bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface moisture and satellite microwave observations in semiarid southern Africa
Authors: Owe M. , van de Griend A. A. , Chang A. T. C.
Published: February 2004

Nimbus 7 scanning multichannel microwave radiometer 6.6‐GHz passive microwave data were studied in relation to large‐scale soil moisture estimates over a 3‐year period in southeastern Botswana. An extensive data base of weekly surface soil moisture measurements was used with meteorological data to estimate pixel average soil moisture on a daily basis. The influence of the vegetation canopy on the surface emissivity was studied by partitioning the data set into classes on the basis of the normalized difference vegetation index. After correcting for the vegetation optical depth, a correlation of r = 0.84 was established between the normalized brightness temperature observations and surface soil moisture for the 3‐year period.
Bacterial transport in porous media: Evaluation of a model using laboratory observations
Authors: Hornberger George M. , Mills Aaron L. , Herman Janet S.
Published: February 2004

The factors that control the transport of bacteria through porous media are not well understood. The relative importance of the processes of dispersion, of immobilization of bacterial cells by various mechanisms (deposition), and of subsequent release of these trapped cells (entrainment) in describing transport has not been elucidated experimentally. Moreover, the variability of the phenomenological coefficients used to model these processes, given changes in such primary factors as grain size, organism, and ionic strength of the water, is unknown. We report results of fitting solutions of an advection‐dispersion equation, modified to account for deposition and entrainment, to breakthrough curves from packed sand columns using two sizes of sand, two ionic strengths of the carrier solution, and two organisms with different sizes. A solution to the advection‐dispersion equation including three processes, that is, dispersion, deposition, and entrainment, provides a match to the data that is superior to that achieved by solutions ignoring one of the processes. Fitted values of the coefficient describing deposition vary in a consistent manner with the control variables (organism, grain size, and ionic strength) and are generally within one order of magnitude of those predicted on the basis of theory.
Analytical solutions for steady state gas flow to a soil vapor extraction well
Authors: Shan Chao , Falta Ronald W. , Javandel Iraj
Published: February 2004

Analytical solutions are developed for modeling steady state gas flow to a single vacuum extraction well in the unsaturated zone. The solutions are applicable to isotropic as well as anisotropic homogeneous subsurface media in which the ground surface is open to the atmosphere. Analytic expressions are given for the gas pressure field, and for the stream function distribution. Streamline travel times are computed using a simple numerical technique employing the analytical solutions. The pressure solution may be used to analyze in situ gas pumping tests to determine average horizontal and vertical gas permeabilities. The stream function solution and streamline travel times may be used to optimize the design of vapor extraction systems for volatile organic chemical removal from the unsaturated zone. Application of the solutions to several examples illustrates the strong effect of screen location and anisotropy on the induced gas flow field, and dimensionless type curves are given for a variety of gas well geometries.
Effect of rainfall errors on accuracy of design flood estimates
Authors: Kuczera George , Williams Brian J.
Published: February 2004

A procedure is presented to explicitly evaluate the effect of estimation errors in the temporal and spatial distribution of rainfall on the uncertainty of calibrated rainfall‐runoff model parameters. The effect of this uncertainty on the reliability of design flood predictions is considered. A case study of the Hacking catchment, south of Sydney, Australia, is presented to illustrate the procedure. For a major storm event a simple stochastic transformed rainfall model is calibrated and validated using kriging and then used to infer the mean and covariance of subareal rainfall. The RORB model, a distributed nonlinear rainfall‐runoff model, is calibrated to this storm event. When allowance is made for uncertainty in the calibration event rainfall, the results indicate that the uncertainty in the calibrated parameters increases, especially in the rainfall excess parameters, and the 90% prediction interval on the 100‐year design flood increases by about 100%.
Bayesian stochastic optimization of reservoir operation using uncertain forecasts
Authors: Karamouz Mohammad , Vasiliadis Haralambos V.
Published: February 2004

Operation of reservoir systems using stochastic dynamic programming (SDP) and Bayesian decision theory (BDT) is investigated in this study. The proposed model, called Bayesian stochastic dynamic programming (BSDP), which includes inflow, storage, and forecast as state variables, describes streamflows with a discrete lag 1 Markov process, and uses BDT to incorporate new information by updating the prior probabilities to posterior probabilities, is used to generate optimal reservoir operating rules. This continuous updating can significantly reduce the effects of natural and forecast uncertainties in the model. In order to test the value of the BSDP model for generating optimal operating rules, real‐time reservoir operation simulation models are constructed using 95 years of monthly historical inflows of the Gunpowder River to Loch Raven reservoir in Maryland. The rules generated by the BSDP model are applied in an operation simulation model and their performance is compared with an alternative stochastic dynamic programming (ASDP) model and a classical stochastic dynamic programming (SDP) model. BSDP differs from the other two models in the selection of state variables and the way the transition probabilities are formed and updated.
A variable aperture fracture network model for flow and transport in fractured rocks
Authors: Nordqvist A. Wille , Tsang Y. W. , Tsang C. F. , Dverstorp Björn , Andersson Johan
Published: February 2004

