bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlated Rating Curve Error in Flood Frequency Inference
Authors: Kuczera George
Published: February 2004

An incremental rating curve error model is proposed to describe the systematic error introduced when a rating curve is extended by methods such as slope‐conveyance, log‐log extrapolation, or fitting to indirect discharge estimates. Extension can introduce a systematic or highly correlated error which is anchored by the more extensively measured part of the rating curve. A likelihood function is developed which explicitly accounts for such error and accepts both gauged and binomial‐censored data. A sampling experiment based on the three‐parameter generalized extreme value distribution was conducted to assess the performance of maximum likelihood quantile estimators. This experiment revealed that substantial, and in some cases massive, degradation in the performance of quantile estimators can occur in the presence of correlated rating curve error (rating error). Comparison of maximum likelihood estimators allowing for and ignoring rating error produced mixed results. As rating error impact and/or information content increased, estimators allowing for rating error tended to perform better, and in some cases significantly better, than estimators ignoring rating error. It is also shown that in the presence of rating error, the likelihood surface may have multiple optima that may result in nonunique solutions for hill‐climbing search methods. Moreover, in the presence of multiple optima and constraints on parameters, the likelihood surface may be poorly described by asymptotic approximations.
Local Contributions to Infiltration Excess Runoff for a Conceptual Catchment Scale Model
Authors: Orlandini Stefano , Mancini Marco , Paniconi Claudio , Rosso Renzo
Published: February 2004

The response of a conceptual soil water balance model to storm events is compared to a detailed finite element solution of the one‐dimensional Richards equation in order to test the capabilities of the former in calculating the local contributions to infiltration excess runoff in a distributed catchment scale model. Local infiltration excess runoff is computed from ground level precipitation using the time compression approximation and a Philip infiltration capacity curve with Brooks‐Corey constitutive equations. The validity of applying the conceptual model for local runoff and soil water balance calculations is investigated by performing numerical experiments over a range of soil types, control volume depths, and initial soil moisture conditions. We find that a good agreement between the conceptual and detailed models is obtained when the gravitational infiltration rate in Philip's formula is set to the saturated hydraulic conductivity, and when percolation from the control volume is updated as a function of the soil moisture content in a stepwise fashion. The comparison between these two models suggests that the simpler (and much less computer‐intensive) conceptual water balance technique could be incorporated into distributed models for large scale complex terrains as an efficient means of retaining consideration of spatial variability effects in catchment scale hydrologie simulations. This is illustrated in an application to the Rio Missiaga catchment in the eastern Italian Alps, where the local contributions to surface and subsurface runoff are routed onto a digital elevation model‐based conceptual transport network via a simple numerical scheme based on the Muskingum‐Cunge method.
The Flushing Flow Problem: Defining and Evaluating Objectives
Authors: Kondolf G. Mathias , Wilcock Peter R.
Published: February 2004

Reservoir releases may be specified for the purpose of maintaining or improving the downstream channel and habitat. A wide variety of ecological or management objectives may be defined for such flushing flows (which may be broadly divided into sediment maintenance and channel maintenance flows). To specify a particular discharge and water volume for a flushing flow requires that the ecological or management objectives be translated into specific physical objectives for which flows can be specified. Flushing objectives that cannot be translated into definable flows are of little practical use, regardless of their intrinsic importance. Once defined, flushing flow objectives may be shown to conflict in some cases. For example, no flushing flow can satisfy the typical sediment maintenance objectives of maximizing sand removal and minimizing gravel loss. A discharge that mobilizes sediment throughout the channel cross section for channel maintenance purposes will often produce comparable transport rates of sand and gravel, thereby eliminating the selective transport of sand needed to reduce the sand content in the bed. Some nonflushing alternatives, such as artificial gravel replenishment and pool dredging, can be used to improve the performance of flushing flows. Selection among these alternatives and specification of a flushing flow discharge and volume depend directly on quantitative estimates of sand and gravel transport as a function of flow rate and volume.
A Physically Based Model for Air‐Lift Pumping
Authors: François Odile , Gilmore Tyler , Pinto Michael J. , Gorelick Steven M.
Published: February 2004

A predictive, physically based model for pumping water from a well using air injection (air‐lift pumping) was developed for the range of flow rates that we explored in a series of laboratory experiments. The goal was to determine the air flow rate required to pump a specific flow rate of water in a given well, designed for in‐well air stripping of volatile organic compounds from an aquifer. The model was validated against original laboratory data as well as data from the literature. A laboratory air‐lift system was constructed that consisted of a 70‐foot‐long (21‐m‐long) pipe, 5.5 inches (14 cm) inside diameter, in which an air line of 1.3 inches (3.3 cm) outside diameter was placed with its bottom at different elevations above the base of the long pipe. Experiments were conducted for different levels of submergence, with water‐pumping rates ranging from 5 to 70 gallons/min (0.32–4.4 L/s), and air flow ranging from 7 to 38 standard cubic feet/min (0.2–1.1 m3 STP/min). The theoretical approach adopted in the model was based on an analysis of the system as a one‐dimensional two‐phase flow problem. The expression for the pressure gradient includes inertial energy terms, friction, and gas expansion versus elevation. Data analysis revealed that application of the usual drift‐flux model to estimate the air void fraction is not adequate for the observed flow patterns: either slug or churn flow. We propose a modified drift‐flux model that accurately predicts air‐lift pumping requirements for a range of conditions representative of in‐well air‐stripping operations.
Optimal Measurement Strategy for Surface Temperature to Determine Sensible Heat Flux From Anisothermal Vegetation
Authors: Sugita Michiaki , Brutsaert Wilfried
Published: February 2004

