bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic operation of a surface water resources system
Authors: Peng Cheng‐shuang , Buras Nathan
Published: February 2004

The management of storage facilities (reservoirs, lakes, and ponds) is an important element of the utilization of a surface water resources system. Dynamic programming (DP) techniques have been used extensively in the analysis and resolution of reservoir systems operation. A major drawback of DP for a multireservoir system is the “curse of dimensionality.” To overcome this problem, this study developed a nonlinear programming (NLP) solution scheme which also integrated the objectives of several stakeholders, objectives which were noncommensurate. The NLP model employed a time series model to generate synthetic inflows into the storage facilities in order to simulate future operations of the system. Two sets of simulations were run, and a statistical test was used to terminate them. The expected values of the decision variables were tabulated and plotted, as well as their standard deviations and probability distributions. The model allows decision makers to adjust operation targets for different alternative operation scenarios.
Kinematic structure of minipermeameter flow
Authors: Tartakovsky Daniel M. , Moulton J. David , Zlotnik Vitaly A.
Published: February 2004

Minipermeameters are rapidly becoming a popular tool for collecting localized measurements of permeability in both laboratory and field studies. While one of the main advantages of minipermeameters is their ability to collect data on various support volumes, there have been only limited attempts to analyze their size and geometry. We define the support volume of minipermeameter measurements as a region containing 90% of the total gas flow, i.e., a region bounded by the 10% streamline. Using our new semianalytical solutions for the Stokes' stream function, we demonstrate that the support volume has a shape of the semitoroid adjacent to the sample surface. Hence there is a blind spot directly below the minipermeameter, which is not probed by the measurement. We demonstrate that the support volume of the minipermeameter measurements decreases with the tip‐seal's ratio (a ratio of the inner tip‐seal radius to the outer tip‐seal radius), while the size of the corresponding blind spot increases.
Magnitude and timing of annual maximum floods: Trends and large‐scale climatic associations for the Blacksmith Fork River, Utah
Authors: Jain Shaleen , Lall Upmanu
Published: February 2004

The magnitude and timing of spring snowmelt floods reflects seasonal snow accumulation and spring temperature patterns. Consequently, interannual variations in regions such as the intermountain West, with snowmelt annual maximum floods, may be related to low‐frequency variations in winter/spring large‐scale climate variability. Changes in the seasonality of basin precipitation and temperature consequent to slow changes in the baseline climate state (e.g., owing to natural climate variations and/or potential global warming trends) may have significant impacts on such floods. A case study of the Blacksmith Fork River in northern Utah that explores such a hypothesis is presented here. Trends and associations in the magnitude and timing of annual maximum floods are documented, their impact on time‐varying estimates of the 100 year flood is assessed, and relationships with known large‐scale, quasi‐oscillatory patterns of climate variability are explored. Evidence for structured low‐frequency variation in flood timing and magnitude and its relation to winter/spring precipitation and temperature and to tropical (El Niño– Southern Oscillation) and extratropical (Pacific Decadal Oscillation) Pacific climate precursors is presented. Mechanisms for these ocean‐atmosphere teleconnections to basin precipitation, temperature, and flood potential are discussed.
Geoelectric characterization of the hydraulic conductivity field and its spatial structure at variable scales
Authors: Purvance David T. , Andricevic Roko
Published: February 2004

