bluuebunny/crossref_metadata_2025 huggingface dataset:
Evapotranspiration of sweet sorghum: A general model and multilocal validity in semiarid environmental conditions
Authors: Rana Gianfranco , Katerji Nader , Perniola Michele
Published: February 2004

A model of canopy resistance rc is presented for the determination of actual crop evapotranspiration using the Penman‐Monteith type of model at hourly and daily scales. Here rc is deduced from weather variables (net radiation, air temperature and humidity, and wind speed) and by an indicator of the crop water status measured on the plants: the predawn leaf water potential (i.e., measured before dawn). The model needs a calibration “per crop” and not “per site”; that is, once it is calibrated for a given crop in a certain locality, then it can be used anywhere without further calibration. The model was tested on sorghum crops in different water conditions at two sites in southern Italy for several years. The results showed that it performs well at hourly, daily, and seasonal scales. In fact, the difference between the cumulative values of the actual evapotranspiration measured and the one calculated by the model ranges between −9% and +11%. We concluded that it is not sensitive to climate and soil type or to crop species and variety.
Introduction to Special Section: Reynolds Creek Experimental Watershed
Authors: Marks Danny
Published: February 2004

To understand how variations in climate, land use, and land cover will impact water supply and water quality, we must have access to long‐term hydrologic and climatic databases. The National Research Council [1999] recognized the value of experimental watersheds as the setting for the development of our current understanding of physical and biological watershed processes. Specifically, data from watersheds that include significant human activities, such as grazing, farming, irrigation, and urbanization, are critical for determining the signature of human‐induced changes on hydrologic processes and the water cycle. One of the primary components of effective watershed research is a sustained, long‐term monitoring and measurement program [National Research Council, 1997].
Hydraulic properties of two‐dimensional random fracture networks following a power law length distribution: 2. Permeability of networks based on lognormal distribution of apertures
Authors: de Dreuzy Jean‐Raynald , Davy Philippe , Bour Olivier
Published: February 2004

The broad length and aperture distributions are two characteristics of the heterogeneity of fractured media that make difficult, and even theoretically irrelevant, the application of homogenization techniques. We propose a numerical and theoretical study of the consequences of these two properties on the permeability of bidimensional synthetic fracture networks. We use a power law for the model of length distribution and a lognormal model for aperture distribution. We have especially studied the two endmost models for which length and aperture are (1) independent and (2) perfectly positively correlated. For the model without correlation between length and aperture we show that the permeability can be adequately characterized by a power‐averaging function whose parameters are detailed in the text. In contrast, for the model with correlation we show that the prevailing parameter is the correlation when the power law length exponent a is lower than 3, whereas the random structure of the network is a second‐order parameter. We also determine the permeability scaling and the scale dependence of the flow pattern structure. Three types of scale effects are found, depending exclusively on the geometrical properties of the network, i.e., on the length distribution parameter a. For a larger than 3, permeability decreases for scales below a definite correlation length and becomes constant above. We show in this case that a correlation between length and aperture does not fundamentally change the permeability model. In all other cases the correlation entails much larger‐scale effects. For a in the range 1–;3 in the case of an absence of correlation and for a in the range 2–3 in the case of correlation, permeability increases and tends to a limit, whereas the flow structure is channeled when permeability increases and tends to homogenize when permeability tends to its limit. We note that this permeability model is consistent with natural observations of permeability scaling. For a in the range 1–2, in the case of correlation, permeability increases with scale with no apparent limit. We characterize the channeled flow pattern, and we show that permeability may increase even when flow is distributed in several independent structures.
Land data assimilation with satellite measurements for the estimation of surface energy balance components and surface control on evaporation
Authors: Boni Giorgio , Entekhabi Dara , Castelli Fabio
Published: February 2004

A variational land data assimilation system is used to estimate latent heat flux and surface control on evaporation. The dynamic equation for surface temperature with energy balance is used as a constraint on the estimation using the adjoint technique. Measurements of land surface temperature from satellite remote sensing are assimilated over two subregions within the Southern Great Plains 1997 hydrology field experiment. The performance of the estimation is linked to the timing of the satellite overpass. During days when the measurements close to the time of peak ground temperature are available, the estimation is adequate. The approach shows that satellite remote sensing of land temperature may be used to provide estimates of components of the surface energy balance and land surface control on evaporation. The latter parameter is related to surface soil moisture, and here they are compared with independent values derived from ground measurements.
Lookup tables for transionospheric effects on signals
Authors: Strickland D. J. , Barnes R. P. , Kochenash A. J. , Jones W. A. , Reilly M. H. , Daniell R. E.
Published: February 2004

