± 9 nstr yr−1across a zone of width ∼125 km in the south (latitude ∼36°N), (2) 25 ± 5 nstr yr−1in the central region (latitude ∼38°N), and (3) 36 ± 1 nstr yr−1across a zone of width ∼300 km in the north (latitude ∼40°N). By construction there is no net extension or shortening perpendicular to SNGV‐CGB relative motion. However, we observe about 8.6 ± 0.5 nstr yr−1extension on average in the direction of shear from southeast to northwest within the Walker Lane belt, comparable to the average east‐west extension rate of 10 ± 1 nstr yr−1across the northern Basin and Range but implying a distinctly different mechanism of deformation from extension on north trending, range‐bounding normal faults. An alternative model for this shear parallel deformation, in which extension is accommodated across a narrow, more rapidly extending zone that coincides with the central Nevada seismic belt, fits the WGB data slightly better. Local anomalies with respect to this simple kinematic model may reveal second‐order deformation signals related to more local crustal dynamic phenomena, but significant improvements in velocity field resolution will be necessary to reveal this second‐order pattern.
Climate, soil, and vegetation controls upon the variability of water balance in temperate and semiarid landscapes: Downward approach to water balance analysis
Authors: Farmer Darren , Sivapalan Murugesu , Jothityangkoon Chatchai
Published: May 2003

Observed differences between water balance for temperate and semiarid catchments can be attributed to variability in the primary controls of the soil profile (soil water storage capacity and permeability), vegetation (surface coverage and water use efficiency), and climate (rainfall and potential evaporation). Motivated by the downward development methodology mooted by Klemes [1983], this paper explores the underlying climate and landscape interactions that cause differences in water balance between a number of temperate and semiarid benchmark catchments around Australia. A systematic parsimonious analysis based around a simple water balance model whose parameters are estimated from available data is used to observe and interpret signatures of annual, intra‐annual, and daily water balance behavior and variability. The essence of the downward analysis is that model complexity evolves from the simplest form to its final quasi‐distributed framework with multiple storages and process interactions in response to inadequacies in signature prediction. Consequently, each step in the process develops insight into local climate and landscape influences and sensitivity to their temporal and spatial variabilities. The results affirm that the relative influence of climate and landscape properties upon catchment response is manifested in a systematic transformation of process sensitivity with increasing timescales (annual, monthly, daily, and hourly). Significantly, drier catchments are shown to be more sensitive to small‐scale perturbation than humid catchments. We conclude that the downward analysis approach is a means to better understand the significance of landscape variability upon catchment response. Consequential knowledge can begin to explain data requirements and requisite model complexity and to provide insight into the ability to reliably predict water balance behavior. These are important first steps toward the longer‐term objective of reliably predicting water balance variability across ungauged basins.
Effective hydraulic conductivity of a randomly heterogeneous porous medium
Authors: Stepanyants Y. A. , Teodorovich E. V.
Published: May 2003

The effective hydraulic conductivity of a randomly heterogeneous isotropic porous medium has been calculated by means of a new perturbative technique, which differs greatly from that commonly used. In the approach developed, we construct the perturbation series for a seepage velocity and then utilize Darcy's law for calculating pressure. On the basis of the analysis of high‐order approximations it is shown that in the general case the effective conductivity in the large‐scale limit does depend on the form of the correlation function. Thus the widespread Landau‐Lifshitz‐Matheron formula, which operates on the conjecture that the first two terms of the perturbation series are those for the Taylor series expansion of an exponential function, proves to be invalid. The calculations were carried out in a space of arbitrary dimension. The dependence of the effective conductivity on the variance has been obtained up to the third‐order approximation inclusive in terms of a conductivity logarithm. For convenience and simplification of series analysis the Feynman diagrammatic technique was developed and utilized. In a one‐dimensional (1D) case our result in the large‐scale limit (when a heterogeneity scale is small enough with respect to the characteristic scale of groundwater flow) gives an exact formula for the effective conductivity.
Sediment detachment by rain power
Authors: Gabet Emmanuel J. , Dunne Thomas
Published: May 2003

In interrill areas, overland flow is often incapable of detaching soil particles so detachment is primarily by raindrop impact. We derive a mathematical expression, rain power (R, W m−2), relating the energy expenditure of raindrops impacting a soil surface to the rate of detachment of soil particles. Rain power incorporates rainfall, hillslope, and vegetation characteristics and is modulated by flow depths. Rainfall simulation experiments on natural hillslopes were performed to measure detachment rates and across‐slope flow depth distributions in surface runoff. Our results indicate that flow depths follow a Poisson distribution, and this observation is used to develop a dimensionless function, A(, d), that accounts for the interaction of flow depths (h) and raindrop diameter (d) in moderating detachment rates. Rain power correlates well with the detachment rate of fine‐grained particles (ψ, g m−2 s−1) so that ψ = 0.011R1.4A(, d) (n = 44, R2 = 0.88, p < 0.005). We generalize this result to represent natural rainfall conditions and present a method for modeling sediment detachment rates and sediment discharge along entire lengths of hillslopes under the range of conditions where detached sediment is transported as wash load. Modeling simulations demonstrate the temporal and spatial variation in detachment rates caused by increases in flow depth.
Determining salinization extent, identifying salinity sources, and estimating chloride mass using surface, borehole, and airborne electromagnetic induction methods
Authors: Paine Jeffrey G.
Published: May 2003

