bluuebunny/crossref_metadata_2025 huggingface dataset:
Improved methodology for parameter inference in nonlinear, hydrologic regression models
Authors: Bates Bryson C.
Published: February 2004

A new method is developed for the construction of reliable marginal confidence intervals and joint confidence regions for the parameters of nonlinear, hydrologic regression models. A parameter power transformation is combined with measures of the asymptotic bias and asymptotic skewness of maximum likelihood estimators to determine the transformation constants which cause the bias or skewness to vanish. These optimized constants are used to construct confidence intervals and regions for the transformed model parameters using linear regression theory. The resulting confidence intervals and regions can be easily mapped into the original parameter space to give close approximations to likelihood method confidence intervals and regions for the model parameters. Unlike many other approaches to parameter transformation, the procedure does not use a grid search to find the optimal transformation constants. An example involving the fitting of the Michaelis‐Menten model to velocity‐discharge data from an Australian gauging station is used to illustrate the usefulness of the methodology.
Size and distance of travel of unconstrained clasts on a streambed
Authors: Church Michael , Hassan Marwan A.
Published: February 2004

We have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers. Distance of movement was scaled using the mean distance of movement of the median size group of the surface material; size was scaled using the median b axis diameter of the subsurface material. The scaled data are fitted with a nonlinear function. Clasts larger than the median of surface material exhibit a steep decline of mobility, with an upper limit for mobility near Di/D50sub = 10. Smaller materials exhibit a less sensitive relation. We suppose that trapping of small clasts affects their travel distance. These results indicate that the movement of free surface stones is size preferred. A subordinate analysis of free, constrained and buried particles reveals that bed surface structure acts to reduce differential movement.
Benefit transfers: Conceptual and empirical issues
Authors: Brookshire David S. , Neill Helen R.
Published: February 2004

The focus of this special section is the conceptual and empirical issues regarding benefit transfer applications. A benefit transfer is the application of monetary values obtained from a particular nonmarket goods analysis to an alternative or secondary policy decision setting. The papers address the ongoing development of the procedures for benefit transfers through a case study approach. This paper attempts to focus the discussion of benefit transfers and highlight the issues discussed in each of the papers.
On separating defensible benefit transfers from “smoke and mirrors”
Authors: Smith V. Kerry
Published: February 2004

Benefit transfer methods increasingly are being applied to value nonmarketed resources for both policy evaluation and natural resource damage litigation. This paper illustrates the need for guidelines for deciding when benefit transfer methods can be used to value changes in environmental resources. It begins by discussing applied economic modeling perspectives and relating them to benefit transfers as tools for evaluating policy. It reviews the history of benefit transfers and summarizes how they are typically undertaken, including the influence of the analyst's judgments on their outcome, by comparing the development of two different analyses that use benefit transfers to consider the same issue: estimating the benefits from limiting industrial effluents discharged into specific rivers. It proposes an agenda for future benefit transfer research: devising strategies for extending available benefit transfer theory, learning from existing research, and formulating transferable versus “portable” modeling strategies.
Vapor transport in unsaturated soil columns: Implications for vapor extraction
Authors: Gierke John S. , Hutzler Neil J. , McKenzie David B.
Published: February 2004

A mathematical model was derived to examine the impact of gas advection, gas diffusion, gas‐water mass transfer, gas‐water partitioning, sorption, and intraaggregate diffusion on subsurface movement of organic vapors. Laboratory experiments were performed to determine the validity of the model and to investigate the impact of the various mechanisms on vapor transport. Columns were packed with a uniform Ottawa sand and an aggregated porous soil material (APSM) to compare transport in different soil structures. Toluene vapor transport was observed in the sand under dry and wet (27% water saturation) conditions. The experiments with the APSM were performed dry and at 67% water saturation. In all the sand and the dry APSM experiments, gas advection and diffusion had the greatest impact. In a wet APSM experiment, intraaggregate (liquid) diffusion was also important to consider for gas velocities greater than approximately 0.05 cm s−1. For both soil materials, sorption of toluene vapors occurred for dry conditions, while vapor sorption was negligible when liquid water was present. These findings imply that vapor extraction performance in moist, aggregated soils will be affected by nonequilibrium transport. Therefore models that are developed for predicting the complete removal of contaminants by vapor extraction must account for nonequilibrium.
Problems of seepage to empty ditch and drain
Authors: Ilyinsky N. B. , Kacimov A. R.
Published: February 2004

Analytical solutions for the maximum area of the cross section of a drain ditch and a mole drain are obtained. In the case of seepage to a ditch from an underflooded soil surface, the extreme curve is represented by a semicircle, with a constant value of flow velocity along the curve. Seepage to a system of empty horizontal drains is studied for one of three possible patterns of water tables. At low values of infiltration to a phreatic surface, the drain of a maximum area is nearly circular.
The influence of microbial activity and sedimentary organic carbon on the isotope geochemistry of the Middendorf Aquifer
Authors: Murphy Ellyn M. , Schramke Janet A. , Fredrickson James K. , Bledsoe Horace W. , Francis A. J. , Sklarew Deborah S. , Linehan John C.
Published: February 2004

