bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical estimation of polynomial generalized covariance functions and hydrologic applications
Authors: Kitanidis Peter K.
Published: February 2008

Minimum‐variance unbiased linear estimation theory has found many applications in the analysis of spatial rainfall and Hydrogeologic data: estimation of point values or areal averages, calculation of variances of estimation error, and network design. However, the important problem of inferring the spatial structure or interdependence of the function of interest has received relatively less attention. Three methods for the estimation of the parameters of Matheron's polynomial generalized covariance function (a useful model for nonstationary functions) are presented and evaluated: Maximum likelihood (ML), minimum variance unbiased quadratic (MVUQ), and minimum norm (least squares). The estimation error covariance matrix of the parameters is calculated with each of these methods. ML and MVUQ estimation have very good properties and, for reasonably large samples, give very similar results. Minimum norm estimation is appropriate for preliminary data analysis. The application of these methodologies is illustrated with a set of rainfall data, and problems of implementation are discussed. It is shown that the estimation of the parameters of the covariance function involves sizeable errors. It is suggested that, in practice, it may be rather difficult to identify the parameters of all but simple models. It is important that future research evaluate the effects of parameter uncertainty in the circulation of estimation error variances of point or areal averages and in the design of hydrologic data collection networks.
Meltwater movement in a deep snowpack: 2. Simulation model
Authors: Jordan Peter
Published: February 2008

A numerical simulation model is developed describing the one‐dimensional flow of meltwater in a snowpack, based on established methods of modeling unsaturated groundwater flow. The results of field observations of snowmelt and meltwater flow are used to determine parameters describing the hydraulic properties of the snowpack, and surface energy balance measurements are used as input to the model. Tests of the performance of the model show a high sensitivity to the saturated hydraulic conductivity of the snowpack and to liquid water content of the snow. Saturated hydraulic conductivity can be measured with acceptable precision by simple field methods; however, measurement of liquid water content is more difficult and constitutes the major limitation affecting calibration of the model. The model is relatively insensitive to exponents in the power law equations relating conductivity, capillary pressure, and liquid water content. Meltwater wave speeds predicted by the model are somewhat slower than those observed in the field. Observations of snowpack layering and horizontal inhomogeneity of flow suggest that these features affect transient storage of meltwater and consequently cause meltwater wave speeds to differ from those predicted by a one‐dimensional model.
Improved parameter inference in catchment models: 2. Combining different kinds of hydrologic data and testing their compatibility
Authors: Kuczera George
Published: February 2008

Often some of the parameters of catchment models fitted to runoff data are poorly determined thereby making the task of developing useful regionalization relationships more difficult. The Bayesian methodology developed in part 1 is extended to utilize several kinds of hydrologic data in parameter inference, the goal being to improve the precision of poorly determined parameters. The concept of compatibility is developed using statistical hypothesis tests. Different kinds of data are said to be compatible if differences between their fitted parameters are not statistically significant. The pooling of incompatible data may undermine the model's ability to predict runoff and also induce bias in the parameters. A hierarchy of three levels of information is introduced to enable systematic checking for compatibility. Finally, a case study is presented. Using data on runoff, soil moisture, and interception, it is shown that substantial reductions in parameter uncertainty can be realized; also the importance of compatibility testing is demonstrated.
Some new procedures for numerical solution of variably saturated flow problems
Authors: Cooley Richard L.
Published: February 2008

Persistent difficulties that arise in forming numerical solutions of variably saturated flow problems include controlling the stability of the nonlinear equation solvers and devising a reliable, yet efficient, method for determining the positions of seepage surfaces. New techniques for addressing these problems are applied to a subdomain finite element discretization of the governing flow equations. A series of test problems demonstrates that the techniques are reliable and efficient for a wide variety of problems.
Aggregation and estimation for low‐order periodic ARMA models
Authors: Vecchia A. V. , Obeysekera J. T. , Salas J. D. , Boes D. C.
Published: February 2008

The aggregated time series resulting from summing over the seasons of a seasonal time series, which is assumed to follow either an AR(1) or ARMA(1, 1) model having periodic (seasonal) parameters, is shown to follow an ARMA(1, 1) model. Parameter estimation for such models is investigated analytically and via simulation. Significant gain in parameter estimation efficiency at the aggregated level is demonstrated when the seasonal data and their model is utilized rather than the aggregated (annual) data and their model.
Water movement into seasonally frozen soils
Authors: Kane Douglas L. , Stein Jean
Published: February 2008

