bluuebunny/crossref_metadata_2025 huggingface dataset:
An investigation of the relationship between ponded and constant flux rainfall infiltration
Authors: Poulovassilis A. , Kerkides P. , Elmaloglou S. , Argyrokastritis I.
Published: February 2004

The physics of the rainfall infiltration under a constant rainfall flux greater than the hydraulic conductivity at saturation are investigated and compared with those pertaining to the infiltration process under flooding. From the arguments developed theoretically the following conclusions are reached: The moisture profile prevailing at the time the soil surface becomes saturated during the rainfall process (t = T) is identical with the profile which has been developed during flooding at an earlier time at which the infiltration rate is equal to the rainfall flux (t = tc < T). The rainfall profile development after T is the same as that of the profile under flooding after tc, the two profiles having a constant time lag equal to (T−tc). The Green and Ampt analysis as has been applied to the constant flux rainfall infiltration presupposes the identity of the profiles mentioned above. In addition to the theoretical arguments, the flow equation was solved numerically for four different soils under the conditions that define the two infiltration processes. The numerical results obtained conform with the theory.
Field scale mass arrival of sorptive solute into the groundwater
Authors: Destouni Georgia , Cvetkovic Vladimir
Published: February 2004

The effect of spatial variability in sorption parameters is investigated for field scale solute transport in the unsaturated zone. The considered sorption reaction is kinetically controlled sorption‐desorption coupled with a parallel linear equilibrium isotherm. The source of solute is assumed to be distributed over a heterogeneous field where the horizontal extent of the source area is significantly larger than the depth to the ground water table. The transport model is expressed in terms of the mass flux and in dimensionless form, where the effect of equilibrium sorption is incorporated in the dimensionless parameters of the kinetic sorption model. The influence of different correlation properties between the sorption parameters and the hydraulic conductivity at saturation, Ks, on the expected field scale mass flux of solute is illustrated. The results indicate that spatially variable sorption rate coefficients with negative or no correlation to Ks may result in an earlier mass arrival and a considerably altered form of the breakthrough curve compared to the case of constant sorption parameters. In addition, a comparison between the flux‐averaged and the resident concentration models indicates that spatial variability in the sorption rate coefficients has similar effects on the temporal variation of the field‐scale resident concentration as on the field scale mass flux of solute.
Large‐scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 2. Analysis of spatial moments for a nonreactive tracer
Authors: Garabedian Stephen P. , LeBlanc Dennis R. , Gelhar Lynn W. , Celia Michael A.
Published: February 2004

A large‐scale natural gradient tracer test was conducted to examine the transport of reactive and nonreactive tracers in a sand and gravel aquifer on Cape Cod, Massachusetts. As part of this test the transport of bromide, a nonreactive tracer, was monitored for about 280 m and quantified using spatial moments. The calculated mass of bromide for each sampling date varied between 85% and 105% of the injected mass using an estimated porosity of 0.39, and the center of mass moved at a nearly constant horizontal velocity of 0.42 m per day. A nonlinear change in the bromide longitudinal variance was observed during the first 26 m of travel distance, but afterward the variance followed a linear trend, indicating the longitudinal dispersivity had reached a constant value of 0.96 m. The transverse dispersivities were much smaller; transverse horizontal dispersivity was 1.8 cm, and transverse vertical dispersivity was about 1.5 mm.
Two‐dimensional modeling of advection‐dominated solute transport in groundwater by the matched artificial dispersivity method
Authors: Syriopoulou Dimitra , Koussis Antonis D.
Published: February 2004

A methodology for modeling, in two dimensions, the transport of dissolved constituents in steady groundwater flow is presented. The model aims at the efficient confrontation of the difficulties associated with advection‐dominated conditions (oscillation‐free resolution of sharp fronts). Employing a spatial splitting algorithm within the framework of the (curvilinear) principal directions of transport formulation, it computes transport along streamlines by a variant of the pseudoviscosity method. The algorithm is subject to mild grid design constraints that afford efficiency in terms of CPU time and storage; its accuracy is tested against analytical solutions. The model is also applied to the simulation of the chloride plume observed under the Borden landfill in Ontario, Canada. Model calibration issues are discussed.
Exact solution for nonlinear, nonhysteretic redistribution in vertical soil of finite depth
Authors: Sander G. C. , Cunning I. F. , Hogarth W. L. , Parlange J.‐Y.
Published: February 2004

We present the first exact solution to nonlinear, nonhysteretic redistribution of water in a vertical column of soil of finite depth. We analyze the effects of gravity, capillarity and initial volume of infiltrated water on the redistribution process. Redistributed profile shapes are found to be consistent with the numerically produced profiles in the absence of hysteresis. Finally, we derive the steady state redistributed profile for arbitrary soil properties.
An evapotranspiration‐interception model for urban areas
Authors: Grimmond C. S. B. , Oke T. R.
Published: February 2004