A three‐dimensional variable aperture fracture network model for flow and transport in fractured rocks was developed. The model generates both the network of fractures and the variable aperture distribution of individual fractures in the network. Before solving for the flow and transport of the whole network, a library of single‐fracture permeabilities and particle transport residence time spectra is first established. The spatially varying aperture field within an individual fracture plane is constructed by geostatistical methods. Then the flow pattern, the fracture transmissivity, and the residence times for transport of particles through each fracture are calculated. The library of transmissivities and frequency distributions of residence times is used for all fractures in the network by a random selection procedure. The solution of flow through the fracture network and the particle‐tracking calculation of solute transport for the whole network are derived from one side of the network to the other. The model thus developed can handle flow and transport from the single‐fracture scale to the multiple‐fracture scale. The single‐fracture part of the model is consistent with earlier laboratory tests and field observations. The multiple‐fracture aspect of the model was verified in the constant aperture fracture limit with an earlier code. The simulated breakthrough curves obtained from the model display dispersion on two different scales as has been reported from field experiments.
A surface energy balance method for partitioning evapotranspiration data into plant and soil components for a surface with partial canopy cover
Authors: Massman W. J.
Published: February 2004

An objective, data‐based method for partitioning evapotranspiration (ET) measurements into soil and plant contributions is proposed and applied to a semiarid shortgrass steppe site in northeastern Colorado. This method requires fairly standard micrometeorological/surface energy balance measurements and relies on a two‐source Penman‐Monteith model of ET and a submodel (proposed and developed in this study) that relates the soil evaporation resistance to the soil Bowen ratio. Assuming that these soil parameters do not vary much during any given day, their daily values are determined by an optimization procedure which exploits the diurnal change in the meteorological data. Once the soil resistance and soil Bowen ratio are determined, the available energy and the ET data are partitioned into soil and plant components and then the canopy resistance and the within‐canopy aerodynamic resistance (herein referred to as the subcanopy resistance) are determined. Comparisons between the present results and other observations made in the shortgrass steppe ecosystem provide support for the validity of the present methods.
Regional surface fluxes under nonuniform soil moisture conditions during drying
Authors: Brutsaert Wilfried , Sugita Michiaki
Published: February 2004

The fifth intensive field campaign of FIFE (First ISLSCP Field Experiment (International Satellite Land Surface Climatology Project)) was conducted in northeastern Kansas during late summer 1989, in order to study the dry down of a surface with prairie vegetation prior to senescence and dormancy. As precipitation became sparse and rare, the surface developed nonuniform features at the scales of the rainfall events. Nevertheless, lower boundary layer measurements used together with remotely sensed surface temperatures on the basis of Monin‐Obukhov similarity allowed the estimation of the flux of sensible heat for a region with characteristic dimensions of 104 m. In contrast, under these conditions it was difficult to aggregate an array of local flux measurements into a true regional mean. On account of the same problem in the aggregation of local measurements of net radiation and ground heat flux, the reliability of the estimation of regional evaporation by the energy budget method was hard to assess.
Natural denitrification in the saturated zone: A review
Authors: Korom Scott F.
Published: February 2004

Denitrification is increasingly recognized for its ability to eliminate or reduce nitrate concentrations in groundwater. With this awareness comes a desire to predict the rate and extent of denitrification in aquifers. The limiting factor in making predictive models, however, is our limited knowledge of the physical characteristics of this process. This review synthesizes the published literature on natural aquifer denitrification. A background section discusses denitrification requirements and dissimilatory nitrate reduction to ammonium, which occurs in environments similar to those where denitrification occurs, and gives a historical perspective on denitrification. Other sections discuss denitrification with organic carbon serving as the electron donor (heterotrophic denitrification) and with reduced inorganic compounds serving as the electron donor (autotrophic denitrification). The section on heterotrophic denitrification is structured around two tables that summarize natural aquifer denitrification rates reported by laboratory studies and natural aquifer denitrification rates reported by field studies. The section on autotrophic denitrification discusses denitrification with reduced iron and reduced sulfur. Thus far, most studies only consider a single electron donor or donor type, whether heterotrophic or autotrophic. This review demonstrates, however, that multiple electron donors may be present in a given aquifer. Future research efforts are recommended to determine the factors affecting the availability of electron donors and their denitrification rates. Additional research is also suggested on how dissolved oxygen affects denitrification rates and on the factors influencing the partitioning of nitrate reduction products to nitrous oxide, a potential contributor to the destruction of the ozone layer, and to ammonium.
Landsat surface temperatures and radio soundings to obtain regional surface fluxes
Authors: Sugita Michiaki , Brutsaert Wilfried
Published: February 2004

