bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport and processing of nitrogen in a tidal freshwater wetland
Authors: Bowden W. B. , Vörösmarty C. J. , Morris J. T. , Peterson B. J. , Hobbie J. E. , Steudler P. A. , Moore B.
Published: February 2004

We combined field, laboratory, and modeling efforts to construct a process‐oriented N budget for a tidal freshwater wetland in eastern Massachusetts. The emergent marsh contained most of the total N in the wetland because of its large area and high N mass per unit area. Total N stored in live marsh plants, dead litter, and inorganic interstitial water were roughly similar and 50–500 × less than total peat N. A simple input‐output budget indicated that most of the N entering the wetland annually passes through the ecosystem unaltered; a relatively small amount of N is imported from the river by the marsh. In the short term, mineralization of peat N is sufficient to satisfy N demands within the marsh. Mineralized N is conserved by plant uptake, microbial immobilization, and litter immobilization. The small net N exchange with the river does provide an important subsidy to the marsh over the long term, offsetting losses due to denitrification and burial. Tidal freshwater wetlands may not appear to be influenced by current water quality and yet may respond to long‐term cumulative nutrient loading, especially when this loading increases because of incremental watershed development.
Daily evaporation over a region from lower boundary layer profiles measured with radiosondes
Authors: Sugita Michiaki , Brutsaert Wilfried
Published: February 2004

Regional daily evaporation was estimated by means of continuous measurements of the available energy flux at the surface and one or more instantaneous determinations of the evaporative fraction (EF). The data were obtained during the First ISLSCP Field Experiment (FIFE) in northeastern Kansas. EF, which is the ratio of the latent heat flux and the available energy flux, was assumed to be constant during the daylight hours; thus it was determined from only a few instantaneous surface flux values calculated on the basis of radiosonde profiles in the surface layer, together with remotely sensed surface temperature. Comparison of 23 estimated and measured daytime evaporation values showed good correlation (r = 0.97), although the evaporation was underestimated by about 5% on average. In the estimation of daily evaporation, nighttime evaporation must also be considered; measurements by means of the eddy correlation method showed that on average it accounted for some 8% of the total daily evaporation.
Stochastic dynamic programming for reservoir optimal control: Dense discretization and inflow correlation assumption made possible by parallel computing
Authors: Piccardi Carlo , Soncini‐Sessa Rodolfo
Published: February 2004

The solution via dynamic programming (DP) of a reservoir optimal control problem is often computationally prohibitive when the proper description of the inflow process leads to a system model having several state variables and/or when a sufficiently dense state discretization is required to achieve numerical accuracy. Thus, to simplify, the inflow correlation is usually neglected and/or a coarse state discretization is adopted. However, these simplifications may significantly affect the reliability of the solution of the optimization problem. Nowadays, the availability of very powerful computers based on innovative architectures (vector and parallel machines), even in the domain of personal computers (transputer architectures), stimulates the reformulation of the standard dynamic programming algorithm in a form able to exploit these new machine architectures. The reformulated DP algorithm and new machines enable faster and less costly solution of optimization problems involving a system model having two state variables (storage and previous period inflow, then taking into account the inflow correlation) and a number of states (of the order of 104) such as to guarantee a high numerical accuracy.
Analytical solutions for one‐dimensional, transient infiltration toward the water table in homogeneous and layered soils
Authors: Srivastava Rajesh , Yeh T.‐C. Jim
Published: February 2004

Analytical solutions describing the transient soil water pressure distributions during one‐dimensional, vertical infiltration toward the water table through homogeneous and two‐layer soils are derived. Exponential functional forms K = Kseαψ and θ = θr + (θs − θr)eαψ are used to represent the hydraulic conductivity and pressure relation and the soil water release curve. Steady state profiles are used as initial conditions. Hydraulic behavior of the soils during wetting and drainage processes is discussed in terms of the pressure head and moisture content profiles and temporal variation of the specific discharge. The solutions provide a reliable means of comparing the accuracy of various numerical methods, especially in very dry layered soils.
Hydrological impacts of changing land management practices in a moderate‐sized agricultural catchment
Authors: Potter Kenneth W.
Published: February 2004

Since the mid‐1930s a variety of soil conservation practices have been applied to agricultural lands throughout the United States. While intended to reduce soil erosion, if effective, these practices should alter the hydrology of streams which drain the treated lands. This hypothesis was explored for the East Branch of the Pecatonica River, a gaged 221 square mile agricultural catchment in southwestern Wisconsin. On the basis of the analysis of peak and daily flow data there has been a decrease in flood peaks and in winter/spring flood volumes and an increase in hydrologic rise times and in the contribution of winter/spring snowmelt events to base flow. These changes do not appear to be due to climatic variations, reservoir construction, or major land use changes. Instead, they appear to have resulted from the adoption of various soil conservation practices, particularly those involving the treatment of gullies and the adoption of conservation tillage.
Fractal structures of river plan forms
Authors: Nikora Vladimir I.
Published: February 2004