A model to calculate evapotranspiration from urban areas over a wide range of meteorological conditions is presented. An evapotranspiration‐interception approach is used because it is necessary to cope with the changing water availability on the surface, during and following rainfall or irrigation. The model is applicable to areas ranging from the size of city blocks to land use zones and time periods of one hour and longer. The modeled evaporation is compared with that from micrometeorological measurements conducted from January to June 1987 in a suburb of Vancouver, British Columbia, Canada. The results show that this approach to modeling urban evapotranspiration provides realistic hourly and daily estimates of the areally averaged latent heat flux and surface water state.
The Emerald Lake Watershed Study: Introduction and site description
Authors: Tonnessen Kathy A.
Published: February 2004

The Emerald Lake Watershed study was organized to investigate the effects of acidic deposition on high‐elevation watersheds and surface waters of the Sierra Nevada, California. Some of the results of this comprehensive study of aquatic and terrestrial ecosystems at a small, headwater basin are presented in four papers in this series. The watershed study site is in Sequoia National Park, on the western slope of the Sierra Nevada. This glacial cirque is located in the upper Marble Fork of the Kaweah River. This 120‐ha watershed ranges from Alta Peak (3416 m) down to Emerald Lake (2400 m). Most of the watershed surface area is exposed granite and granodiorite rocks, with limited coverage (about 20%) by thin, acidic soils. The hydrology of the basin is dominated by snowmelt runoff during March–June. Emerald Lake, a glacial tarn, is 2.72 ha in area, with a maximum depth of 10.5 m. Surface waters are poorly buffered and dominated by calcium and bicarbonate. Most of the yearly precipitation falls as dilute snow (pH 5.2–5.4), with acidic rain storms sampled during May–October.
Glacial ice composition: A potential long‐term record of the chemistry of atmospheric deposition, Wind River Range, Wyoming
Authors: Naftz David L. , Rice James A. , Ranville James R.
Published: February 2004

During a reconnaissance study, ice samples were collected from Knife Point glacier to determine if glaciers in the Wind River Range could provide a long‐term record of the chemical composition of wet deposition. Eight annual ice layers comprising the years 1980–1987 were identified. The concentration of calcium, chloride, and sulfate in the annual‐weighted wet deposition samples collected at the National Atmospheric Deposition Program (NADP) station near Pinedale, Wyoming, showed a significant, positive correlation to the concentration of the same major ions in composite samples from the annual ice layers. Results of the study imply that continuous ice cores reaching to the deeper parts of glaciers in the Wind River Range could provide long‐term records of the chemical composition of wet deposition.
Surface‐subsurface water interactions in an alluviated mountain stream channel
Authors: Castro Nancy Mathews , Hornberger George M.
Published: February 2004

Data from an instream tracer experiment performed in North Fork Dry Run, Shenandoah National Park, Virginia, illustrate that the transport of water and conservative solute in the stream is greatly influenced by interaction with water present in both the relatively shallow gravel bed and in the relatively deeper alluvial infill material. The shape of concentration‐time curves from the field tracer experiment exhibit “storage” of tracer in the subsurface‐short‐term storage imposed by exchange with the gravel bed and long‐term storage by exchange with the deeper alluvial infill. Time series models fit the observed concentration data very well. Estimates of mixing volumes generated by the time series analysis indicate that a large volume of the gravel bed participates in rapid mixing with the surface stream. At one point along the study reach a second‐order time series model is supported by the data, suggesting two independent, parallel flow paths. Observations of tracer in wells more than 10 m from the stream and the very extended recession of the tracer in the stream where it flows over a bedrock ledge support the contention that the deeper alluvium plays an important role in transient storage of solute in North Fork Dry Run. The work implies that physically‐based models to predict transport and storage in alluviated mountain catchments may have to account for interactions between the stream and large portions of its floodplain.
Redistribution of Water and Solute Following Infiltration From a Surface Drip Source
Authors: Clothier Brent , Sauer Tom , Scotter Dave
Published: February 2004

Laboratory observations of the redistribution of water and solute, following one‐dimensional and three‐dimensional constant flux infiltration, reveal distinctly different patterns of water and chemical transport. The geometry‐induced enhancement of capillarity over gravity in three dimensions is thought responsible for the rapid attainment during redistribution of a near‐constant water content θ*, some radial distance r around the dripper. This renders simple the prediction of the maximum radial extent of convective solute redistribution, r*. Nevertheless, molecular diffusion about r* is the dominant mode of solute transport during the weak convective redistribution in three dimensions.
Nonlinear Dynamics of Soil Moisture at Climate Scales: 2. Chaotic Analysis
Authors: Rodriguez‐Iturbe Ignacio , Entekhabi Dara , Lee Jae‐Soo , Bras Rafael L.
Published: February 2004