Microorganisms present in deep Atlantic coastal plain sediments affect the geochemical evolution of groundwater and its chemical and isotopic composition, yet the factors controlling their origin, distribution, and diversity are poorly understood. The evolution of the groundwater chemistry, the fractionation of stable carbon isotopes, and the groundwater age are all indicators of the inorganic and microbial reactions occurring along a given flow path from groundwater recharge to groundwater discharge. In this study, tritium, 14C, and groundwater chemistry along three flow paths of the Middendorf aquifer in South Carolina were analyzed. The 14C ranged from 89 percent modern carbon (pmC) in the recharge zone to 9.9 pmC in the distal borehole; the δ13C remained relatively constant at ∼−22‰, suggesting microbial oxidation of organic carbon. Carbon isotope analyses of particulate organic carbon from core sediments and groundwater chemistry were used to model the carbon chemistry; the groundwater ages obtained from 14C ranged from modern to 11,500 years B.P. The highest frequencies of occurrence, numbers, and diversity of aerobic and anaerobic bacteria were found in boreholes near the recharge zone where the calculated ages were <1000 years B.P. The transport of microorganisms from the recharge zone may be responsible for this distribution as well as the electron acceptors necessary to support this diverse community of bacteria. The presence of both aerobic heterotrophs and anaerobic sulfate‐ and iron‐reducing bacteria in the core sediments suggested the occurrence of anaerobic microsites throughout this otherwise aerobic aquifer. The highest in situ microbial respiration rate, as determined by modeling, was found along a flow path near the recharge area. It is likely that the electron acceptors necessary for supporting a diverse microbial community are depleted by the time the groundwater residence time in the Middendorf aquifer exceeds several hundred years.
Optimal operation of a multisource and multiquality regional water system
Authors: Mehrez Abraham , Percia Carlos , Oron Gideon
Published: February 2004

A mathematical model for optimal real‐time operation of a regional water system was developed and examined. The regional system consists of numerous water sources with various water qualities and a variety of consumers with different flow and quality requirements. In order to satisfy the different consumer constraints, nodal flow and solute balances were defined. Pump and valve‐on and valve‐off situations were defined by continuous nonlinear equations. The model was solved numerically for a short time period by a projected Lagrangian method for the water supply system in the Arava Rift Valley, Israel. The global uniqueness of optimal solution was proven
Gravity‐driven groundwater flow and slope failure potential: 1. Elastic Effective‐Stress Model
Authors: Iverson Richard M. , Reid Mark E.
Published: February 2004

Hilly or mountainous topography influences gravity‐driven groundwater flow and the consequent distribution of effective stress in shallow subsurface environments. Effective stress, in turn, influences the potential for slope failure. To evaluate these influences, we formulate a two‐dimensional, steady state, poroelastic model. The governing equations incorporate groundwater effects as body forces, and they demonstrate that spatially uniform pore pressure changes do not influence effective stresses. We implement the model using two finite element codes. As an illustrative case, we calculate the groundwater flow field, total body force field, and effective stress field in a straight, homogeneous hillslope. The total body force and effective stress fields show that groundwater flow can influence shear stresses as well as effective normal stresses. In most parts of the hillslope, groundwater flow significantly increases the Coulomb failure potential Φ, which we define as the ratio of maximum shear stress to mean effective normal stress. Groundwater flow also shifts the locus of greatest failure potential toward the slope toe. However, the effects of groundwater flow on failure potential are less pronounced than might be anticipated on the basis of a simpler, one‐dimensional, limit equilibrium analysis. This is a consequence of continuity, compatibility, and boundary constraints on the two‐dimensional flow and stress fields, and it points to important differences between our elastic continuum model and limit equilibrium models commonly used to assess slope stability.
A model of transient, topographically driven, saturated subsurface flow
Authors: Jackson C. Rhett , Cundy Terrance W.
Published: February 2004

A two‐dimensional, finite difference model of saturated subsurface hillslope flow, capable of handling varied topography, is developed. The model uses rainfall, elevation data, saturated conductivities, and available soil porosity to predict saturated thicknesses in time and space over the hillslope. The model is tested against piezometric data collected in a steep hillslope hollow in the Cascade Range of Washington state during the winter wet season. The model closely tracks the timing and magnitude of piezometric response.
Characterization of suspended particles collected in groundwater under natural gradient flow conditions
Authors: Ronen D. , Magaritz M. , Weber U. , Amiel A. J. , Klein E.
Published: February 2004