Many floods of record in cold regions are produced by snow ablation. Accurate prediction of snowmelt runoff events by using computer models hinges upon an accurate understanding of the hydrologic role of frozen soils. This paper discusses the redistribution of soil moisture that takes place during the development of seasonal frost in a silty loam soil and the infiltration characteristics of this seasonally frozen soil. Depending upon the total (water and ice) soil moisture conditions near the ground surface, a wide range of infiltration rates can exist. This disparity in the infiltration rate results in considerable variation in the amount of snowmelt runoff.
Nonparametric Tests for Trend in Water Quality
Authors: van Belle Gerald , Hughes James P.
Published: February 2008

Recently, several nonparametric tests for trends in water quality have been proposed. These tests have been developed because the assumptions of classical parametric methods (i.e., normality, linearity, independence) are usually not met by typical water quality data. Additional idiosyncrasies of the data, such as missing values, censored data, and seasonality, compound the analysis problem. The nonparametric methods are more flexible and can handle these problems more easily. However, information on relative power of the various nonparametric procedures has not been reported in the hydrologic literature. For this reason, two classes of procedures, intrablock methods (procedures that compute a statistic, such as Kendall's tau, for each block or season and then sum these to produce a single overall statistic), and aligned rank methods (procedures that remove the block effect from each datum, sum the data over blocks, and then create a statistic from these sums) are examined in detail. Results from the statistical literature show that aligned rank methods are asymptotically more powerful than intrablock methods. In contrast, intrablock methods are more adaptable. Procedures to analyze more general models, including multistation designs and models, which include a trend‐season or trend‐site interaction, are developed by using Kendall's tau and intrablock methods. Finally, some recommendations on the analysis of data sets with missing values and multiple values per season values are presented.
Seasonal Fluctuations in the Carbon Dioxide Partial Pressure in a Cave Atmosphere
Authors: Troester Joseph W. , White William B.
Published: February 2008

Carbon dioxide partial pressures were measured at 2‐week intervals in a segment of cave passage isolated by a water trap from mixing with outside air. CO2 levels fluctuated from 0.03 to 0.4 vol % with maxima in the summer and minima in the winter. The CO2 measured directly in the atmosphere and the CO2 partial pressure calculated from the pH and alkalinity of the underground stream have similar seasonal profiles.
An Advection‐Diffusion Concept for Solute Transport in Heterogeneous Unconsolidated Geological Deposits
Authors: Gillham R. W. , Sudicky E. A. , Cherry J. A. , Frind E. O.
Published: February 2008

In layered permeable deposits with flow predominately parallel to the bedding, advection causes rapid solute transport in the more permeable layers. As the solute advances more rapidly in these layers, solute mass is continually transferred to the less permeable layers as a result of molecular diffusion due to the concentration gradient between the layers. The interlayer solute transfer causes the concentration to decline along the permeable layers at the expense of increasing the concentration in the less permeable layers, which produces strongly dispersed concentration profiles in the direction of flow. The key parameters affecting the dispersive capability of the layered system are the diffusion coefficients for the less permeable layers, the thicknesses of the layers, and the hydraulic conductivity contrasts between the layers. Because interlayer solute transfer by transverse molecular diffusion is a time‐dependent process, the advection‐diffusion concept predicts a rate of longitudinal spreading during the development of the dispersion process that is inconsistent with the classical Fickian dispersion model. A second consequence of the solute‐storage effect offered by transverse diffusion into low‐permeability layers is a rate of migration of the frontal portion of a contaminant in the permeable layers that is less than the groundwater velocity. Although various lines of evidence are presented in support of the advection‐diffusion concept, more work is required to determine the range of geological materials for which it is applicable and to develop mathematical expressions that will make it useful as a predictive tool for application to field cases of contaminant migration.
Bed Load Grain Velocities and Sediment Transport Rates
Authors: Bridge J. S. , Dominic D. F.
Published: February 2008

Theoretical consideration of the dynamics of bed load sediment transport leads to expressions for bed grain velocity and sediment transport rate as, respectively, Ub = a(U* − U*c) and ib = (a/tan α) (U* − U*c) (т0−тc), agreeing in form with many previous analyses. The grain velocity equation agrees well with all experimentally determined velocities of single grains moving over fixed rough beds if a is allowed to vary with bed roughness and saltation height. Values of U*c for such overriding grains are substantially less than normally accepted values. The sediment transport rate equation predicts plane bed experimental data well, over five orders of magnitude of ib. Values of a/tan α increase significantly from lower to upper stage plane beds due to an increase in a with suspended sediment concentration but also due to a decrease in tan α with increasing transport stage and grain size (as predicted by Bagnold).
Double‐Porosity Models for a Fissured Groundwater Reservoir With Fracture Skin
Authors: Moench Allen F.
Published: February 2008

