bluuebunny/crossref_metadata_2025 huggingface dataset:
Percolation theory and its application to groundwater hydrology
Authors: Berkowitz Brian , Balberg Isaac
Published: February 2004

The theory of percolation, originally proposed over 30 years ago to describe flow phenomena in porous media, has undergone enormous development in recent years, primarily in the field of physics. The principal advantage of percolation theory is that it provides universal laws which determine the geometrical and physical properties of the system. This survey discusses developments and results in percolation theory to date, and identifies aspects relevant to problems in groundwater hydrology. The methods of percolation theory are discussed, previous applications of the theory to hydrological problems are reviewed, and future directions for study are suggested.
A comparison of selection at list time and time‐stratified sampling for estimating suspended sediment loads
Authors: Thomas Robert B. , Lewis Jack
Published: February 2004

Time‐stratified sampling of sediment for estimating suspended load is introduced and compared to selection at list time (SALT) sampling. Both methods provide unbiased estimates of load and variance. The magnitude of the variance of the two methods is compared using five storm populations of suspended sediment flux derived from turbidity data. Under like conditions, the SALT coefficient of variation was 1.4–7.7 times that of time‐stratified sampling. Time‐stratified sampling performs well if the range of sediment flux in each stratum is small. This requirement can be met by using small sample sizes in many short strata. Theoretically, SALT sampling has the potential for smaller sampling variance; however, it is difficult to select an auxiliary variable that predicts flux well under diverse flow conditions. An “optimum” auxiliary variable formed from the largest storm performed about as well as time‐stratified sampling for the larger storms. Time‐stratified sampling ensures that specimens are collected in all storms, facilitating load estimation for individual storms. In contrast, SALT can better allocate sampling resources over different size storms, enabling efficient estimation of the total load for longer periods. Because time‐stratified sampling is less sensitive to the way measurements are allocated to different parts of the population, it is preferred for estimating storm loads of multiple constituents from the same sample.
The influence of precipitation and temperature on seasonal streamflow in California
Authors: Cayan Daniel R. , Riddle Laurence G. , Aguado Edward
Published: February 2004

We examine the influence of climate parameters on seasonal streamflow in watersheds over a range of elevations in California and Oregon. Effects of precipitation, temperature, and snow water content (SWC) are diagnosed using linear regression models and categoric composites. Most of the models explain over 60–80% of the seasonal streamflow variability. The models and the composites provide insight into the climate influences which drive the individual watersheds. Low (warmer) basins have little snow and little memory of prior seasons' climate variability. High (cooler) basins, with more snow, have longer memories. Precipitation has the greatest influence on streamflow variations in spring. Temperature is important in spring in the mid and high elevations. By late spring, SWC accounts for nearly all of the summer streamflow variation at mid and high elevations, but earlier in the runoff season, precipitation and temperature add variance. The variations in surface climate parameters, including streamflow, are generally controlled by atmospheric circulation anomalies with a spatial scale much larger than those in watersheds. This explains why little skill was lost by broadening the scale of temperature and precipitation predictors to regional climate areas.
Microbial production of CO2 in unsaturated geologic media in a mesoscale model
Authors: Hendry M. J. , Lawrence J. R. , Zanyk B. N. , Kirkland R.
Published: February 2004

The production of microbially derived CO2 was investigated in the 3.6 m thick unsaturated geologic media of a mesoscale model system. The mesoscale model, which was 2.4 m diameter × 4.6 m high, was filled with a medium grained lacustrine sand, capped with an Ap (0.11 m) and B (0.20 m) soil profile, and maintained under steady state moisture conditions for 343 days. A near steady state soil gas CO2 concentration profile (>100 times greater than atmospheric) developed after 130 days implying that geochemical stability was rapidly attained. The presence of metabolically active microorganisms, consumption of O2 and dissolved organic carbon, and carbon 13 data indicated that the CO2 was microbially produced. At steady state, the calculated and modeled CO2 flux to the atmosphere (about 0.9 mol m−2 day−1) were greater than the measured CO2 flux (0.1 mol m−2 day−1) and greater than the calculated CO2 flux to the water table (0.02 mol m−2 day−1). The difference between the calculated and measured fluxes of CO2 to the atmosphere was attributed to spatial variability in the gas diffusion coefficient and reflected the difficulty in estimating CO2 production, even under steady state conditions. Laboratory estimates of microbial CO2 fluxes (1.8 and 2.9×103 mol m−2 day−1 ) were much greater than those determined from measured and calculated fluxes. Measured microbial respiration rates suggested that between 50 and 70% of the respired CO2 was produced in the unsaturated geologic medium beneath the Ap and B, and that there was sufficient CO2 production potential by bacteria to account for the respired CO2 determined from the flux calculations. These observations indicated that long‐term experimentation in mesoscale models can overcome some of the difficulties encountered in field studies and provide insight into biogeochemical processes in natural systems.
Muskingum flood routing model for multiple tributaries
Authors: Khan M. Hassanuzzaman
Published: February 2004