Lookup tables have been constructed that address group delay and RF carrier phase advance for transionospheric signal propagation referenced to a ground‐based transmitter. The tables are based on parameterizations of ray‐tracing results using the Reilly ray trace/ICED, Bent, Gallagher (RIBG) model with the magnetic field turned off. The quantities of interest are incremental group and phase path lengths referenced to straight‐line distance between transmitter and receiver and are designated by Δlg and Δlp (as defined, both quantities are positive). Equations are presented that relate these quantities to group delay and ionospheric Doppler shifts. Tables currently in use are three‐dimensional, expressed in terms of frequency (f), elevation angle (θ), and straight‐line total electron content (TECSL). At high frequencies, Δlg and Δlp can be accurately expressed by a straight‐line propagation formula that is proportional to TECSL. With decreasing frequency, these quantities become greater than their straight‐line counterparts due to additional refraction effects, including ray bending. Extensive ray‐tracing runs have been performed under a variety of ionospheric conditions to produce scatterplots of Δlg and Δlp versus TECSL for given pairs of [f, θ] values. The current ranges in these variables are from 20 to 100 MHz and 5° to 90°, respectively. The tables contain coefficients from fourth‐degree polynomial fits to the distributions of points within the scatterplots. Where scatter is problematic, the introduction of a fourth parameter, slab thickness, is shown to significantly reduce this scatter. Magnetic field effects are discussed and are shown to have a small effect on table coefficients. Discussion is also included on the use of the tables with a model ionosphere.
Fast labile carbon turnover obscures sensitivity of heterotrophic respiration from soil to temperature: A model analysis
Authors: Gu Lianhong , Post Wilfred M. , King Anthony W.
Published: February 2004

Labile carbon, although often a small fraction of soil organic carbon (SOC), significantly affects heterotrophic respiration at short timescales because of its rapid decomposition. However, in the current literature, most soil respiration measurements are interpreted without simultaneous information on labile carbon pool dynamics. Sensitivity of soil respiration to temperature is routinely derived directly from field observations, and such relationships have been used to extrapolate effects of global change (e.g., warming) on carbon emission from SOC. Here we use a multipool SOC model to demonstrate the impacts of seasonal fluctuations of labile carbon pools on interpretation of soil respiration measurements. We find that labile carbon pool sizes vary widely in response to seasonal changes in representative plant material inputs and temperature even though the model is operating at equilibrium in terms of annual means. Convolution of the dynamics of fast turnover carbon pools and temporal progression in temperature lead to misrepresentation and misinterpretation of the heterotrophic respiration‐temperature relationships estimated from bulk soil CO2 exchanges. Temperature sensitivity is overestimated when the variations of labile carbon pools and temperature are in phase and underestimated when they are out of phase. Furthermore, with normally used observation time windows (weeks to a year), temperature sensitivity is more likely to be underestimated. A distortion of temperature sensitivity (Q10) from 2 (actual, sensitive dependence on temperature) to nearly 1 (false, no dependence on temperature) is shown. Applying estimated temperature sensitivity parameter back into the model considerably overestimates soil carbon storage at equilibrium. Our findings indicate that caution must be taken when soil respiration‐temperature relationships are evaluated based on bulk soil observations and when sensitivity of soil respiration to temperature estimated directly under field conditions is used to predict future carbon cycle‐climate feedbacks.
Modulational and decay instabilities of Alfvén waves in a multicomponent plasma
Authors: Hertzberg M. P. , Cramer N. F. , Vladimirov S. V.
Published: February 2004

The parallel propagating transverse modes in a plasma are known to be exact solutions of the MHD and multifluid equations. Here we investigate these waves in a multicomponent plasma consisting of two ion‐like species, which are allowed to be fully mobile. In this paper we perturb these exact solutions and investigate modulational or decay instabilities where the pump wave is modified by the daughter waves in regions of instability. In a warm plasma, the parallel modes can interact with transverse and acoustic modes. A tenth‐degree polynomial dispersion equation is obtained, describing the interaction of the pump with two acoustic modes and sideband‐beat‐waves. In the low‐temperature regime the effect of a second ion species is to reduce the usual modulational instability, and a new decay instability arises. In the high‐temperature regime, two new decay instabilities, narrow in wave number, are found. All of these instabilities involve the additional modes associated with the introduction of a second heavy species.
Global change in the thermosphere: Compelling evidence of a secular decrease in density
Authors: Emmert J. T. , Picone J. M. , Lean J. L. , Knowles S. H.
Published: February 2004