Using an example from an oil field in the semiarid Red River basin in Texas, we show that electromagnetic (EM) methods are useful in locating salinized soil and water, determining salinization extent, identifying likely salinity sources, and estimating the total mass of chloride within a saline‐water plume. Each of these aspects assists in managing salinization and assessing its impact. We used ground EM instruments to establish salinization boundaries and determine the range of electrical conductivity, airborne measurements to locate potential sources and quantify the lateral extent and intensity of salinization, and borehole measurements and time domain EM soundings to determine salinization depth and relate ground conductivity to chloride content. We estimated infiltration volume and total chloride mass in the plume from EM data and an empirical, site‐specific chloride:conductivity ratio established from well data. Because the measured conductivity of water strongly correlates with total dissolved solids concentration, mass estimation could be extended to any ionic constituent that covaries linearly with total dissolved solids concentration. EM methods owe their success to the large increase in electrical conductivity that occurs where highly conductive, saline water infiltrates geologic materials having naturally low conductivities.
Effective permeability of fractured porous media in steady state flow
Authors: Bogdanov I. I. , Mourzenko V. V. , Thovert J.‐F. , Adler P. M.
Published: May 2003

Flow in fractured porous media was first investigated by Barenblatt and Zheltov [1960] and Barenblatt et al. [1960] by means of the double‐porosity model. A direct, exact, and complete numerical solution of the flow in such media is given in this paper for arbitrary distributions of permeabilities in the porous matrix and in the fracture network. The fracture network and the porous matrix are automatically meshed; the flow equations are discretized by means of the finite volume method. This code has been so far applied to incompressible fluids and to statistically homogeneous media which are schematized as spatially periodic media. Some results pertaining to random networks of polygonal fractures are presented and discussed; they show the importance of the percolation threshold of the fracture network and possibly of the porous matrix. Moreover, the influence of the fracture shape can be taken into account by means of the excluded volume.
Determining the sources of suspended sediment in a forested catchment in southeastern Australia
Authors: Motha J. A. , Wallbrink P. J. , Hairsine P. B. , Grayson R. B.
Published: May 2003

Knowledge is scarce regarding the relative contributions from harvested areas and unsealed roads to sediments in forested catchments. We investigate the source composition of suspended sediment using sediment tracers and an improved multivariate mixing model. Sediment samples were collected during six rainfall events. Geochemical and radiometric tracer properties were corrected for particle size and organic content as well as conservativeness during erosion and sediment delivery. The mixing model incorporates variability of the tracer properties, using a Monte Carlo simulation technique. Mean sediment contribution from the undisturbed forest was 50–70%; harvested areas, gravel‐surfaced roads, and ungravelled roads contributed 5–15%, 6–14%, and 12–25%, respectively. The unsealed roads contribute 20 to 60 times more sediment than the undisturbed forest and about 10 times more sediment than the harvested areas on a per unit area basis. Harvested areas contribute 1 to 5 times greater sediment than the undisturbed forest. These results support other studies that identify unsealed roads as important sediment sources in forested catchments.
Aquifer parameter identification using the extended Kalman filter
Authors: Leng C. H. , Yeh H. D.
Published: May 2003

An approach using the extended Kalman filter (EKF) and cubic spline is proposed to identify the aquifer parameters in both confined and unconfined aquifer systems. The cubic spline applied to the observation data can generate interpolated data with uniform time intervals and facilitates the implementation of EKF. The EKF combined with the Theis solution or Neuman's model, using the interpolated drawdown data produced by cubic spline, can optimally determine the parameters through the recursive process. The proposed approach can quickly identify the parameters, using only part of observed drawdown data, and the obtained parameters are shown to have good accuracy. Thus length of time of pumping tests may be shortened. Comparisons of results from nonlinear least squares combined with finite difference Newton's method (NLN) and EKF show that the EKF allows a wider range of initial guess values than NLN and have the accuracy of the results on the same order of magnitude as that of NLN. When determining the aquifer parameters, the identification process of specific yield reflects the effect of gravity drainage on the drawdown curve and conforms to the physical nature of an unconfined aquifer. Furthermore, this study shows that EKF can be successfully applied to analyze the drawdown data even with white noises or temporally correlated noises.
Solute spreading in nonstationary flows in bounded, heterogeneous unsaturated‐saturated media
Authors: Lu Zhiming , Zhang Dongxiao
Published: May 2003