The basis of existing methods for the quantitative description of the plan geometry of rivers is determination of the characteristics of individual meander bends (macroforms). This approach is effective when one is working with a few well‐defined macroforms. However, the plan pattern of many rivers is really determined at the next higher geometric level, the morphologically homogeneous river section, which may comprise many tens of macroforms, often with several superimposed scales. At this level, macroform parameters alone do not seem adequate to describe the geometry of the section. New indices are required. The concept of fractals affords a natural way of defining such indices. The presence of hierarchical features (superimposed sinuosity on various scales, braiding with superimposed bars of a range of sizes, etc.) suggests fractal behavior, at least over certain ranges of spatial scale. In this paper we demonstrate fractal behavior for 46 river sections in Moldavia from measurements made from existing topographic maps. We also introduce the notion of internal and external fractal scales that limit the range of fractal behavior; here the internal and external scales are the width of the river channel B and of its valley floor B0, respectively. Using this idea together with elements of fractal geometry, we obtain a relation among the sinuosity, B, B0, and the fractal dimension D of the river bed pattern. We propose the use of D as a new and informative parameter for describing the internal structure of the plan pattern of both single‐thread and multithread rivers.
Infiltration and downslope unsaturated flows in concave and convex topographies
Authors: Philip J. R.
Published: February 2004

The solution of the nonlinear unsaturated flow equation is given for a problem of infiltration (and associated subsurface flows) into concave and convex topographies made up of homogeneous isotropic soil with uniform initial moisture content. The analysis embraces concave and convex surfaces which are parts of the surfaces of circular cylinders and spheres. We thus model concave and convex slopes (of both ridges and isolated hills), valley bottoms and hollows, and ridgecrests and hillcrests. The major geometrical influence on the flow processes is the slope angle. Only when σ−1 (σ = surface total curvature) is less than about 10 times the characteristic infiltration length lgrav (or 10 times the sorptive length) does the perturbation due to surface curvature need to be taken into account. Accordingly, the earlier analysis of infiltration on planar hillslopes has wide applicability to slopes with surface curvature.
A three‐dimensional field study of solute transport through unsaturated, layered, porous media: 1. Methodology, mass recovery, and mean transport
Authors: Ellsworth T. R. , Jury W. A. , Ernst F. F. , Shouse P. J.
Published: February 2004

A field experiment is reported which monitored the three‐dimensional movement of cubic solute plumes through an unsaturated, loamy sand soil. The plumes were created with one of two methods, a two‐dimensional flux application and an initial resident distribution. Soil coring was used to sample resident concentrations for the three solutes studied. The data were analyzed using the method of moments. In addition to the solute transport experiments, a detailed set of physical properties of the field was obtained by excavating three pits to a depth of 5.0 m and also by taking soil cores throughout the study area. This paper explains the experimental methodology, summarizes the relevant site characteristics, and describes the observed transport based on the zeroth and first order spatial moments. Mass balance varied between 78 and 138%. The field‐averaged gravimetric water content and dry bulk density were used to accurately predict the mean vertical plume displacements. The plumes spread relatively little in the horizontal direction.
The reciprocity principle and an analytical solution for Darcian flow in a network
Authors: Barker J. A.
Published: February 2004

A proof is given that the reciprocity principle applies to Darcian flow in a network of heterogeneous branches with either no‐flow or fixed‐head boundary conditions. Formulae are obtained for the Laplace transform of the drawdown in the special case of homogeneous branches; this provides a new method for accurately solving some flow problems in networks.
Automatic, real‐time monitoring of soil moisture in a remote field area with time domain reflectometry
Authors: Herkelrath W. N. , Hamburg S. P. , Murphy Fred
Published: February 2004

A multiplexing time domain reflectoinetry (TDR) system for real‐time monitoring of volumetric soil moisture content was developed. The system was tested at a remote field site in the Hubbard Brook Experimental Forest in New Hampshire. The average value of soil moisture content in the top 500 mm of soil was measured every 4 hours for 1 year at 12 locations within a 12‐ by 18‐m plot. The system functioned well except when the air temperature dropped below −15°C, which caused the data logger tape recorder to stop. Calibrations run on undisturbed soil cores did not compare well with published curves developed for mineral soils, probably because of high soil organic matter content. The standard error of estimate of soil moisture content, indicated by the calibrations, was 0.02 cm3/cm3. The standard deviation of repeated moisture content measurements made in the field was 0.003 cm3/cm3. The effect of cable length on the TDR signal was investigated. It was found that long cables tend to attenuate the signal, ultimately making the measurement impractical. However, cable length had little effect on the calibration up to a length of 27 m. The coefficient of variation of the moisture content measurements taken at any given time ranged from 0.12 to 0.21 during the test period. As predicted by a stochastic analysis of soil moisture flow in heterogeneous soil, the spatial variability of the measurements decreased as average soil moisture increased.
Rate‐limited sorption and nonequilibrium transport of organic chemicals in low organic carbon aquifer materials
Authors: Brusseau M. L. , Larsen T. , Christensen T. H.
Published: February 2004