Theories of flow to a well in a double‐porosity groundwater reservoir are modified to incorporate effects of a thin layer of low‐permeability material or fracture skin that may be present at fracture‐block interfaces as a result of mineral deposition or alteration. The commonly used theory for flow in double‐ porosity formations that is based upon the assumption of pseudo–steady state block‐to‐fissure flow is shown to be a special case of the theory presented in this paper. The latter is based on the assumption of transient block‐to‐fissure flow with fracture skin. Under conditions where fracture skin has a hydraulic conductivity that is less than that of the matrix rock, it may be assumed to impede the interchange of fluid between the fissures and blocks. Resistance to flow at fracture‐block interfaces tends to reduce spatial variation of hydraulic head gradients within the blocks. This provides theoretical justification for neglecting the divergence of flow in the blocks as required by the pseudo–steady state flow model. Coupled boundary value problems for flow to a well discharging at a constant rate were solved in the Laplace domain. Both slab‐shaped and sphere‐shaped blocks were considered, as were effects of well bore storage and well bore skin. Results obtained by numerical inversion were used to construct dimensionless‐type curves that were applied to well test data, for a pumped well and for an observation well, from the fractured volcanic rock terrane of the Nevada Test Site.
Maximum Likelihood Estimates for the Parameters of Mixture Distributions
Authors: Leytham K. M.
Published: February 2008

Maximum likelihood estimates for the parameters of a mixture of two normal distributions are presented in terms of an expectation‐maximization algorithm. Small sample properties of the parameter estimates are explored using Monte Carlo simulation. Although parameters estimated from unclassified data are inaccurate, quantiles derived from the fitted distributions are only slightly less accurate than quantiles estimated from classified data.
An Application of the Geostatistical Approach to the Inverse Problem in Two‐Dimensional Groundwater Modeling
Authors: Hoeksema Robert J. , Kitanidis Peter K.
Published: February 2008

The geostatistical approach to the estimation of transmissivity from head and transmissivity measurements is developed for two‐dimensional steady flow. The field of the logarithm of transmissivity (log‐transmissivity) is represented as a zero‐order intrinsic random field; its spatial structure is described in this application through a two‐term covariance function that is linear in the parameters θ1 and θ2. Linearization of the discretized flow equations allows the construction of the joint covariance matrix of the head and log transmissivity measurements as a linear function of θ1 and θ2. In this particular application the coefficient matrices are calculated numerically in a noniterative fashion. Maximum likelihood estimation is employed to estimate θ1 and θ2 as well as additional parameters from measurements. Linear estimation theory (cokriging) then yields point or block‐averaged estimates of transmissivity. The approach is first applied to a test case with favorable results. It is shown that the application of the methodology gives good estimates of transmissivities. It is also shown that when the transmissivities are used in a numerical model they reproduce the head measurements quite well. Results from the application of the methodology to the Jordan aquifer in Iowa are also presented.
A Simulation Analysis of Thermal Effects on Evaporation From Soil
Authors: Milly P. C. D.
Published: February 2008

The Richards equation, which expresses the conservation of water in an isothermal soil, has a more general form in a nonisothermal soil. In using the latter, it is necessary to know soil temperature, and modeling becomes considerably more complicated. A detailed, numerical simulation model quantifies the thermal effects for two hypothetical soils under two climates. During characteristic 4‐day climatic sequences, in a season of soil heating, diffusion of vapor due to thermally induced vapor concentration gradients suppresses evaporation. The suppression is greatest (5–15% in this set of experiments) under arid conditions. Under these conditions, such thermal vapor diffusion also distorts the usual diurnal pattern of evaporation. Evaporation is generally more sensitive to isothermal than to thermal vapor diffusion. Variations in time and depth of the soil temperature cause corresponding variations in the water transport coefficients. These, in turn, result in biases (2–5%) and diurnal distortions of evaporation rates. Liquid flow attributable to the dependence of matric potential on temperature accounts for about 1% of evaporation in our experiments. In simulations of 1 month duration for each combination of soil and climate, the joint neglect of all thermal effects mentioned above introduces an error of only about 1% in the average evaporation rate and does not distort its time distribution significantly.
Flow to a Partially Penetrating Well in a Double‐Porosity Reservoir
Authors: Dougherty D. E. , Babu D. K.
Published: February 2008

Analysis of flow in a fractured porous reservoir forms the basis for investigations of chemical and energy transport in such media. Numerical models are often employed to analyze these geohydrologic systems. In this paper a well hydraulics problem is solved using the Laplace transformation and the double‐porosity concept. The transient solution is obtained by numerical inversion of the Laplace transform. Solutions to slug test problems indicate that the head response due to fracturing is distinct from the response due to partial penetration or skin effect. An alternative to the commonly used van Everdingen model of skin effect is given. No method for identifying fractured porous reservoir parameters from slug tests has been developed. The results of this paper may be applied to test numerical models of flow in fractured porous media.
Origin and Distribution of Carbon Dioxide in the Unsaturated Zone of the Southern High Plains of Texas
Authors: Wood Warren W. , Petraitis Michael J.
Published: February 2008