It is commonly assumed in stochastic solute (advective) transport models that either the velocity field is stationary (statistically homogeneous) or the mean flow is unidirectional. In this study, using a Lagrangian approach, we develop a general stochastic model for transport in variably saturated flow in randomly heterogeneous porous media. The mean flow in the model is multidirectional, and the velocity field can be nonstationary (with location‐dependent statistics). The nonstationarity of the velocity field may be caused by statistical nonhomogeneity of medium properties or complex boundary configurations. The particle's mean position is determined using the mean Lagrangian velocity. Particle spreading (the displacement covariances) is expressed in terms of the state transition matrix that satisfies a time‐varying dynamic equation whose coefficient matrix is the derivative of the mean Lagrangian velocity field. In the special cases of stationary velocity fields the transition matrix becomes the identical matrix, and our model reduces to the well‐known model of Dagan [1984]. For nonstationary but unidirectional flow fields our model reduces to that of Butera and Tanda [1999] and Sun and Zhang [2000]. The validity of the transport model is examined by comparisons with Monte Carlo simulations for the following three cases: transport in a mean gravity‐dominated flow, in an unsaturated flow with a water table boundary, and in a saturated‐unsaturated flow. An excellent agreement is found between our model results and those from Monte Carlo simulations.
Radar frequency dielectric dispersion in sandstone: Implications for determination of moisture and clay content
Authors: West L. Jared , Handley Kirk , Huang Yi , Pokar Magdeline
Published: May 2003

The relationship between dielectric permittivity, water saturation, and clay content for the Sherwood Sandstone from NE England was characterized as part of a wider study of the vadose zone moisture dynamics and pollution vulnerability of this aquifer. Dielectric permittivity was measured over the full range of saturation levels, for various lithologies ranging from clean medium‐grained sandstone to fine‐grained sandstone containing up to 5% clay, using a specially constructed dielectric cell. Dielectric constant, Kr, is largely independent of frequency between 350 MHz and 1000 MHz. Below 350 MHz, Kr of fine‐grained, clay‐rich sandstone shows frequency dispersion. Tests on physical models of the sandstone consisting of a fine fraction of Ottawa Sand and montmorillonite clay indicate that the clay minerals within the sandstone are responsible for its frequency dispersive behavior. These tests also show that increasing pore fluid salinity increases dielectric dispersion at the lower end of the frequency range, which indicates that this arises from the interfacial Maxwell‐Wagner mechanism associated with platy clay particles. Water saturated sand:clay mixtures show very low dielectric constants at high frequencies (over 650 MHz). This effect is independent of salinity and probably results from the layered geometric arrangement of solids, bound and free water within the swelling clay. The complex refractive index method (CRIM) with a mineral dielectric constant of about 5 provides a good match to the water saturation versus dielectric constant data for all Sherwood Sandstone lithologies at frequencies between 350 and 1000 MHz and for clay‐poor sandstone at lower frequencies. Below 350 MHz the presence of a few percent of clay in some Sherwood Sandstone lithologies raises their CRIM best fit mineral dielectric constant substantially.
A thermodielectric analyzer to measure the freezing and moisture characteristic of porous media
Authors: Bittelli Marco , Flury Markus , Campbell Gaylon S.
Published: May 2003

The freezing and moisture characteristics of porous media are difficult to measure. We have designed an instrument to measure the freezing characteristic of a porous medium in the range −20 to 0°C. Because of the similarity between the freezing characteristic and the moisture characteristic, the data obtained can be used to infer the moisture characteristic from about −30 to −23,600 J kg−1. Temperatures and unfrozen water contents are measured with a thermistor and a spiral‐shaped transmission line connected to an oscillator, respectively. Temperatures are converted to water potential using the Clapeyron equation. Experimental results obtained with the freezing technique were compared with vapor pressure methods. The four test media used, a silt loam soil, a silty‐clay loam, a clay, and a sandstone, showed good agreement between freezing and standard methods. The freezing technique described here has a water potential resolution of about −11 J kg−1. An entire characteristic can be determined within 24 hours.
Physical and hydraulic properties of volcanic rocks from Yucca Mountain, Nevada
Authors: Flint Lorraine E.
Published: May 2003