An extended version of the Muskingum flood routing model is developed for application to river systems where multiple tributaries, gaged at upstream stations, unite to a single channel downstream. The Muskingum storage equation is redefined to include the tributary flows as independent inflows to the channel system by increasing the number of x parameters. The routing equation is accordingly modified to contain tributary flows as individual inflows through an increased number of coefficients. An efficient method of analysis is presented which yields direct estimates of the routing coefficients. The model automatically takes into account any effect of ungaged tributary flows and/or lateral inflow in the prediction of outflow. Application of the model, using the method of analysis presented, has been demonstrated on the Meghna river system in Bangladesh, in both real time forecasting and reconstruction modes. The results thus obtained are encouraging.
Analytical solutions for radial pressure distribution including the effects of the quadratic‐gradient term
Authors: Chakrabarty Chayan , Ali S. M. Farouq , Tortike W. S.
Published: February 2004

This study provides a quantitative analysis of the effects of neglecting the quadratic‐gradient term in solving the diffusion equation governing the transient pressure distribution during high pressure‐gradient injection of compressible liquids in porous media. Mathematical solutions of the two‐dimensional cylindrical‐symmetry nonlinear diffusion equation are derived by using the Laplace transform. A fully penetrating well bore in a homogeneous and isotropic porous medium is considered. The analysis accounts for well bore storage and incorporates a wide range of boundary conditions. Analytical early‐ and late‐time solutions are also presented for some cases. Quantitative deviations from existing linear solutions are related to a dimensionless group, α, which is proportional to the fluid compressibility; the higher the magnitude of α, greater is the deviation of the nonlinear solutions from the linear ones. The linear pressure and rate solutions are generally within 0.5% of the corresponding nonlinear solutions for the constant pressure inner boundary. However, for the constant discharge‐ rate condition, the error may be as high as 10% (within the ranges of a and dimensionless radius and time considered). The error may be even higher for higher injection rates in flow systems with smaller transmissivity.
MS‐2 and poliovirus transport in porous media: Hydrophobic effects and chemical perturbations
Authors: Bales Roger C. , Li Shimin , Maguire Kimberly M. , Yahya Moyasar T. , Gerba Charles P.
Published: February 2004

In a series of pH 7 continuous‐flow column experiments, removal of the bacteriophage MS‐2 by attachment to silica beads had a strong, systematic dependence on the amount of hydrophobic surface present on the beads. With no hydrophobic surface, removal of phage at pH 5 was much greater than at pH 7. Release of attached phage at both pH values did occur, but was slow; breakthrough curves exhibited tailing. Poliovirus attached to silica beads at pH 5.5 much more than at pH 7.0, and attachment was also slowly reversible. Time scales for phage and poliovinis attachment were of the order of hours. The sticking efficiency factor (α), reflecting microscaie physicochemical influences on virus attachment, was in the range of 0.0007–0.02. Phage release was small but measurable under steady state conditions. Release was enhanced by lowering ionic strength and by introducing beef extract, a high‐ionic‐strength protein solution. Results show that viruses experience reversible attachment/detachment (sometimes termed sorption), that large chemical perturbations are needed to induce rapid virus detachment, and that viruses should be quite mobile in sandy porous media. Even small amounts of hydrophobic organic material in the porous media (≥0.001%) can retard virus transport.
Estimation of two‐phase relative permeability relationships for organic liquid contaminants
Authors: Demond Avery H. , Roberts Paul V.
Published: February 2004

Although the constitutive relationship of relative permeability as a function of saturation has been examined extensively in soil science and petroleum engineering, there is little information about this relationship for many systems encountered at hazardous waste sites. This lack of data has led to the use of estimation methods for relative permeability without verifying that they are indeed applicable to systems representative of hazardous waste sites. To assess the accuracy of methods commonly used for the prediction of two‐phase relative permeabilities, drainage and imbibition relative permeabilities were measured for several organic liquid‐water systems. A comparison of these measurements with estimates generated with five common methods showed that these methods are limited in their predictive capabilities. They estimated the relative permeability to the aqueous phase well, but failed to predict the low permeability to the organic liquid. For the relative permeability to the organic liquid to reach the predicted value of unity at the residual aqueous phase saturation, the effective permeabilities must be normalized by the permeability measured in the presence of residual water. The common estimation techniques make no provision for hysteresis. Since the aqueous phase showed little hysteresis, these estimations methods can be applied to both drainage and imbibition processes for this phase. However, the organic liquid phase showed hysteresis, and thus these techniques may only be appropriately applied to drainage for the non wetting phase.
Economic failure plagues developing countries' public irrigation: An assurance problem
Authors: Easter K. William
Published: February 2004