Thermal infrared radiances recorded by the thematic mapper of the Landsat 5 satellite, in combination with wind velocity and temperature profiles recorded by radiosondes, were used to derive surface fluxes of sensible heat during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). FIFE was conducted in 1987 and 1989 in a strongly dissected tallgrass prairie area of the Flint Hills in northeastern Kansas. The scale of the surface temperature measurements was made compatible with the effective fetch of the boundary layer profile measurements by aggregation. The methods of calculation were based on known Monin‐Obukhov flux‐profile relationships and on bulk similarity formulations for the atmospheric boundary layer. The surface temperatures were used both with and without atmospheric correction. The latent heat fluxes (evaporation) were derived from those of sensible heat by means of the energy budget. The calculated fluxes were generally in good agreement with the means of the fluxes measured in the field by all (22 in 1987 and 14 in 1989) ground stations in the 15×15 km experimental area.
Determining and interpreting the order of a two‐state Markov Chain: Application to models of daily precipitation
Authors: Gregory J. M. , Wigley T. M. L. , Jones P. D.
Published: February 2004

We illustrate the result that lumping together some of the states of a many‐state first‐order Markov chain does not in general give a first‐order Markov chain with a smaller number of states. If a series generated in this way is nevertheless assumed to have been produced by a two‐state Markov chain, standard statistical procedures (using the Akaike and Bayesian information criteria) may indicate that it should be fitted by a higher order than first. Stochastic models based on a Markov chain are often used to model precipitation series. It is normal to classify days as “dry” and “wet” and fit a two‐state process. In some cases, second‐ or higher‐order chains are preferred by reference to information criteria. In the light of the foregoing, we suggest that this might be because a many‐state process, possibly of only first order, would actually be a better choice than a two‐state process.
Representation of grid block permeability in coarse scale models of randomly heterogeneous porous media
Authors: Durlofsky Louis J.
Published: February 2004

The two basic approaches for the assignment of coarse scale grid block permeabilities from highly detailed permeability descriptions are sampling, where the permeability of the coarse scale grid block is simply specified as the fine scale permeability evaluated at the grid block center (no averaging is performed), and scaleup or homogenization, where the coarse scale permeability is determined through some procedure which appropriately averages the fine scale data. The intent of this paper is to quantitatively compare these two approaches, through extensive numerical calculations, for the modeling of single phase flow through two‐dimensional realizations of randomly heterogeneous porous media, with heterogeneity quantified through the correlation length (/) and the variance of the log of permeability (σ2ϕ). Several different flow scenarios and two scaleup techniques (local and global) are considered over a range of l and σϕ. It is seen that for flows driven by smoothly varying boundary conditions, a sampling procedure is more accurate than global scaleup when the length scale of a typical grid block is less than l, independent of σϕ. For flows driven by source terms, by contrast, more resolution is required for a sampling approach to be preferable to global scaleup, particularly at high variances. Local scaleup is shown to be preferable to either sampling or global scaleup in essentially all cases, though its use for more general permeability fields requires additional numerical calculations.
Large‐scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 3. Hydraulic conductivity variability and calculated macrodispersivities
Authors: Hess Kathryn M. , Wolf Steven H. , Celia Michael A.
Published: February 2004

Hydraulic conductivity (K) variability in a sand and gravel aquifer on Cape Cod, Massachusetts, was measured and subsequently used in stochastic transport theories to estimate macrodispersivities. Nearly 1500 K measurements were obtained by borehole flowmeter tests and permeameter analyses of cores. The geometric mean for the flowmeter tests (0.11 cm/s) is similar to that estimated from other field tests. The mean for the permeameter tests (0.035 cm/s) is significantly lower, possibly because of compaction of the cores. The variance for the flowmeter (0.24) is also greater than that for the permeameter (0.14). Geostatistical analyses applying negative exponential models with and without nuggets reveal similar spatial correlation structures for the two data sets. Estimated correlation scales range from 2.9 to 8 m in the horizontal and from 0.18 to 0.38 m in the vertical. Estimates of asymptotic longitudinal dispersivity (b.35–0.78 m) are similar in magnitude to that observed in the natural gradient tracer test (0.96 m) previously conducted at this site.
Statistical analysis of spatial variability in unsaturated flow parameters
Authors: Russo David , Bouton Moshe
Published: February 2004