The soil moisture balance equation over large spatial territories is analyzed at seasonal time scales. Local recycling of precipitation and dynamic effects of soil moisture in the generation and modification of mesoscale circulations are explicitly included in the parameterization which also incorporates time delays in the above effects. It is shown that the dynamics of the equation is quite complex, being capable of exhibiting fixed point, limit cycle, and chaotic type of behavior. The implications of this dynamics are important for the interpretation and modeling of hydrologie phenomena.
A Generalization of the Richards Equation Within Extended Irreversible Thermodynamics
Authors: Del Río P. J. A. , De Haro M. López
Published: February 2004

From an extended irreversible thermodynamic formalism a wavelike equation for the water potential in soils is obtained. This avoids the problem of the infinite velocity of transmission of disturbances in the water potential as implied by the Richards equation.
Response of Ned Wilson Lake Watershed, Colorado, to Changes in Atmospheric Deposition of Sulfate
Authors: Campbell Donald H. , Turk John T. , Spahr Norman E.
Published: February 2004

The Ned Wilson Lake watershed responds directly and rapidly to changes in precipitation inputs of sulfate, which has important implications for effects of acid deposition on the aquatic system. Chemistry at three precipitation collection sites and three watershed sites (a pond, a lake, and a spring) has been monitored in and near the Flattops Wilderness Area in northwestern Colorado beginning in 1981–1983. Bulk snowpack concentration of sulfate in the watershed and volume‐weighted annual mean concentration of sulfate in precipitation at two nearby sites generally decreased from 1981 to 1985, were small through 1987, and increased in 1988–1989. Changes in concentration of sulfate at the watershed sites are controlled by precipitation inputs. Responsiveness of the individual sites was dependent on their position along the hydrologic flow path. The fastest response was in the pond, which has a hydrologic residence time of less than 1 year; over 90% of the variance in concentration of sulfate in the pond was explained by changes in concentration in precipitation. The lake has a hydrologic residence time of 1 to 4 years; a regression model of the concentration of sulfate in the lake, as a function of the concentration in the lake during the previous year and the concentration in precipitation, explained 87% of the variance in concentration of sulfate in the lake. The hydrologic response time of the spring is unknown; it was not responsive to changes in concentration of sulfate in precipitation. The recent increase of sulfate concentration in precipitation and in the pond and lake is evidence for a rapid rather than a delayed response, which could not be determined when only a decreasing trend in sulfate concentration was reported in 1982–1987. Watersheds of this type are sensitive to acidification (acid‐neutralizing capacity less than 60 μeq L−1), and these results indicate conservative behavior of sulfate. This is important in predicting effects of future changes in atmospheric deposition, which could potentially be caused by anthropogenic emissions or climatic change.
A Semidiscrete Model for Water and Solute Movement in Tile‐Drained Soils: 1. Governing Equations and Solution
Authors: Kamra S. K. , Singh Sita Ram , Rao K. V. G. K. , Van Genuchten M. Th.
Published: February 2004

A finite element model has been developed to simulate solute transport in tile‐drained soil‐aquifer systems. Water flow in the unsaturated zone and to drains in the saturated zone was assumed to be at steady state. The model considers the transport of nonreactive solutes, as well as of reactive solutes whose behavior can be described by a distribution coefficient. The exact‐in‐time numerical solution yields explicit expressions for the concentration field at any future point in time without having to compute concentrations at intermediate times. The semidiscrete method involves the determination of an eigensystem of eigenvalues and eigenvectors of the coefficient matrix. The eigensystem may be complex (i.e., it may have imaginary components) due to asymmetry created by the convection term in the governing convection‐dispersion equation. The proposed approach facilitates long‐term predictions of concentrations in drainage effluents and of salt distributions in soil and groundwater. The accuracy of the model was verified by comparing model results with those based on an analytical solution for two‐dimensional solute transport in groundwater.
Comparing Simulated and Experimental Hysteretic Two‐Phase Transient Fluid Flow Phenomena
Authors: Lenhard R. J. , Parker J. C. , Kaluarachchi J. J.
Published: February 2004

A hysteretic model for two‐phase permeability (k)‐saturation (S)‐pressure (P) relations is outlined that accounts for effects of nonwetting fluid entrapment. The model can be employed in unsaturated fluid flow computer codes to predict temporal and spatial fluid distributions. Consideration is given to hysteresis in S‐P relations caused by contact angle, irregular pore geometry, and nonwetting fluid entrapment effects and to hysteresis in k‐S relations caused by nonwetting fluid entrapment effects. An air‐water flow experiment is conducted with a 72‐cm vertical soil column where the water table is fluctuated to generate scanning S‐P paths. Water contents are measured via a gamma radiation system, and water pressures are measured via pressure transducers connected to ceramic tensiometers inserted in the soil column. Computer simulations of the experiment employing the hysteretic k‐S‐P model and a nonhysteretic k‐S‐P are compared with measured water contents and pressures. Close agreement is found between experimental water contents and those predicted by a numerical code employing the hysteretic k‐S‐P relations. When nonhysteretic k‐S‐P constitutive relations are utilized, there is poor agreement between measured and predicted water saturations of the scanning paths. Only one more parameter is needed to model two‐phase hysteretic fluid behavior than to model nonhysteretic behavior. Results of this study suggest that consideration should be given to effects of hysteresis in k‐S‐P relations to accurately predict fluid distributions.
The Genetic Algorithm and Its Application to Calibrating Conceptual Rainfall‐Runoff Models
Authors: Wang Q. J.
Published: February 2004