The rate‐limited sorption and nonequilibrium transport of several hydrophobic organic chemicals in three low‐organic carbon (<0.025%) aquifer materials was investigated. Results of miscible displacement experiments performed at two pore water velocities and with very low solution‐phase concentrations (30–60 μg L−1) were analyzed using a first‐order mass transfer nonequilibrium model, as well as a model employing the local equilibrium assumption. Results of the analyses revealed sorption to be significantly rate limited, possibly by a diffusion‐limited mechanism. The impact of rate‐limited sorption on transport was dependent upon pore water velocity. The experiments performed at a faster velocity (∼1 cm/h) could be successfully simulated only with the nonequilibrium model, whereas the equilibrium model was adequate for the slower‐velocity (∼0.2 cm/h) experiments. Comparison of experimental results to those reported in the literature revealed that time scale has a significant impact on the degree of nonequilibrium observed in, and on the values of rate constants determined from, experiments. Regression equations were developed for the two kinetics‐associated parameters contained in the nonequilibrium model: first‐order mass transfer constant and fraction of instantaneous sorption.
Reliability Improvements in Design of Water Distribution Networks Recognizing Valve Location
Authors: Bouchart F. , Goulter I.
Published: February 2004

Water distribution networks can fail either by the actual demand at one or more nodes exceeding the design demands, or by a pipe between two nodes failing. The implications of each type of failure can be assessed by the shortfall in supply caused by a failure event together with the probability of occurrence of the event, and can be represented by the expected volume of deficit. Converting the implications of the two failure types into these commensurate units permits them to be added directly to give a single consistent measure of reliability. The assessment of shortfall for the pipe failure mode is derived from the observation that when a pipe breaks, a section of pipe must be isolated by valves to permit the repair to be made. Isolating the pipe also isolates the customers who withdraw water from that section of pipe. Thus the shortfall in supply in this case is based on the amount of supply (number of customers) cut off by isolating the pipe for repair. The measure extends previous reliability parameters by recognizing that in reality demand occurs along links rather than being concentrated solely at nodes at the ends of the links which is the normal assumption for both simulation and optimization models. If reliability of the network is found to be unsatisfactory, it can be improved in two ways. One is to increase the design demand at nodes so that the probability of actual demand's exceeding the design value is reduced. The other is to add more valves so that the length of pipe which has to be isolated in order to repair a pipe is reduced, thereby, reducing the number of customers who must have their supply cut off during a repair. Use of the two methods to determine and, if necessary, to improve reliability is demonstrated by their application to an example network.
Stochastic modeling of large‐scale flow in heterogeneous unsaturated soils
Authors: Polmann Donald J. , McLaughlin Dennis , Luis Steve , Gelhar Lynn W. , Ababou Rachid
Published: February 2004

A simulation experiment is used to test the stochastic theory of unsaturated flow proposed by Mantoglou and Gelhar (1987a, b, c). Tension means and variances derived from this theory are compared to tension distributions obtained from a detailed three‐dimensional model developed by Ababou (1988). The synthetically generated unsaturated hydraulic conductivity functions used in the detailed model vary significantly over relatively small distances. Results from a simulated infiltration event indicate that the stochastic theory's predictions reproduce the overall trend of the heterogeneous tension distribution. Deviations from the predicted mean generally lie within confidence intervals derived from the predicted variance. The stochastic theory predicts less vertical moisture movement and somewhat more horizontal spreading than a comparable deterministic analysis. This reflects the fact that the effective hydraulic conductivity function used in the stochastic approach is anisotropic. The magnitude of this anisotropy increases with increasing tension.
Estimating changes in recreational fishing participation from national water quality policies
Authors: Ribaudo Marc O. , Piper Steven L.
Published: February 2004

The complete evaluation of the offsite effects of national policies or programs that affect levels of agricultural nonpoint source pollution requires linking extensive water quality changes to changes in recreational activity. A sequential decision model is specified to describe an individual's decisions about fishing. A participation model for recreational fishing that includes a water quality index reflecting regional water quality is developed and estimated as a logit model with national level data. A visitation model for those who decide to fish that also includes the water quality index is estimated using ordinary least squares. The water quality index is found to be significant in the participation model but not in the visitation model. Together, the two models provide a means of estimating how changes in water quality might influence the number of recreation days devoted to fishing. The model is used to estimate changes in fishing participation for the Conservation Reserve Program.
Hysteresis and state‐dependent anisotropy in modeling unsaturated hillslope hydrologic processes
Authors: McCord James T. , Stephens Daniel B. , Wilson John L.
Published: February 2004