The poor performance of many government or public irrigation systems is well documented. This study uses a model including internal and external assurance, commitment, and fairness to explain the performance of irrigation in the Philippines, Sri Lanka, Nepal, and Maharashtra State in India. The suggested approach is superior to ones based on the concept of “free rider” or government failure. The analysis shows that the Philippines and Maharashtra have developed ways to improve assurance concerning the actions of government and other irrigators which has improved irrigation performance. In contrast, Sri Lanka and Nepal have provided little assurance in their government irrigation systems and performance is poor.
All coliforms are not created equal: A comparison of the effects of water source and in‐house water contamination on infantile diarrheal disease
Authors: VanDerslice James , Briscoe John
Published: February 2004

Storing drinking water in the home is common in the developing world. Several studies have documented increased concentrations of fecal coliforms during household storage. This has led to the belief that in‐house water contamination is an important transmission route for enteric pathogens and, moreover, that improving water source quality is not warranted until that quality can be maintained in the home. We contend that in‐house water contamination does not pose a serious risk of diarrhea because family members would likely develop some level of immunity to pathogens commonly encountered in the household environment. Even when there is no such immunity, transmission of these pathogens via stored water may be inefficient relative to other household transmission routes, such as person‐to‐person contact or food contamination. A contaminated water source poses much more of a risk since it may introduce new pathogens into the household, The effects of water source and in‐house contamination on diarrheal disease are estimated for 2355 Filipino infants. The results confirm our hypothesis: contaminated water sources pose a serious risk of diarrhea while contamination of drinking water in the home does not. Water boiling is shown to eliminate the risk of diarrhea due to water source contamination. The results imply that improvements in water source quality are more important than improving water storage practices.
Water source as a housing characteristic: Hedonic property valuation and willingness to pay for water
Authors: North J. H. , Griffin C. C.
Published: February 2004

Using data from a large representative sample of rural households in one region of the Philippines, we estimate the determinants of the rental value of dwellings using the bid‐rent approach to the hedonic price model. Our particular interest is in the relative valuation these households place on owning a private source of water and distance to a public or communal source. We find that low‐, middle‐, and high‐income households value an in‐house piped water source highly relative to other characteristics of their homes. Middle‐ and high‐income households value a deep well or piped water in the yard, although at a substantially lower level than piped water in the house. It is somewhat surprising to find that, except in the case of high‐income families, households appear to gain little or no utility from having a communal source of water, such as a river, lake, or public tap, closer to their homes. As a consequence, public water policies that emphasize improving the quality and proximity of communal sources would be inappropriate for the region represented by this sample.
Rural water supply in Kerala, India: How to emerge from a low‐level equilibrium trap
Authors: Singh Bhanwar , Ramasubban Radhika , Bhatia Ramesh , Briscoe John , Griffin Charles C. , Kim Chongchun
Published: February 2004

Large quantities of financial and human resources have been devoted to improving rural water supplies in developing countries over the past two decades. Many projects have been successful, but many have failed to meet the needs of the intended beneficiaries. Evidence of the failures lies in the unused and poorly maintained systems that are scattered throughout rural areas of the developing world. The current situation in water supply in rural Kerala, India, reflects this general observation and can be described as a “low‐level equilibrium trap.” Water systems provide a low level of service with few yard taps. The monthly tariff for water from household connections is low. With few connectors and low tariffs, little revenue is generated beyond subsidies provided by the government. The water authority can afford to maintain the system up to a level at which the reliability of service is low, forcing consumers to supplement piped water from traditional sources. This study analyzes contingent valuation data collected in three areas of Kerala to evaluate the possibility of lifting the system out of this trap. The analysis shows that by making a few critical policy changes, encouraging private connections and financing those connections through higher tariffs, the system can ratchet up to a “high‐level equilibrium” in which there are many connectors, monthly revenues are greatly increased, and consumer welfare improves. Such a system would be better financed, making it possible to improve the reliability and quality of the service.
Confirmation of rate‐dependent behavior in water retention during drainage in nonswelling porous materials
Authors: Constantz Jim
Published: February 2004