The genetic algorithm is a search procedure based on the mechanics of natural selection and natural genetics, which combines an artificial survival of the fittest with genetic operators abstracted from nature. In this paper, a genetic algorithm for function optimization is introduced and applied to calibration of a conceptual rainfall‐runoff model for data from a particular catchment. All seven parameters of the model are optimized. The results show that the genetic algorithm can be efficient and robust.
A Stochastic Analysis of Pumping Tests in Laterally Nonuniform Media
Authors: Butler James J.
Published: February 2004

Conventional pumping test analysis methodology assumes that aquifer transmissivity is invariant in space. The ramifications of this assumption are examined for hypothetical units whose variations in transmissivity are considered to be reasonable representations of variations possible in natural systems. The dependence of pumping test transmissivity on spatial (angular and radial) and temporal location of observations and the method of drawdown analysis is assessed. The dependence on the angular position of an observation well appears of little significance. Dependence on radial position, however, can be strong. The dependence on the interval of time used in the pumping test analysis is only important in highly variable systems. Methods of drawdown analysis that yield identical estimates in uniform units yield differing estimates in nonuniform ones as a result of a difference in data‐fitting procedures. A reasonable estimate of transmissivity at a regional scale can be obtained from a Cooper‐Jacob analysis applied at a large duration of pumpage. In general, conventional approaches for pumping test analysis should be viable in the nonuniform aquifers of the type considered here.
A Robust‐Resistant Approach to Interpret Spatial Behavior of Saturated Hydraulic Conductivity of a Glacial Till Soil Under No‐Tillage System
Authors: Mohanty B. P. , Kanwar R. S. , Horton R.
Published: February 2004

A central Iowa glacial till soil under no‐tillage condition was studied for its spatial behavior of saturated hydraulic conductivity (K) at the surface soil layers. Hydraulic conductivity measurements both in situ and in the laboratory were made at two depths of 15 and 30 cm at regular intervals of 4.6 m on two perpendicular transects crossing each other at the center of the field. Simplified split‐window median polishing in conjunction with a robust semivariogram estimator were used to examine the spatial structure of the glacial till material. Results of this study indicated a nested structure of K at 30 cm depth. Soil clustering at the experimental site at intervals of 20 m, in addition to the soil microheterogeneity, contributed to variation in K, with an overall range of spatial dependence of K up to 60 m. Medians of split windows of 23 m width were found to be the “solo representatives” or “summary points” of the soil clusters contributing to spatial structure. In situ and laboratory measurements for K showed consistency in their trends even though some parametric variations were observed. K values observed near the soil surface at a depth of 15 cm were dominated by white noise and directional trends.
Sensitivity Analysis of Infiltration, Exfiltration, and Drainage in Unsaturated Miller‐Similar Porous Media
Authors: Kabala Z. J. , Milly P. C. D.
Published: February 2004

The effects of soil heterogeneity on the transport of moisture in the unsaturated zone are studied by means of functional sensitivities, which are more suitable than elementary (or conventional) sensitivities for analyzing continuous systems. The earlier described algorithm is applied to three problems, drainage, infiltration, and exfiltration, for two Miller‐similar soils (a randomly generated heterogeneous Yolo light clay and a homogeneous one). It reveals the difference between drainage, on the one hand, and both infiltration and exfiltration on the other. The scaled flux sensitivity profiles, i.e., the functional derivatives of fluxes with respect to Miller‐similarity parameter λ(x), are flat for drainage, demonstrating that drainage is equally sensitive to changes in λ(x) at all points of the profile and hence that the similarity parameter has to be modeled well in the whole soil column. Analogous sensitivity profiles for infiltration and exfiltration problems show, significant delay in propagation of functional sensitivities in the soil column (with respect to the moisture front), allowing thus for less stringent modeling of the similarity parameter for the bottom of the column. Infiltration problems seem to be most difficult to deal with, because they are most sensitive to the similarity parameter.
A Model for Simulating Transport of Reactive Multispecies Components: Model Development and Demonstration
Authors: Yeh Gour‐Tsyh , Tripathi Vijay S.
Published: February 2004

This paper presents the development and demonstration of a two‐dimensional finite‐element hydrogeochemical transport model, HYDROGEOCHEM, for simulating transport of reactive multispecies solutes. The model is designed for application to heterogeneous, anisotropic, saturated‐unsaturated media under transient or steady state flow conditions. It simulates the chemical processes of complexation, dissolution‐precipitation, adsorption‐desorption, ion exchange, redox, and acid‐base reaction, simultaneously. A set of four example problems are presented. The examples illustrate the model's ability to simulate a variety of reactive transport problems. Important results presented include a depiction of the propagation of multiple precipitation‐dissolution fronts, a display of the large errors in model response if the number of iterations between the hydrologic transport and chemical equilibrium modules is limited to one, an illustration of the development of greater concentration of contaminants In groundwater away from a waste site than near the source, and a demonstration of the variation in distribution coefficients of more than 6 orders of magnitude.
Evaluation of liquid and vapor water flow in desert soils based on chlorine 36 and tritium tracers and nonisothermal flow simulations
Authors: Scanlon Bridget R.
Published: February 2004