This paper describes a series of soil water tracer experiments and approaches taken to numerically model the flow and transport behavior observed in the field experiments. These experimental and numerical results strongly suggest that current widely held views and commonly applied modeling approaches are flawed in many cases for unsaturated flow, and provide strong supporting evidence for a variable, state‐dependent anisotropy in the hydraulic conductivity of an unsaturated medium. This phenomenon has been previously postulated in a number of independent theoretical and experimental investigations. In general, the previous studies identify layered heterogeneity as a primary cause of the macroscopic anisotropy. In addition, we show how hysteresis in the soil moisture characteristics (θ‐ψ relationship) can cause a lexturally homogeneous porous media profile to behave anisotropically under transient unsaturaied conditions. Recognizing that both of these factors (layered heterogeneity and capillary hysteresis) contribute the anisotropic behavior observed in the tracer experiments, we attempt to quantify the relative magnitude of their contributions in a numerical modeling investigation. For the numerical modeling study we use a finite element flow and transport code, and introduce a simple procedure for incorporating variable anisotropy into a predictive numerical model. To determine the relative magnitude of textural heterogeneity and capillary hysteresis as causes of the observed macroscopic anisotropy, we employ a diagnostic modeling approach. The results of the diagnostic modeling study indicate that textural heterogeneity is by far the most important contributor to the variable macroscopic anisotropy observed at the field site. The diagnostic simulations further show that the variable anisotropy approach is well suited to modeling field‐scale problems. Subsequently, a sensitivity analysis was performed to determine how climate, geologic and topographic structure, and media lithology affect flow and transport behavior when soils were specified to have a variable macroscopic anisotropy. The results of this study clearly indicate that variable state‐dependent anisotropy is a real and significant process at the field site and that modeling with consideration of variable anisotropy strongly affects model predictions.
A nonstationary spectral method for solving stochastic groundwater problems: Unconditional analysis
Authors: Li Shu‐Guang , McLaughlin Dennis
Published: February 2004

Stochastic analyses of groundwater flow and transport are frequently based on partial differential equations which have random coefficients or forcing terms. Analytical methods for solving these equations rely on restrictive assumptions which may not hold in some practical applications. Numerically oriented alternatives are computationally demanding and generally not able to deal with large three‐dimensional problems. In this paper we describe a hybrid solution approach which combines classical Fourier transform concepts with numerical solution techniques. Our approach is based on a nonstationary generalization of the spectral representation theorem commonly used in time series analysis. The generalized spectral representation is expressed in terms of an unknown transfer function which depends on space, time, and wave number. The transfer function is found by solving a linearized deterministic partial differential equation which has the same form as the original stochastic flow or transport equation. This approach can accomodate boundary conditions, spatially variable mean gradients, measurement conditioning, and other sources of nonstationarity which cannot be included in classical spectral methods. Here we introduce the nonstationary spectral method and show how it can be used to derive unconditional statistics of interest in groundwater flow and transport applications.
A coupled channel network growth and hillslope evolution model: 1. Theory
Authors: Willgoose Garry , Bras Rafael L. , Rodriguez‐Iturbe Ignacio
Published: February 2004

This paper presents a model of the long‐term evolution of catchments, the growth of their drainage networks, and the changes in elevations within both the channels and the hillslopes. Elevation changes are determined from continuity equations for flow and sediment transport, with sediment transport being related to discharge and slope. The central feature of the model is that it explicitly differentiates between the sediment transport behavior of the channels and the hillslopes on the basis of observed physics, and the channel network extension results solely from physically based flow interactions on the hillslopes. The difference in behavior of channels and hillslopes is one of the most important properties of a catchment. The flow and sediment transport continuity equations in the channel and the hillslope are coupled and account for the long‐term interactions of the elevations in the hillslope and in the channels. Sediment transport can be due to fluvial processes, creep, and rockslides. Tectonic uplift may increase overall catchment elevations. The dynamics of channel head advance, and thus network growth, are modeled using a physically based mechanism for channel initiation and growth where a channel head advances when a channel initiation function, nonlinearly dependent on discharge and slope, exceeds a threshold. This threshold controls the drainage density of the basin. A computer implementation of the model is introduced, some simple simulations presented, and the numerics of the solution technique described.
Secondary Water Recovery by Air Injection: 3. Evaluation of Feasibility
Authors: Moridis George J. , Reddell Donald L.
Published: February 2004

Numerical simulation indicated that secondary water recovery by air injection could be technically feasible in aquifers with thick unsaturated zones, confining top layers, high ratio of horizontal‐to‐vertical permeability, and low intrinsic permeability, and high irreducible air saturations, i.e., clayey textures. Injection at the top of the unsaturated zone, venting the well to the atmosphere immediately after the end of air injection, low‐to‐moderate injection rates and large volumes of injected air seemed to enhance water recovery. Simulated water recovery volumes were significantly lower than the recovery estimates based on water level elevations in observation wells during field tests. The model helped interpret some of the observations made during the field tests of secondary water recovery and helped identify areas of inadequacy in available information needed for representative numerical simulations.
The Laplace Transform Finite Difference Method for Simulation of Flow Through Porous Media
Authors: Moridis George J. , Reddell Donald L.
Published: February 2004