For a surface covered by vegetation, the scalar roughness for sensible heat zoh (or equivalently, any bulk heat transfer coefficient or surface resistance) depends directly on the way in which the effective or bulk surface temperature θs is measured. One undesirable feature of any such parameter like zoh is that when θs is measured radiometrically under conditions of solar heating of the surface, it tends to depend strongly on solar elevation. An analysis of experimental surface heat flux and related data herein confirms that this dependency can be weakened by making the radiometric surface temperature measurements obliquely, that is, by properly weighting the temperature of the upper layers of the canopy; these are the more direct heat exchangers with the turbulent air than the lower layers. It is also shown, however, that such oblique radiometric measurements produce lower values of θs, which in turn, require larger values of zoh, in the bulk heat transfer equation. Unfortunately, lower θs and larger zoh result in larger relative error in heat flux calculations. Therefore, under certain conditions, it may be preferable to design a compromise strategy and to accept some dependency of zoh on solar elevation, in order to maintain a sufficiently low error level in the calculated heat flux. In any event, this error level can be reduced considerably by making use of air temperature measurements at higher levels above the ground.
Semianalytical Solution for Solute Transport in Porous Media With Multiple Spatially Variable Reaction Processes
Authors: Xu Linlin , Brusseau Mark L.
Published: February 2004

A small‐perturbation semianalytical solution is derived for solute transport in porous media with multiple spatially variable reaction processes. Specific reactions of interest include reversible sorption, reversible mass transfer, and irreversible transformation (such as radioactive decay, hydrolysis reactions with fixed pH, and biodegradation). Laplace transform is employed to eliminate the time derivatives in the linear transport equations, and the transformed equations are solved analytically. The transient solution is ultimately obtained by use of an efficient quotient‐difference inversion algorithm. Results indicate that spatial variation of transformation constants for the solution phase and the sorbed phases decreases the global rate of mass loss and enhances solute transport. If the sorbed‐phase transformation constant is spatially uniform but not zero, a similar effect is observed when there is spatial variation of the equilibrium sorption coefficient. The global rate of mass loss and apparent retardation are decreased when the spatial variability of the sorbed‐phase transformation constant is positively correlated with the spatial variability of the equilibrium sorption coefficient and increased for a negative correlation.
Stochastic Modeling of Multidimensional Precipitation Fields Considering Spectral Structure
Authors: Yoo Chulsang , Valdés Juan B. , North Gerald R.
Published: February 2004

A multidimensional stochastic precipitation model with major emphasis on its spectral structure is proposed. As a hyperbolic type of stochastic partial differential equation, this model is characterized by a small set of easily estimable parameters. These characteristics are similar to those of the noise‐forced diffusive precipitation model, but the representation of the physical and statistical features of the precipitation field is similar to that of the Waymire–Gupta–Rodriguez‐Iturbe (WGR) precipitation model. The derivation was based on the autoregressive process considering advection and diffusion, the dominant statistical and physical characteristics of the precipitation field propagation. The model spectrum showed a good match with the Global Atlantic Tropical Experiment spectrum. This model was also compared with the WGR model and the noise‐forced diffusive precipitation model both analytically and through applications such as the sampling error estimation from spaceborne sensors and rain gauges. The sampling error from spaceborne sensors based on the proposed model was similar to that of the noise‐forced diffusive precipitation model, but much smaller than that of the WGR model. Similar results were also obtained in the estimation of the sampling error from rain gauges.
Determinants of Diarrheal Disease in Jakarta
Authors: Alberini Anna , Eskeland Gunnar S. , Krupnick Alan , McGranahan Gordon
Published: February 2004

Using data from a Jakarta household survey, we estimate a model of household defensive behavior and illness allowing for some risk factors that are hidden to the analyst but, perhaps, known to the household. As predicted by a general preference‐based theoretical model, defensive behavior (washing hands after toilet) intensifies with exposure to contaminants, income, opportunity, and education. Diarrheal disease, in turn, is determined by these factors and by defensive behavior. We also find evidence, though weak, in support of findings by VanDerslice and Bnscoe [1993] that pathogens are less harmful if they originate from within the household. Households are affected by water company decisions and management strategies, in particular, the frequency of interruptions, which interfere with defensive behavior. These findings link the inadequate supply of water for domestic use, a publicly provided good, and the private actions of the household, which are driven by its preferences for consumption goods and health, resource constraints, and knowledge of the sources of contagion. Such findings are therefore important in guiding the intersectoral allocations of public funds as well as the balance between private and public budgets.
Modeling Evapotranspiration and Surface Energy Budgets Across a Watershed
Authors: Flerchinger G. N. , Hanson C. L. , Wight J. R.
Published: February 2004