A database of physical and hydraulic properties was developed for rocks in the unsaturated zone at Yucca Mountain, Nevada, a site under consideration as a geologic repository for high‐level radioactive waste. The 5320 core samples were collected from 23 shallow (<100 m) and 10 deep (500–1000 m) vertical boreholes. Hydrogeologic units have been characterized in the unsaturated zone [Flint, 1998] that represent rocks with ranges of welding, lithophysae, and high and low temperature alteration (as a result of the depositional, cooling, and alterational history of the lithostratigraphic layers). Lithostratigraphy, the hydrogeologic unit, and the corresponding properties are described. In addition, the physical properties of bulk density, porosity, and particle density; the hydraulic properties of saturated hydraulic conductivity and moisture retention characteristics; and the field water content were measured and compiled for each core sample.
A physically based interpolation method for fluvially eroded topography
Authors: Niemann Jeffrey D. , Bras Rafael L. , Veneziano Daniele
Published: May 2003

Interpolation of elevation data from fluvially eroded topography is difficult for available methods because of the inherent roughness and drainage properties of the fine‐scale surface. We propose a new method that produces an interpolation surface by adapting a simulated topography to a given set of point elevation data. The simulated surface is produced by a simple model that rapidly simulates topography in dynamic equilibrium including the effects of tectonic uplift, fluvial incision, and hillslope diffusion. Agreement between the simulated topography and the elevation data is improved by iteratively adjusting a spatially varying erodability parameter. When the iterative procedure terminates, the simulated surface is in close but not in exact agreement with the observations. Exact agreement can usually be produced by distorting the simulated surface in a way that does not introduce local depressions. Interpolation surfaces generated by the proposed method have realistic drainage patterns without unrealistic depressions. In addition, the textural properties of the interpolation surfaces, including the distributions of slopes and curvatures, are better than those generated by other interpolation methods.
Estimation of flood inundation probabilities as conditioned on event inundation maps
Authors: Romanowicz Renata , Beven Keith
Published: May 2003

The generalized likelihood uncertainty estimation (GLUE) methodology is applied to the problem of predicting the spatially distributed, time‐varying probabilities of inundation of all points on a floodplain. Advantage is taken of the relative independence of different effective conveyance parameters to minimize the simulations required. Probability estimates are based on conditioning predictions of Monte Carlo realizations of a distributed quasi‐two‐dimensional flood routing model using maps of maximum inundation and aerial photographs of flooding in the area. The methodology allows posterior distributions of conveyance parameters to be estimated and maps of inundation potential probabilities to be drawn up for flood events of different magnitudes. The results suggest that combining information from different magnitude events should be done with care, as the distributions of effective parameter values may vary with event magnitude. The value of accurate topographic information that is consistent with mapped inundation is also highlighted. The methodology can be used to obtain dynamic probabilities of floodplain inundation in real time forecasting.
Valley incision by debris flows: Evidence of a topographic signature
Authors: Stock J. , Dietrich W. E.
Published: May 2003

The sculpture of valleys by flowing water is widely recognized, and simplified models of incision by this process (e.g., the stream power law) are the basis for most recent landscape evolution models. Under steady state conditions a stream power law predicts that channel slope varies as an inverse power law of drainage area. Using both contour maps and laser altimetry, we find that this inverse power law rarely extends to slopes greater than ∼0.03 to 0.10, values below which debris flows rarely travel. Instead, with decreasing drainage area the rate of increase in slope declines, leading to a curved relationship on a log‐log plot of slope against drainage area. Fieldwork in the western United States and Taiwan indicates that debris flow incision of bedrock valley floors tends to terminate upstream of where strath terraces begin and where area‐slope data follow fluvial power laws. These observations lead us to propose that the steeper portions of unglaciated valley networks of landscapes steep enough to produce mass failures are predominately cut by debris flows, whose topographic signature is an area‐slope plot that curves in log‐log space. This matters greatly as valleys with curved area‐slope plots are both extensive by length (>80% of large steepland basins) and comprise large fractions of main stem valley relief (25–100%). As a consequence, valleys carved by debris flows, not rivers, bound most hillslopes in unglaciated steeplands. Debris flow scour of these valleys appears to limit the height of some mountains to substantially lower elevations than river incision laws would predict, an effect absent in current landscape evolution models. We anticipate that an understanding of debris flow incision, for which we currently lack even an empirical expression, would substantially change model results and inferences drawn about linkages between landscape morphology and tectonics, climate, and geology.
Vertical sorting in bed forms: Flume experiments with a natural and a trimodal sediment mixture
Authors: Blom Astrid , Ribberink Jan S. , de Vriend Huib J.
Published: May 2003