The distribution of anthropogenic 36Cl and 3H was used along with numerical flow simulations to evaluate the relative importance of liquid and vapor flow in the shallow unsaturated zone of an area within the Chihuahuan Desert of Texas. Chlorine 36 is nonvolatile and is restricted to liquid phase flow, whereas tritiated water is volatile and can move in both liquid and vapor phases. Tritium penetrated 1 m deeper than 36Cl, although 3H fallout occurred later than that of 36Cl. Deeper penetration of 3H relative to that of 36Cl was attributed to enhanced downward movement of 3H in the vapor phase. The moisture flux calculated from the 36Cl/Cl peak at 0.5‐m depth was 1.4 mm yr−1, whereas that based on the 3H peak at 1.4‐m depth was 7 mm yr−1. The difference in moisture fluxes between the two tracers suggests a vapor flux of approximately 6 mm yr−1. The vapor flux hypothesis was tested using nonisothermal liquid and vapor flow simulations with the computer code SPLaSHWaTr. Simulations of 5‐day periods in the winter and summer were conducted to represent the extremes in temperature gradients. The calculated vapor flux was two to eight orders of magnitude greater than the liquid flux for the periods simulated. Predicted vapor fluxes were upward in the top 0.04 m of the unsaturated zone in the summer and winter in response to steep water potential gradients induced by surface evaporation. Below the evaporation front, from depths of 0.15 to 1 m, downward vapor fluxes in the summer were much greater than generally upward vapor fluxes in the winter. These results suggest an annual net downward vapor flux that is consistent with the chemical tracer data.
A theoretical approach for modeling unsaturated flow in spatially variable soils: Effective flow models in finite domains and nonstationarity
Authors: Mantoglou Aristotelis
Published: February 2004

Models of unsteady unsaturated flow in large‐scale heterogeneous soils are developed considering finite flow domains and nonstationarity of the soil properties and flow characteristics. The problem is cast into a more general stochastic framework than the originally proposed stationary spectral framework (Mantoglou and Gelhar, 1987a, b, c). The methodology considers the three dimensionality of the local governing flow equation, the nonlinear dependence of the local output on the local soil properties, as well as the effect of finite flow domains and nonstationarity of the soil properties and flow characteristics. The large‐scale model representation is in the form of a partial differential equation, with large‐scale “effective” parameters, subject to a set of initial and boundary conditions. The effective model parameters are related to a set of fluctuation covariance equations obtained by using a linearized fluctuation equation. This set of covariance equations and the corresponding large‐scale model of the system are coupled and must be solved simultaneously. Particular cases of interest where stationarity in two or three spatial dimensions occurs are investigated, and, using spectral representations, the dimensionality of the covariance equations is reduced. Simple closed‐form and practical expressions for the effective parameters valid in specific situations are presented, and illustrative examples are discussed. The theory and the models presented provide a more complete view of the large‐scale unsaturated flow problem and can prove useful for evaluation of unsaturated flow phenomena of paramount importance in practical applications, for example, for predicting the movement of liquid wastes in the unsaturated zone.
Distributed Snowmelt Simulations in an Alpine Catchment: 1. Model Evaluation on the Basis of Snow Cover Patterns
Authors: Blöschl G. , Kirnbauer R. , Gutknecht D.
Published: February 2004

This paper presents an attempt at deterministically modeling spatially distributed snowmelt in an alpine catchment. The basin is 9.4 km2 in area and elevations range from 1900 to 3050 m above sea level. The model makes use of digital terrain data with 25 m grid spacing. Energy balance components are calculated for each grid element taking topographic variations of solar radiation into account. For each grid element albedo and snow surface temperatures are simulated. Model performance is evaluated on the basis of snow cover depletion patterns as derived from weekly air photographs. The use of spatially distributed data allows for addressing individual model components. Results indicate that the basic model assumptions are realistic. Model inadequacies are shown to arise from processes not included in the model such as avalanching and long wave emission from surrounding terrain as well as inaccurate model parameters.
Tracer Transport in Fractures: Analysis of Field Data Based on a Variable‐Aperture Channel Model
Authors: Tsang C. F. , Tsang Y. W. , Hale F. V.
Published: February 2004