We derive secular trends in upper thermospheric density from the historical orbital elements of 27 long‐lived, near‐Earth space objects. The results cover all levels of solar activity during the period 1996–2001, and each object indicates a long‐term decrease of total mass density. The observed trends increase with increasing height from 200–700 km and are largest for solar minimum conditions; average values range from −2% to −5% per decade. The trends are largely independent of geomagnetic activity, local time, latitude, and season. We examine several possible sources of error in our results and conclude that none of them can individually account for the observed trends. The trends that we obtain are qualitatively, and in some cases quantitatively, consistent with available theoretical predictions of density decreases associated with the cooling effect of increased greenhouse gas concentrations.
Nonparametric variable kernel estimation with historical floods and paleoflood information
Authors: Guo Sheng Lian
Published: February 2004

Extending a data record back in time using historical or paleoflood data has the potential to provide a considerable amount of information on very large floods. Parametric estimation methods are readily applicable to flood frequency analysis when historical or paleoflood estimates are available. However, all parametric approaches need an assumption about the underlying parent distribution, which is never known exactly. In recent years nonparametric methods of estimating probability density functions have been developed. Each of these involves the use of a suitable smoothing function known as a kernel. A new nonparametric variable kernel estimation model is proposed. Results obtained from a limited amount of real data and from simulation experiments show that quantiles estimated by the nonparametric method are better than those estimated by some selected parametric models both in descriptive ability and in predictive ability. The uncertainty in the choice of the threshold value of perception of a historical flood is also discussed.
A new formulation for transmissivity estimation with improved global convergence properties
Authors: Tucciarelli Tullio , Ahlfeld David P.
Published: February 2004

A new formulation is proposed for the estimation of transmissivity in confined aquifers based on head measurements. This formulation consists of a formulation which minimizes a weighted residual norm of the gradient of the measured and observed head. The weighting is the square of the estimated transmissivity. We show by theoretical analysis and numerical experimentation the improved global convergence properties and characteristics of stability under head measurement uncertainty exhibited by the new formulation. We conclude that the new formulation may be superior to conventional head residual formulations or may be useful as an added term to conventional formulations. The formulation is further demonstrated by a field scale application to the Friuli Aquifer in Italy.
Information transfer with censored data: Some large‐sample results
Authors: Clarke Robin T.
Published: February 2004

This paper presents some new results for the transfer and extension of information at sites with short hydrological records. The results refer particularly to the transfer and extension of annual flood data. The methods described make use of incomplete or “censored” data such as may be supplied by people living near a river or from records collected for nonhydrological purposes, and they constitute extensions to methods already described in the literature. For two specific censoring configurations, the gain in information is assessed analytically; it is shown that, under certain conditions, the gain in information can be substantial, but in general the gain is small, particularly where scale parameters are estimated. For other configurations of censored data, an analytical formulation is not possible, and integrals are given from which information gain may be assessed by numerical calculation. Another result extends the use of flood peaks exceeding some threshold value (“peaks over a threshold,” POTs); probabilistic models of flood frequency using POTs are, in effect, fitted using censored data. The value of the results presented in the paper is likely to be restricted by assumptions about the probabilistic structure of flood sequences: in one case that their distribution (at different sites) is bivariate lognormal, and, for the POT model, that floods occur as a Poisson process with POTs exponentially distributed. The results show, in strictly qualitative terms, the circumstances in which information might be gained by the use of censored data at two sites, and how large (or small) the gains might be.
Subice layering and origin of acidic waters in a small boreal lake during the spring runoff
Authors: Roberge Jean , Jones H. Gerald
Published: February 2004

In a small boreal forest lake, the subice circulation of waters from five distinct origins (lake premelt water, lakeshore snowpack meltwater, ice cover meltwater, groundwater and hillslope through‐flow water) was studied during the spring melt period. A network of collared access holes was used to monitor the temperature, conductivity and pH profiles of the water column beneath the ice cover. The study also included the tracing of snowbank meltwaters and through‐flow waters with rhodamine. Conductivity and pH appeared to be efficient parameters for discriminating most of the water types considered. During the spring melt three distinct low‐pH episodes were recorded at the lake outflow. The first episode (pH 5) occurred in the early melt period; snow and ice meltwaters layered beneath the ice forming a stratum ≈20 cm thick. It moved toward the outlet under laminar flow conditions (observed average velocity, 1m/h). Two subsequent pH episodes of pH 5.3 and 5.1 were observed during the massive melt period. These episodes were due to peak flows of hillslope through flow which entered the lake and formed a layer 20–40 cm thick in the water column (observed average velocity, greater than 20 m/h). The ice cover thus not only modified the nearshore water quality by inducing thermal layering but also strongly influenced the quality of the outflow waters due to the segregation and rapid evacuation of the through flow and surface meltwaters.
Stream chemistry in the eastern United States: 2. Current sources of acidity in acidic and low acid‐neutralizing capacity streams
Authors: Herlihy Alan T. , Kaufmann Philip R. , Mitch Mark E.
Published: February 2004