This paper describes how to begin with extensive geoelectric measurements of an aquifer along with a few hydraulic measurements and end with an estimate of the aquifer's hydraulic conductivity field and spectrum at variable scales. It is based on a preponderance of theoretical and empirical evidence indicating that electrical and hydraulic (eh) conductivities of aquifers are linearly correlated on a bilogarithmic scale. The steps required for a geoelectric characterization are scaling up, calibration, and spectrum estimation. Scaling up estimates electrical conductivity at variable scales from the apparent resistivity measurement. Calibration equilibrates coincident, equal‐scale eh conductivities using the theoretical eh conductivity correlation. Combined, scaling up and calibration provide a variable‐scale expression for the hydraulic conductivity field in terms of a geoelectric measurement. From this expression the hydraulic conductivity field spectrum is calculated. These steps are applied to borehole data collected at the Department of Energy Central Nevada Testing Area and the U.S. Geological Survey Cape Cod site. In each case a statistically significant eh conductivity correlation is observed. The paper ends by conjecturing that a horizontal exploitation of the eh conductivity correlation offers for the first time an inexpensive means of characterizing the hydraulic conductivity field at a high resolution and over a large area.
Temporal moment analysis of tracer discharge in streams: Combined effect of physicochemical mass transfer and morphology
Authors: Gupta A. , Cvetkovic V.
Published: February 2004

In environmental applications it is often of interest to predict the rates at which contaminant mass is discharged at a given cross section of streams and rivers. We present a Lagrangian methodology for evaluating tracer discharge (mass per unit time) at specified control cross sections (CCS) of streams. The key transport processes included in the analysis are advection, degradation/decay, and kinetically controlled mass transfer in storage zones and in bed sediment. The transport in the bed sediment is described as a diffusion process, where the tracer may sorb onto the sediment. We have derived a general solution for tracer discharge in the Laplace domain wherefrom temporal moments are computed. The derived solutions may account for deterministic changes in morphological characteristics along stretches of streams. The results are illustrated for zeroth and first two moments where we show the combined effect of advection, degradation, physicochemical mass transfer, and morphology. For illustrative purposes, we assume morphology to change downstream following power laws suggested by Langbein [1947] and Leopold and Maddock [1953]. The moments depend nonlinearly on the downstream distance, following power laws that reflect the power laws for the hydraulic geometry. We define two main dimensionless parameters, namely, kinetic storage parameter α* and bed parameter M, that control the amount of tracer mass eventually discharged at any given CCS. For M ≈≥ 0.3 the most dominant mechanism that controls the amount of ultimately discharged tracer mass is the exchange with the bed sediment. Once estimated from field data for stretches of specified streams, the dimensionless parameters can be used in the derived expressions for predictive purposes.
Risk‐based in situ bioremediation design using a noisy genetic algorithm
Authors: Smalley J. Bryan , Minsker Barbara S. , Goldberg David E.
Published: February 2004

Risk‐based corrective action (RBCA) is rapidly becoming the method of choice for remediating contaminated groundwater. In this paper, a management model is presented that simultaneously predicts risk and proposes cost‐effective options for reducing risk to acceptable levels under conditions of uncertainty. The model combines a noisy genetic algorithm with a numerical fate and transport model and an exposure and risk assessment model. The noisy genetic algorithm uses sampling from parameter distributions to assess the performance of candidate designs. Results from an application to a site from the literature show that the noisy genetic algorithm is capable of identifying highly reliable designs from a small number of samples, a significant advantage for computationally intensive groundwater management models. For the site considered, time‐dependent costs associated with monitoring and the remedial system were significant, illustrating the potential importance of allowing variable cleanup lengths and a realistic cost function.
A formula for predicting advection‐dispersion in the vadose zone at uneven drainage intervals
Authors: Bidwell Vincent J.
Published: February 2004

A computational method is presented for predicting one‐dimensional advective‐dispersive solute transport with linear equilibrium sorption, for complex solute input histories associated with uneven intervals of transporting water flux. The suggested application is monitoring of leached solutes from land use and near‐surface transformations, which are assumed to be transported through the vadose zone to the underlying groundwater without further degradation. Real‐time forecasts of solute concentration at the groundwater surface provide a smooth feedback signal to operational management of land use for protection of groundwater, as a surrogate for groundwater monitoring which has unacceptable transport lag. The conceptual basis is the series of mixing cells as an analogue for advection‐dispersion, in the form of a linear system for which cumulative pore‐water drainage is the continuous index. This mixing‐cell formula is unconditionally stable for any interval of drainage and assures conservation of solute mass. A demonstration example is presented.
Testing a full‐range soil‐water retention function in modeling water potential and temperature
Authors: Andraski B. J. , Jacobson Elizabeth A.
Published: February 2004