Microscale Eulerian variations in the flux, mineralogical composition and size of suspended particles have been found in a contaminated sandy aquifer under natural gradient flow conditions () during an 8‐month study period. Particle variability has been detected along a 16‐m saturated section of the aquifer at a scale of centimeters and meters in the vertical and horizontal dimensions, respectively. The average concentration of particles in groundwater varied between 1 and 40 mg/L, but high concentrations of up to 5000 mg/L were determined in specific 3‐cm vertical intervals of the aquifer. The particles were primarily composed of CaCO3(11–57%), quartz (7–39%) and clays (8–43%). Most of the particles were within the 140–3000 nm size range with size modes varying from 310 to 660 nm. The large amounts of suspended particles are considered to be related to high inputs of dissolved organic carbon into groundwater, from sewage effluents which have been used for agricultural irrigation since the early 1960s. As a result of organic matter biodegradation in the saturated zone, anoxic conditions developed and thepCO2content of groundwater increased dramatically (pCaCO2= 10−1.8to 10−1.3atm). It is postulated that part of the carbonate cement of the rocks dissolved and detrital CaCO3, quartz and clay were released as colloidal particles. Part of the clay particles could have also been transported through the unsaturated zone into groundwater after mobilization from surface layers as a result of the intermittent input of water of different chemical quality. In the prevailing anoxic conditions of groundwater at the study site (dissolved oxygen concentrations of <1 mg/L) colloidal stability is enhanced by organic matter coating of particles.
Effective and efficient global optimization for conceptual rainfall‐runoff models
Authors: Duan Qingyun , Sorooshian Soroosh , Gupta Vijai
Published: February 2004

The successful application of a conceptual rainfall‐runoff (CRR) model depends on how well it is calibrated. Despite the popularity of CRR models, reports in the literature indicate that it is typically difficult, if not impossible, to obtain unique optimal values for their parameters using automatic calibration methods. Unless the best set of parameters associated with a given calibration data set can be found, it is difficult to determine how sensitive the parameter estimates (and hence the model forecasts) are to factors such as input and output data error, model error, quantity and quality of data, objective function used, and so on. Results are presented that establish clearly the nature of the multiple optima problem for the research CRR model SIXPAR. These results suggest that the CRR model optimization problem is more difficult than had been previously thought and that currently used local search procedures have a very low probability of successfully finding the optimal parameter sets. Next, the performance of three existing global search procedures are evaluated on the model SIXPAR. Finally, a powerful new global optimization procedure is presented, entitled the shuffled complex evolution (SCE‐UA) method, which was able to consistently locate the global optimum of the SIXPAR model, and appears to be capable of efficiently and effectively solving the CRR model optimization problem.
Spatial averaging of transmissivity in heterogeneous fields with flow toward a well
Authors: Desbarats A. J.
Published: February 2004

Steady state nonuniform flow in a heterogeneous medium is investigated using a combined numerical‐empirical approach. The objective of the study is to determine the effective transmissivity of a square field with a constant‐head external boundary and a constant‐rate well at its center. Transmissivity at the point scale is modeled as an isotropic and multivariate lognormal spatial random function. Block transmissivities are defined using an empirical scaling up of point support values within the field. The scaling process is a weighted spatial geometric averaging where log transmissivities are weighted by the inverse square of their distance from the well. Block transmissivities thus obtained and true effective values calculated using a numerical flow model are compared for realizations a finely discretized transmissivity field. The principal finding of this study is that effective transmissivities obtained from the numerical flow model and block values obtained by spatial averaging are in excellent agreement for low to moderate variances of log transmissivity and moderately eccentric field geometries. The geostatistical model for point scale transmissivity and the deterministic spatial averaging law are combined in order to create a geostatistical model for transmissivity at the block scale. Exact expressions are derived for the ensemble moments of block‐averaged transmissivities and well drawdowns in both the nonconditional case and the conditional case when transmissivity at the well bore is known. The ensemble mean of block‐averaged transmissivity is found to decrease from the ensemble arithmetic mean toward the ensemble geometric mean as the field size becomes large compared to the integral range of spatial correlation. This result contradicts earlier published work. Numerical results presented in this study also disagree with the analytical results of a recent stochastic analysis of specific discharge in radial systems.
On the North Atlantic Circulation
Authors: Schmitz William J. , McCartney Michael S.
Published: February 2004