A variable‐aperture channel model is used as the basis to interpret data from a 3‐year tracer transport experiment in fractured rocks. The data come from the so‐called Stripa‐3D experiment performed by Neretnieks and coworkers. Within the framework of the variable‐aperture channel conceptual model, tracers are envisioned as traveling along a number of variable‐aperture flow channels, whose properties are related to the mean
and standard deviation σ of the fracture aperture distribution. Two methods are developed to address the presence of strong time variation of the tracer injection flow rate in this experiment. The first approximates the early part of the injection history by an exponential decay function and is applicable to the early time tracer breakthrough data. The second is a deconvolution method involving the use of Toeplitz matrices and is applicable over the complete period of variable injection of the tracers. Both methods give consistent results. These results include not only estimates of
and σ, but also the number of channels involved in the tracer transport and their Peclet numbers and dispersivities. An interesting and surprising observation is that the data indicate that for each channel the Peclet number increases with the mean travel time; i.e., dispersivity decreases with mean travel time. The meaning of this trend is discussed in terms of the strong heterogeneity of the flow system.
Optimal reservoir operation for irrigation of multiple crops
Authors: Vedula S. , Mujumdar P. P.
Published: February 2004

A model for the optimal operating policy of a reservoir for irrigation under a multiple crops scenario using stochastic dynamic programming (SDP) is developed. Intraseasonal periods smaller than the crop growth stage durations form the decision intervals of the model to facilitate irrigation decisions in real situations. Reservoir storage, inflow to the reservoir, and the soil moisture in the irrigated area are treated as state variables. An optimal allocation process is incorporated in the model to determine the allocations to individual crops when a competition for water exists among them. The model also serves as an irrigation scheduling model in that at any given intraseason period it specifies whether irrigation is needed and, if it is, the amount of irrigation to be applied to each crop. The impact on crop yield due to water deficit and the effect of soil moisture dynamics on crop water requirements are taken into account. A linear root growth of the crop is assumed until the end of the vegetative stage, beyond which the root depth is assumed to be constant. The applicability of the model is demonstrated through a case study of an existing reservoir in India.
The stream function of potential theory for a dual‐pipe subirrigation‐drainage system
Authors: Kirkham Don , Horton Robert
Published: February 2004

An exact mathematical solution to Laplace's equation is presented for appropriate boundary conditions associated with the problem of dual‐pipe subirrigation and drainage. The solution can be used to determine a flow net within the groundwater flow region and the associated water table shape. The solution is general. The effects of several hydraulic and geometrical parameters on the groundwater system, such as thickness of saturated zone, position of subirrigation and drainage pipes, heads in the subirrigation and drainage pipes, crop evapotranspiration, fraction of inflowing subirrigation water that exits through the drains, and the aquifer hydraulic conductivity system are evaluated. Calculations are presented showing how pipe spacing affects the shape of the water table. For example, with hydraulic conductivity of 10 m/d, evapotranspiration of 0.01 m/d, drainpipe radius of 0.05 m, and subirrigation pipe radius of 0.0375 m, calculations show that the maximum water table elevation for a pipe spacing of 40 m is 0.64 m greater than for a pipe spacing of 16 m when 40% of the input subirrigation water volume is being removed from the system by drainage. Finally, the general mathematical solution can be used to predict chemical movement as well as water flow through the system.
Long‐term heat storage in the Great Lakes
Authors: Croley Thomas E.
Published: February 2004

Practical estimation of long‐term daily Great Lakes evaporation requires one‐dimensional (depth) models of heat storage and mixing. Conceptual models are preferable to physical models for small‐computer simulations that are multiple, continuous, and long. This paper describes a new conceptual superposition model of heat storage to extend an existing evaporation model along the lake depth. The resulting daily model is recalibrated to remotely sensed surface water temperatures and is used to illustrate anew seasonal heating and cooling cycles, heat‐temperature hysteresis, water column turnovers, and mixed‐layer developments. It is used as well to compare the vertical distribution of temperatures with independent bathythermograph data. The time occurrence structure of evaporation on the Great Lakes is investigated, and the effects of summertime initial conditions on subsequent wintertime behavior of evaporation are simulated. Impacts of perceived large‐lake thermodynamic behavior are analyzed, and suggestions are made for further research.
Stable isotopic study of the groundwater of the Martha Brae River Basin, Jamaica
Authors: Ellins K. K.
Published: February 2004

The hydrology of a small karst drainage basin in Jamaica, the Martha Brae River basin, was examined using stable isotopes. Variations in the isotopic composition of the groundwaters sampled and their positions relative to the local meteoric water line on a δD/δ18O diagram permitted the identification of two distinct groundwater types. The isotopic data also provided evidence that the most productive portion of the aquifer is divided by a major fault, which impedes groundwater flow. Information regarding the mechanisms and elevation of recharge was inferred from the δD versus δ18O relationships and differences in isotopic composition, respectively.
Groundwater withdrawal from a hill with uniformly sloped seepage faces
Authors: Maas C.
Published: February 2004