Core scale estimates of soil parameters of the Gardner‐Russo and van Gemachten models of the hydraulic conductivity and water retention functions were obtained for 417 undisturbed soil cores taken from a wall of a trench (20 m long, 2.5 m deep), using a procedure based on inverse problem methodology. These estimates were used to evaluate the first two statistical moments of the underlying random space functions (RSFs), using the restricted maximum likelihood estimation procedure, coupled with the weighted least squares procedure, to estimate parameters of models of the covariance and the drift functions of the pertinent RSFs. The fitted models were used to evaluate the mean and covariance functions of the hydraulic conductivity and water rentention functions for given water saturations. Covariance functions of log‐saturated conductivity (log Ks) and the “shape” parameters of the Gardner‐Russo and van Genuchten models exhibited statistical anisotropy characterized by aspect ratios that vary between 3 to 4. Correlation scales of log Ks were larger than those of the “shape” parameters. Consequently, the product of the variance of log unsaturated conductivity and its correlation scale remained essentially invariant for a considerable range of water saturation. The implications of these results regarding stochastic modeling of transport in heterogeneous porous formations and possible applications of the results of this study are discussed briefly.
Measurements in a Braided River chute and lobe: 1. Flow pattern, sediment transport, and channel change
Authors: Ferguson R. I. , Ashmore P. E. , Ashworth P. J. , Paola C. , Prestegaard K. L.
Published: February 2004

This paper and its companion (Ashworth et al., this issue) discuss measurements of channel change and associated flow and sediment transport processes in a representative chute‐and‐bar reach within a proglacial gravel‐bed river, the Sunwapta River in the Canadian Rockies. During a week in which water discharge through the reach increased then decreased, a sediment wave passed into and partly along the reach. At first the chute aggraded, then sediment was eroded from the chute and deposited in a prograding lobe to one side of the original bar head between two distributaries. Measurements of velocity, shear stress, and gravel transport rate revealed day‐to‐day changes in the divergent pattern of flow and sediment transport. The off‐center location of the lobe reflected an initial asymmetry in the pattern of flow and sediment transport, but as the lobe grew, the flow and transport gradually switched away to the other distributary. Width‐averaged bed load transport rates do not agree well with average rates inferred from bar head deposition or from volumes and spatial patterns of scour and fill; the indirect estimates are considered more reliable than those based on direct sampling for necessarily brief durations. By the end of the study the new lobe had almost accreted onto the original bar head, supporting the idea that most braid bars are of compound origin. Temporal and spatial patterns in grain size distributions of the bed, bed load, and deposited sediment are discussed in the companion paper.
Relation of nickel concentrations in tree rings to groundwater contamination
Authors: Yanosky Thomas M. , Vroblesky Don A.
Published: February 2004

Increment cores were collected from trees growing at two sites where groundwater is contaminated by nickel. Proton‐induced X ray emission spectroscopy was used to determine the nickel concentrations in selected individual rings and in parts of individual rings. Ring nickel concentrations were interpreted on the basis of recent concentrations of nickel in aquifers, historical information about site use activities, and model simulations of groundwater flow. Nickel concentrations in rings increased during years of site use but not in trees outside the contaminated aquifers. Consequently, it was concluded that trees may preserve in their rings an annual record of nickel contamination in groundwater. Tulip trees and oaks contained higher concentrations of nickel than did sassafras, sweet gum, or black cherry. No evidence was found that nickel accumulates consistently within parts of individual rings or that nickel is translocated across ring boundaries.
Sensitivity of time domain reflectometry measurements to lateral variations in soil water content
Authors: Knight J. H.
Published: February 2004

The spatial responses of two types of time domain reflectometry (TDR) probes used for measuring the water content of soils and other porous materials are examined. We investigate the effect of spatial variation in the water content of the sample on the measured value of the water content for both the coaxial cylinders probe and the two parallel wires probe. The response of the instrument to water content distributions that are perturbed slightly from a uniform distribution is calculated, using the conventional electrostatic treatment of electromagnetic wave propagation in the transverse electro‐magnetic (TEM) mode. Under this small perturbation assumption, we show that the spatial weighting function is approximately proportional to the distribution of electromagnetic energy between the electrodes for uniform water content, calculated from a solution of Laplace's equation for each probe type. For coaxial probes, most of the energy (and hence most of the measurement sensitivity) is concentrated around the inner cylinder in a “skin effect” if the ratio of the radii of the inner and outer cylinders is too small. For parallel wire probes, most of the measurement sensitivity is close to the wires if the wire diameter is too small compared to the spacing between them. This can cause significant errors if there is an air gap close to the wires or soil around the wires has been compacted by the process of inserting them into the soil.
Stochastic analysis of dispersion in unsteady flow in heterogeneous aquifers
Authors: Rehfeldt Kenneth R. , Gelhar Lynn W.
Published: February 2004