Two sets of flume experiments were conducted to examine grain size selective transport and vertical sorting in conditions with migrating bed forms and bed load transport. In the two sets of experiments we used a sediment mixture from the river Rhine and a trimodal mixture, respectively. The vertical sorting profiles showed a downward coarsening trend within the bed forms, and in some experiments an essentially immobile coarse bed layer formed underneath the migrating bed forms. Three mechanisms contribute to the formation of such a coarse bed layer: (1) the avalanching process at the lee face, (2) conditions of partial transport in which a relatively large amount of coarse material does not participate in the transport process, and (3) the winnowing of fines from the trough surface and subsurface. The experiments show that vertical sorting fluxes not only occur through net degradation or aggradation but also through the migration of bed forms and through the variability in trough elevations. This is contradictory to the way vertical sorting processes are modeled in most existing sediment continuity models for nonuniform sediment. The present study is therefore also a plea for modifying existing sediment continuity models to account for vertical sorting processes other than through net aggradation or degradation.
Numerical simulations of preasymptotic transport in heterogeneous porous media: Departures from the Gaussian limit
Authors: Trefry M. G. , Ruan F. P. , McLaughlin D.
Published: May 2003

The objective of this work is to determine whether conventional solute macrodispersion theories adequately predict the behavior of individual subsurface plumes over time and length scales relevant to practical risk assessment and remediation activities. This issue is studied with a set of high‐resolution numerical simulations of conservative tracers moving through saturated two‐dimensional heterogeneous conductivity fields. The simulation experiments are designed to mimic long‐term field studies. Spatially correlated statistically stationary lognormal conductivity realizations are generated with a Fourier transform procedure. Steady state velocity solutions are calculated for these fields using an accurate Darcian solver. Velocity contour plots reveal the presence of disconnected networks of preferential pathways over a range of correlation lengths. Reverse flow cells are rare. The velocity probability density functions have exponential tails and strong longitudinal asymmetries. Solute concentrations are derived from the simulated velocity fields with an accurate taut‐spline transport code that minimizes numerical dispersion. The resulting plumes are tracked for travel distances of over one hundred spatial correlation lengths, corresponding to scales of practical interest. First moments and macrodispersivities, which measure the location and extent of the plume, are reasonably well approximated by conventional Fickian theories but continue to vary after long travel distances. This temporal variability highlights the slow convergence of the plume moments to Gaussian limits. Calculations of the relative dilution index, which is a measure of the plume mixing state, indicate strongly non‐Gaussian behavior. Other measures of plume structure suggested by anomalous dispersion theories also reveal the persistence of non‐Gaussian behavior after long travel distances. These measures appear to be more sensitive to non‐Gaussian behavior than the spatial moments. Taken together, the simulation results suggest that conservative solute plumes moving through statistically stationary random media may not converge to Gaussian limits even after traveling hundreds of log conductivity correlation scales.
Spatial evaluation of alternative nonpoint nutrient regulatory instruments
Authors: Whittaker Gerald , Färe Rolf , Srinivasan Raghavan , Scott David W.
Published: May 2003

Recent work has shown that because of differences in the physical environment across an area the choice of an instrument for regulation of nonpoint source agricultural pollution is an empirical question. The objective of this study is an empirical comparison of an economic incentive policy (an input tax) for reduction of agricultural fertilizer application with a command and control policy. Although a “first‐best” policy would be administered at the farm level, transactions costs practically require a single policy for the whole area. In an application to the Columbia plateau, a 300% input tax is shown to be preferred to a mandated 25% reduction in nitrogen fertilizer on financial and policy efficiency grounds.
Global‐local optimization for parameter structure identification in three‐dimensional groundwater modeling
Authors: Tsai Frank T.‐C. , Sun Ne‐Zheng , Yeh William W.‐G.
Published: May 2003

This paper develops two global‐local optimization methods for three‐dimensional parameter structure identification in groundwater modeling. Parameter structure identification is formulated in terms of solving a generalized inverse problem, which allows for a determination of an appropriate level of parameter structure complexity, and the identification of its pattern and the associated parameter values. The structure complexity is determined by calculating the parameter structure error measured in the prediction space or management space, while the structure pattern and the associated parameter values are identified simultaneously by minimizing the fitting residual measured in the observation space. The former requires solving a continuous maximum‐minimum (max‐min) optimization problem, and the latter requires solving a combinatorial optimization problem. In this paper, Voronoi zonation is used to parameterize the unknown distributed parameter. For each given level of structure complexity a sequential global‐local optimization method, which consists of a genetic algorithm, a quasi‐Newton method, and local search, is developed to solve the combinatorial problem. In addition, the continuous max‐min problem is transformed to a hierarchical optimization problem and solved by a hybrid global‐local method. Sensitivities of state variables to the unknown parameters are calculated by the sensitivity equations. The validity and applicability of the proposed methodology are demonstrated by numerical experiments. We have shown that the choice of an objective function in model application impacts the determination of the parameter structure complexity.
When good statistical models of aquifer heterogeneity go bad: A comparison of flow, dispersion, and mass transfer in connected and multivariate Gaussian hydraulic conductivity fields
Authors: Zinn Brendan , Harvey Charles F.
Published: May 2003