A new kind of free boundary is introduced, namely, the projection on the horizontal plane of the interzonal boundary between a seepage face and a phreatic surface. This “seepage boundary” is demonstrated to show certain mathematical similarities to a phreatic surface, viewed in a vertical plane. A variation on Zhukovsk's method is employed to derive a formula for the deformation of the seepage boundary on the side of a hill due to groundwater withdrawal. The modified Zhukovski function, being many valued, is handled by Strack's extension of the Schwartz‐Christoffel transformation. It is finally pointed out that more complicated problems could be solved by a modified hodograph method.
Model building and judgment: Implications for benefit transfers with travel cost models
Authors: McConnell K. E.
Published: February 2004

Benefit transfer is the process by which researchers take recreational demand models or other models which are estimated for one site or region and apply them to another site or region. Model estimation is frequently viewed as a simple exercise in classical statistics. Here it is argued that, while standard hypothesis testing plays a role in model estimation, it may be less important than the researcher's judgment about how the model ought to work. The power of economics lies principally with the logic of theory, and then with the strength of empirical evidence. These arguments are illustrated using the travel cost approach to estimating the demand for recreation. There are several major research issues (for example, the role of substitute prices) which influence the final estimates of benefits but which must be determined at least in part by the researcher. The implication for benefit transfer is that this is also within the realm of judgment for the researcher, but that it will be more acceptable when judgment is recognized. That is, the original benefit estimates are tailored for a specific application. They cannot be treated as if they came from a strictly random process. Recognizing this tailoring process will put the researcher in a better position for transferring model results.
Sorptivity and macroscopic capillary length relationships
Authors: Warrick A. W. , Broadbridge P.
Published: February 2004

The relationship of sorptivity S and macroscopic capillary length λc is explored. Sorptivity is the proportionality constant between cumulative intake, expressed as a length, and the square root of time for sorption into an unsaturated soil. The capillary length is defined as the integral of the unsaturated hydraulic conductivity with respect to matric potential normalized by the difference between the conductivity at the two limits of integration. The approximation b = λc(θwct ‐ θdry)(Kwct ‐ Kdry)S−2 was found previously to have bounds of 0.5 and π/4, with 0.55 as a good overall approximation. Those results were for two diffusivity functions, an exponential D = D0 exp (Bθ) and for D = D0(θb ‐ θ)−2. We now extend the investigation to four additional cases, two for which S and λc are analytical and two described by the empirical hydraulic functions of Brooks and Corey (1964) and by van Genuchten (1980). The representative value b = 0.55 was found to be generally correct. For those functions with a finite D value at the wettest value, a refinement from the 0.55 value is offered. The results are believed of value for future parameter estimation problems, including two previous applications related to time of ponding and disk infiltrometers.
Base flow response from nonpoint source contamination: Simulated spatial variability in source, structure, and initial condition
Authors: Duffy Christopher J. , Lee Do‐Hun
Published: February 2004

Numerical experiments are applied to the problem of modeling base flow water quality originating from nonpoint or spatially distributed sources of groundwater contamination in an idealized stream‐aquifer setting. The experiments examine the dependence of base flow concentration on stochastic spatial variability of the nonpoint source, the initial aquifer concentration field, and the hydraulic conductivity field. The deterministic effects of local dispersive mixing, aquifer aspect ratio, and equilibrium sorption are also investigated. For the simulated conditions, base flow concentration is found to be independent of second‐spatial moments of the above random parameter fields and independent of the magnitude of local dispersive mixing. The experiments demonstrate the primary importance of defining the mean flow field and the space average concentration when determining “effective” field parameters. Input‐output behavior is represented by a nondiffusive integral balance model with exponential or Weibull‐type kernel. Parameters of the integral balance model are shown to be adequately represented by space averages.
Simulation of large‐scale transport of variable density and viscosity fluids using a stochastic mean model
Authors: Welty Claire , Gelhar Lynn W.
Published: February 2004

Previous results of stochastic analysis of variable density and viscosity fluids in heterogeneous isotropic porous media show that longitudinal macrodispersivity is a function of mean concentration and concentration gradient. The dependence of macrodispersivity on mean concentration and concentration gradient gives rise to a nonlinear mean solute transport equation. To evaluate this behavior, a one‐dimensional finite difference formulation of the mean solute transport equation that incorporates the analytically derived expression for longitudinal macrodispersivity is used to simulate a step input problem. This approach allows effects of a spatially and temporally varying macrodispersivity on breakthrough curves to be observed and effects on dispersed‐zone width to be calculated. Simulation results show that, for solute concentrations of environmental significance, density effects give rise to macrodispersivities up to several orders of magnitude larger (for the unstable (fingering) case) or smaller (for the stable case) than would be predicted by analysis of an ideal tracer, and the smaller the velocity, the greater the effect. In miscible secondary petroleum recovery, where the displaced and displacing fluids differ in concentration by as much as 100%, viscosity effects have the predominant influence on macrodispersivity; in this case, simulations illustrate that viscous fingering effects give rise to macrodispersivities that are several orders of magnitude larger than predicted by classical analysis. Qualitatively these results agree with fine‐grid numerical simulations of viscous fingering and laboratory column experiments in that similar trends are observed for the effects of displacement orientation, velocity, and density and viscosity contrasts on dispersed‐zone width.
Linearized time‐dependent infiltration from a shallow pond
Authors: Pullan A. J.
Published: February 2004