In the water retention process in porous material, changes in water content are assumed to be independent of the rate at which the matric potential varies. Periodically, researchers have presented results that do not support this assumption, indicating that water retention may be rate‐dependent under certain conditions. In the present study, long‐term experiments were performed on five porous materials in which core samples were drained at different rates down to matric potentials of −50 kPa. Comparisons of these results with previous results confirm that slight rate‐dependent behavior occurs during drainage of porous materials in this matric potential range. Specifically, new and previous results indicate that on average 4–5% more water was retained for a slow, multistep drainage versus a fast, one‐step drainage treatment. For the present study, average results for individual materials were variable, ranging from a 1 to a 10% difference in water content due to treatment. Several possible mechanisms for the observed rate‐dependent behavior are discussed. The only plausible mechanism is related to variations in pore water salt concentrations that induce differences in the pore water surface tension for fast versus slow drainage. Regardless of the mechanism, results suggest that rate‐dependent behavior during water retention may contribute to an enhanced temperature dependence of water retention, due to higher rates of water redistribution at higher temperatures.
Modeling moisture movement in revegetating waste heaps: 1. Development of a finite element model for liquid and vapor transport
Authors: Connell L. D. , Bell P. R. F.
Published: February 2004

A management concern with waste dumps is the movement of Ieachate into local groundwater systems. This paper presents a model to predict the saturated‐unsaturated flow of water within such waste dumps. The model predicts moisture movement when the surface of the dump is bare and when it has revegetated. For the bare state, the evaporation rate is related to a number of complex processes, including atmospheric transport, radiation balance, and subsurface water supply in either liquid or vapor forms. Transpiration from the revegetated heap is related to the leaf area index and rooting depth, both of which change as plants grow. Mechanisms influencing vapor transport are considered in some detail, and a nonequilibrium approach employed to explain combined liquid and vapor movement. The mathematical problem consists of solving a system of coupled, nonlinear, partial differential equations which are related dynamically to the surface boundary conditions. The boundary conditions are determined by a combination of meteorological variables as well as the type and amount of plant cover. A moving node finite element method is presented which has the capability of automatically relocating nodes to areas of numerical difficulty. This method is validated through comparison with a proven solution to the unsaturated flow equation. The next paper in this series presents the application of the model to oil shale waste dumps.
Transport of volatile organic compounds across the capillary fringe
Authors: McCarthy Kathleen A. , Johnson Richard L.
Published: February 2004

Physical experiments were conducted to investigate the transport of a dissolved volatile organic compound (trichloroethylene, TCE) from shallow groundwater to the unsaturated zone under a variety of conditions including changes in the soil moisture profile and water table position. Experimental data indicated that at moderate groundwater velocities (0.1 m/d), vertical mechanical dispersion was negligible and molecular diffusion was the dominant vertical transport mechanism. Under these conditions, TCE concentrations decreased nearly 3 orders of magnitude across the capillary fringe and soil gas concentrations remained low relative to those of underlying groundwater. Data collected during a water table drop showed a short‐term increase in concentrations throughout most of the unsaturated zone, but these concentrations quickly declined and approached initial values after the water table was returned to its original level. In the deep part of the unsaturated zone, the water table drop resulted in a long‐term decrease in concentrations, illustrating the effects of hysteresis in the soil moisture profile. A two‐dimensional random walk advection‐diffusion model was developed to simulate the experimental conditions, and numerical simulations agreed well with experimental data. A simpler, one‐dimensional finite‐difference diffusion‐dispersion model was also developed. One‐dimensional simulations based on molecular diffusion also agreed well with experimental data. Simulations which incorporated mechanical dispersion tended to overestimate flux across the capillary fringe. Good agreement between the one‐ and two‐dimensional models suggested that a simple, one‐dimensional approximation of vertical transport across the capillary fringe can be useful when conditions are appropriate.
Stochastic dynamic programming models for water quality management
Authors: Cardwell Hal , Ellis Hugh
Published: February 2004

This paper presents optimization models for waste load allocation from multiple point sources which include both parameter (Type II) and model (Type I) uncertainty. These optimization models employ more sophisticated water quality simulation models, for example, in the case of dissolved oxygen modeling, QUAL2E and WASP4, than is typically the norm in studies on the optimization of waste load allocation. Variability in selected input parameters to the water quality simulation models gives rise to stochastic dynamic programming approaches. Two types of reliability and feasibility attributes are highlighted, associated with the management options that are generated. Several dissolved oxygen simulation models are incorporated into the optimization procedures to explore the effects of Type I uncertainty on control decisions. Information from simultaneous consideration of multiple simulation models is aggregated in the dynamic programming framework through two regret‐based formulations. By accommodating both model and parameter uncertainty in the modeling framework, trade‐offs can be generated between the two so as to assess their influence on control decisions. The models are applied to a waste load allocation problem for the Schuylkill River in Pennsylvania.
Water right transactions: Market values and price dispersion
Authors: Colby Bonnie G. , Crandall Kristine , Bush David B.
Published: February 2004