We examined anion composition in National Stream Survey (NSS) data in order to evaluate the most probable sources of current acidity in acidic and low acid‐neutralizing capacity (ANC) streams in the eastern United States. Acidic streams that had almost no organic influence (less than 10% of total anions) and sulfate and nitrate concentrations indicative of evaporative concentration of atmospheric deposition were classified as acidic due to acidic deposition. These acidic streams were located in small (<30 km2) forested watersheds in the Mid‐Atlantic Highlands (an estimated 1950 km of stream length) and in the Mid‐Atlantic Coastal Plain (1250 km). Acidic streams affected primarily by acidic deposition but also influenced by naturally occurring organic anions accounted for another 1180 km of acidic stream length and were located in the New Jersey Pine Barrens, plateau tops in the Mid‐Atlantic and Southeast Highlands, and the Florida Panhandle. The total length of streams acidic due to acid mine drainage in the NSS (4590 km) was about the same as the total length of acidic streams likely affected by acidic deposition (4380 km). Acidic streams whose acid anion composition was dominated by organics were located in Florida and the Mid‐Atlantic Coastal Plain. In Florida, most of the acidic streams were organic dominated, whereas about half of the streams in the Mid‐Atlantic Coastal Plain were organic dominated. Organic‐dominated acidic streams were not observed in the Mid‐Atlantic and Southeast Highlands.
The response of water level in a well to a time series of atmospheric loading under confined conditions
Authors: Furbish David Jon
Published: February 2004

The water level in a well that penetrates a confined aquifer can fluctuate in response to changes in atmospheric pressure. The response varies with the well casing and screen dimensions, the transmissivity and compressibility of the aquifer, and to a small extent, its storativity. Recently this loading‐response problem has been solved in terms of a frequency response function that characterizes how attenuation and phase shifts in the response signal vary with frequency. The counterpart of this solution in the time domain is an impulse response function. This solution has the immediate appeal that it can be used directly to filter raw loading and water level records via serial convolution, where the response at any time is a weighted aggregate of the previous loading signal. The impulse response function derives from well‐known solutions to the slug test problem. When based on the Cooper‐Bredehoeft‐Papadopulos solution, the response function is precise but computationally intensive. Based on the Hvorslev solution, the response function is computationally simple, but applicable only to aquifers with small storativity. Simulations and a field example clearly illustrate the dominating influence of transmissivity in dampening and lagging the response of a confined aquifer to a loading signal, and how both effects increase with higher frequency. The results also illustrate the weak influence of storativity in modulating the response signal.
The estimation of fluid flow properties from the response of water levels in wells to the combined atmospheric and Earth tide forces
Authors: Ritzi Robert William , Sorooshian Soroosh , Hsieh Paul A.
Published: February 2004

The water level in an open well tapping a confined formation is influenced by natural forces including the solid Earth tide (SET) and atmospheric pressure variation (APV). The spectral method is used to analytically derive a model for well response to both random and periodic components of the combined SET and APV forcings (CSA). An inverse theory and algorithm are developed in order to provide improved results when using the model to estimate the hydraulic parameters associated with a given formation. An examination of the response surface of the estimation criterion reveals a uniqueness problem in estimating storativity (S). Since there is little correlation between the transmissivity (T) and S estimators, a good estimate for T is still possible independent of having accurate knowledge of S. An estimate of T is possible only if the data contain sufficient information so that the analysis occurs within an identifiability window. The CSA estimation methodology is compared to individual SET and APV schemes. The CSA scheme gives the greatest probability that sufficient information is contained in a data record so that T is indeed identifiable. The results of applications to synthetic data indicate that the CSA scheme gives a T estimate with the most precision and also that it requires collecting fewer observations. These results are discussed in light of practical considerations when designing data collection procedures.
Modeling three‐dimensional flow about ellipsoidal inhomogeneities with application to flow to a gravel‐packed well and flow through lens‐shaped inhomogeneities
Authors: Fitts Charles R.
Published: February 2004

Analytic functions are superimposed to model three‐dimensional steady groundwater flow in regions containing one or more Inhomogeneities shaped like prolate or oblate ellipsoids of revolution. Each function and the sum of such functions are solutions of Laplace's equation, the governing differential equation for steady groundwater flow. The functions are implemented in a manner that provides exact continuity of flow across the entire boundary of each inhomogeneity. In general, continuity of head is provided at specified control points on the boundary and is approximated between control points. For the case of one inhomogeneity in a uniform flow field, it turns out that there is exact continuity of head across the entire surface of the inhomogeneity. The method is implemented in a computer program written by the author. Two applications are demonstrated: (1) flow to a gravel‐packed well and (2) flow through a series of lens‐shaped inhomogeneities. The examples demonstrate that the approximation of continuity of head can be made acceptable for many problems. A possible application of the technique would be testing various theories regarding contaminant migration and dispersion by simulating flow and chemical diffusion through large numbers of lens‐shaped inhomogeneities.
Effects of an organic mat filter on artificial recharge with turbid water
Authors: Schuh W. M.
Published: February 2004