Partial pressures of CO2, O2, N2, and Ar were monitored at two locations in the Ogallala aquifer system on the Southern High Plains of Texas. Samples were collected monthly during parts of 1980–1981 from nine depths ranging from 0.6 to 36 meters below land surface. PCO2 was observed to be greater at depth than in the active soil zone and thus appears to contradict the normal process in which CO2 is generated in the soil zone and diffuses upward to the atmosphere and downward to the water table. The δ13C of the CO2 gas was quite uniform and averaged −17.9 per mil. PO2 declined with depth, suggesting in situ generation of CO2 by the oxidation of carbon. Several hypotheses were considered to explain the origin of the CO2 at depth. It was concluded that the most probable hypothesis was that dissolved and particulate organic carbon introduced by recharging water was oxidized to CO2 by the aerobic microbial community that utilized oxygen diffusing in from the atmosphere. This hypothesis is consistent with the CO2 concentration profile, calculated production profile of CO2, δ13C values of CO2 gas, caliche, soil humic acid fraction, and dissolved carbonate in groundwater. The abundance of CO2, its concentration profile, and its probable origin provide information for evaluating the observed complex sequence of caliche dissolution and precipitation known to occur in the aquifer.
The Effect of Tortuosity on Fluid Flow Through a Single Fracture
Authors: Tsang Y. W.
Published: February 2008

Calculations to investigate the effect of path tortuosity and connectivity on fluid flow rate through a single rough fracture were carried out. The flow paths are represented by electrical resistors placed on a two‐dimensional grid, and the resistances vary as the inverse of the fracture aperture cubed. The electric current through the circuit bears a one‐to‐one correspondence to the fluid flow rate. Both fracture apertures derived from measurements and from hypothetical analytic functions were used in a parameter study to investigate the dependence of tortuosity on fracture roughness characteristics. It was found that the more small apertures there are in the aperture distribution, the larger is the effect of tortuosity. When the fraction of contact area between the fracture surfaces rises above 30%, the aperture distributions are invariably large at small apertures, and the effect of fracture roughness and flow path tortuosity depresses flow rate from the value predicted by the parallel plate representation of a fracture by three or more orders of magnitude. The impact of these results on the calculations and measurements in fracture hydrology is discussed.
From Excess Infiltration to Aquifer Recharge: A Derivation Based on the Theory of Flow of Water in Unsaturated Soils
Authors: Morel‐Seytoux Hubert J.
Published: February 2008

An approximate unit hydrograph of aquifer recharge due to effective percolation is derived from a theory describing flow of water in unsaturated soils. The parameters appearing in the expression of the unit hydrograph have physical meaning. Comparison of the theory with field observations indicates that the theory works well. A new tool for estimation of the temporal variation of aquifer recharge from that of excess infiltration is now available. The tool is practical (simple) and physical and apparently works well, as evidenced by comparisons with field observations.
A Borehole Methodology for Hydrogeochemical Investigations in Fractured Rock
Authors: Bottomley D. J. , Ross J. D. , Graham B. W.
Published: February 2008

Research into the development of borehole techniques for hydrogeochemical investigations in fractured rock has resulted in the construction of a geochemical probe controlled and operated with an umbilical cable system. This probe has been field tested to a depth of 600 m in a multilevel ported casing at the Chalk River Nuclear Laboratories (CRNL). The probe provides in situ measurements of pH, redox potential, dissolved oxygen, temperature, conductivity, and pressure while allowing groundwater sample collection both at surface and downhole. The multilevel ported casing preserves the subsurface hydrogeochemical zonation and facilitates sampling at various depths. In the CRNL borehole, dilute Na/HCO3 and Na/Cl waters ranging in age from 500 to 12,000 years have been observed in gneissic rock at depths of 85 to 350 m. However, accurate hydrogeochemical characterization of the low‐permeability metagabbro present from 400 to 600 m has been frustrated by contamination from casing installation water.
A Reinvestigation of the Analytical Solution for Drawdown Distributions in a Finite Confined Aquifer
Authors: Chen Chia‐Shyun
Published: February 2008