Transport of mass and energy between and within soils, canopies, and the atmosphere is an area of increasing interest in hydrology and meteorology. On arid and semiarid rangelands, evapotranspiration (ET) can account for over 90% of the precipitation, making accurate knowledge of the surface energy balance particularly critical. Recent advances in measurement and modeling have made the accurate estimate of ET and the entire surface energy balance possible. The Simultaneous Heat and Water (SHAW) model, a detailed physical process model capable of simulating the effects of a multispecies plant canopy on heat and water transfer, was applied to 2 years of data collected for three vegetation types (low sagebrush, mountain big sagebrush, and aspen) on a semiarid watershed. Timing and magnitude of ET from the three sites differed considerably. Measured and simulated ET for approximately 26 days of measurement in 1990 were 41 and 44 mm, respectively, for the low sagebrush, 74 and 69 mm for the mountain big sagebrush, and 85 and 89 mm for the aspen. Simulated and measured cumulative ET for up to 85 days of measurement at the three sites in 1993 differed by 3–5%. Simulated diurnal variation in each of the surface energy balance components compared well with measured values. Model results were used to estimate total ET from the watershed as a basis for a complete water budget of the watershed.
Design Optimization for Multiple Management Period Groundwater Remediation
Authors: Rizzo Donna M. , Dougherty David E.
Published: February 2004

A technique for obtaining a (nearly) optimal scheme using multiple management periods has been developed. The method has been developed for very large scale combinatorial optimization problems. Simulated annealing has been extended to this problem. An importance function is developed to accelerate the search for good solutions. These tools have been applied to groundwater remediation problems at Lawrence Livermore National Laboratory (LLNL). A deterministic site‐specific engineering‐type flow and transport model (based on the public domain code SUTRA) is combined with the heuristic optimization technique. The objective is to obtain the time‐varying optimal locations of the remediation wells that will reduce concentration levels of volatile organic chemicals in groundwater below a given threshold at specified areas on the LLNL site within a certain time frame and subject to a variety of realistic complicating factors. The cost function incorporates construction costs, operation and maintenance costs for injection and extraction wells, costs associated with piping and treatment facilities, and a performance penalty for well configurations that generate flow and transport simulations that exceed maximum concentration levels at specified locations. The resulting application reported here comprises a huge optimization problem. The importance function detailed in this paper has led to rapid convergence to solutions. The performance penalty allows different goals to be imposed on different geographical regions of the site; in this example, short‐term off‐site plume containment and long‐term on‐site cleanup are imposed. The performance of the optimization scheme and the effects of various trade‐offs in management objectives are explored through examples using the LLNL site.
Stochastic Indicators for Waste Site Characterization
Authors: Christakos George , Hristopulos Dionissios T.
Published: February 2004

Site characterization is an important prerequisite of risk assessment and remediation strategies. Evaluation of the health effects of groundwater and soil contamination depends on the adequate analysis of spatial heterogeneity, exceedance levels, and uncertainties. In this work we formulate and calculate stochastic indicators that provide a rigorous characterization of exposure levels in sites with heterogeneous contaminant distributions and offer valuable information for a cost‐effective cleanup analysis. These site indicators are general and can be used for different types and distributions of groundwater and soil contaminants. Important properties of the stochastic indicators are examined which can evaluate the potential for contamination at large scales, and improve understanding of threatened and damaged ecosystems. Analytically tractable formulas are derived that allow the practical estimation of site indicators on the basis of experimental data. Scale and modeling effects on contaminant level analysis are examined in terms of the stochastic indicators. Site cleanup costs depend directly on inferred characteristics of the stochastic indicators, which thus can play an important role in waste site management. Applications are discussed that offer insight regarding certain aspects of stochastic site characterization. Analytical methods of site characterization are compared to numerical simulations. It is shown that the latter can provide a practical alternative to the former, but they could lead to inaccurate results if they are not interpreted carefully.
Discrimination of Fractional Brownian Movement and Fractional Gaussian Noise Structures in Permeability and Related Property Distributions With Range Analyses
Authors: Liu H. H. , Molz F. J.
Published: February 2004