Total recharge was 81% larger for a sandy basin with an organic mat filter than for the same basin with a clean, fully renovated surface, but without the mat. Surface impedance was larger for check areas without organic mat, while areas with the mat exhibited impedance increases to greater depths. Experimental results indicated a likely trade‐off between short‐term recharge enhancement and an eventual larger depth of renovation to offset deeper sediment penetration under the mat. A substantial decrease in hydraulic impedance and a corresponding increase in infiltration rate were measured between 90 and 400 hours and indicated that despite large sediment influx, soil hydraulic conductivity was larger under the organic mat than for the same basin positions on clean sands using clean water. The large decrease in impedance is attributed to microbial transformation of O2 to CO2 which increased water‐filled large porosity.
Snow hydrology of a headwater Arctic basin: 1. Physical measurements and process studies
Authors: Kane Douglas L. , Hinzman Larry D. , Benson Carl S. , Liston Glen E.
Published: February 2004

The hydrologic cycle of an arctic watershed is dominated by such physical elements as snow, ice, permafrost, seasonally frozen soils, wide fluctuations in surface energy balance, and phase change of snow and ice to water. Hydrologic data collected over a 5‐year period (1985–1989) from an arctic watershed in northern Alaska are presented. Processes related to snow accumulation, redistribution, ablation, evaporation and subsequent snowmelt‐generated runoff are discussed. The total water content of the snowpack at the end of winter has comprised between 28 and 40% of the annual precipitation. Redistribution of snow on the ground by the wind is a major factor in increasing the snowmelt runoff. Much of the redistributed snow accumulates in the valley bottom along the stream and also along depressions on the hillslopes. These depressions are small surface drainage channels that are referred to as water tracks. Partitioning of the snowmelt into runoff, evaporation and increased soil water storage in the active layer was carried out on the plot and watershed scale. Over a 5‐year period, the volume of snowmelt runoff varied from 50 to 66% of the average watershed snowpack, while evaporation varied from 20 to 34% and soil moisture storage increased between 10 and 19%. Much greater variation in these hydrologic components occurred at the plot scale.
On the meaning of randomness in stochastic environmental models
Authors: Zielinski Przemyslaw A.
Published: February 2004

This paper describes and explains the differences between two different approaches toward stochastic water quality modeling. Two groups of models, namely, models based on differential equations with coefficients being either random variables (random variable models) or stochastic processes (stochastic process models), are analyzed. The reasoning behind these two approaches is discussed and a numerical example illustrating the possible consequences in estimating stochastic characteristics of the solution process using either of these two approaches is presented and discussed.
Heuristic operating policies for reservoir system simulation
Authors: Johnson Sharon A. , Stedinger Jery R. , Staschus Konstantin
Published: February 2004

Reservoir system simulation models are widely used to determine a system's firm water yield, average yield, or hydropower capacity. Most such models use heuristic guidelines to define the system's operating policy. Alternatively, optimization can be used within the simulation to identify a reasonable operating strategy. In this paper a theoretical justification is provided for several heuristic operating guidelines, including the widely used space rule. The guidelines are expressed as a mathematical objective function and combined with constraints on system operation to yield one‐period optimization submodels that can be used to determine releases within a simulation. Use of these one‐period optimization models improved the simulated operation of the Central Valley Project in California over the critical period of record and provided reasonable policies for other hydrologic scenarios.
Analysis of soil water dynamics in time and space by use of pattern recognition
Authors: Zhang Tielin , Berndtsson Ronny
Published: February 2004

This paper presents a method to use pattern recognition in the analysis of soil water dynamics in time and space simultaneously. Space‐time correlation structures are generated by use of a numerical model for the one‐dimensional unsaturated flow equation. By applying the method to various soil parameter combinations (unsaturated hydraulic conductivity and dispersion coefficient) and length of infiltration periods it is shown how space‐time dependence of soil water changes. The method clearly visualizes effects of changes in infiltrated input and soil heterogeneities which are not noticeable in the original soil water time series. Application of the method to field data reveals small‐scale heterogeneities and vertical and horizontal variability in hydraulic properties. The method possesses special advantages when analyzing time‐ and space‐dependent properties simultaneously. Since the method gives a statistical measure of the dependent property that varies within the space‐time field, it can be used to interpolate the fields to points where observations are not available, to estimate spatial or temporal averages from discrete observations, and to define regions (spatial or temporal) where observations are the most efficient.
Statistical estimation and interpretation of trends in water quality time series
Authors: Zetterqvist Lena
Published: February 2004