The growing interest in water market transactions among policymakers and water users has created a need for understanding market price behavior and the heterogeneous nature of water rights. This article analyzes characteristics of markets, water rights, and water transactions to identify attributes which affect market values and price dispersion within regional markets. Based on econometric analysis of market data, water commodity and transaction characteristics are shown to be significantly related to market prices. Price dispersion that is unexplained by commodity attributes is related to characteristics of the regional market, including the number and size distribution of potential traders, costs of obtaining market information, and heterogeneity of water commodities traded.
Compositional simulation of groundwater contamination by organic compounds: 1. Model development and verification
Authors: Sleep Brent E. , Sykes Jonathan F.
Published: February 2004

A compositional simulator is developed for application to the analysis of contamination and remediation of groundwater systems. Simultaneous flow of three fluid phases (water, gas, and organic) is modeled. Interphase partitioning and transport of an arbitrary number of organic and inorganic components can be simulated. Phase densities are functions of pressure and phase composition. The model includes several numerical options, ranging from fully implicit with first‐order upstream weighting to implicit in pressure, explicit in saturations and concentration with third‐order upstream weighting. The model is verified to the extent possible with analytical solutions for simplified cases of multiphase flow and contaminant transport. The accuracy and efficiency of the various numerical options used in the model are illustrated.
Compositional simulation of groundwater contamination by organic compounds: 2. Model applications
Authors: Sleep Brent E. , Sykes Jonathan F.
Published: February 2004

The flow and transport of organic compounds in variably saturated porous media is investigated using a compositional simulator. The simulator incorporates a number of numerical options to maximize computational efficiency and accuracy. The effect of field scale heterogeneities on the movement of organic compounds is demonstrated. The influence of infiltrating wetting fronts on gas phase transport of volatile organic compounds is shown to be significant. The long‐term fate of a subsurface spill of a three‐component dense organic liquid is simulated. Three‐dimensional simulations of soil vacuum extraction demonstrate the difficulty in removing dissolved organic compounds from the saturated zone with this process.
Ownership and sources of inefficiency in the provision of water services
Authors: Lambert David K. , Dichev Dimo , Raffiee Kambiz
Published: February 2004

The relative efficiency of service provision by publicly versus privately owned enterprises has been the subject of considerable theoretical debate and empirical investigation for at least the last hundred years. This research investigates the relative efficiencies of publicly versus privately owned water utilities. The publicly owned enterprises are found to be more efficient overall, as well as in the technical efficiency associated with the employment of labor, capital, energy, and material inputs. No significant differences are found in scale efficiencies between the two classes of enterprise. The greatest level of inefficiency results from the overuse of capital relative to the most efficient firms in the sample, while discrepancies in labor use was the least among the four inputs considered.
Local choice and wastewater treatment plant performance
Authors: Schwarz Gregory E. , Mcconnell Virginia D.
Published: February 2004

Wastewater treatment plant effluent can be a major contributor to water pollution in the United States, yet we know little about how actual effluent quality is determined. Although Federal government regulations specify uniform minimum treatment levels for these plants, actual treatment levels vary considerably from this standard. In addition, there is evidence of substantial excess capacity at treatment plants. This paper examines the economic and technical relations between actual operating and design characteristics of plants. We specify and test among alternative models of what determines actual plant performance relative to design performance, including one in which plants have the incentive to “overinvest” and operate with permanent excess capacity due to large Federal subsidies on capital costs. The paper then explores the determinants of actual effluent quality using a supply and demand model in which effluent quality is fully endogenous, allowing treatment levels to be a function of both the costs of control, water quality characteristics, and other regional or regulatory preferences for control. Finally, the model with fully endogenous effluent quality is compared to one in which actual levels of plant effluent quality are technologically determined by the design characteristics of the plant and the exogenous properties of the incoming waste stream.
Global optimum tree solution for single‐source looped water distribution networks subjected to a single loading pattern
Authors: Sonak Vilas V. , Bhave Pramod R.
Published: February 2004

A methodology, based on linear programming (LP) is developed for obtaining the global optimum tree solution for single‐source looped water distribution networks (WDNs) subjected to a single loading pattern. Initially, a multiple‐link loop is considered, and it is shown that the cost‐discharge curve is concavoconvex: convex for local minima corresponding to trees and concave for discharges in between. The logic is then extended to P loop networks for which the cost‐discharge surface is a (P+1)‐dimensional hypersurface. A tree of the looped WDN is initially selected, optimized by LP, and successively modified, considering one loop at a time, so that the solution jumps on the hypersurface, from one local minimum to a better one. The procedure is continued until no further improvement in local optimum solution occurs; thus the final solution is the global optimum tree solution. The optimization procedure is illustrated through a one‐source, 31‐demand node, three‐loop WDN, for which only 23 trees, out of possible 1048 trees, were required to be optimized for obtaining the global optimum tree solution. Since each link had to carry a minimum specified flow, one more iteration was carried out in the end to satisfy the minimum‐flow constraint. Even though the tree solution is global optimum, the final solution satisfying the minimum‐flow constraint cannot be claimed to be necessarily the global optimum.
Groundwater as a nonpoint source of atrazine and deethylatrazine in a river during base flow conditions
Authors: Squillace Paul J. , Thurman E. M. , Furlong Edward T.
Published: February 2004