Recent studies have shown that a useful scheme to characterize subsurface heterogeneity is to use fractional Brownian motion (fBm) and fractional Gaussian noise (fGn). Commonly used techniques to detect these fractal distributions include rescaled range, spectral, and semivariogram analyses. Although rescaled range analysis is considered to be superior to spectral or semivariogram techniques, it cannot be used to clearly discriminate fGn and fBm from a given data set in some cases. On the basis of the fact that both adjusted range and rescaled range analyses, for a true fGn data set, should yield the same slope value in the relevant log‐log plots, a simple scheme is developed to discriminate fGn and fBm using a combination of the two analyses. The usefulness of the proposed scheme is evaluated with one‐dimensional computational experiments and by an application to a data set of observed air permeability logs. Lack of clear discrimination may have contributed to inconsistent past results, and it is recommended that old data sets be reanalyzed.
Sensitivity for Correlated Input Variables and Propagated Errors in Evapotranspiration Estimates From a Humid Region
Authors: Ahn Hosung
Published: February 2004

A sensitivity and uncertainty analysis of the Penman‐Brutsaert evapotranspiration (ET) model was conducted using meteorological data collected from the humid South Florida region. Since net radiation and relative humidity were found to be correlated (ρ = −0.74) in this region, a method was developed to analyze the sensitivity with the correlation effect of these two independent variables. After conducting sensitivity analyses with and without the correlation effect, the results were evaluated using conditional probability density functions. Both theoretical and computed results show that the conditional probability with the correlation effect increases in proportion to the increasing absolute correlation coefficient of two independent variables. This finding suggests that the sensitivity analysis with the correlation effect represents more adequately a given set of meteorological data in South Florida than that without the correlation effect. After defining the random errors of the independent variables based on multiple measurements, the ET model error as well as the propagated error caused by erroneous independent variables are presented.
Lognormal Distribution Model for Unsaturated Soil Hydraulic Properties
Authors: Kosugi Ken'ichirou
Published: February 2004

The soil water retention model developed by Kosugi was modified to be compatible with Mualem's model in order to derive an analytical expression for the relative hydraulic conductivity Kr. The modified water retention model is to be derived by applying a lognormal distribution law to the soil pore radius distribution function. Parameters of this retention model have physical significance on the water content (θ)– capillary pressure (ψ) curve and are related directly to the statistics of the pore radius distribution. The accuracy of the resulting combined water‐retention‐hydraulic‐conductivity model is verified for observed data sets for six soils. Results showed that the proposed model produces acceptable matches with observed water retention curves and adequate predictions of hydraulic conductivities in five out of six cases. The θ − ψ and Kr− ψ (or Kr − θ) curves generated by this model are generally similar to those generated by van Genuchten's model.
A numerical model of heat and water movement in furrow‐sown water repellent sandy soils
Authors: Yang Bangjie , Blackwell Paul S. , Nicholson David F.
Published: February 2004

Water repellent soils have thin layers of hydrophobic organic matter on the surfaces of soil particles. The hydrophobic organic matter makes these soils difficult to wet in opening rains after dry periods. This can cause losses of crop and pasture production and encourage land degradation by wind and water erosion. Wetting patterns in water repellent soils are irregular and incomplete, and no successful model of heat and water movement in water repellent soils has been reported. Furrow sowing improves crop production on water repellent sandy soils. Seeding machinery forms ridges and furrows. Rain is easily shed from dry ridges and runoff accumulates in the furrow (water harvesting). Ponded water infiltrates easily into the soil around the seed. The water movement in furrow‐sown water repellent sandy soils is more regular. This regularity makes it possible to develop a model. A numerical model of heat and water movement in furrow‐sown water repellent sand was developed and validated to help optimize the design of furrow sowing. The model used aerodynamic resistance, soil surface moisture and temperature, and surface evaporative resistance to estimate evaporation from each surface element. Wind speed and radiation was varied by calculation, according to the direction of the Sun or wind to the ridge direction and height. Movement of water and heat in the soil was determined by the initial conditions, boundary conditions, and coupled functions for heat and water movement, as used in many other models. Water repellency effects on drying were included by including the appropriate moisture and unsaturated hydraulic conductivity characteristics. Application of the finite element method to a two‐dimensional cross section of a ridge and furrow system allows water to be excluded from a soil region beneath a ridge. Field experiments were used to estimate surface evaporative resistance characteristics and to validate the model. The model has been successfully used to suggest optimized designs for ridge and furrow systems to minimize high soil temperatures and evaporation in Western Australia.
Linear Response Concept Combining Advection and Limited Rock Matrix Diffusion in a Fracture Network Transport Model
Authors: Barten W.
Published: February 2004

This paper presents the conceptual and mathematical basis of a model for transport of nuclides in the geosphere between the near field of a radioactive waste repository and high‐conductivity fractures that are assumed to be closely connected to the biosphere. The geosphere is modeled as a network of fractures that are mapped onto a network of channels. The balance equations and boundary conditions on the scale of a single rectangular channel are given. The effects of one‐dimensional matrix diffusion in addition to advection, linear sorption in the fractures and the rock matrix, and radioactive decay are considered. A hierarchical linear response concept using Laplace transformation techniques is developed for solution of the balance equations (1) on the scale of a channel network, (2) in the individual channels, and (3) in the rock matrix adjacent to the channels. Examples with unlimited and limited matrix diffusion and for additional rock layers are discussed.
Analysis of the Cape Cod tracer data
Authors: Ezzedine Souheil , Rubin Yoram
Published: February 2004