Recent work has emphasized development of full‐range water‐retention functions that are applicable under both wet and dry soil conditions, but evaluation of such functions in numerical modeling has been limited. Here we show that simulations using the Rossi‐Nimmo (RN) full‐range function compared favorably with those using the common Brooks‐Corey function and that the RN function can improve prediction of water potentials in near‐surface soil, particularly under dry conditions. Simulations using the RN function also improved prediction of temperatures throughout the soil profile. Such improvements could be important for calculations of liquid and vapor flow in near‐surface soils and in deep unsaturated zones of arid and semiarid regions.
Modeling of rainfall time series and extremes using bounded random cascades and levy‐stable distributions
Authors: Menabde Merab , Sivapalan Murugesu
Published: February 2004

A new model for simulation of rainfall time series is proposed. It is shown that both the intensity and duration of individual rainfall events can be best modeled by a “fat‐tailed” Levy‐stable distribution. The temporal downscaling of individual events is produced by a new type of a bounded random cascade model. The proposed rainfall model is shown to successfully reproduce the statistical behavior of individual storms as well as, and in particular, the statistical behavior of annual maxima. In contrast, a model based on a gamma distribution for rainfall intensity substantially underestimates the absolute values of extreme events and does not correctly reproduce their scaling behavior. Similarly, a model based on self‐similar random cascade (as opposed to the bounded cascade) substantially overestimates the extreme events.
Thermal mediation in a natural littoral wetland: Measurements and modeling
Authors: Andradóttir Hrund Ó. , Nepf Heidi M.
Published: February 2004

As a river flows through shallow littoral regions such as wetlands, forebays, and side arms, the temperature of the water is modified through atmospheric heat exchange. This process, which we call thermal mediation, can control the initial fate of river‐borne nutrient and contaminant fluxes within a lake or reservoir. This paper presents temperature observations that demonstrate the occurrence of thermal mediation and directly support the theoretical results derived by Andradóttir and Nepf [2000]. The measurements show that the wetland warms the river inflow by approximately 1–3°C during summer and fall nonstorm conditions. Less thermal mediation occurs during storms, both because the residence time is significantly reduced and because the wetland circulation shifts from laterally well mixed (low flows) to short‐circuiting (storms). The dead‐zone model can simulate both these regimes and the transition between the regimes and is therefore a good choice for wetland modeling.
A simple surface temperature assimilation scheme for use in land surface models
Authors: Lakshmi Venkataraman
Published: February 2004

This paper examines the utilization of surface temperature as a variable which can be assimilated in off‐line land surface hydrological models. The connection between the surface temperature and evapotranspiration is utilized in making adjustments to the model‐computed surface soil moisture. This adjustment is a function of the difference between the model‐computed and the observed surface temperature. Comparisons between the model‐computed and satellite‐observed surface temperatures have been carried out. The assimilation of surface temperature is carried out twice a day (corresponding to the A.M. and P.M. overpass of the NOAA 10) over the Red‐Arkansas basin in the southwestern United States (31°50′N‐36°N, 94°30′W‐104°30′W) for a period of 1 year (August 1987 to July 1988). The soil moisture estimates resulting from the assimilation of surface temperature have a closer agreement with the values derived from the special sensor microwave imager than those from simulations without surface temperature assimilation. Assimilation reduces the effect of errors in precipitation and/or shortwave radiation on simulated soil moistures.
Hydraulic characterization and upscaling of fracture networks based on multiple‐scale well test data
Authors: Niemi Auli , Kontio Kimmo , Kuusela‐Lahtinen Auli , Poteri Antti
Published: February 2004