Alluvial groundwater adjacent to the main stem river is the principal nonpoint source of atrazine and deethylatrazine in the Cedar River of Iowa after the river has been in base flow conditions for 5 days. Between two sites along a 116‐km reach of the Cedar River, tributaries contributed about 25% of the increase in the atrazine and deethylatrazine load, whereas groundwater from the alluvial aquifer contributed at least 75% of the increase in load. Within the study area, tributaries aggregate almost all of the discharge from tile drains, and yet the tributaries still only contribute 25% of the increase in loads in the main stem river. At an unfamned study site adjacent to the Cedar River, the sources of atrazine and deethylatrazine in the alluvial groundwater are bank storage of river water and groundwater recharge from areas distant from the river. Atrazine and deethylatrazine associated with bank storage water will provide larger concentrations to the river during early base flow conditions. After the depletion of bank storage, stable and smaller concentrations of atrazine and deethylatrazine, originating from groundwater recharge, continue to be discharged from the alluvial aquifer to the river; thus these results indicate that alluvial aquifers are an important nonpoint source of atrazine and deethylatrazine in rivers during base flow.
Comparison of Penman‐Monteith, Shuttleworth‐Wallace, and Modified Priestley‐Taylor Evapotranspiration Models for wildland vegetation in semiarid rangeland
Authors: Stannard David I.
Published: February 2004

Eddy correlation measurements of sensible and latent heat flux are used with measurements of net radiation, soil heat flux, and other micrometeorological variables to develop the Penman‐Monteith, Shuttleworth‐Wallace, and modified Priestley‐Taylor evapotranspiration models for use in a sparsely vegetated, semiarid rangeland. The Penman‐Monteith model, a one‐component model designed for use with dense crops, is not sufficiently accurate (r2 = 0.56 for hourly data and r2 = 0.60 for daily data). The Shuttleworth‐Wallace model, a two‐component logical extension of the Penman‐Monteith model for use with sparse crops, performs significantly better (r2 = 0.78 for hourly data and r2 = 0.85 for daily data). The modified Priestley‐Taylor model, a one‐component simplified form of the Penman potential evapotranspiration model, surprisingly performs as well as the Shuttle worth‐Wallace model. The rigorous Shuttleworth‐Wallace model predicts that about one quarter of the vapor flux to the atmosphere is from bare‐soil evaporation. Further, during daylight hours, the small leaves are sinks for sensible heat produced at the hot soil surface.
The effect of spatial correlations on the accessibility characteristics of three‐dimensional cubic networks as related to drainage displacements in porous media
Authors: Ioannidis M. A. , Chatzis I. , Sudicky E. A.
Published: February 2004

Percolation theory is used in this paper to study the general accessibility characteristics of bond site correlated networks in the presence or absence of spatial correlations among the sites. Spectral methods are employed to generate networks in which the sites are spatially correlated according to exponential or Gaussian autocovariance functions. Monte Carlo simulations are performed for bond‐correlated site percolation with and without trapping, as related to drainage‐type displacements of water by oil (or air) and air by mercury, respectively. For the correlation modes studied, it is found that spatial correlations result in significant reduction of the site percolation threshold with a concomitant modification of the site accessibility. However, the bond accessibility characteristics and the bond percolation threshold are not significantly affected. The simulated drainage capillary pressure curves become more gradual and the residual wetting phase saturation associated with oil (or air)‐water displacements is significantly decreased when spatial correlations among the pore bodies exist.
Cross‐correlated random field generation with the direct Fourier Transform Method
Authors: Robin M. J. L. , Gutjahr A. L. , Sudicky E. A. , Wilson J. L.
Published: February 2004

This paper presents a computer algorithm that is capable of cogenerating pairs of three‐dimensional, cross‐correlated random fields. The algorithm produces random fields of real variables by the inverse Fourier transform of a randomized, discrete three‐dimensional spectral representations of the variables. The randomization is done in the spectral domain in a way that preserves the direct power and cross‐spectral density structure. Two types of cross spectra were examined. One type specifies a linear relationship between the two fields, which produces the same correlation scales for both variables but different variances. The second cross spectrum is obtained from a specified transfer function and the two power spectra, and it produces fields with different correlation scales. For both models the degree of correlation is specified by the coherency. A delay vector can also be specified to produce an out‐of‐phase correlation between the two fields. The algorithm is very efficient computationally, is relatively easy to use, and does not produce the lineation problems that can be encountered with the turning bands method. Perhaps most important, this random field generator is capable of co‐generating cross‐correlated random fields.
Transport and accumulation of radionuclides and stable elements in a Missouri River Reservoir
Authors: Callender Edward , Robbins John A.
Published: February 2004