An analysis of the Cape Cod test was performed using several first‐ and higher‐order theoretical models. We compare conditional and unconditional solutions of the transport equation and employ them for analysis of the experimental data. We consider spatial moments, mass breakthrough curves, and the distribution of the solute mass in space. The concentration measurements were also analyzed using theoretical models for the expected value and variance of concentration. The theoretical models we employed are based on the spatial correlation structure of the conductivity field, without any fitting of parameters to the tracer data, and hence we can test the predictive power of the theories tested. The effects of recharge on macrodispersion are investigated, and it is shown that recharge provides a reasonable explanation for the enhanced lateral spread of the Cape Cod plume. The compendium of the experimental results presented here is useful for testing of theoretical and numerical models.
A governing equation for fluid flow in rough fractures
Authors: Ge Shemin
Published: February 2004

Fluid flow in a rock fracture bounded by two irregular surfaces is complex even under a laminar flow regime. The major factor causing deviation of predicted fracture flow behavior from the ideal parallel plate theory is the nature of nonparallel and nonsmooth geometry of fracture surfaces. Important questions on the validity of the cubic law and the Reynolds equation for complicated fracture geometries have been studied by many researchers. The general conclusion from these efforts is that the cubic law is valid provided that an appropriate average aperture can be defined. Many average apertures have been proposed, and for some cases, some work better than others. Nonetheless, to date, these efforts have not converged to form a unified definition on the fracture aperture needed in the cubic law, which stimulates the current effort to develop a general governing equation for fracture flow from a fundamental consideration. In this study, a governing equation stemming from the principle of mass conservation and the assumption that the cubic law holds locally is derived for incompressible laminar fluid flow in irregular fractures under steady state conditions. The equation is formulated in both local and global coordinates and explicitly incorporates two vectorial variables of fracture geometry: true aperture and tortuosity. Under the assumption of small variations in both tortuosity and aperture, the governing equation can be reduced to the Reynolds equation. Two examples are provided to show the importance and generality of the new governing equation in both local and global coordinate systems. In a simple fracture with two nonsmooth and nonparallel surfaces, the error in permeability estimation can be induced using the Reynolds equation with the apparent aperture and can reach 10% for a 25° inclination between the fracture surfaces. In a fracture with sinusoidal surfaces, the traditional method can cause significant errors in both permeability and pressure calculation.
Electrical structure beneath the eastern collapsed flank of Piton de la Fournaise volcano, Reunion Island: Implications for the quest for groundwater
Authors: Descloitres Marc , Ritz Michel , Robineau Bernard , Courteaud Michel
Published: February 2004

Time domain electromagnetic (TDEM) and tensor audiomagnetotelluric (AMT) data have been acquired at several locations on the eastern flank of Piton de la Fournaise volcano (Reunion Island) within a depressed area called Grand Brulé, interpreted as a collapse structure. The survey objectives were (1) to provide a geophysical estimate of the subsurface structure and (2) to evaluate the possibility of detecting aquifers in a volcanic environment not very known. The TDEM and the AMT data collected along two E‐W traverses orthogonal to coastline on the northern and southern edges of Grand Brulé were interpreted with one‐dimensional layered models. From the surface downward, the geoelectrical sections reveal two major units: very resistive, young lava flows (dry) and a shallow conductor (<500 m) which is probably primarily attributable to a clayey, poorly permeable base. A notable exception to this pattern is seen at sites close to the coast, where we found three‐layered structures. There is an intermediate layer of resistivity of about 100–200 ohm m between the top resistive layer and bottom conductive layer that represents a probable freshwater lens in the southern part and an alluvial fan with resistivities substantially higher (200 ohm m) in the northern part of Grand Brulé. It is suggested that the 200 ohm m layer, interpreted as a buried paleoriver, corresponds to a drainage structure.
Influence of transverse mixing on the breakthrough of sorbing solute in a heterogeneous aquifer
Authors: Berglund Sten , Fiori Aldo
Published: February 2004

The temporal moments associated with the breakthrough of a sorbing solute in a heterogeneous aquifer are investigated assuming an anisotropic distribution of hydraulic conductivity and deterministic linear sorption kinetics. The expressions for the temporal moments are based on a solution to the advection‐dispersion equation with constant coefficients which implies that they are valid in the large‐time regime. For the evaluation of the “macrodispersion” coefficient we use the asymptotic results of Fiori [1996], who quantified the effect of pore‐scale dispersion on nonreactive solute and showed that porescale dispersion affects field‐scale transport primarily through transverse mixing between flow paths. The present study shows that linear sorption kinetics decrease the effect of transverse mixing on the temporal moments as compared to the nonreactive case. It is concluded that in the considered large‐time regime, the pure‐advection approach to quantify field‐scale spreading is appropriate under the conditions encountered in most natural formations.
Sensitivity of long‐term bare soil infiltration simulations to hydraulic properties in an arid environment
Authors: Stothoff Stuart A.
Published: February 2004