A new numerical method, the Laplace transform finite difference (LTFD) method, was developed to solve the partial differential equation (PDE) of transient flow through porous media. LTFD provides a solution which is semianalytical in time and numerical in space by solving the discretized PDE in the Laplace space and numerically inverting the transformed solution vector. The effects of the traditional treatment of the time derivative on accuracy and stability are rendered irrelevant because time is no longer a consideration. For a single time step, LTFD requires no more than eight matrix solutions and an execution time eight times longer than the analogous finite difference (FD) requirement without an increase in storage. This disadvantage is outweighed by an unlimited time step size without any loss of accuracy, a superior accuracy, and a stable, nonincreasing round off error. Thus, a problem in standard FD format may require several hundred time steps and matrix inversions between the initial condition and the desired solution time, but LTFD requires only one time step and no more than eight matrix inversions to achieve a more accurate result.
Unsaturated Flow Theory Including Interfacial Phenomena
Authors: Gray William G. , Hassanizadeh S. Majid
Published: February 2004

The macroscopic porous medium equations for mass, momentum, and energy transport for air, water, and solid phases and the interfaces between these phases are examined in light of the second law of thermodynamics. Attention is focused on the momentum balance for the water phase. Appropriate forms of the momentum balance are obtained, in general, for the slow flow situation and for the case when the water phase completely wets the solid. This last case suggests that the relative wettability of the water and air phases is an important dependent thermodynamic variable which contributes to the hysteretic nature of the capillary pressure versus saturation curve.
A Continuously Varying Parameter Model of Downstream Hydraulic Geometry
Authors: Rhoads Bruce L.
Published: February 2004

Few studies have attempted to examine explicitly the factors that produce variation in hydraulic geometry parameters. A continuously varying parameter model of downstream hydraulic geometry is formulated to address this issue. Parameter variation is viewed as a function of channel sediment characteristics and flood magnitude. Results for the Missouri River basin show that these factors are strongly associated with variation in hydraulic geometry coefficients and exponents. The results also indicate that bivariate relationships are biased by these associations. The analysis yields site‐specific parameters that uniquely describe the relationship between active channel form and mean discharge at a particular location within the basin. The continuously varying parameter approach has several advantages: (1) it isolates the separate influences of each explanatory variable on each hydraulic geometry parameter, (2) it maintains the precision of parameter estimates by including the entire data set in the analysis, and (3) it allows testing of the conception that hydraulic geometry parameters vary continuously rather than discretely, yet permits analysis of discrete change through the use of dummy variables.
Preliminary Analysis of the Hydrologic and Geochemical Controls on Acid‐Neutralizing Capacity in Two Acidic Seepage Lakes in Florida
Authors: Pollman Curtis D. , Lee T. M. , Andrews W. J. , Sacks L. A. , Gherini S. A. , Munson R. K.
Published: February 2004

In late 1988, parallel studies of Lake Five‐O (pH 5.14) in the Florida panhandle and Lake Barco (pH 4.50) in north central Florida were initiated to develop hydrologic and major ion budgets of these lakes as part of an overall effort to improve understanding of the hydrologic, depositional, and biogeochemical factors that control acid‐neutralizing capacity (ANC) in seepage lakes. Preliminary findings from these studies indicate that earlier perceptions of lake hydrology and mechanisms of ANC regulation in Florida seepage lakes may have to be revised. The traditional perspective of seepage lakes in the Florida panhandle views these systems as dominated by precipitation inputs and that ANC regulation is due largely to in‐lake processes. Our results for Lake Five‐O show modest to steep hydraulic gradients almost entirely around the lake. In addition, the horizontal hydraulic conductivity of the surficial aquifer is high (8–74 m day−1), indicating that large quantities of groundwater flow into Lake Five‐O. Calculations of net groundwater flow from hydrologic budgets also indicate that groundwater may comprise at least 38 to 46% of the total inflow. For Lake Barco, net flow estimates of the minimum groundwater inflow range from 5 to 14% of total inflow. Enrichment factor and ion flux calculations for Lake Five‐O and Lake Barco indicate that terrestrial as well as in‐lake processes contribute significantly to ANC regulation. The extent that terrestrial processes contribute to ANC generation is directly related to the magnitude of groundwater inflow as well as the degree of ion enrichment or depletion that occurs in the surficial aquifer. Net ANC generation in both study lakes was dominated by anion retention (NO3− and SO42−). Where previous studies concluded that in‐lake reduction was the primary sink for SO42−, our preliminary calculations show that adsorption of SO42− within the watershed is perhaps twice as important as in‐lake reduction as a source of ANC. Net base cation enrichment in both lakes was negligible.
Mathematical Simulation of the Subsidence of Ravenna
Authors: Gambolati Giuseppe , Ricceri Giuseppe , Bertoni Werter , Brighenti Giovanni , Vuillermin Enzo
Published: February 2004