Several long sediment cores from the Cheyenne River Embayment of Lake Oahe, a 250‐km‐long Missouri River reservoir in South Dakota, have been analyzed for radionuclides and stable elements. The combination of fine‐scale sampling and rapid sedimentation produces radionuclide distributions that can be used to estimate the detailed chronology of particle transport processes in the Oahe reservoir system. A self‐consistent and quantitative treatment of the 137Cs data suggests processes to which characteristic times may be associated. Times that characterize system‐wide processes include (1) an integration time of several years reflecting retention of the sediment‐bound tracer in regions within or external to the reservoir, (2) a relaxation time of approximately 15 years reflecting a decreasing rate of sediment accumulation ascribed to shoreline stabilization, (3) a time of a few months characterizing the breadth of riverine signatures in cores due to integration effects in the Cheyenne River system and deltaic deposits, and (4) times of a few years associated with propagation of riverine load signatures along the embayment. The distribution of total sedimentary arsenic confirms the validity of the variable sedimentation model. In 1977, a tailings retention facility was built at the Homestake Mine site, and the unrestricted input of As ceased. As a result of this remedial action, the concentration of sedimentary As decreased dramatically. In the upper section of the core, above the depth represented by the year 1976, the concentration of As decreases tenfold. In this same core the distribution of lithologically discriminating chemical elements, calcium and vanadium, relate to major flow events in the Cheyenne River basin. Because there is minimal diagenesis of chemical constituents in these rapidly accumulating sediments, stable element signatures, in addition to radiotracers, may be used to reconstruct hydrologic events in drainage basins that contribute sediment to lakes and reservoirs.
A scaling model of a storm hyetograph
Authors: Koutsoyiannis Demetris , Foufoula‐Georgiou Efi
Published: February 2004

Empirical evidence suggests that statistical properties of storm rainfall at a location and within a homogeneous season have a well‐structured dependence on storm duration. To explain this dependence, a simple scaling model for rainfall intensity within a storm was hypothesized. It was shown both analytically and empirically that such a model can explain reasonably well the observed statistical structure in the interior of storms, thus providing an efficient parametrization of storms of varying durations and total depths. This simple scaling model is also consistent with, and provides a theoretical basis for, the concept of mass curves (normalized cumulative storm depth versus normalized cumulative time since the beginning of a storm) which are extensively used in hydrologic design. In contrast, popular stationary models of rainfall intensity are shown unable to capture the duration dependent statistical structure of storm depths and are also inconsistent with the concept of mass curves.
Evaluation of a distributed catchment scale water balance model
Authors: Troch Peter A. , Mancini Marco , Paniconi Claudio , Wood Eric F.
Published: February 2004

The validity of some of the simplifying assumptions in a conceptual water balance model is investigated by comparing simulation results from the conceptual model with simulation results from a three‐dimensional physically based numerical model and with field observations. We examine, in particular, assumptions and simplifications related to water table dynamics, vertical soil moisture and pressure head distributions, and subsurface flow contributions to stream discharge. The conceptual model relies on a topographic index to predict saturation excess runoff and on Philip's infiltration equation to predict infiltration excess runoff. The numerical model solves the three‐dimensional Richards equation describing flow in variably saturated porous media, and handles seepage face boundaries, infiltration excess and saturation excess runoff production, and soil driven and atmosphere driven surface fluxes. The study catchments (a 7.2‐km2 catchment and a 0.64‐km2 subcatchment) are located in the North Appalachian ridge and valley region of eastern Pennsylvania. Hydrologic data collected during the MACHYDRO 90 field experiment are used to calibrate the models and to evaluate simulation results. It is found that water table dynamics as predicted by the conceptual model are close to the observations in a shallow water well and therefore, that a linear relationship between a topographic index and the local water table depth is found to be a reasonable assumption for catchment scale modeling. However, the hydraulic equilibrium assumption is not valid for the upper 100 cm layer of the unsaturated zone and a conceptual model that incorporates a root zone is suggested. Furthermore, theoretical subsurface flow characteristics from the conceptual model are found to be different from field observations, numerical simulation results, and theoretical baseflow recession characteristics based on Boussinesq's groundwater equation.
Colloid‐facilitated groundwater contaminant transport
Authors: Corapcioglu M. Yavuz , Jiang Shiyan
Published: February 2004