The suitability of Yucca Mountain, Nevada, for emplacement of a high‐level nuclear waste geologic repository is currently being evaluated. Assessments of the repository performance suggest that the uncertainty in infiltration rates strongly affects predicted repository performance. Most of the ground surface over the potential repository footprint is characterized by shallow to deep colluvium/alluvium overlying densely fractured, welded tuffs. In order to identify characteristic behavior of infiltration that might be expected at the site, two idealizations of this situation are examined: an effectively semi‐infinite column of alluvium and a two‐layer column of alluvium over a fractured impermeable matrix. For each idealization the impact of hydraulic properties is assessed. Examining the sensitivity of bare soil simulator predictions for an effectively semi‐infinite column, it is found that decreasing the air entry pressure while holding all other parameters at a fixed level tends to increase both the long‐term average moisture content and the long‐term average net infiltration flux for homogeneous media. In contrast, increasing the van Genuchten scale parameter (m=1 − 17sol;n) or decreasing the porosity tends to decrease the average soil moisture but increase the infiltration. Most interestingly, three regimes are found for permeability. For relatively high permeabilities, there is a trend toward increasing average infiltration and increasing average moisture content with decreasing permeability. For relatively low permeabilities, vapor transport dominates over liquid transport, runoff and evaporation overwhelm infiltration, and the soil becomes very dry with essentially no infiltration flux. Between the extreme cases of high and low permeability, there is a zone where decreasing permeability results in decreased infiltration but increased moisture content, which is explained by the capacity of more permeable media to maintain surface wetness for longer periods of time after precipitation, thus allowing longer periods of evaporation at the potential rate. When there is a highly permeable fractured matrix underlying the alluvium, the depth of the alluvial cover has a dominant impact on infiltration. With little or no cover, more than half of the precipitation results in net infiltration. When alluvial cover is between depths of 25–50 cm and roughly 5 m, there is little or no net infiltration. Once the alluvial cover increases beyond 10 m, the semi‐infinite infiltration rate is recovered. The hydraulic properties of the fracture continuum have little impact on the infiltration behavior, as long as the equivalent fracture porosity is above a minimum value.
New approximate analytical technique to solve Richards Equation for arbitrary surface boundary conditions
Authors: Parlange J.‐Y. , Barry D. A. , Parlange M. B. , Hogarth W. L. , Haverkamp R. , Ross P. J. , Ling L. , Steenhuis T. S.
Published: February 2004

A general approximation for the solution to the one‐dimensional Richards equation is presented. It applies to arbitrary soil properties and boundary conditions but only uniform initial conditions at current stage. The result is very accurate (within 2%) when the diffusivity is constant, suggesting that the present general formulation is reliable, since the approximation becomes increasingly accurate as the soil‐water diffusivity approaches a delta function.
Real‐time reservoir operation for irrigation
Authors: Mujumdar P. P. , Ramesh T. S. V.
Published: February 2004

A short‐term real‐time reservoir operation model is developed for irrigation of multiple crops. Two major features of the model distinguish it from the earlier models which address the problem of reservoir operation for irrigation: its capability to consider interdependence of crop water allocations across time periods and its ability to provide an adaptive release policy. Integration of reservoir operation with on‐farm utilization of water by crops is achieved through the use of two models: An operating policy model, which optimizes reservoir releases across time periods in a year, and an allocation model, which optimizes irrigation allocations across crops within a time period. The model is applied to a case study of an existing reservoir in India.
Groundwater detection monitoring using combined information from multiple constituents
Authors: Small Mitchell J.
Published: February 2004

A Bayesian methodology is developed for groundwater leak detection at waste disposal sites. The method requires specification of the prior probability of a leak at the facility and updates this probability based on the proportion of constituent observations above a threshold value (e.g., the analytical detection limit) in the monitoring versus the background samples. The information content of a chemical measurement is characterized by the probability of occurrence of elevated concentrations of the chemical in a monitoring well, given the presence or absence of a facility leak. The evidence from multiple constituents is combined, assuming independence of errors, into a single aggregate estimate for the posterior probability of a leak. The procedure allows expert knowledge on site conditions and chemical behavior in the groundwater to be systematically incorporated in the regulatory evaluation of leak detection monitoring data. Applications using hypothetical and actual monitoring data from a hazardous waste facility demonstrate the procedure and highlight needs for further research.
Soil water depletion by oak trees and the influence of root water uptake on the moisture content spatial statistics
Authors: Katul Gabriel , Todd Philip , Pataki Diane , Kabala Zbigniew J. , Oren Ram
Published: February 2004