A new, speculative, and, we hope, provocative summary of the North Atlantic circulation is described, including both horizontal currents (wind‐driven) and the primarily (thermohaline) meridional flows that involve the transformation of warm to cold water at high latitudes. Our picture is based on a synthesis of a variety of independent investigations that are contained in the literature as opposed to a presentation of the results of one technique or the point of view of one author. We describe a thermohaline cell (the so‐called thermohaline conveyor belt) that is concentrated within the Atlantic and Southern oceans (rather than essentially global), with the most important upwelling sites being in the circumpolar and the equatorial current regimes. We concentrate on deep water formation and its replacement relative to intermediate‐water formation. It has been pointed out recently that the formation of 13 Sv (1 Sv = 106m³ s−1) of southward flowing North Atlantic Deep Water is compensated for in the upper ocean by northward cross‐equatorial transport. We suggest that this thermocline layer flow passes through the Straits of Florida, transits the Gulf Stream system on its inshore side, and exits through the North Atlantic Current system after recirculation and modification. There is now a clear observational basis for the structure of recirculating gyres on the southern and northern sides of the Gulf Stream. We suggest a recirculation for the North Atlantic Current as well. We also describe a C‐shaped component to the southern Gulf Stream recirculation and identify a roughly 10‐Sv circulation in the eastern North Atlantic associated with the Azores Current. Recirculations play an important role in deep boundary current regimes and in water mass formation and modification. The transport of the deep western and northern boundary currents in the North Atlantic Ocean may be boosted (roughly doubled or tripled) by counterclockwise recirculating gyres and by additions of modified bottom or intermediate water. While the North Atlantic is the most completely observed ocean, there are still significant gaps in our knowledge of its circulation.
Coastal metabolism and the oceanic organic carbon balance
Authors: Smith S. V. , Hollibaugh J. T.
Published: February 2004

Net organic metabolism (that is, the difference between primary production and respiration of organic matter) in the coastal ocean may be a significant term in the oceanic carbon budget. Historical change in the rate of this net metabolism determines the importance of the coastal ocean relative to anthropogenic perturbations of the global carbon cycle. Consideration of long‐term rates of river loading of organic carbon, organic burial, chemical reactivity of land‐derived organic matter, and rates of community metabolism in the coastal zone leads us to estimate that the coastal zone oxidizes about 7 × 1012 moles C/yr. The open ocean is apparently also a site of net organic oxidation (∼16 × 1012 moles C/yr). Thus organic metabolism in the ocean appears to be a source of CO2 release to the atmosphere rather than being a sink for atmospheric carbon dioxide. The small area of the coastal ocean accounts for about 30% of the net oceanic oxidation. Oxidation in the coastal zone (especially in bays and estuaries) takes on particular importance, because the input rate is likely to have been altered substantially by human activities on land.
Streamflow and sediment transport responses to snow fencing a rangeland watershed
Authors: Sturges David L.
Published: February 2004

The effects of snow fencing a big sagebrush rangeland watershed were evaluated within a paired watershed study framework. The fence, 3.8 m tall and 800 m long, increased snow water storage 78% and snowmelt runoff 129%. Flow duration was lengthened 33 days. An average of 39% of channel snow appeared as Streamflow while 54% of additional snow deposited by the snow fence appeared as streamflow. The proportion of the snowpack appearing as Streamflow was directly related to the size of the snowpack. Placement of the fence 38 m upwind of an incised channel that was the primary source of streamflow maximized the water yield increase. The concentration of filtrable sediment and bed load transport were unaffected by treatment. Discharge of filtrable sediment increased in proportion to the increase in water yield. Filtrable sediment content averaged 6 mg L−1, and about 25% of sediment weight was contributed by organic matter. Sediment transport after fencing averaged 0.275×103 kg km −2 yr−l, with filtrable sediment accounting for most of the transport.
Groundwater quality management under uncertainty: stochastic programming approaches and the value of information
Authors: Wagner Janet M. , Shamir Uri , Nemati Hamid R.
Published: February 2004

A stochastic optimization model for containment of a plume of groundwater contamination through the installation and operation of pumping wells is developed. It considers explicitly uncertainty about hydraulic conductivity in the aquifer and seeks to minimize the expected total cost of operating the pumping wells plus the recourse cost incurred when containment of the contaminant plume is not achieved. Four different formulations of the model are examined, ranging from simply replacing all uncertain parameters by their expected values to a full stochastic programming with recourse model involving nonsymmetric linear quadratic penalty functions. The full stochastic programming with recourse model, which minimizes the expected total costs over a number of realizations of outcomes of the random parameters, is nonlinear and possibly nonconvex and is solved by an extension of the finite generation algorithm. The value of information about the uncertain parameters is defined through the differences between the values of the optimal solutions to the different formulations. A sample problem is solved using all four formulations. The results indicate that the explicit incorporation of uncertainty does make a difference in the solutions obtained. The work indicates that stochastic programming with recourse is a useful tool in management under uncertainty, and that it can be used with reasonable computational resources for problems of moderate size.
Soil moisture measurement using portable dielectric probes and time domain reflectometry
Authors: Brisco B. , Pultz T. J. , Brown R. J. , Topp G. C. , Hares M. A. , Zebchuk W. D.
Published: February 2004