A finite aquifer is an aquifer in which drawdown variations reflect the influence of the exterior boundary. By making use of Laplace transform the transient solution for drawdown distributions in a finite confined aquifer surrounded by constant‐head boundary is reinvestigated. The development of the solution was facilitated by using the available steady state solution for drawdown distributions in a leaky artisian aquifer with constant leakage. The solution obtained consists of the Theis solution and other functions which are responsible for the influence of the exterior boundary. Methods of evaluating the solution are given, and results are presented in type curves. A time criterion is determined approximating the duration during which an ideal aquifer (i.e., the aquifer is homogeneous, isotropic, and of uniform thickness) can property be analyzed as an infinite one. This relationship interrelates time with the given hydrogeological properties.
A hydrologically useful station precipitation model: 2. Case studies
Authors: Georgakakos Konstantine P. , Bras Rafael L.
Published: February 2008

A physically based model of precipitation at a point, developed by the authors in an accompanying paper, is used to make rainfall and snow predictions at locations in the United States and Venezuela. The model is utilized in off‐line and real‐time modes. The latter involves the use of a Kalman filter. Results indicate that the parameterization suggested here is quite robust, comparing favorably with locally calibrated linear regression alternative models. Ideas for future work are given.
On unifying the concepts of scale, instrumentation, and stochastics in the development of multiphase transport theory
Authors: Cushman John H.
Published: February 2008

A generalized theory of multiphase transport is presented which combines the concepts of scale, instrumentation, stochastics, and time series with the development of transport equations. By defining the filtering process as a convolution of a measure P with a field property ψ, we are able to exploit the Fourier transform to place plausible restrictions on an instrument in frequency space so as to make its measurement relevant to its physical environment. An ideal instrument is defined which filters out high‐frequency noise (corresponding to short distances) and yet does not alter the structure of low frequencies. Using ideal instruments we successively filter out lower frequency noise in a multiscale, multiphase environment. Formulas are developed to relate the autocorrelation of a field property on one scale of motion to that on any other scale while taking into account the types of instruments used in the measuring process. An equation relating the integral scale on one scale of motion to the integral scale on any other scale of motion is developed. Power spectra are developed which relate spectra on different scales to the measuring instrument used. By successively applying filtering theorems, a hierarchy of multiscale transport equations is developed. Filtered properties in the transport equations are mass averages. Different properties are allowed to be measured by different instruments and different instruments are allowed on different scales of motion and for different phases. The concept of a wide sense stationary, ergodic process is introduced to develop mass average autocorrelations and spectra over scales of motion as a function of measuring devices.
An approach to modelling radionuclide migration in a medium with strongly varying velocity and block sizes along the flow path
Authors: Neretnieks Ivars , Rasmuson Anders
Published: February 2008

Radionuclides escaping from a repository in crystalline rock for spent nuclear fuel will migrate with the seeping water in the fissures. Most radionuclides will be retarded by sorption on the rock surfaces and by diffusion into the rock matrix. Available surface for sorption and residence time are two prime variables which influence the radionuclide movement. The water velocity may vary very much along a flow path, especially if the flow path enters a strongly fissured zone (lineament). In this paper the radionuclides in a stream tube with an arbitrary velocity along the flow path have been investigated, and a numerical scheme based on the integrated finite difference method (IFDM) is proposed for practical calculations. The transport mechanisms considered are advection and longitudinal dispersion and the retardation mechanisms used are instantaneous sorption in a portion of the rock (surface sorption) and diffusion into the rock matrix and sorption on the micropore surfaces. An attempt is also made to account for blocks of various sizes and shapes in introduction of a “pseudobody.” This method accounts for the large sorption surface but small volume of small blocks and small surface area but large volume of large blocks. Any block size didtrubution can be handled.
An extended identifiability in aquifer parameter identification and optimal pumping test design
Authors: Yeh William W‐G. , Sun Ne‐Zheng
Published: February 2008

An extended identifiability, called “δ identifiability”, is developed for groundwater modeling and management. Some fundamental concepts are developed for establishing a criterion in connection with the problem of optimal experimental design, such as the design of an optimum pumping test to assist aquifer parameter identification. In practice, the quantity and quality of data prohibits a unique solution of the inverse problem of parameter identification. The proposed “δ identifiability” is based upon the concept of weak uniqueness. The determination of the admissibility of a given design is formulated as a nonlinear programming problem. The original constrained problem is transformed into solving a sequence of unconstrained problems by a penalty function method. Numerical experiments are conducted to illustrate the proposed concept and algorithm.
Algorithms for Optimizing Hydropower System Operation
Authors: Grygier Jan C. , Stedinger Jery R.
Published: February 2008