Three approaches to trend analysis of water quality time series are discussed: (1) seasonal model, with a test for trend based on ranks of observations, with observations assumed to be m dependent; (2) transfer function noise model, in which covariate series may be included by means of transfer functions, with the remaining noise modeled as a seasonal autoregressive moving average process; and (3) component model, with the noise decomposed into series which describe trends, and irregular and seasonal variation. Models are studied with regards to their ability to include covariate series, possibility of interpretation of trends, treatment of seasonal variation and serial dependence, and robustness for outliers. We regard the component model being the most realistic and the most informative of the three approaches. Models are applied to series of monthly phosphorus concentration in the Ljungbyån River in Southern Sweden.
Multiple‐Peak Response to Tracer Injection Tests in Single Fractures: A Numerical Study
Authors: Moreno Luis , Tsang Chin Fu
Published: February 2004

Under certain conditions, when pulse tracer injection tests are performed in a single fracture with a regional flow field, the breakthrough curves may display multiple peaks. Furthermore, the shape of these curves may change when the injection flow rate is varied. In this paper the conditions under which the breakthrough curve may present multiple peaks are analyzed numerically using stochastically generated fractures. The dispersivities of these peaks are also calculated. It is found that the dispersivity is small for each of the peaks and that the dispersivities of the different peaks in a breakthrough curve are quite similar. The results presented in this paper may be equally applied to tracer tests in a two‐dimensional strongly heterogeneous medium.
Using Kalman Filtering to Improve and Quantify the Uncertainty of Numerical Groundwater Simulations: 2. Application to Monitoring Network Design
Authors: Yangxiao Zhou , Te Stroet Chris B. M. , Van Geer Frans C.
Published: February 2004

Integrating Kalman filtering and a deterministic groundwater flow model not only improves and quantifies the uncertainty of numerical groundwater simulation, but also provides the dynamic relation between the variance of the estimation error and the measurement strategy. This relation is applied to Zhengzhou city and Spannenburg for the analysis and design of networks for monitoring groundwater levels. For both cases, the network is designed in such a way that the network density is minimized under the constraint of given threshold values for the standard deviations of the estimation errors. Several network alternatives are analyzed and a best alternative is selected by trial and error. In the case of Zhengzhou city, much attention is also paid to the sensitivity analysis of the deterministic and stochastic parameters of the standard deviation of the estimation error. The results indicate that the density of the network depends on the characteristics of geohydrological systems. The Zhengzhou city and Spannenburg cases are compared and the difference between them is discussed using a hypothetical example. It is found that the difference in the behavior of the estimation variance in time is attributable to the characteristic response time of the system on which the network density and observation frequency depend.
Transport in heterogeneous porous media: Prediction and uncertainty
Authors: Rubin Yoram
Published: February 2004

This study presents a method for obtaining risk‐qualified estimates of various statistics that characterize the migration of a nonreactive tracer in heterogeneous porous media. The steps required to get these estimates are as follows: (1) the solute body is assumed to be composed of a large number of imaginary solute particles; (2) a Lagrangian description of the migration of the solute particles in space is adopted; (3) the Lagrangian description is transformed into an Eulerian one by a quasi‐linear method, whereby the translation of the particles in space is simulated through successive, spatially correlated displacements; (4) the plume's various statistics are obtained by averaging over the ensemble of simulated particles, and, lastly, (5) simultaneous simulation of the motion of two particles in space entails the spatial moments of the plume and their variances, and in addition, the concentration is obtained qualified by its variance. The model is compared with numerical simulations and is found to compare quite favorably even for log conductivity variances as large as 2.6. This study shows that (1) after a certain travel time, the concentration variance becomes largest next to the plume center and gets smaller with distance from the plume center; (2) the concentration coefficient of variation grows with distance from the plume center and prediction of the low concentration on the periphery of the plume is subject to the largest uncertainty; (3) the concentration coefficient of variation grows with time while the concentration variance gets smaller with time; and (4) interpretation of field data depends on the interpolation method and in particular on the size of the averaging domain.
Immiscible Displacement in Porous Media: Stability Analysis of Three‐Dimensional, Axisymmetric Disturbances With Application to Gravity‐Driven Wetting Front Instability
Authors: Glass R. J. , Parlange J.‐Y. , Steenhuis T. S.
Published: February 2004