Portable dielectric probe (PDP) and time domain reflectometry (TDR) instruments are increasingly being used for rapid in situ determination of soil moisture contents. These techniques provide alternatives for acquiring soil moisture data in field experiments where the logistics of gravimetric sampling can restrict the number of samples obtained. The real part of the dielectric constant of soils, measured by P, L, C, and X band PDPs, was empirically related to soil water content for soils with a wide range of textures. The results were compared to TDR measurements as well. The P and L band probes and the TDR produced very comparable and accurate results, while the C and X band probes showed sensitivity to variability within a soil, the measurement technique, and soil texture. Both TDR and PDP instruments may be useful for field measurement campaigns, as they sample different depths and volumes. In general the TDR is suitable for 0–5 cm or deeper layers while the PDP can be used for measuring layers of the order of 1 cm in thickness.
Optimal adjacent pipe diameters in water distribution networks with reliability constraints
Authors: Khang Do Ba , Fujiwara Okitsugu
Published: February 2004

This paper considers optimization models for water distribution network problems with reliability constraints. Conditions are presented for an optimal solution where each link consists of at most two pipe segments with adjacent diameters. A simple example is also given to illustrate the case where the proposed conditions do not hold and three nonconsecutive pipe diameters may be selected for a link in the optimal solution.
Contaminant transport in layered porous media: 2. Applications
Authors: Tang Yi , Aral Mustafa M.
Published: February 2004

In a companion paper, analytical solutions describing the fate of a solute in a layered aquifer system are given. In the study presented here, these closed form solutions are utilized in solution of several problems of relevance to layered aquifer analysis. Initially, the example problems discussed here are primarily chosen for the purpose of verification of some of these analytical solutions. These applications are later extended to other aquifer‐aquitard configurations, to demonstrate the range of problems that may be analyzed using these procedures. Thus, in this sense, the problems discussed here cover only a sample of cases which may be studied utilizing the solutions developed earlier. In the examples discussed here, the importance of advective transport in the vertical direction is emphasized for contaminant transport analysis in a layered system.
Accuracy of operator splitting for advection‐dispersion‐reaction problems
Authors: Valocchi Albert J. , Malmstead Michael
Published: February 2004

An operator‐splitting approach is often used for the numerical solution of advection‐dispersion‐reaction problems. Operationally, this approach advances the solution over a single time step in two stages, one involving the solution of the nonreactive advection‐dispersion equation and the other the solution of the reaction equations. The first stage is usually solved with a finite difference, finite element, or related technique, while the second stage is normally solved with an ordinary differential equation integrator. The only generally published guidelines on numerical accuracy suggest that the discretization errors associated with each stage must be small in order to achieve high accuracy of the overall solution. However, in this note we demonstrate that there is an inherent mass balance error present in the operator‐splitting algorithm for problems involving continuous mass influx boundary conditions. The mass balance error does not exist for instantaneous mass input problems. These conclusions are based upon analysis of a simple first‐order decay problem for which each stage of the calculation can be performed analytically (i.e., without discretization error). For this linear decay problem we find that the product of the first‐order decay coefficient (k) times Δt must be less than approximately 0.1 in order for the mass balance error to be less than 5%. We also present a variant of the normal operator‐splitting algorithm in which the order of solving the advection‐dispersion and reaction operators is reversed at each time step. This modification reduces the mass balance error by more than a factor of 10 for a wide range of kΔt values.
On spectral measure and Monte Carlo Approaches to flow in heterogeneous media
Authors: Deng Fei‐Wen , Cushman John H.
Published: February 2004

A pedagogical approach to the spectral integral method as applied to flow problems is presented. The deterministic Fourier analog to the random spectral integral is highlighted, thus illustrating the close ties between the two. The general solution to the deterministic flow problem is used to illustrate that a weakly stationary conductivity field does not in general lead to a weakly stationary head field. Several alternative Monte Carlo methods are presented and compared to illustrate the problems associated with dropping terms of the form ƒ′h′ from the flow equation, where ƒ and h are respectively perturbations in the conductivity and head fields.
Late winter snow‐landscape relationships in the subarctic near Hoarfrost River, Great Slave Lake, Northwest Territories, Canada
Authors: Timoney Kevin , Kershaw G. Peter , Olesen Dave
Published: February 2004