We describe the upscaled groundwater flow and solute transport characteristics of two‐dimensional hydraulic conductivity fields with three fundamentally different spatial textures and consider the conditions under which physical mobile–immobile domain mass transfer occurs in these fields. All three fields have near‐identical lognormal univariate conductivity distributions, as well as near‐identical isotropic spatial covariance functions. They differ in the pattern by which high‐ or low‐conductivity regions are connected: the first field has connected high‐conductivity structures; the second is multivariate log‐Gaussian and, hence, has connected structures of intermediate value; and the third has connected regions of low conductivity. We find substantially different flow and transport behaviors in the three different fields. Flow and transport in the multivariate log‐Gaussian field are consistent with stochastic theory. The field with connected high‐conductivity paths has an effective conductivity greater than the geometric mean and large variations in fluid velocity. It produces significant mass transfer behavior (i.e., tailing) when the conductivity variance is large and, depending on the system parameters, this mass transfer is driven by either diffusion or advection. In the field with connected low‐conductivity regions, the effective conductivity is below the geometric mean and transport is well characterized by the advection–dispersion model with a dispersivity smaller than that in the multivariate log‐Gaussian field. Thus, physical mobile–immobile domain mass transfer may occur in smooth hydraulic conductivity fields with univariate log‐Gaussian density functions if the variability in conductivity is sufficient and the high values are more connected than modeled by the multivariate log‐Gaussian distribution.
Multiscaling fractional advection‐dispersion equations and their solutions
Authors: Schumer Rina , Benson David A. , Meerschaert Mark M. , Baeumer Boris
Published: May 2003

The multiscaling fractional advection‐dispersion equation (ADE) is a multidimensional model of solute transport that encompasses linear advection, Fickian dispersion, and super‐Fickian dispersion. The super‐Fickian term in these equations has a fractional derivative of matrix order that describes unique plume scaling rates in different directions. The directions need not be orthogonal, so the model can be applied to irregular, noncontinuum fracture networks. The statistical model underlying multiscaling fractional dispersion is a continuous time random walk (CTRW) in which particles have arbitrary jump length distributions and finite mean waiting time distributions. The meaning of the parameters in a compound Poisson process, a subset of CTRWs, is used to develop a physical interpretation of the equation variables. The Green's function solutions are the densities of operator stable probability distributions, the limit distributions of normalized sums of independent, and identically distributed random vectors. These densities can be skewed, heavy‐tailed, and scale nonlinearly, resembling solute plumes in granular aquifers. They can also have fingers in any direction, resembling transport along discrete pathways such as fractures.
Influence of step composition on step geometry and flow resistance in step‐pool streams of the Washington Cascades
Authors: MacFarlane William A. , Wohl Ellen
Published: May 2003

Step‐pool streams dissipate flow energy primarily through spill resistance. We compared the geometry, step characteristics, and flow hydraulics of 20 step‐pool reaches without large woody debris (LWD) to 20 step‐pool reaches with LWD. Non‐LWD streams exhibited significantly shallower flows, lower steps, shorter step spacings, greater percentages of water‐surface drop created by steps, larger grain sizes, and smaller Darcy‐Weisbach friction factors. Grain resistance was negligible in both stream types. Form resistance created by irregularities in the channel shape associated with steps contributed more to the total flow resistance in LWD reaches. Although both stream types showed poor correlation between step height and flow resistance, the significant positive correlation between flow resistance and step height/length ratio in the non‐LWD reaches demonstrates the increasing effect of spill resistance with increasing step height. The lack of such a trend in the LWD‐loaded reaches suggests that spill resistance was highly influenced by a few large log steps in these reaches. LWD creates deep pools and increases flow resistance along step‐pool streams. It thus stabilizes channels and stores sediment in steep headwater streams recently scoured by debris flows.
Experimental analysis on the effects of variable apertures on tracer transport
Authors: Lee Jaehyoung , Kang Joe M. , Choe Jonggeun
Published: May 2003

Aperture distributions have been measured using replicas of apertures to investigate the effect of variable aperture on solute transport in five transparent fracture samples. The coefficient of variation (Cv), correlation length (λ), and anisotropy ratio (AR) were used to quantify the aperture distributions. Step‐type tracer tests showed that dispersion is significantly influenced by both λ andCv. Dispersivity (α) increases with λ andCv. The magnitude of α is proportional to a polynomial function of natural logarithm ofCv2and is linearly proportional to λ and the total sill measured from the semivariogram. The effect of AR is insignificant to those of the other parameters on α. Despite assumptions of the small perturbation theory developed byGelhar[1993], the prediction of α from the theory is similar to α measured in this study. If the small perturbation theory is valid and reliable data for the tailing in a breakthrough curve are obtained, the analysis of the curve by the moment method gives a more reliable result than by the conventional fitting of a one‐dimensional analytical solution to the curve.
Geochemical response to variable streamflow conditions in contaminated and uncontaminated streams
Authors: Nagorski Sonia A. , Moore Johnnie N. , McKinnon Temple E. , Smith David B.
Published: May 2003