As water infiltrates downward into an air‐filled, water wettable porous medium, the wetting front which forms may become unstable and allow the formation of downward moving fingers within the vadose zone. In this paper we first review stability criteria and estimated finger widths determined from linear stability theory in two‐dimensional systems. Two approaches reported in the literature which employ different formulations for the interfacial boundary conditions, yield different estimates of the finger width. We extend the analyses to investigate finger diameter in three‐dimensional systems by considering axisymmetric disturbances. Results of the three‐dimensional analyses are illustrated through comparison to previously reported experimental results in three‐dimensional systems. Because either approach gives similar results for low system fluxes, in practice, it probably will not matter which formulation is used. However, one approach represents the data better and contains only traditionally measured porous media properties.
Stochastic Analysis of the Effects of Fluid Density and Viscosity Variability on Macrodispersion in Heterogeneous Porous Media
Authors: Welty Claire , Gelhar Lynn W.
Published: February 2004

Both porous medium heterogeneities and fluid density and viscosity contrasts affect solute transport in miscible fluid displacement. The effects of interaction of these processes on large‐scale mixing are evaluated using spectral‐based perturbation theory. A three‐dimensional, statistically isotropic, exponential log permeability autocovariance is used to represent the spatial variability of the porous medium. State equations linearly relating log density and log viscosity perturbations to concentration perturbations represent the density and viscosity variability and strongly couple the flow and solute transport perturbation equations. Analytical expressions for longitudinal macrodispersivity, derived for one‐dimensional mean solute transport, are functionally dependent on mean displacement distance, mean concentration and concentration gradient, density and viscosity differences, mean velocity, gravity, and correlation scale and variance of the log permeability process. Transient analysis shows that longitudinal macrodispersivity grows exponentially in time (or mean displacement distance) without bound for the case where instabilities due to viscous or gravity fingering arise (the “unstable” or “fingering” case) and that it grows at early time then decreases exponentially to an asymptotic value close to that of local dispersivity for the case where density or viscosity contrasts produce a stabilizing effect (the “stable” case).
Simulation of non‐Gaussian space random functions for modeling transport in groundwater
Authors: Rubin Yoram , Journel Andre G.
Published: February 2004

The recent developments and successful applications of covariance‐related stochastic methods to groundwater field experiments suggest that further efforts in developing spatial characterization methods are warranted. One area that deserves further attention is that of characterizing multimodal distributions, such as fractured rock masses, dolomite rocks with dissolution channels, and sand‐shale formations. Traditionally used covariance functions may not be enough to fully characterize spatial continuity in applications where the pattern of spatial continuity depends on the specific magnitude level of the attribute, for example, hydraulic conductivity. Alternative models based on multiple indicator covariances or mixture of populations offer greater flexibility. Applying such models to the case of perfect stratification, a stochastic simulation is proposed which allows generating different realizations, all sharing the same vertical conductivity covariance yet differing by other spatial statistics and leading to widely different transport characteristics. A mere covariance‐based approach would have failed to distinguish between these alternatives. Investigation of tracer transport in bimodal stratified formations suggests a dependence of the mean advection velocity on the dispersion coefficients as a result of a mechanism that enhances longitudinal dispersion directly proportional to lateral pore scale dispersion and entails a diversion of a substantial portion of the plume into the low‐conductivity layers. This mechanism may not be evident when the class‐specific spatial structures of the conductivities are ignored.
A Comparison of Four Inverse Approaches to Groundwater Flow and Transport Parameter Identification
Authors: Keidser Allan , Rosbjerg Dan
Published: February 2004

Four different methods of parameterizing spatially varying log transmissivities in an inverse approach are compared with respect to prediction accuracy of simulated flow and transport. Transport parameter estimation is included by two‐stage feedback optimization. In stage one the log transmissivities are estimated by fitting both head and concentration data, given initial values of the source concentration and the dispersivities. In stage two, the source concentration and the dispersivities are estimated by fitting the concentration data. With the updated transport parameters, final estimates of the log transmissivities are obtained by repeating the optimization of stage one. The formulated objective functions are minimized using Levenberg‐Marquardt's algorithm. The models are applied to synthetic two‐dimensional transport problems in steady state flow regimes. The “true” log transmissivity fields are generated by the turning bands method, thereby incorporating spatial variability. The test cases differ in the input variances of the generated fields and with respect to the amount and accuracy of “observed” transmissivity data.
Resistance to Overland Flow on Desert Pavement and Its Implications for Sediment Transport Modeling
Authors: Abrahams Athol D. , Parsons Anthony J.
Published: February 2004