Late winter sampling of snowpack depth and density was conducted in five subarctic forest and forest‐tundra landscape types north of the east arm of Great Slave Lake, Northwest Territories between March 28 and April 9, 1990. Tree and sampling density were nonnormal in distribution in study sites with stems <1500 ha−1, and normally distributed in dense forest. For the Hoarfrost River area, average snow depth, density, and water equivalent were 40.7 ± 7.9 cm, 0.17 ± 0.05 g cm3, and 6.58 ± 1.99 cm, respectively. Variation in snowpack depth and density was low within each site, indicative of relatively homogeneous snow conditions. Snow density was negatively correlated with tree stem density. For the Hoarfrost River area, both snow depth and variability in snow density increased with tree stem density. Variability in snow depth increased with variability in snow density. Snow depth was more variable in forest‐tundra than in forest sites.
A boundary integral technique for multiple‐front simulation of incompressible, immiscible flow in porous media
Authors: Stothoff S. A. , Pinder G. F.
Published: February 2004

A boundary integral formulation is developed for flow of two immiscible, incompressible phases in a porous medium. With the boundary integral formulation, large computational domains may be considered with little impact on accuracy or computational effort, as the boundary integral mesh is placed only along contours of phase saturation and the domain boundaries. Time stepping is performed by updating the position of the contours based on phase fluxes obtained with a total flux formulation. Capillary pressure gradients are treated by interpolating saturation between contours using a separate boundary integral step; accordingly, the formulation is not restricted to the special form of the constitutive relationships implied in the quasi‐linear approximation to unsaturated flow. Examples of one‐dimensional and radial two‐phase flow are simulated, with excellent agreement to the corresponding analytical solutions. Vertical imbibition of trichoroethylene into an initially pristine aquifer is simulated with both a finite element simulator and the boundary integral simulator, with good agreement between the two methods.
A mathematical model of hillslope and watershed discharge
Authors: Stagnitti Frank , Parlange Jean‐Yves , Steenhuis Tammo S. , Parlange Marc B. , Rose Calvin W.
Published: February 2004

A mathematical water balance model describing major hydrological processes operating within wet forested watersheds is proposed. The model is capable of predicting hillslope and watershed discharge, evapotranspiration demands, hillslope moisture status, and surface and subsurface flow rates. It is based on soil physical principles and requires the following input variables: average hillslope angle and width, average soil depth, precipitation, average daily evaporation rates, effective saturated hydraulic conductivity, soil moisture holding capacity and initial moisture content. These variables are often easily measured from field studies. However, in some cases, the absence of field data may require that some of the variables in the model, e.g., saturated hydraulic conductivity, be estimated or calibrated from hillslope hydrograph records. The watershed model is composed of two submodels: a storage model and a hillslope model. The storage model describes the dynamic variation in water table elevation in recharge zones and the hillslope model is used to predict runoff and seepage through flow from surrounding hillsides. Application of the model is illustrated on a small watershed located in North Madison, Connecticut.
Pressurized steady state flows from spherical cavities
Authors: Pullan A. J.
Published: February 2004

Coupled steady state saturated‐unsaturated flow from buried spherical cavities is investigated using a combined boundary element‐finite element method. Earlier work on such pressurized coupled flows has been for simple two‐dimensional cavities. The analysis of a simple three‐dimensional flow allows the effect of dimensionality to be investigated. Detailed results for a range of pressure heads and dimensionless cavity radii and the size of the saturated regions as a function of these two parameters is illustrated. While these results show how greatly diminished the effect of gravity is in three as opposed to two dimensions, they also illustrate that the effect of gravity cannot in general be ignored. The results indicate that the ratio of the total flow from a cavity under hydrostatic pressure to the flow obtained analytically by ignoring the water depth is just a linear function of the dimensionless cavity radius. It is also shown that, while the size of the saturated region does not increase uniformly with the additional applied pressure, the relationship between the total flux from the cavity and the applied pressure is linear.
The treatment of nonparticipants in travel cost analysis and other demand models
Authors: Hellerstein Daniel
Published: February 2004

When estimating demand models, such as those used in travel cost analysis, the procedures used to collect data must be considered. When the statistical assumptions underlying the chosen model are inconsistent with the sample design, biased results will be produced. In this paper, three types of bias related to the improper treatment of nonparticipants are reviewed: censoring, truncation, and endogenous stratification. Methods that control for these sources of bias are considered in the context of both continuous and count models.
Estimating net solar radiation using Landsat Thematic Mapper and digital elevation data
Authors: Dubayah R.
Published: February 2004

A radiative transfer algorithm is combined with digital elevation and satellite reflectance data to model spatial variability in net solar radiation at fine spatial resolution. The method is applied to the tall‐grass prairie of the 16 × 16 km2FIFE site (First ISLSCP Field Experiment) of the International Satellite Land Surface Climatology Project. Spectral reflectances as measured by the Landsat thematic mapper (TM) are corrected for atmospheric and topographic effects using field measurements and accurate 30‐m digital elevation data in a detailed model of atmosphere‐surface interaction. The spectral reflectances are then integrated to produce estimates of surface albedo in the range 0.3–3.0 μm. This map of albedo is used in an atmospheric and topographic radiative transfer model to produce a map of net solar radiation. A map of apparent net solar radiation is also derived using only the TM reflectance data, uncorrected for topography, and the average field‐measured downwelling solar irradiance. Comparison with field measurements at 10 sites on the prairie shows that the topographically derived radiation map accurately captures the spatial variability in net solar radiation, but the apparent map does not. The regional means for the entire site, as estimated from field measurements by themselves, from the topographic model, and from the apparent net solar radiation map are nearly equal, although the variance is an order of magnitude larger for the topographic model.
A theoretical model of the effects of timber harvesting on slope stability
Authors: Sidle Roy C.
Published: February 2004