Successive linear programming, an optimal control algorithm, and a combination of linear programming and dynamic programming (LP‐DP) are employed to optimize the operation of multireservoir hydrosystems given a deterministic inflow forecast. The algorithm maximize the value of energy produced at on‐peak and off‐peak rates, plus the estimated value of water remaining in storage at the end of the 12‐month planning period. The LP‐DP algorithm is clearly dominated: it takes longer to find a solution and produces significantly less hydropower than the other two procedures. Successive linear programming (SLP) appears to find the global maximum and is easily implemented. For simple systems the optimal control algorithm finds the optimum in about one fifth the time required by SLP but is harder to implement. Computing costs for a two‐reservoir, 12‐month deterministic problem averaged about seven cents per run using optimal control and 37 cents using successive linear programming.
Hillside Seepage: An Analytical Solution to a Nonlinear Dupuit‐Forchheimer Problem
Authors: Yates S. R. , Warrick A. W. , Lomen D. O.
Published: February 2008

An exact analytical solution is developed for steady flow in a sloping Dupuit aquifer. Constant areal accretion in the form of recharge from the surface and/or a leaky substratum occurs. An integrating factor is used to solve the nonlinear equation subject to upper and lower boundary conditions of known hydraulic head or flow. The field problem of interest is for shallow soil overlying a sloping bedrock.
Stochastic Analysis of Unsaturated Flow in Heterogeneous Soils: 2. Statistically Anisotropic Media With Variable α
Authors: Yeh T.‐C. Jim , Gelhar Lynn W. , Gutjahr Allan L.
Published: February 2008

Steady unsaturated flow in heterogeneous soil with an arbitrarily oriented mean hydraulic gradient is analyzed using spectral solutions of the stochastic perturbation equation which describes the capillary pressure head ψ. The unsaturated hydraulic conductivity is related to ψ by K = Ks exp (−αψ), where Ks is the saturated hydraulic conductivity and α is a soil parameter, and both Ks and α are treated as three‐dimensional statistically homogeneous, anisotropic random fields. Analytical results are obtained for the capillary pressure head variance and the effective (mean) unsaturated hydraulic conductivity. The head variance depends upon the degree of anisotropy of the ln Ks covariance; when α is random, the head variance increases with mean capillary pressure head. The effective hydraulic conductivity for arbitrary orientation of the mean hydraulic gradient J is shown to have tensorial properties, but its components depend on the magnitude and direction of J and the orientation of the stratification in the soil. When α is random, the degree of anisotropy of the effective conductivity depends strongly on mean capillary pressure.
Water Balance Models in One‐Month‐Ahead Streamflow Forecasting
Authors: Alley William M.
Published: February 2008

Techniques are tested that incorporate information from water balance models in making 1‐month‐ahead streamflow forecasts in New Jersey. The results are compared to those based on simple autoregressive time series models. The relative performance of the models is dependent on the month of the year in question. The water balance models are most useful for forecasts of April and May flows. For the stations in northern New Jersey, the April and May forecasts were made in order of decreasing reliability using the water‐balance‐based approaches, using the historical monthly means, and using simple autoregressive models. The water balance models were useful to a lesser extent for forecasts during the fall months. For the rest of the year the improvements in forecasts over those obtained using the simpler autoregressive models were either very small or the simpler models provided better forecasts. When using the water balance models, monthly corrections for bias are found to improve minimum mean‐square‐error forecasts as well as to improve estimates of the forecast conditional distributions.
Quadratic model for reservoir management: Application to the Central Valley Project
Authors: Mariño Miguel A. , Loaiciga Hugo A.
Published: February 2008

A quadratic optimization model is applied to a large‐scale reservoir system to obtain operation schedules. The model has the minimum possible dimensionality, treats spillage and penstock releases as decision variables and takes advantage of system‐dependent features to reduce the size of the decision space. An efficient and stable quadratic programming active set algorithm is used to solve for the optimal release policies. The stability and convergence of the solution algorithm are ensured by the factorization of the reduced Hessian matrix and the accurate computation of the Lagrange multipliers. The quadratic model is compared with a simplified linear model and it is found that optimal release schedules are robust to the choice of model, both yielding an increase of nearly 27% in the total annual energy production with respect to conventional operation procedures, although the quadratic model is more flexible and of general applicability. The adequate fulfillment of other system functions such as flood control and water supply is guaranteed via constraints on storage and spillage variables.
Stochastic Optimization of a Multireservoir Hydroelectric System: A Decomposition Approach
Authors: Pereira M. V. F. , Pinto L. M. V. G.
Published: February 2008

A computational scheme that is able to determine at each stage the most economical generation decision for each plant of a hydrothermal system is presented. The algorithm is based on the stochastic extension of Benders decomposition and can be implemented from already existing operation models. The results can be used in the weekly or monthly generation scheduling activities in real‐time operation. A case study with 37 reservoirs of the Brazilian system is presented and discussed.
Brahmaputra River, Assam, India: Physiography, Basin Denudation, and Channel Aggradation
Authors: Goswami Dulal C.
Published: February 2008