The dispersion of a solute plume resulting from unsteady flow in three‐dimensional, heterogeneous porous media was analyzed using stochastic continuum theory. Asymptotic stochastic solutions of the perturbed unsteady flow and solute transport equations were used to construct the macroscopic dispersive flux and evaluate the resulting macrodispersivity tensor in terms of a three‐dimensional, statistically anisotropic input covariance describing the hydraulic conductivity and an input covariance describing the temporal variability of the mean hydraulic gradient. The flow equation was approximated by neglecting the influence of internal storage due to medium and fluid compressibility (specific storage, SS ≈ 0). The predictive expression for the macrodispersivity tensor is the sum of two components: a spatial variability component identical to previous steady state theory and an unsteady component. Two special cases of unsteady flow were examined: variation only in the magnitude and variation only in the direction of the hydraulic gradient. Gradient magnitude variation produces a slightly larger longitudinal macrodispersivity than does the steady flow case, whereas gradient direction variation produces a significantly larger transverse macrodispersivity. Longitudinal and horizontal transverse macrodispersivities predicted with the unsteady stochastic theory were shown to be of a magnitude similar to observed values from the Borden, Cape Cod, and Columbus tracer experiments.
Estimating shear velocity and roughness length from velocity profiles
Authors: Bergeron Normand E. , Abrahams Athol D.
Published: February 2004

In turbulent boundary layer flows, shear velocity u*, and roughness length z0 are commonly derived from semilogarithmic flow velocity profiles by fitting a straight line by ordinary least squares regression to the profile and calculating estimates of u*, and z0 from the slope and intercept of the computed regression equation. However, it is not clear from the literature whether the appropriate regression is of flow velocity u on the logarithm of height above the bed In z or of ln z on u. In order to calculate estimates of u* and z0, the true or structural relation between u and In z must be established. Because u is generally observed with much greater error than is In z, the structural relation may be estimated by regressing u on ln z; regressing ln z on u is incorrect. An analysis of 24 stream channel flow velocity profiles indicates that even in situations where the correlation between u and ln z exceeds 0.9, performing the incorrect regression can result in the considerable overestimation of u* and z0.
Efficient sequential sampling strategies for environmental monitoring
Authors: Mukhopadhyay Nitis , Bendel Robert B. , Nikolaidis Nikolaos P. , Chattopadhyay Saibal
Published: February 2004

Assessments of resources at risk to anthropogenic pollution require extensive environmental monitoring. In addition, such assessments are required to have a long‐term monitoring component in order to evaluate not only the status but also the trend of the resources at risk to ecological stresses. There is a need to identify statistical methodologies that would provide effective and cost‐saving environmental monitoring designs, since such monitoring surveys are very expensive. In this paper the purely sequential, accelerated sequential, and three‐stage procedures are evaluated as effective fixed‐precision sampling procedures for environmental monitoring. Current monitoring designs utilize a sampling methodology where each resource is assigned a population inclusion probability, with the intent of describing the distribution of the whole population of resources at risk to anthropogenic environmental stresses. This study assumes that existing designs accurately describe the population distribution. A simultaneous fixed‐precision estimation procedure is developed as an efficient method of estimating practically relevant percentiles of the cumulative distribution function, using water quality data from the Eastern Lake Survey as a lake population distribution. Accelerated sequential and three‐stage procedures are shown to be better alternatives to the purely sequential procedure, requiring fewer sampling operations without any substantial loss of efficiency. Depending upon the precision required, all procedures showed potential reductions in sample size by as much as 60%. These types of designs for environmental monitoring are expected to be advantageous in national monitoring efforts directed toward the assessment of the status and trends of various ecological indicators.
Fingered flow in two dimensions: 2. Predicting finger moisture profile
Authors: Selker J. , Parlange J.‐Y. , Steenhuis T.
Published: February 2004

An exact integral solution for the moisture profile in growing fingers in sandy soil is derived from Richards' equation. The solution provides moisture content along a finger as a function of position and time and provides applicable results, including the calculation of the asymptotic matric potential of a growing finger and a method of obtaining the unsaturated conductivity in a single experiment. The solution is verified experimentally through comparison with measurements of matric potential and moisture content using high‐speed tensiometers.
Recreation benefits of instream flow: Application to Montana's Big Hole and Bitterroot Rivers
Authors: Duffield John W. , Neher Christopher J. , Brown Thomas C.
Published: February 2004

Allocation of water between instream uses such as recreation and consumptive uses such as irrigation is an important public policy issue in the western United States. One basis for identifying appropriate levels of instream flows is maximization of net economic benefits. A general framework for estimating the recreational value of instream flows was developed and applied to Montana's Big Hole and Bitterroot rivers. The paper also provides a synthesis of methods for interpreting covariate effects in dichotomous choice contingent valuation models. Precision of the estimates is examined through a simulation approach. The marginal recreational value of instream flow in these rivers is in the range of $50 per acre foot (1 acre foot equals 1233.5 m3) for recreation at low‐flow levels plus $25 per acre foot for downstream hydroelectric generation. These values indicate that at some flow levels, gains may be achieved on the study rivers by reallocating water from consumptive to instream uses.
Flood frequency analysis of simulated flows
Authors: Bradley A. Allen , Potter Kenneth W.
Published: February 2004