The space‐time statistical structure of soil water uptake by oak trees was investigated in a 3.1‐m‐diameter closed top chamber using a three‐dimensional measurement grid of soil moisture and pressure, and measurements of tree transpiration. Using the time domain reflectometery (TDR) measured moisture content, resistance block measured soil water pressure, and a compact constant head permeameter measured saturated hydraulic conductivity, the soil hydraulic properties for the chamber were first estimated. Then, dimensionless statistical measures that utilize the soil water pressure were proposed and used to assess the relative importance of lateral to vertical flow. On the basis of the measured statistical properties of the soil‐water pressure, it was found that the vertical flow is at least an order of magnitude larger than the lateral flow, and thus a one‐dimensional flow approximation to continuity was utilized. Using continuity and a first‐order Taylor series expansion of the Buckingham‐Darcy vertical flux about the spatial mean moisture content state, an approximate relation for the time variation of the spatial mean moisture content was derived and tested with the TDR measurements. Despite a large spatial coefficient of variation in the TDR measured moisture content (which was also shown to be comparable to reported values from larger‐scale field experiments), good agreement between mean moisture content predictions and measurements were found for two separate drying cycles. The approximate Taylor series flux expansion was utilized for deriving an analogous relation for the time variation of the spatial moisture content variance. The resultant variance budget was used to assess the role of root water uptake on the spatial variability of moisture content. It was found that the root uptake component, which resulted from a covariance between the root water uptake and moisture content spatial perturbations, is comparable to the contribution from soil hydraulic properties and soil water redistribution. One of the main findings in this study is that root water uptake is central to the moisture content spatial variance dissipation especially for dry soil moisture conditions. These results were further investigated using Monte‐Carlo simulations.
CO2aq‐dependent photosynthetic 13C fractionation in the ocean: A model versus measurements
Authors: Rau G. H. , Riebesell U. , Wolf‐Gladrow D.
Published: February 2004

A theoretical model of CO2aq‐dependent phytoplankton carbon isotope fractionation (єp) and abundance (δ13Corg) is compared to observed isotopic trends with temperature and [CO2aq] in the ocean. It is shown that the model's δ13Corg response to surface ocean temperature and to [CO2aq] can simulate observed trends when the other independent model variables (phytoplankton cell growth rate, cell size, cell membrane CO2 permeability, and enzymatic isotope fractionation) are held at realistic, constant values. The possible contribution made by each of these variables to the residual scatter in δ13Corg about its trends with temperature and [CO2aq] is quantified, thus estimating a maximum isotopic sensitivity to changes in each of these variables. The model response to growth rate and especially cell size, however, appears to be unrealistically high. This may occur because the net isotopic effects of such factors may be attenuated through dependent and isotopically offsetting variations among variables. The model's indicated sensitivity to such factors as CO2 permeability, enzymatic fractionation, cell size, and cell surface area/volume provides mechanisms whereby changes in species composition can play a significant role in affecting observed variations in oceanic δ13Corg. Overall, the model is consistent with earlier suggestions that marine δ13Corg and єp variability can be explained by carbon isotope fractionation evoked by CO2aq‐dependent phytoplankton. This has important implications for interpreting carbon isotopic variability encountered in plankton and their organic constituents in the present‐day ocean and in the marine sedimentary record.
Dissolved organic matter and terrestrial‐lotic linkages in the Central Amazon Basin of Brazil
Authors: McClain Michael E. , Richey Jeffrey E. , Brandes Jay A. , Pimentel Tania P.
Published: February 2004

We evaluate the hypothesis that decomposition and adsorption reactions operating in upland soils of headwater catchments control the concentration and composition of dissolved and fine particulate organic matter in rivers of the Amazon basin. In two contrasting first‐order catchments characteristic of the central Amazon basin, we analyzed plant, litter, soil, groundwater, and stream water chemistry. Our results indicate that clear and persistent differences exist in the concentration and elemental composition of dissolved organic matter (DOM) in stream waters and groundwaters from the two catchments, due mainly to corresponding differences in soil texture and chemistry. Within the more oxide and clay rich Oxisols underlying terra firme forest, groundwater DOM concentrations were uniformly low (120 μMC) and C/N ratios averaged 10. Conversely, within the oxide and clay deficient Spodosols underlying campinarana forest, groundwater DOM concentrations were greatly elevated (3000 μMC), and C/N ratios averaged near 60. We found that, in the terra firme/Oxisol terrain, the majority of DOM contributions to the stream derived from the riparian zone, while in the campinarana/Spodosol terrain, upland groundwater contributions could account for the concentration and composition of DOM in the stream. The implications of our findings are that in the terra firme terrains which dominate the region, upland soil profiles are not the site of definitive processes which impart compositional signatures to organic matter carried by the largest rivers of the Amazon basin, as was hypothesized. Instead, we suggest that definitive reactions are focused primarily in the river corridor.
Gaseous emissions and oxygen consumption in hydroelectric dams: A case study in French Guyana
Authors: Galy‐Lacaux Corinne , Delmas Robert , Jambert Corinne , Dumestre Jean‐François , Labroue Louis , Richard Sandrine , Gosse Philippe
Published: February 2004