Land subsidence at Ravenna is the result of aquitard and reservoir compaction caused, respectively, by extensive groundwater withdrawals from the unconsolidated Quaternary basin and gas production from a number of pre‐Quaternary pools scattered over the area. Water pumpage paralleled the postwar industrial development of Ravenna until the middle seventies when consumption was drastically curtailed owing to the economic crisis and the activation of a new aqueduct. Gas production started in 1952. The exploitation of several reservoirs is currently under way and the search for new fields is still in progress. Geodetic records indicate that the maximum cumulative subsidence over the period 1950–1986, including a natural geologic settlement of perhaps 2 mm/yr, has been 1.30 m in the industrial zone of Ravenna. In 1980 the municipality promoted a reconnaissance study with the primary aim of providing the information base needed to reconstruct the actual occurrence, understand correctly the physical behavior and produce the essential input data to a mathematical model which realistically relates the subsidence of the city to groundwater withdrawal and gas removal with an emphasis on their respective influences. The results from the three‐dimensional numerical simulations, performed with the aid of mixed finite element, finite difference and integral models, show that the primary responsibility for the regional land sinking should be placed on the subsurface water overdraft which occurred until the middle 1970s. Gas withdrawal plays a role restricted to the area overlying each reservoir with a magnitude depending on the depth of burial, thickness of mineralized rocks and overall volumetric production. A major environmental impact may be expected where the gas subsidence bowl is intersected by the Adriatic coastline.
Modeling Base Flow Soil Water Residence Times From Deuterium Concentrations
Authors: Stewart Michael K. , McDonnell Jeffrey J.
Published: February 2004

Three approaches to determining mean soil water residence times in a steep headwater catchment were investigated. The deuterium concentrations of soil water collected from 11 suction cup samplers at the Maimai M8 catchment were determined weekly for 14 weeks and the results compared with those of rainfall in the same period. Deuterium variations in the suction samples were considerably delayed and diminished compared with the rainfall, indicating significant storage times and mixing with soil water. Soil matrix water at shallow levels (∼200 mm depth) in unsaturated soils was relatively responsive to fresh input, but deeper water and water near the stream subject to occasional water table rises showed much less variation. Steady state and non‐steady state exponential models gave similar mean residence times, ranging from 12 to more than 100 days for different locations. Three groups of soil water response were defined, comprising shallow, medium and deep (near‐stream) soil locations based on the mean residence times. The nonsteady models revealed considerable week‐to‐week and longer variations in mean residence time for shallow soil (SL4), but indicated that steady state models could adequately represent the system in the overall period investigated. In the third approach, model types and parameters that gave the best fits to the soil water deuterium concentrations were determined. Exponential and especially dispersion models were the most satisfactory. Weighting the input (rainfall δD) partially or fully with the amount of rainfall gave much worse fits than with the unweighted input, showing that much of the rainfall bypasses the soil matrix. The best fitting dispersion model (designated DM2) yielded the most accurate mean residence times: 13 days for shallow soil (SL4), 42 days for soil at 400 mm depth (SL5), both at midslope locations, and 63 days for soil at 800 mm depth near the stream (SL2). Capillary flow was important for the unsaturated shallow soil (SL4), while advection and hydrodynamic dispersion (mixing) were more dominant for the periodically saturated (SL5) and the generally saturated (SL2) soils.
An Experimental Investigation of the Effect of Idealized Heterogeneity on the Dispersion of Miscible Fluids
Authors: Haselow John S. , Greenkorn Robert A.
Published: February 2004

Laboratory experiments on dispersion of miscible fluids in a porous medium have been performed. Dimensionally scaled two‐dimensional models were packed with both homogeneous and idealized heterogeneous permeability distributions. Longitudinal dispersion coefficients were calculated by mapping the flow into the streamline‐equipotential coordinate system. The results of these experiments indicate that the dispersion coefficient is not a function of the spatial scale of the system provided that the permeability distribution is preserved as the system dimension is increased. Also, it was found that an inclusion of permeability differing from the surrounding medium could result in both Gaussian and non‐Gaussian tracer distributions depending upon the location of the inclusion, and systems with heterogeneous permeability distributions sometimes resulted in dispersion similar to that for homogeneous media. In some instances, the dispersion everywhere within the system appeared to be Fickian, but when the dispersion was monitored at the withdrawal point from the system, the dispersion appeared to be non‐Fickian.
Combining Rain Gages With Satellite Measurements for Optimal Estimates of Area‐Time Averaged Rain Rates
Authors: North Gerald R. , Shen Samuel S. P. , Upson Robert B.
Published: February 2004