When conventional statistical techniques are used with model‐simulated flows to estimate flood frequencies for different basin scenarios or for regulated flow conditions, the resulting estimates can appear inconsistent with the simulated flood data. In this paper we describe a new approach for flood frequency analysis of model‐simulated flows. Rather than fitting a probability distribution directly to simulated peak discharges, separate distributions are estimated for runoff volumes and for peak discharges conditioned on runoff volumes. The latter estimation is based on rainfall‐runoff modeling using historical rainfall data for the basin of interest as well as data from extreme storms that have occurred in the region. The modeling of extreme events forces consistency in the resulting flood frequency estimates. An example illustrates the proposed method.
Energy balance concept in the evaluation of water table management effects on corn growth: Experimental investigation
Authors: Kalita Prasanta K. , Kanwar Rameshwar S.
Published: February 2004

The effects of water table management practices (WTMP) on corn growth in 1989 and 1990 at two field sites, Ames and Ankeny, Iowa, were evaluated by calculating crop water stress index (CWSI) and monitoring plant physiological parameters during the growing seasons. Experiments were conducted on field lysimeters at the Ames site by maintaining water tables at 0.3‐, 0.6‐, and 0.9‐m depths and in a subirrigation field at the Ankeny site with 0.2‐, 0.3‐, 0.6‐, 0.9‐, and 1.1‐m water table depths, and periodically measuring leaf and air temperature, transpiration rate, stomatal conductance, and photosynthetically active radiation (PAR) using leaf chamber techniques. Net radiation of canopy was estimated using the leaf energy balance equation and leaf chamber measurements and then correlated with PAR. Analysis of data revealed that net radiation, leaf air temperature differential, transpiration rate, stomatal conductance, and CWSI were strongly related to WTMP during vegetative and flowering stages of corn growth. Excess water in the root zone with a water table depth of 0.2 m caused the maximum crop water stress and ceased crop growth. Both water and oxygen could be adequately maintained for favorable crop growth by adopting the best WTMP. Results indicate that plant physiological parameters and CWSI could be used to evaluate the effectiveness of WTMP and develop the best WTMP for corn growth in the humid region.
Climate and energy exchange at the snow surface in the Alpine Region of the Sierra Nevada: 2. Snow cover energy balance
Authors: Marks Danny , Dozier Jeff
Published: February 2004

A detailed evaluation of surface climate and energy exchange at the snow surface in a small alpine watershed, typical of much of the southern Sierra Nevada, is presented for the 1986 water year. Measurements of snowfall, meteorological and snow cover conditions, and snow cover ablation, described in part 1 of this paper (Marks et al., this issue), are used to characterize the climate. Each form of energy transfer, radiation, sensible and latent heat flux, soil heat flux, and heat flux by mass advection, is evaluated separately to determine how its magnitude changes during the snow season. These are then combined to approximate a snow cover energy balance and determine the relative importance of each form of energy transfer in the seasonal energy and mass balance of the snow cover. Radiation and sensible and latent heat flux dominate the snow cover energy balance throughout the snow season. During snowmelt, radiation accounts for between 66 and 90% of the energy available for melt. Sensible and latent heat transfer during this time are of approximately equal magnitude but are usually of opposite sign and therefore cancel. Calculated sublimation during the entire snow season accounted for the loss of about 20% (approximately 50 cm snow water equivalent) of the mass of the snow cover. This experiment shows that energy and mass transfer can be adequately monitored at a remote site using a combination of measured and modeled parameters and that the energy balance of the snow cover in the alpine zone of the Sierra Nevada is dominated by net radiation during snowmelt.
Sensitivity of a basin evolution model to the nature of runoff production and to initial conditions
Authors: Ijjász‐Vásquez Ede J. , Bras Rafael L. , Moglen Glenn E.
Published: February 2004

This work investigates the effects of modifying the runoff production mechanism used in the Willgoose‐Bras‐Rodiguez‐Iturbe basin evolution model from Hortonian to subsurface saturation. The modified model also simulates basins that reproduce well common geomorphological statistics. This result confirms the ability of the model to adapt to different hydrological environments. The most important difference between the original and the modified model appears in the evolution of the hypsometric curve, particularly in the transition between saturated and unsaturated nodes. The role of mass movement due to diffusive processes (such as rain splash and rock slides) is more significant in the modified model due to decreased role of wash in unsaturated regions. A measure is developed to quantify the sensitivity of the model to initial conditions. Evolutions were found to separate in time following a power law, explaining the apparent randomness present in catchments simulated with only slight perturbations in their initial conditions. This apparent randomness is the result of the weakly chaotic features of the model.
The apparent conductivity for steady unsaturated flow in periodically fractured porous media
Authors: Martinez M. J. , Dykhuizen R. C. , Eaton R. R.
Published: February 2004