Methane, carbon dioxide, and hydrogen sulfide emissions from the hydroelectric dam of Petit Saut on the Sinnamary River in French Guyana have been measured over a 2 year period. Since the beginning of the reservoir filling (January 1994), 300 km2 of tropical forest have been submerged. Emissions of CH4 by diffusion and by bubbling into the atmosphere or by degassing of the water released into the river, as well as the stock of dissolved gases in the lake, and their temporal evolutions were determined. Maximum emissions of 800 t CH4 per day were reached in February 1995, corresponding to dissolved CH4 concentrations of 14 mg4L−1 in the water column. The biological oxidation of methane results in a strong oxygen consumption in lake and river waters. Total emissions of CH4 and CO2 from January 1994 to December 1995 were calculated from the whole data set, which also allows us to calculate the total carbon loss since reservoir filling. About 10% of the carbon stored in soil and vegetation was released in gaseous form within 2 years.
Indicators of nitrate export from forested watersheds of the mid‐Appalachians, United States of America
Authors: Williard Karl W. J. , DeWalle David R. , Edwards Pamela J. , Schnabel Ronald R.
Published: February 2004

Soil net nitrogen mineralization and nitrification rates were studied on nine undisturbed, forested watersheds in an effort to explain large variations in nitrate export in streamflow within the mid‐Appalachian region. Rates of soil net nitrogen mineralization and net nitrification were measured in the upper 10 cm of mineral soil over a 5‐week summer incubation period (June–July) using nine buried bags in each of the three major soil types on each watershed. Watersheds with high, medium, and low nitrate export rates exhibited high, medium, and low mean net nitrogen mineralization and net nitrification rates, respectively. Exchangeable calcium (an index to site fertility), C/N ratios, and soil moisture content together explained 63% of the variation in soil nitrogen mineralization rates, and exchangeable calcium and soil moisture content explained 61% of the variation in soil nitrification rates using multiple regression analysis. The variation in watershed nitrate export was best explained by total nitrogen in the upper 10 cm of mineral soil (explained 46%) and the percentage of mineralization due to nitrification (explained 42%). Estimated rates of wet and dry atmospheric deposition of nitrogen were not significantly correlated with watershed nitrate export. Results from this study demonstrate that soil nitrogen pools and dynamics are the most critical factors controlling nitrate export from forested watersheds in the mid‐Appalachians. Long‐term changes in site fertility, C/N ratios, and soil moisture, which largely control microbial nitrogen cycling, should have a significant effect on long‐term trends in nitrate leaching.
Seasonal variations in the atmospheric O2/N2 ratio in relation to the kinetics of air‐sea gas exchange
Authors: Keeling Ralph F. , Stephens Britton B. , Najjar Raymond G. , Doney Scott C. , Archer David , Heimann Martin
Published: February 2004

Observations of seasonal variations in the atmospheric O2/N2 ratio are reported at nine baseline sites in the northern and southern hemispheres. Concurrent CO2 measurements are used to correct for the effects of land biotic exchanges of O2 on the O2/N2 cycles thus allowing the residual component of the cycles due to oceanic exchanges of O2 and N2 to be calculated. The residual oceanic cycles in the northern hemisphere are nearly diametrically out of phase with the cycles in the southern hemisphere. The maxima in both hemispheres occur in summer. In both hemispheres, the middle‐latitude sea level stations show the cycles with largest amplitudes and earliest phasing. Somewhat smaller amplitudes are observed at the high‐latitude stations, and much smaller amplitudes are observed at the tropical stations. A model for simulating the oceanic component of the atmospheric O2/N2 cycles is presented consisting of the TM2 atmospheric tracer transport model [Heimann, 1995] driven at the lower boundary by O2 fluxes derived from observed O2 saturation anomalies in surface waters and by N2 fluxes derived from the net air‐sea heat flux. The model is optimized to fit the observed atmospheric O2/N2 cycles by adjusting the air‐sea gas‐exchange velocity, which relates O2 anomaly to O2 flux. The optimum fit corresponds to spatially and temporally averaged exchange velocities of 24±6 cm/hr for the oceans north of 31°N and 29±12 cm/hr for the oceans south of 31° S. These velocities agree to within the uncertainties with the gas‐exchange velocities expected from the Wanninkhof [1992] formulation of the air‐sea gas‐exchange velocity combined with European Centre for Medium‐Range Weather Forecasts winds [Gibson et al., 1997] but are larger than the exchange velocities expected from the Liss and Merlivat [1986] relation using the same winds. The results imply that the gas‐exchange velocity for O2, like that of CO2, may be enhanced in the open ocean by processes that were not systematically accounted for in the experiments used to derive the Liss and Merlivat relation.