The problem of optimally combining data from an array of point rain gages with those from a low Earth‐orbiting satellite to obtain space‐time averaged rain rates is considered. The mean square error due to sampling gaps in space‐time can be expressed as an integral of a filter multiplied by the space‐time spectral density of the rain rate field. It is shown that for large numbers of gages or large numbers of overpasses the two estimates can be regarded as orthogonal in the sense that the optimal weighting is the same as for independent estimators, i.e., the weights are inversely proportional to the error variances that would occur in the single‐component case. The result involving point gages and satellite overpasses appears to hold under quite general conditions. The result is interesting since most other design combinations do not exhibit the orthogonality property.
Transport of Chloride Through an Unsaturated Field Soil
Authors: Roth K. , Jury W. A. , Flühler H. , Attinger W.
Published: February 2004

A chloride tracer was applied to the surface of a vegetable field and then leached downward by rainfall and irrigation. Tracer concentrations in a vertical two‐dimensional region down to a depth of 2.4 m were monitored with suction cups that, were installed horizontally from a tunnel. The uniformly applied tracer pulse split into a slowly moving main pulse and a series of fast pulses. The first of the fast pulses reached a depth of 2.2 m after an infiltration of just 31 mm of natural rainfall, whereas the peak of the main pulse was still at a depth of 0.84 m by the end of the experiment after an infiltration of 0.853 m. The movement of the main pulse can be described by a convection‐dispersion process in a homogeneous medium, provided that time is replaced by cumulative infiltration. However, the values of the parameters that produce a maximum agreement between the model and the observed main pulse have no physical basis, and consequently prediction of solute movement, based on measurements of soil properties, is not possible.
The Deposition, Composition, and Potential Sources of Major Ionic Solutes in Rain of the Central Amazon Basin
Authors: Lesack Lance F. W. , Melack John M.
Published: February 2004

Major solute concentrations were measured for 123 individual rainstorms greater than 1 mm that occurred over a continuous 18‐month period (September 1983 through February 1985) in the central Amazon Basin. The sum of concentrations was low (volume‐weighted mean, 39 μeq/L), mean pH was 4.9, and the dominant source of acidity was organic acids. Analyses of interactions among solute concentrations in storms of differing size, frequency, and season of occurrence permitted evaluation of how rainfall variation affects depositional rates of solutes. The 20% of least frequent, large‐volume storms accounted for about 75% of the water and almost 60% of the solutes deposited annually. Depositional rates of total N and P, Na+, and SO42− did not differ substantially as rainfall varied, while Ca2+, Mg2+, Cl−, NO3− and PO43− did. Wet deposition of N and P varied less than 20%, despite a threefold intraannual variation of rainfall. Comparison between event and bulk precipitation samples indicated that dry deposition was minor except for P. Ionic ratios relative to Na+, R mode and Q mode factor analyses of rainwater composition, and differences in deposition among seasons provide evidence for the seasonal‐scale importance of biogenic emissions of aerosols and reduced S and N gases from the Amazon rain forest to the atmosphere.
Effect of Catchment‐Scale Subsurface Mixing on Stream Isotopic Response
Authors: McDonnell J. J. , Stewart M. K. , Owens I. F.
Published: February 2004

A 3.8‐ha watershed on the west coast of New Zealand was instrumented with suction lysimeters and automatic water samplers to determine the relationship between subsurface isotopic and chemical concentrations to those of rainfall and resulting streamflow. A t test showed that ±2‰ represented a significant difference between successive sample deuterium values. Eleven rainfall episodes were subdivided into two categories: (1) two events where stream isotopic composition did not deflect >2‰ from prestorm values, and (2) four events which demonstrated new water flushing. Detailed analysis of one 47‐mm rainfall (9.8‐mm runoff) event showed that old water dominated stream water exiting the watershed by 90% using a standard two‐component hydrograph separation for deuterium (corroborated by Cl and electrical conductivity). Three‐component hydrograph separation indicated that 12–16% was in the form of soil water, with <5% as on‐channel precipitation and 80% groundwater. Analysis of over 1000 water samples revealed systematic trends in soil water and groundwater isotopic composition both in a downslope and downprofile direction. Between‐storm suction lysimeter deuterium data showed a systematic dampened response to temporally variable rainfall deuterium concentrations. Multivariate cluster analysis revealed three distinct soil water/groundwater groupings, with respect to soil depth and geographic position within the watershed. Within‐storm suction lysimeter sampling preserved similar groupings, indicating that the subsurface reservoir is poorly mixed on short time scales. Understanding subsurface mixing response to rainfall should greatly improve models of episodic stream response and partitioning of storm flow into waters of different age.
Chemistry of dissolved organic carbon and organic acids in two streams draining forested watersheds
Authors: David Mark B. , Vance George F. , Kahl Jeffrey S.
Published: February 2004