Seventy‐three overland flow experiments were performed on eight runoff plots located on a gentle piedmont hillslope in Walnut Gulch Experimental Watershed, southern Arizona. These plots were sited on gravel‐covered desert pavement surfaces where overland flow occurs during natural rainstorms. Multiple regression analyses indicate that the Darcy‐Weisbach friction factor ƒƒ can be well predicted by the Reynolds number and gravel concentration and size, with gravel concentration being by far the single best predictor. Grain resistance ƒƒ', calculated using Savat's algorithm, is found to be typically less than 10% of ƒƒ. Given that sediment transport capacity is controlled by grain shear stress τ' rather than total shear stress τ and that the ratio of τ' to τ is the same as that of ƒƒ to ƒƒ, sediment transport modeling on the gravel‐covered hillslope under study must be based on τ' rather than τ to avoid gross overestimation of transport capacity. This conclusion is relevant to all gravel‐covered desert hillslopes and probably also to most humid hillslopes and agricultural lands where plant stems, litter, and tilled surfaces replace gravel as sources of form resistance and ensure that grain resistance remains relatively small.
Nonlinear Dynamics of Soil Moisture at Climate Scales: 1. Stochastic Analysis
Authors: Rodriguez‐Iturbe Ignacio , Entekhabi Dara , Bras Rafael L.
Published: February 2004

Hydrologic and climatic time series are often characterized by smaller fluctuations around persistent anomalies. Occasional transitions occur between these prolonged runs of above or below average conditions. A statistical‐dynamical model is developed to represent the surface hydrology of large continental regions. The model has physically based components, and it includes a parameterization for the local recycling of precipitation. It is shown that stochastic fluctuations in this equation may give rise to separate and distinct statistical modes with noise‐induced transitions between these modes. In an example situation the characteristic long‐term probability distribution of soil moisture states is shown to be bimodal. The implications of this result are important for the interpreting and the statistical modeling of hydrologic records.
Theoretical Prediction of Electrical Conductivity in Saturated and Unsaturated Soil
Authors: Mualem Y. , Friedman S. P.
Published: February 2004

A conceptual model is proposed for the prediction of the electrical conductivity of bulk soil in saturated and in unsaturated states. The model is based on the hypothesis that the tortuosity factor affecting the bulk soil electrical conductivity is identical to that defined for prediction of the soil hydraulic conductivity. Simple mathematical functions are derived to forecast the soil electrical conductivity as a function of the water content. Experimental results of seven previous studies carried out on 26 soils were used for validation tests. The model's efficiency as a predictive tool, in cases where limited experimental data are available, is found to be generally good.
Applicability of the Steady State Flow Assumption for Solute Advection in Field Soils
Authors: Destouni Georgia
Published: February 2004

A comparison between solute travel times predicted by a transient and a steady state flow model is made. Data for five different soil profiles with detailed measurements of their hydraulic properties and their variation with depth are used. Daily measurements of meteorological data are used as input parameters in the transient simulations that include snow and frost dynamics, interception of precipitation, and evapotranspiration. The parameters of the steady state flow model are related to the measured soil properties and the hydrological characteristics of each transient simulation. Furthermore, the influence of solute injection time on the predicted travel time is analyzed, and the effect of root water uptake on the applicability of the steady state flow assumption for solute advection is investigated. The results indicate that the steady state flow model may provide estimates of the mean solute advection that are compatible with those of the transient flow model. The constant rate of recharge in the steady state flow model should then be interpreted as the average annual effective infiltration (i.e., infiltration minus actual evapotranspiration). When root water uptake is accounted for, an arithmetic depth‐averaging of the soil parameters appears to yield steady state estimates of arrival time that are closest to the transient predictions. When root water uptake is neglected, a harmonic depth‐averaging of the soil parameters provides the best steady state results. The discrepancy between the arrival times predicted with the two flow models decreases with the travel distance from the soil surface.
Secondary Water Recovery by Air Injection: 1. The Concept and the Mathematical and Numerical Model
Authors: Moridis George J. , Reddell Donald L.
Published: February 2004

An air‐driven secondary water recovery operation was proposed in order to recover water stored in the unsaturated zone of depleted aquifers and currently unavailable with conventional techniques. The operation employed a single‐well serving as both a production and injection facility and consisted of three stages: (1) an air injection stage, (2) a “recovery” stage, and (3) a “production” (pumping) stage. The concept of the operation was based on the hysteretic behavior of the capillary pressures and the relative permeabilities of air and water. Hysteresis was counted upon to prevent water, forced into the saturated zone during injection, from migrating back to the unsaturated zone. A general mathematical model was developed and numerically approximated using a simultaneous two‐phase, three‐dimensional finite difference model in both Cartesian and cylindrical coordinates. Two types of nonlinearities were identified: (1) pressure‐related weak nonlinearities and (2) saturation‐related strong nonlinearities. Techniques to alleviate the resulting instabilities, as well as methods for the establishment of stable initial conditions and the treatment of boundary conditions, were developed.