The influence of horizontal fractures on the steady seepage of moisture in variably saturated porous media is analyzed by analytical and numerical means. The fractures are assumed to contain many open (dry) regions, and to be distributed periodically in two dimensions. The dry regions of the fracture form a barrier to moisture flow through the geologic medium. An idealized two‐dimensional model that maximizes the barrier effect of the fractures is analyzed. The results of the analysis quantify the effect of the dry regions of the fractures on global water flow through the fractured medium. An apparent conductivity is determined such that the fractured system can be replaced by a homogeneous medium for describing steady unsaturated flow. An asymptotic analysis yields an analytic expression for the apparent hydraulic conductivity through such a system in the limit of small sorptive number (fracture spacing divided by a characteristic capillary suction) for the intact matrix material. The apparent hydraulic conductivity for arbitrary spacing and sorptive number is determined by numerical means. The numerical model accounts for variable hydraulic conductivity as a function of the local pressure head, whereas the asymptotic solution represents the limit of constant conductivity. The numerical results confirm the analytical solution as a lower bound on the apparent hydraulic conductivity.
Irrigation management and investment under saline, limited drainage conditions: 3. Policy analysis and extensions
Authors: Knapp Keith C.
Published: February 2004

Water demand is characterized for the cotton‐cotton‐tomatoes rotation considered in the previous paper (Knapp, this issue (b)). Demand is found to be price inelastic. However, currently low water prices imply that fairly moderate increases in water prices can result in large water savings. A marginal cost curve for source control of drain water emissions is constructed. Moderate increases in drainage emissions fees can result in large reductions in drain water emissions with relatively small impacts on income. Management response and income effects from increased water salinity are estimated. The model is also extended to consider drain water reuse. Typical optimal management with reuse is low‐volume, low‐quality water on first‐year cotton, improved quality and quantity on second‐year cotton, and the highest quality water on salt‐sensitive tomatoes. An approach to maximization of multifield farm‐level returns is proposed using decentralized pricing and the field‐level optimization model.
Prediction in partial duration series with generalized pareto‐distributed exceedances
Authors: Rosbjerg Dan , Madsen Henrik , Rasmussen Peter Funder
Published: February 2004

As a generalization of the common assumption of exponential distribution of the exceedances in partial duration series the generalized Pareto distribution has been adopted. Estimators for the parameters are presented using estimation by both method of moments and probability‐weighted moments. The corresponding estimators for the T‐year event are given and approximate expressions for bias and variance of the estimators are derived in both cases. Using the mean square error of the T‐year event estimator as a performance index it is shown that the method of moments is preferable to the probability‐weighted moments. Maintaining the generalized Pareto distribution as the parent exceedance distribution the T‐year event is estimated assuming the exceedances to be exponentially distributed. For moderately long‐tailed exceedance distributions and small to moderate sample sizes it is found, by comparing mean square errors of the T‐year event estimators, that the exponential distribution is preferable to the correct generalized Pareto distribution despite the introduced model error and despite a possible rejection of the exponential hypothesis by a test of significance. For moderately short‐tailed exceedance distributions (with physically justified upper limit) the correct exceedance distribution should be applied despite a possible acceptance of the exponential assumption by a test of significance.
Field study of dispersion in a heterogeneous aquifer: 2. Spatial moments analysis
Authors: Adams E. Eric , Gelhar Lynn W.
Published: February 2004

Analysis is performed of a 20‐month natural gradient tracer study in the saturated zone of a highly heterogeneous aquifer. Graphical presentation of concentration distributions versus time and spatial moments analysis reveal dramatically non‐Gaussian behavior and a systematic mass loss. Implications of the mass loss on plume moments is analyzed through sensitivity studies. The moments data are interpreted by applying two simple models: (1) pure advection from a continuous source, and (2) advection plus dispersion in a converging nonuniform flow field. A longitudinal dispersivity of 5–10 m is estimated from the latter model and is somewhat larger than the value of about 1.5 m calculated by Rehfeldt et al. (this issue) using the stochastic theory of Gelhar and Axness (1983) based on independent measurements of the spatial variation of hydraulic conductivity. The dispersivity of 5–10 m is an order of magnitude larger than values measured at recently studied field sites (Borden and Cape Cod) with less heterogeneity, but an order of magnitude lower than would be computed from the moments data if the flow is presumed to be uniform.
A three‐dimensional transport model study of PSC processing during EASOE
Authors: Chipperfield M. P. , Cariolle D. , Simon P.
Published: February 2004