The Brahmaputra River in Assam, India, characterized by high seasonal variability in flow, sediment transport, and channel configuration, experienced a secular period of aggradation from 1971 to 1979. The suspended load budget indicates an overall aggradation of the 607‐km Assam reach of the Brahmaputra by about 16 cm during that period, with about 70% of the suspended sediment inflow into the reach being retained in the channel. Expressed as a percentage of the change in storage for the different reaches, computed errors due to sampling variability in sediment discharge generally lie within about 5–15% and do not appear to be large enough to affect the conclusions drawn from the suspended load budget. For a 145‐km reach of the Brahmaputra, an alternative method based on measurement of channel cross sections suggests 21 cm of aggradation, somewhat more than estimated by the suspended load budget. Based on the suspended load carried by trans‐Himalayan rivers, the present rate of denudation of the eastern Himalayas is estimated to be 73–157 cm/103years. The average rate for the last 2–3 million years, estimated from the volume of alluvial fill in the Brahmaputra valley in Assam, the sediment yield of the Himalayan rivers, and assuming a total yield to deposition ratio of 1.4 (present study), is 3 cm/103years. The current high rate of denudation of the Himalayas may be attributed mainly to the rapid uplift of the mountain system, recent earthquake activity, and high susceptibility of geologic formations to erosion by running water coupled with the effectiveness of the monsoon rainfall regime.
Stability Analysis of Water Movement in Unsaturated Porous Materials: 3. Experimental Studies
Authors: Diment G. A. , Watson K. K.
Published: February 2008

In the first paper of this series a detailed theory was presented in which the principles of hydrodynamic stability analysis were used to develop a linear perturbation equation for vertical water movement with nonsharp fronts. The second paper of the series discussed the application of this analysis to the stability of several soil water systems, some of which were potentially unstable. These included redistribution following infiltration, infiltration into a scale heterogeneous medium, and infiltration into a fine‐over‐coarse stratified profile. Although instability was expected for these systems and a noticeable trend toward instability was present, the numerical results did not predict the occurrence of such a condition. There was evidence to suggest that small amounts of initial water could have a significant effect on the damping out of instabilities that would occur under dry initial conditions. The present paper studies the effect of initial water content and grain size on wetting front instability by using a series of simple experiments. Results are presented for redistribution following infiltration for three porous materials, initially in a dry condition, and for three wetting depths prior to the onset of the redistribution phase. In addition, for the coarsest of these porous materials, a similar series of infiltration‐redistribution experiments is conducted with varying amounts of initial water present. Results are also presented for the effect of initial water content on infiltration into a stratified profile. The experiments show quite clearly that even small amounts of initial water have a strongly inhibiting effect on the development of frontal instability patterns.
On the Components of Flow Resulting From Abstraction in a Two‐Layered Confined Aquifer With No Cross Flow
Authors: Wikramaratna R. S.
Published: February 2008

The problem of abstraction from a well penetrating a two‐layered confined aquifer, each layer being independently homogeneous, isotropic, and artesian, is considered. Three components of the total abstraction are identified: the contribution from well storage and contributions from flow in each of the aquifer layers. For the case of a constant total abstraction rate, analytical expressions are derived for each of these components. The transient variation of the three components is investigated by evaluating the expressions numerically with different sets of parameter values.
Experimental Investigation of Solute Transport in Stratified Porous Media: 1. The Nonreactive Case
Authors: Sudicky E. A. , Gillham R. W. , Frind E. O.
Published: February 2008

The migration of a nonreactive solute in layered porous media was studied under controlled laboratory conditions by injecting a tracer into a thin sand layer bounded by silt layers. The experimental results show a delay in the breakthrough of the tracer in the effluent of the permeable sand layer along with a substantial decrease in concentrations after a 1‐m travel distance. The strongly dispersed and skewed shape of the breakthrough curves for the layered system is in contrast to the symmetric and weakly dispersed concentration patterns typically associated with homogeneous media. Because of the tailing of the breakthrough measurements, a departure from classical Fickian transport is apparent. Simulations of the experiments demonstrate that these effects are the result of a transient redistribution of the tracer across the strata by transverse molecular diffusion and that local longitudinal dispersion is only of secondary importance as a spreading process in such systems. These findings are consistent with recent theoretical descriptions of dispersion in stratified aquifers.
Analytical and Approximate Solutions to Radial Dispersion From an Injection Well to a Geological Unit With Simultaneous Diffusion Into Adjacent Strata
Authors: Chen Chia‐Shyun
Published: February 2008