The concentration, major fractions, and contribution of dissolved organic carbon (DOC) to stream chemistry were examined in two paired streams (East and West Bear Brooks) draining upland catchments in eastern Maine. Concentrations of DOC in East and West Bear Brooks were 183 ± 73 and 169 ± 70 μmol C L−1 (mean plus or minus one standard deviation), respectively. Organic acids were the dominant fractions of the stream DOC, with hydrophilic and hydrophobic organic acids representing 44 and 39% of total DOC (average of both streams), respectively. East and West Bear Brooks had mean organic anion concentrations of about 20 μeq L −1 during episodic events and 12–13 μeq L−1 during base flow. During the episodic events, the maximum organic anion values were 51 and 36 μeq L−1, respectively, for East and West Bear Brooks. Although SO42− was the dominant stream anion, followed by Cl−, organic anions were also determined to be an important component of these stream waters, especially during storm events. This illustrates that even in streams with low DOC, such as those studied here, organic anions can contribute significantly to stream acidity.
Comparison of Isothermal and Isobaric Water Retention Paths in Nonswelling Porous Materials
Authors: Constantz Jim
Published: February 2004

Water in porous media near the Earth's surface is subject to large fluctuations in pore water pressure and temperature, often causing significant changes in the degree of saturation. Quantitative comparisons of isothermal versus isobaric water retention are necessary to accurately predict changes in saturation due to the coupled influence of changes in the pore water matric potential ψ and temperature T. Yet there is a lack of experimental measurements of isobaric (i.e., constant ψ) water retention, inhibiting comparisons between isobaric and isothermal processes. In the present study the influence of the chronological sequence of changes in ψ and T is examined to determine whether the volumetric water content θ for any given final ψ‐T condition is independent of the ψ‐T sequence, when θ changes monotonically. To obtain the necessary data for these comparisons, isothermal water retention experiments were performed over a range in ψ from 0 to −100 kPa, and isobaric water retention experiments were performed at 20° and 80°C on core samples of a sandy soil and a nonwelded tuff. Results provide further evidence that the effect of T on ψ is considerably greater than estimates based on pore water capillary theory. For these materials the thermal enhancement of ψ was 4–12 times greater than capillary theory would predict. The effect of T on θ during isobaric water retention was several times greater for drying (warming) conditions than for wetting (cooling) conditions at a given ψ, resulting in net losses in θ ranging from 3 to 30%. For a given final ψ at 80°C, virtually identical θ values were obtained regardless of the chronological sequence of isothermal and isobaric drainage paths for both materials. This confirms the validity of unique θ (ψ, T) surfaces describing monotonic changes in θ as functions of both ψ and T in nonswelling porous materials. Determination of these θ (ψ, T) response surfaces for drying and wetting should yield water retention envelopes, useful in modeling water retention in near‐surface environments where both ψ and T vary.
Convergent Radial Dispersion: A Note on Evaluation of the Laplace Transform Solution
Authors: Moench Allen F.
Published: February 2004

A numerical inversion algorithm for Laplace transforms that is capable of handling rapid changes in the computed function is applied to the Laplace transform solution to the problem of convergent radial dispersion in a homogeneous aquifer. Prior attempts by the author to invert this solution were unsuccessful for highly advective systems where the Peclet number was relatively large. The algorithm used in this note allows for rapid and accurate inversion of the solution for all Peclet numbers of practical interest, and beyond. Dimensionless breakthrough curves are illustrated for tracer input in the form of a step function, a Dirac impulse, or a rectangular input.
Several Duality Theorems for Interlocking Ridge and Channel Networks
Authors: Werner C.
Published: February 2004

Many fluvially eroded terrains display an intimate interaction of their patterns of channel and ridge lines. Under the assumption that there is exactly one outer ridge link positioned between any two neighboring outer channel links, the paper analyzes such interlocking networks by investigating their dual graphs. Several formal relationships are established which translate into a set of both existing and new theorems describing the close interdependency of channel and ridge parameters: Interlocking networks are of equal magnitude, the link numbers of the ridge and channel paths enclosing channel and ridge subnetworks have the same mean value, the link number of the boundary path enclosing a subnetwork is equal to the number of boundary paths passing through the root of the subnetwork, there exists a one‐to‐one correspondence between the inner nodes of the two networks as well as between their paths, and every such pair of corresponding ridge and channel paths delineates an area that is closed with regard to slope runoff.
Modeling water erosion due to overland flow using physical principles: 2. Rill flow
Authors: Hairsine P. B. , Rose C. W.
Published: February 2004

The model of soil erosion on a plane slope introduced by Hairsine and Rose (this issue) is further developed by considering rill flow. The rills are considered to be parallel and incised in a homogeneous soil mass and have sediment and water flux contributed to them orthogonally. The erosive action of water flowing down these rills is described using the concept of stream power, which is assumed to be uniformly distributed around the wetted perimeter. The apportioning of the erosion and deposition processes within a rill is then considered to be influenced by the spatial extent of a deposited layer. The theory is developed for trapezoidal rills, with triangular rills and rectangular rills as special cases. Equations describing sediment continuity are presented, and numerical and analytical solutions are described for the entrainment‐limited and transport‐limited situations. The experiments of Meyer and Harmon (1985) are examined using this theory, and consistency of evaluated parameters is demonstrated within this data set.