An infinite slope stability model is proposed which incorporates changes in root cohesion and vegetation surcharge through several timber management cycles along with the stochastic influence of rainfall on pore water pressure. Recovery of rooting strength and tree surcharge following timber harvest are simulated by a sigmoid relationship, while root deterioration of harvested vegetation is described by an exponential decay function. The effects of long‐term timber management on probability of failure are simulated by overlaying the impacts of a prior vegetation removal on a more recent removal. For each year the critical pore water pressure (ucrit) needed to trigger slope failure is computed. An empirical function relating piezometric level to antecedent rainfall, storm intensity, and total precipitation is presented to assess the probability of occurrence of ucrit, based on historical rainfall records for a site in coastal Alaska. Simulations of probability of failure indicate that alternate thinnings and clear‐cuts and clear‐cuts alone produce less stable conditions than shelterwood harvesting systems and partial cuts. Repeated harvesting cycles with progressively shorter rotations, reduced regeneration potential of new vegetation, and destruction of understory vegetation during logging or site preparation can all increase the probability of failure. These analyses can provide land managers with options for acceptable vegetation management strategies on potentially unstable hillslopes.
The validity of a simple statistical model for estimating fluvial constituent loads: An Empirical study involving nutrient loads entering Chesapeake Bay
Authors: Cohn Timothy A. , Caulder Dana L. , Gilroy Edward J. , Zynjuk Linda D. , Summers Robert M.
Published: February 2004

We consider the appropriateness of “rating curves” and other log linear models to estimate the fluvial transport of nutrients. Split‐sample studies using data from tributaries to the Chesapeake Bay reveal that a minimum variance unbiased estimator (MVUE), based on a simple log linear model, provides satisfactory load estimates, even in some cases where the model exhibited significant lack of fit. For total nitrogen (TN) the average difference between the MVUE estimates and the observed loads ranges from −8% to + 2% at the four sites. The corresponding range for total phosphorus (TP) is −6% to +5%. None of these differences is statistically significant. The observed variability of the MVUE load estimates for TN and TP, which ranges from 7% to 25% depending on the case, is accurately predicted by statistical theory.
Note on the sorptivity for mixed saturated‐unsaturated flow
Authors: Parlange J.‐Y. , Barry D. A. , Parlange M. B. , Haverkamp R.
Published: February 2004

An approximate, but general, analytical result for the sorptivity of a soil is derived. It provides an estimate of the sorptivity when ponding occurs at the soil surface, or when the diffusivity is infinite at saturation. Two earlier results appear as limiting cases of this general result. Its accuracy is also checked by comparison with the only exact solution available at present.
Discrete fracture network interpretation of field tracer migration in sparsely fractured rock
Authors: Dverstorp Björn , Andersson Johan , Nordqvist Wille
Published: February 2004

Field tracer migration in sparsely fractured rock is analyzed with a discrete fracture network model that has been calibrated on fracture and flow data from the Stripa‐3D experiment in Sweden. Comparison between transport simulations in the flow‐calibrated discrete model and tracer tests of the field experiment confirms the fracture network parameters obtained from the flow analysis except for the variance of the fracture conductivities which had to be increased. The discrete model reproduces the uneven spatial distribution of flow and tracers, the complex dispersive behavior, and channeling effects that have been observed in the field experiment. As a result of flow channeling, transport occurs along preferential paths whose transport properties may substantially deviate from the medium average properties. For example, porosity and wetted fracture surface area available to sorption may be reduced by almost 2 orders of magnitude compared to the medium average values.
Physical basis for a time series model of soil water content
Authors: Parlange Marc B. , Katul Gabriel G. , Cuenca Richard H. , Kavvas M. Levent , Nielsen Donald R. , Mata Michael
Published: February 2004

A first‐order autoregressive Markovian model AR(1) is formulated on the basis of the hydrologic budget and soil water transport equation. The model predictions compared well with neutron probe measurements of soil moisture content, and the statistical moments were conserved. The applied water events were white noise in structure, and the random shocks generated from the flow dynamics simplifications have a statistical mean of zero and were uncorrelated for all time lags. The derived AR(1) model parameter is used to compute the mean diffusivity of the soil, which is in agreement with reported lab measurements and field estimates obtained from cumulative evaporation measurements made with two large lysimeters.
Physically based hydrologic modeling: 2. Is the concept realistic?
Authors: Grayson Rodger B. , Moore Ian D. , McMahon Thomas A.
Published: February 2004