Seasonal variations in stream inorganic geochemistry are not well documented or understood. We sampled two mining‐impacted and two relatively pristine streams in western Montana over a 12‐month period, collecting samples every 4 weeks, with supplemental sampling (at least weekly) during spring runoff. We analyzed all samples for dissolved (operationally defined as <0.2 μm) and total recoverable concentrations. Generally, the trace elements (Al, As, Cu, Fe, Mn, and Zn) did not correlate linearly with streamflow, while the major elements (e.g., Ca, K, and Mg) did. Suspended sediment, total recoverable metals, and H+ followed clockwise hysteresis rotations, driven by short‐term flushing events during the very early stages of spring runoff. Mining‐impacted sites had higher concentrations of many trace elements than did relatively pristine sites. One of the mining impacted sites exhibited strong geochemical responses to spring rain events in the basin. The results underscore the need to sample streams frequently during changing hydrologic and climatic conditions in order to accurately monitor surface water quality and to determine solute and particulate loads (both contaminant and noncontaminant).
Inverse modeling of interbed storage parameters using land subsidence observations, Antelope Valley, California
Authors: Hoffmann Jörn , Galloway Devin L. , Zebker Howard A.
Published: May 2003

We use land‐subsidence observations from repeatedly surveyed benchmarks and interferometric synthetic aperture radar (InSAR) in Antelope Valley, California, to estimate spatially varying compaction time constants, τ, and inelastic specific skeletal storage coefficients, S*kv, in a previously calibrated regional groundwater flow and subsidence model. The observed subsidence patterns reflect both the spatial distribution of head declines and the spatially variable inelastic skeletal storage coefficient. Using the nonlinear parameter estimation program UCODE we estimate compaction time constants between 3.8 and 285 years. The S*kv values are estimated by linear estimation and range from 0 to almost 0.09. We find that subsidence observations over long time periods are necessary to constrain estimates of the large compaction time constants in Antelope Valley. The InSAR data used in this study cover only a three‐year period, limiting their usefulness in constraining these time constants. This problem will be alleviated as more SAR data become available in the future or where time constants are small. By incorporating the resulting parameter estimates in the previously calibrated regional model of groundwater flow and land subsidence we can significantly improve the agreement between simulated and observed land subsidence both in terms of magnitude and spatial extent. The sum of weighted squared subsidence residuals, a common measure of model fit, was reduced by 73% with respect to the original model. However, the ability of the model to adequately reproduce the subsidence observed over only a few years is impaired by the fact that the simulated hydraulic heads over small time periods are often not representative of the actual aquifer hydraulic heads. Errors in the simulated hydraulic aquifer heads constitute the primary limitation of the approach presented here.
Evaluating the carbon cycle of a coupled atmosphere‐biosphere model
Authors: Delire Christine , Foley Jonathan A. , Thompson Starfley
Published: May 2003

We investigate how well a coupled biosphere‐atmosphere model, CCM3‐IBIS, can simulate the functioning of the terrestrial biosphere and the carbon cycling through it. The simulated climate is compared to observations, while the vegetation cover and the carbon cycle are compared to an off‐line version of the biosphere model IBIS forced with observed climatic variables. The simulated climate presents some local biases that strongly affect the vegetation (e.g., a misrepresentation of the African monsoon). Compared to the off‐line model, the coupled model simulates well the globally averaged carbon fluxes and vegetation pools. The zonal mean carbon fluxes and the zonal mean seasonal cycle are also well represented except between 0° and 20°N due to the misrepresentation of the African monsoon. These results suggest that, despite regional biases in climate and ecosystem simulations, this coupled atmosphere‐biosphere model can be used to explore geographic and temporal variations in the global carbon cycle.
Depth‐dependent elemental compositions of particulate organic matter (POM) in the ocean
Authors: Schneider Birgit , Schlitzer Reiner , Fischer Gerhard , Nöthig Eva‐Maria
Published: May 2003

For the determination of the elemental composition of particulate organic material (POM) and its impact on the marine carbon cycle, we assembled C:N data for POM from many different sources into a single data collection for joint evaluation. The data set contains 10,200 C:N values, encompassing all major oceans and trophic levels, showing that C:N ratios are highly variable with values below the traditional Redfield ratio (C:N = 6.6) to values greatly exceeding it. On a global mean, C:N ratios of marine sinking particles from the surface water amount to 7.1 ± 0.1, and there is a systematic increase of C:N ratios with depth of 0.2 ± 0.1 units per 1000 m. The discrepancy with results from analyses of dissolved nutrient fields, yielding constant C:N ratios close to the Redfield value, can be explained by the implicit depth averaging caused by depth variations of the surfaces under consideration. Additionally, due to preferential remineralization of nitrogen compared to carbon, the C:N ratio of the dissolving component, which is seen on dissolved nutrient fields, is smaller than the C:N ratio of the remaining particles. For carbon flux estimations, elevated and depth dependent C:N ratios should be implemented in biogeochemical models to correctly represent relative strengths of downward carbon and nitrogen fluxes.
Air‐sea flux of bromoform: Controls, rates, and implications
Authors: Quack Birgit , Wallace D. W. R.
Published: May 2003