A 3D transport model has been used to investigate the extent of processing of air by polar stratospheric clouds (PSCs) during the northern winter 1991–92 and the potential for subsequent ozone destruction. The model is forced using the winds and temperatures from the European Centre for Medium Range Weather Forecasts analyses and the transport of tracers is performed using a non‐diffusive scheme, which is important when dealing with localised regions of PSCs. Model experiments have been performed from late November 1991 to the end of February 1992 with tracers introduced in the model to indicate the amount of air which has been processed by PSCs and the amount of time that this perturbed air has subsequently been exposed to sunlight. In the model experiments PSC processing started sporadically in December. By early January air in the polar lower stratosphere had been extensively processed. The occurrence of large‐scale PSCs terminated with a minor warming in January. The inferred chemical ozone loss in the model experiments, while sensitive to some assumed parameters, is small and limited by the availability of sunlight.
A 3D transport model study of chlorine activation during EASOE
Authors: Chipperfield M. P. , Cariolle D. , Simon P.
Published: February 2004

A 3D chemical transport model (CTM) has been used to study the behaviour of stratospheric trace species during the EASOE campaign. The model contains a comprehensive gas phase chemistry scheme as well as a treatment of heterogeneous reactions on PSCs and sulphate aerosols. The CTM is forced using the ECMWF analyses providing realistic meteorological conditions throughout the model simulations. Experiments have been performed to examine the evolution of chlorine species throughout the winter and to estimate the magnitude of chemical ozone loss. Heterogeneous reactions on PSCs lead to large (over 1.5ppbv) abundances of active chlorine in the model polar lower stratosphere in early January. The level of active chlorine then decreases from mid January as PSCs become less frequent. In the model PSCs are more efficient at activating chlorine than the sulphate aerosols although the latter cause extensive denoxification which maintains the high ClOx levels after processing. Despite the high abundances of ClO and BrO the model results show that the opportunity for chemical ozone destruction during this time was limited by the lack of sunlight.
Ground‐based FTIR measurements of vertical column densities of several trace gases above Spitsbergen
Authors: Notholt J. , Schrems O.
Published: February 2004

During the EASOE campaign ground‐based FTIR measurements have been performed in March 1992 at Ny‐Ålesund (Spitsbergen, 79°N, 12°E) to derive column amounts of several trace gases. For the first part of the measurement campaign Ny‐Ålesund was situated inside the polar vortex.

The obtained concentrations of N2O, CH4 and HF inside the vortex are consistent with subsidence. The ratio of HClstrat/HF varied from about 2.0 inside to about 2.8 outside the vortex. Inside the vortex low values for NO2 and high values for HNO3 were found. The O3 concentrations inside the vortex are slightly lower than what was observed outside the vortex.
Magnetic interpretation along the Sudbury Structure Lithoprobe Transect
Authors: Hearst Robert , Morris William , Thomas Michael
Published: February 2004

Magnetic modelling of the Sudbury Structure (SS) has been undertaken using the geometrical and lithological framework provided by high‐resolution seismic reflection data. This initial constraint was required because of the complex magnetic properties exhibited by rock units of the SS. Locally remanent magnetisation (NRM) dominates over induced magnetisation, and the orientation and amplitude of the NRM vector varies both between and within individual rock units. The use of measured magnetic susceptibility and NRM values afforded an important constraint in the modelling process, and provided a valuable insight into the genesis of the various magnetic anomalies.

The magnetic anomaly profile along the Lithoprobe Transect across the SS appears to arise from three main sources: a) a broad regional magnetic anomaly ascribed to a more magnetic layer of the Levack Gneiss Complex subjacent to the base of the Sudbury Igneous Complex (SIC); b) a prominent magnetic high associated with the southern contact between the Onwatin and Onaping Formations, believed to reflect a zone of hydrothermal mineralisation; and c) a magnetic high associated with the southern contact between the SIC and the Huronian mafics is related to the juxtaposition of rock units produced by northward directed thrusting and an enhanced NRM signature in the basal unit of the South Range Norite.
A geomagnetic chronology for antarctic ice accumulation
Authors: Mazaud Alain , Laj Carlo , Bender Michael
Published: February 2004

The curve of 10Be flux versus age in the Vostok ice core largely covaries with a synthetic curve calculated assuming that a large fraction of 10Be production rate is modulated by changes in the geomagnetic field intensity. With this assumption, it is also possible to derive a precise chronology for the Vostok core. This chronology is in good agreement with a very recent glaciological chronology obtained for Vostok ice core, and improves the correspondence of climate events recorded in Antarctic ice with correlative events recorded in deep sea sediments. Therefore, the idea that changes in the 10Be flux at Vostok may arise from variations in Earth's geomagnetic dipole field has to be seriously considered. This would imply that a large portion of the 10Be deposited there is derived from lower latitudes.