Colloidal particles or dissolved organic matter (DOM) can act as carriers to enhance the transport of contaminants in groundwater by reducing retardation effects. When either of these materials is present, the system can be treated as consisting of three phases: an aqueous phase, a carrier phase, and the stationary solid matrix phase. The contaminant may be present in either or all of these phases. In the work reported, a mathematical model was developed to describe the transport and fate of the contaminant and carrier material in a porous medium. The model is based on mass balance equations describing the transport and fate of the contaminant and carrier in a three‐phase medium. Colloid/contaminant and colloid/matrix mass transfer mechanisms are represented by first‐order kinetics. Equilibrium partitioning of DOM acting as a carrier of the contaminant introduces a significant simplification in the model formulation. For a constant DOM concentration a much smaller retardation coefficient can be obtained in the three‐phase system than the coefficient obtained in a conventional advective/dispersive transport equation for a two‐phase system. The modified retardation coefficient reflects the presence of the mobile carrier by incorporating both the sorption of the contaminant and capture of the carrier on the solid matrix. Numerical solutions for the model were obtained by using a finite difference scheme to provide estimates of contaminant and carrier concentrations. Significant sensitivities to model parameters, particularly the rate constants of carrier capture and sorption were discovered. The numerical results of the DOM carrier effect matched favorably with experimental data reported in the literature.
Low fluid pressure within the Pierre Shale: A transient response to erosion
Authors: Neuzil C. E.
Published: February 2004

Naturally transient flow (NTF) occurs when a groundwater regime fails to accommodate geologic changes. This paper describes a conspicuous NTF signature entirely within a low‐permeability shale in an eroded basin. Flow in the shale appears to be a lagging hydrodynamic response to mechanical rebound and cooling from erosion; careful, long‐term pressure measurements revealed a consistent pattern of low hydraulic head with the minimum within the shale. Available data are sufficient to rule out alternate causes of the pressure regime, and to construct a meaningful quantitative model of the effects of erosion. The shale behaves as a Kelvin substance in tests, but theoretical considerations suggest that its deformation during erosion mimics elastic behavior, permitting the model to be based on poroelasticity. Pressure patterns similar to that observed can be produced by incorporating into the model independent estimates of the shale's hydraulic, mechanical, and thermal properties and a reconstruction of the area's erosion history. The results confirm that local permeability is between 10−21 and 10−20 m2 (hydraulic conductivity between 10−14 and 10−13 m/s), in contrast to the higher regional permeability of 2×10−16 m2 (hydraulic conductivity of 2× 0−9 m/s) determined in an earlier study. Exploration techniques employed in this study could reveal similar NTF regimes in the future.
A Markov Chain Flow Model for flood forecasting
Authors: Yapo Patrice , Sorooshian Soroosh , Gupta Vijai
Published: February 2004

This paper presents a new approach to streamflow forecasting, based on a Markov chain model for estimating the probabilities that the one‐step ahead streamflow forecast will be within specified flow ranges. With the new approach, flood forecasting is possible by focusing on a preselected range of streamflows. In addition, the approach introduces a multiobjective (two‐criterion) function for the assessment of model performance. The two criteria are (1) the probability of issuing a false alarm and (2) the probability of failing to predict a flood event. The goal is to minimize both criteria simultaneously. Three versions of the model are presented: a first‐order Markov chain model, a second‐order Markov chain model, and a first‐order Markov chain with rainfall as exogenous input model. These models compared favorably to time series models, using data from two watersheds (a semiarid watershed and a temperate watershed), when evaluated in terms of the multiobjective performance criterion.
The Value of clean water: The public's willingness to pay for boatable, fishable, and swimmable quality water
Authors: Carson Richard T. , Mitchell Robert Cameron
Published: February 2004

This paper presents the findings of a study designed to determine the national benefits of freshwater pollution control. By using data from a national contingent valuation survey, we estimate the aggregate benefits of meeting the goals of the Clean Water Act. A valuation function is estimated which depicts willingness to pay as a function of water quality, income, and other variables. Several validation checks and tests for specific biases are performed, and the benefit estimates are corrected for missing and invalid responses. The two major policy implications from our work are that the benefits and costs of water pollution control efforts are roughly equal and that many of the new policy actions necessary to ensure that all water bodies reach at least a swimmable quality level will not have positive net benefits.
Hillslope drainage with sudden drawdown: Closed form solution and laboratory experiments
Authors: Sanford William E. , Parlange J.‐Yves , Steenhuis Tammo S.
Published: February 2004

Theory is developed for hillslope drainage and is applied to the condition of sudden drawdown at the aquifer outlet. The equations for prediction of cumulative outflow volume, flow rate, and water table profile are based on the Boussinesq approximation for subsurface water flow above a sloping impermeable layer. The advantages of the theory presented here are that the assumption of a long, thin aquifer is unnecessary; the same equations apply to both sloping and horizontal aquifers; and the equations contain only parameters that are easily measured or estimated: slope, drainable porosity, saturated hydraulic conductivity, and layer thickness. The accuracy of the outflow equations is illustrated by a series of laboratory experiments.