Bromoform (CHBr3) is the largest single source of atmospheric organic bromine and therefore of importance as a source of reactive halogens to the troposphere and lower stratosphere. The sea‐to‐air flux, originating with macroalgal and planktonic sources, is the main source for atmospheric bromoform. We review bromoform's contribution to atmospheric chemistry, its atmospheric and oceanic distributions and its oceanic sources and sinks. We have reassessed oceanic emissions, based on published aqueous and airborne concentration data, global climatological parameters, and information concerning coastal and biogenic sources. The goals are to attempt an estimate of the global source strength and partly to identify key regions that require further investigation. The sea‐to‐air flux is spatially and temporally variable with tropical, subtropical and shelf waters identified as potentially important source regions. We obtain an annual global flux of bromoform of ∼10 Gmol Br yr−1 (3–22 Gmol Br yr−1). This estimate is associated with significant uncertainty, arising from data precision and coverage, choice of air‐sea exchange parameterizations and model assumptions. Anthropogenic sources of ∼0.3 (to 1.1) Gmol Br yr−1 (as CHBr3) can be locally significant, but are globally negligible. Our estimate of the global oceanic source is three to four times higher than recent estimates based on the modeling of atmospheric sinks. The reasons for this discrepancy could lie with the limited regional and temporal data available and the broad assumptions that underlie our flux calculations. Alternatively, atmospheric sink calculations, often made on the basis of background CHBr3 levels, may neglect the influence of strong but highly localized sources (e.g., from some coastal and shelf regions). The strongly variable and poorly characterized source of CHBr3, together with its short atmospheric lifetime, complicates model‐based estimation of the distribution of reactive Br resulting from its atmospheric degradation. An integrated program of marine and atmospheric observations, atmospheric modeling and mechanistic studies of oceanic bromoform production is required to better constrain present and future Br delivery to the atmosphere.
Global patterns of the isotopic composition of soil and plant nitrogen
Authors: Amundson Ronald , Austin A. T. , Schuur E. A. G. , Yoo K. , Matzek V. , Kendall C. , Uebersax A. , Brenner D. , Baisden W. T.
Published: May 2003

We compiled new and published data on the natural abundance N isotope composition (δ15N values) of soil and plant organic matter from around the world. Across a broad range of climate and ecosystem types, we found that soil and plant δ15N values systematically decreased with increasing mean annual precipitation (MAP) and decreasing mean annual temperature (MAT). Because most undisturbed soils are near N steady state, the observations suggest that an increasing fraction of ecosystem N losses are 15N‐depleted forms (NO3, N2O, etc.) with decreasing MAP and increasing MAT. Wetter and colder ecosystems appear to be more efficient in conserving and recycling mineral N. Globally, plant δ15N values are more negative than soils, but the difference (δ15Nplant‐δ15Nsoil) increases with decreasing MAT (and secondarily increasing MAP), suggesting a systematic change in the source of plant‐available N (organic/NH4+ versus NO3−) with climate. Nitrogen isotopes reflect time integrated measures of the controls on N storage that are critical for predictions of how these ecosystems will respond to human‐mediated disturbances of the global N cycle.
Environmental control of leaf area production: Implications for vegetation and land‐surface modeling
Authors: Cowling Sharon A. , Field Chris B.
Published: May 2003

Leaf surface area per unit ground cover (leaf area index, LAI) is one of the major controls on plant productivity and biospheric feedbacks on atmospheric energy and water exchanges. Nearly all vegetation and land‐surface models include parameterizations of LAI, however not much research currently focuses on the validation of simulated responses of LAI to environmental change. The objective of our research was to quantitatively review the plant science literature to extract information on the response of LAI to variations in soil moisture, soil fertility and atmospheric CO2. Our synthesis confirms that LAI is likely co‐limited by a number of resources, including water, nitrogen and light. Atmospheric CO2 influences LAI in much the same manner as other plant resources. When CO2 supply is strongly limiting to gross primary production (i.e., at relatively low CO2 concentrations), LAI is strongly correlated with CO2, whereas when CO2 is abundant, LAI sensitivity to CO2 dramatically decreases. Such a nonlinear relationship between leaf area production and atmospheric CO2 may introduce a potential bias for global change modeling, particularly in the simulation of low‐density vegetation that has the potential to significantly increase canopy size without inducing self‐shading.