Future directions for physically based, distributed‐parameter models intended for use as hydrologic components of sediment and nutrient transport models are discussed. The attraction of these models is their potential to provide information about the flow characteristics at points within catchments, but current representations in process‐based models are often too crude to enable accurate, a priori application to predictive problems. The difficulties relate to both the perception of model capabilities and the fundamental assumptions and algorithms used in the models. In addition, the scale of measurement for many parameters is often not compatible with their use in hydrologic models. The most appropriate uses of process‐based, distributed‐parameter models are to assist in the analysis of data, to test hypotheses in conjunction with field studies, to improve our understanding of processes and their interactions and to identify areas of poor understanding in our process descriptions. The misperception that model complexity is positively correlated with confidence in the results is exacerbated by the lack of full and frank discussion of a model's capability /limitations and reticence to publish poor results. This may ultimately diminish the opportunity to advance understanding of natural processes because the managers of research resources are given the impression that the answers are already known and are being provided by models. Model development is often not carried out in conjunction with field programs designed to test complex models, so the link with reality is lost.
A geochemical transport model for redox‐controlled movement of mineral fronts in groundwater flow systems: A case of nitrate removal by oxidation of pyrite
Authors: Engesgaard Peter , Kipp Kenneth L.
Published: February 2004

A one‐dimensional prototype geochemical transport model was developed in order to handle simultaneous precipitation‐dissolution and oxidation‐reduction reactions governed by chemical equilibria. Total aqueous component concentrations are the primary dependent variables, and a sequential iterative approach is used for the calculation. The model was verified by analytical and numerical comparisons and is able to simulate sharp mineral fronts. At a site in Denmark, denitrification has been observed by oxidation of pyrite. Simulation of nitrate movement at this site showed a redox front movement rate of 0.58 m yr−1, which agreed with calculations of others. It appears that the sequential iterative approach is the most practical for extension to multidimensional simulation and for handling large numbers of components and reactions. However, slow convergence may limit the size of redox systems that can be handled.
Thermal energy storage in an unconfined aquifer: 1. Field Injection Experiment
Authors: Palmer Carl D. , Blowes David W. , Frind Emil O. , Molson John W.
Published: February 2004

A thermal injection and storage experiment was conducted to investigate the feasibility of storing thermal energy in shallow unconfined aquifers near the water table. Heated water was injected into a shallow aquifer and plume temperatures were monitored over a 141‐day period by means of a dense array of bundle‐type piezometers. The highly detailed data, which provide the three‐dimensional temperature distribution within the aquifer, give good insight into the physical processes of aquifer thermal energy storage, and provide an excellent basis for the verification of simulation models. The experimental data also allow the physical processes of heat advection, dispersion, retardation, buoyancy and boundary heat loss to be quantified. In a companion paper, a three‐dimensional density‐dependent groundwater flow and thermal transport model is developed and validated using the results of the thermal injection experiment.
Bubble plume modeling for lake restoration
Authors: Wüest Alfred , Brooks Norman H. , Imboden Dieter M.
Published: February 2004

A steady bubble plume model is developed to describe a weak air (or oxygen) bubble injection system used for the restoration of deep stratified lakes. Since the model is designed for two modes of operation, i.e., oxygenation and artificial mixing, gas exchange between water and bubbles has to be included. The integral model is based on the entrainment hypothesis and a variable buoyancy flux determined by the local plume properties and the ambient water column. Fluxes of eight properties are described by nonlinear differential equations which can be numerically integrated. In addition, five equations of state are used. The model leaves open two initial conditions, plume radius and plume velocity. Model calculations with real lake water profiles demonstrate the range of applicability for both modes of operation. The model agrees reasonably well with field data and with laboratory experiments conducted by various investigators.
Irrigation management and investment under saline, limited drainage conditions: 1. Model formulation
Authors: Knapp Keith C.
Published: February 2004

A dynamic optimization model for irrigation management and investment of an individual field is formulated. The model is applicable to situations where soil salinity is a current or potential problem and where drainage flows incur costs. The crop rotation is prespecified and several alternate irrigation systems are available for investment. These differ in application uniformity and cost. The field is assumed to be spatially variable in soil salinity and infiltrated water. The joint distribution of these two variables evolves over time in response to annual volumes of irrigation water and type of irrigation system. Social net benefits are defined as crop revenue net of irrigation costs, production costs, and environmental/disposal costs associated with percolation of water below the root zone. The optimization problem is to choose annual applied water volumes and investment in irrigation systems to maximize the present value of social net benefits.