Time‐dependent quasi‐linearized infiltration with constant diffusivity from an axisymmetric shallow flat pond into a homogeneous semi‐infinite porous medium is investigated. Use of the Laplace transform reduces the equation to a form that can be solved using the boundary element method and inverted without difficulty. Previous attacks on this problem have been limited to solutions for large time because of the lack of a closed form expression for the axisymmetric free space Green's function. Here an alternative technique for evaluating the axisymmetric free space Green's function is presented, which enables solutions to be obtained at any time. Time‐varying fluxes are presented for a range of cavity sizes. The numerical results for small time match those predicted from one‐dimensional infiltration models, while the large time results approach the appropriate steady state values. The detailed numerical results indicate that approximate analytic results for fluxes from buried sources can be extended to include the shallow flat pond. This, together with earlier approximations for steady state fluxes, allows the flux from a shallow flat pond to be simply estimated at any time.
Optimal maintenance‐related decision making for deteriorating water distribution systems: 2. Multilevel decomposition approach
Authors: Li Duan , Haimes Yacov Y.
Published: February 2004

A water distribution system often consists of a large number of components. However, funds are, in general, not available to completely update/upgrade the deteriorating water distribution system's components. Limited funds have to be optimally distributed among the system's components on the basis of the contribution of each component to the overall system's availability. The large dimension and high degree of nonlinearity of models of deteriorating large‐scale water distribution networks make the maintenance‐related problem difficult to handle as a whole. In this paper a multilevel approach is proposed for water distribution systems that maximizes the availability of the overall system under a given cost constraint. This decomposition method tackles a large‐scale nonlinear programming problem by solving a set of linear programming problems with a smaller dimension iteratively, thus greatly reducing a large‐scale system's complexity.
Legal mechanisms for protecting riparian resource values
Authors: Lamb Berton L. , Lord Eric
Published: February 2004

Riparian resources include the borders of rivers, lakes, ponds, and potholes. These border areas are very important for a number of reasons, including stream channel maintenance, flood control, aesthetics, erosion control, fish and wildlife habitat, recreation, and water quality maintenance. These diverse functions are not well protected by law or policy. We reviewed law and policies regarding endangered species habitat designation, land use planning, grazing management, water allocation, takings, and federal permits and licenses, along with the roles of federal, state, and local governments. We discuss the politics of implementing these policies, focusing on the difficulties in changing entrenched water and land use practices. Our review indicates a lack of direct attention to riparian ecosystem issues in almost all environmental and land use programs at every level of government. Protection of riparian resource values requires a means to integrate existing programs to focus on riparian zones.
Stable isotope composition of water vapor as an indicator of transpiration fluxes from rice crops
Authors: Brunel J. P. , Simpson H. J. , Herczeg A. L. , Whitehead R. , Walker G. R.
Published: February 2004

Measurements of the stable isotope composition (δ2H and δ18O) of water vapor and associated micrometeorological parameters were made before and after full establishment of a rice crop in southeastern Australia. The aim of the experiment was to gain a better understanding of stable isotope variations of water vapor near the ground surface in response to local evaporation, local transpiration, regional scale vapor transport and the vertical stability of the atmospheric boundary layer. Vapor samples were collected at several heights within 9 m of the water surface during two separate sampling periods. The δ2H values of the water vapor ranged over more than 60‰, reflecting major rapid changes in regional air mass sources, as well as variations in the stability of the lowest 10 m of the atmosphere. The influence of tropical and higher‐latitude air masses resulted in local vapor compositions which were relatively enriched and depleted, respectively, in heavy isotopes. Vertical gradients in heavy isotope abundances were very large during stable conditions (as much as Δδ2H = –27‰ from 0.8 to 8.4 m), as the result of mixing between transpired water and regional air vapor. Transpiration fluxes calculated from the water vapor δ2H gradient ranged from 5 to 7 mm d−1, which was in good agreement with one‐dimensional aerodynamic energy budget calculations of daytime vapor fluxes.
Atomic theories of phyllosilicates: Quantum chemistry, statistical mechanics, electrostatic theory, and crystal chemistry
Authors: Bleam William F.
Published: February 2004

This is a review of quantum chemical, statistical‐mechanical, electrostatic, and crystal chemical studies of phyllosilicates. Atomic‐level studies have examined the structure and bonding within phyllosilicate layers, the effects of cation substitution on bonding and charge distribution, the structure of water, and the distribution of cations in the interlayer, and the bonding between phyllosilicate layers. The major needs for future research are statistical‐mechanical simulations of ion exchange reactions, electron transport in transition‐metal‐containing phyllosilicates, structure and bonding at phyllosilicate edges, and mechanisms of crystal growth and dissolution.