Analytical and approximate solutions are developed for radial dispersion in aquifers with simultaneous diffusion into adjacent strata. Contaminants from the injection well are transported within the main aquifer by advection and mechanical dispersion, assuming a steady state and radially diverging ground‐water flow field. The leakage of contaminants from the main aquifer to the aquitards is accounted for by molecular diffusion. A mathematical model consisting of two coupled differential equations is proposed to investigate concentration distributions in the main aquifer as well as in the adjacent aquitards. By making use of Laplace transforms, analytical solutions valid for small time periods are obtained without difficulty. To complement these analytical solutions, concentration distributions for intermediate and large time intervals are determined approximately by numerically inverting the appropriate transformed solution in the Laplace domain with the Stehfest algorithm. Excellent agreement exists between analytical solutions and approximate solutions for small time intervals, supporting the validity of approximate solutions for intermediate and large time periods. It is shown that the diffusive leakage may significantly delay contaminant movement within the main aquifer at relatively large time periods. The solutions can be applied to study radial dispersion in granular aquifers bounded by relatively low permeability aquitards, or in planar fractures contained in porous formations.
Solution to the Kinematic Wave Approach to Overland Flow Routing With Rainfall Excess Given by Philip's Equation
Authors: Cundy Terrance W. , Tento Scott W.
Published: February 2008

A solution to the kinematic wave equations for overland flow routing where the lateral inflow term is determined from a constant rainfall rate and the infiltration model of Philip is presented. The solution utilizes the method of characteristics, and while it is not completely analytical, it is reduced to a simple numerical procedure which does not require the use of finite differences. The analysis presented deals with both the rising and falling stages. Example calculations, water surface profiles, and hydrographs are shown.
Errors and Parameter Estimation in Precipitation‐Runoff Modeling: 2. Case Study
Authors: Troutman Brent M.
Published: February 2008

A case study is presented which illustrates some of the error analysis, sensitivity analysis, and parameter estimation procedures reviewed in the first part of this paper. It is shown that those procedures, most of which come from statistical nonlinear regression theory, are invaluable in interpreting errors in precipitation‐runoff modeling and in identifying appropriate calibration strategies.
Assessment of Long‐Term Salinity Changes in an Irrigated Stream‐Aquifer System
Authors: Konikow Leonard F. , Person Mark
Published: February 2008

Changes in salinity in groundwater and surface water in the Arkansas River valley of southeastern Colorado are primarily related to irrigation practices. A solute transport model was applied to an 11‐mile reach of the valley to compute salinity changes in response to spatially and temporally varying stresses. The model was calibrated in 1973 using detailed field measurements made during 1971 and 1972. In 1973 the calibrated model was used to predict that a gradual long‐term increase in groundwater salinity of about 2–3% per year would occur if the observed irrigation practices continued. The study area was resampled during the winter of 1982 to help evaluate if any long‐term changes in salinity are actually occurring. Nonparametric and parametric statistical tests were used to help assess the significance of observed changes in groundwater salinity. These tests indicate that a statistically significant increase in salinity occurred between the winters of 1971 and 1972 (the model calibration period). However, a comparison of the winter 1972 and winter 1982 data indicates that no significant net change in salinity has occurred during this 10‐year period. An analysis of the few available historical data (1895, 1923, 1959–1961, and 1964) supports the hypothesis that groundwater salinity in this irrigated area has reached a long‐term dynamic equilibrium in response to irrigation practices. The model predictions of long‐term salinity increases were invalid probably because the calibration period occurred during a short‐term annual trend of increasing salinity in the river (and hence in leaky irrigation canals and in applied irrigation water), which was not representative of the long‐term trend.
A Policy Evaluation Tool: Management of a Multiaquifer System Using Controlled Stream Recharge
Authors: Danskin Wesley R. , Gorelick Steven M.
Published: February 2008

A model for the optimal allocation of water resources was developed for a multiaquifer groundwater and surface water system near Livermore, California. The complex groundwater system was analyzed using a transient, quasi‐three‐dimensional model that considers the nonlinear behavior of the unconfined aquifer. The surface water system consists of a reservoir that discharges water to three streams which in turn recharge the upper aquifer. Nonlinear streamflow‐recharge relationships were developed based upon synoptic field measurements of streamflow. The management model uses constrained optimization to minimize the cost of allocating surface water subject to physical and economic restrictions. Results indicate that a combined hydrologic and economic management model can be used to evaluate management practices of a complex hydrogeologic system. Questions can be posed which either would be impossible or extremely difficult to solve without the management model. We demonstrate the utility of such a model in three areas. First, the efficiency of intra‐basin water allocations is evaluated. Second, critical factors that control management decisions of the basin are identified. Third, the influence of economic incentives that can best satisfy the conflicting objectives of various water users is explored.