Hydraulic properties and upscaling characteristics of low‐permeability fractured rock are analyzed based on systematic well test data from three different measurement scales. First, tests are simulated in a large number of geological fracture network realizations, and the acceptable fracture transmissivity distribution parameters that produce the observed statistics of the two smallest measurement scales, i.e., 2‐m and 10‐m scales, are defined. Instead of a single value, a range of acceptable parameter values can be found to produce the observed result. Second, upscaling simulations are carried out with the calibrated networks. These indicate that the investigated system cannot be properly modeled by means of a continuum tensor presentation but would better be represented by means of “equivalent fracture” statistics. Third, the conductive characteristics of the calibrated 30‐m network blocks are compared to well test results from the same scale. The results from this preliminary analysis indicate that one‐dimensional borehole observations interpreted with standard continuum‐based methods may considerably underestimate the three‐dimensional conductive characteristics of heterogeneous, noncontinuum fractured media.
Calibration of a two‐dimensional finite element flood flow model using satellite radar imagery
Authors: Horritt Matthew S.
Published: February 2004

The application of numerical models of free surface flow to fluvial flood prediction is currently hampered by the lack of distributed calibration and validation data. We address this shortcoming by the use of satellite‐borne synthetic aperture radar imagery to map a flood on a 15‐km reach of the river Thames, England. A finite element numerical model of shallow water flow is constructed over the reach, and analysis shows that floodplain friction is the dominant factor affecting inundation extent when compared to channel friction, turbulence parameterization, and downstream boundary conditions. The area of the domain correctly classified by the model is maximized with the constraint of uniform floodplain friction (77% at maximum), compared with 70% for a simple planar model of the water free surface. A simple distributed calibration scheme is also tested, with a minor improvement over the uniform parameterization.
Toward improved calibration of hydrologic models: Combining the strengths of manual and automatic methods
Authors: Boyle Douglas P. , Gupta Hoshin V. , Sorooshian Soroosh
Published: February 2004

Automatic methods for model calibration seek to take advantage of the speed and power of digital computers, while being objective and relatively easy to implement. However, they do not provide parameter estimates and hydrograph simulations that are considered acceptable by the hydrologists responsible for operational forecasting and have therefore not entered into widespread use. In contrast, the manual approach which has been developed and refined over the years to result in excellent model calibrations is complicated and highly labor‐intensive, and the expertise acquired by one individual with a specific model is not easily transferred to another person (or model). In this paper, we propose a hybrid approach that combines the strengths of each. A multicriteria formulation is used to “model” the evaluation techniques and strategies used in manual calibration, and the resulting optimization problem is solved by means of a computerized algorithm. The new approach provides a stronger test of model performance than methods that use a single overall statistic to aggregate model errors over a large range of hydrologie behaviors. The power of the new approach is illustrated by means of a case study using the Sacramento Soil Moisture Accounting model.
Multifractal scaling of the intrinsic permeability
Authors: Boufadel Michel C. , Lu Silong , Molz Fred J. , Lavallee Daniel
Published: February 2004

Existing fractal studies dealing with subsurface heterogeneity treat the logarithm of the permeability K as the variable of concern. We treat K as a multifractal and investigate its scaling and fractality using measured horizontal K data from two locations in the United States. The first data set was from a shoreline sandstone near Coalinga, California, and the second was from an eolian sandstone [Goggin, 1988]. By applying spectral analyses and computing the scaling of moments of various orders (using the double trace moment method [Lavallee, 1991; Lavallee et al., 1992]), we found that K is multiscaling (i.e., scaling and multifractal). We also found that the so‐called universal multifractal (UM) [Schertzer and Lovejoy, 1987] model (essentially a log‐Levy multifractal), was able to reproduce the multiscaling behavior reasonably well. The UM model has three parameters: α, σ, and H, representing the multifractality index, the codimension of the mean field, and the “distance” to stationary multifractal, respectively. We found (α = 1.7, σ = 0.23, H = 0.22) and (α = 1.6, σ = 0.11, H = 0.075) for the shoreline and eolian data sets, respectively. The fact that α values were less than 2 indicates that the underlying statistics are non‐Gaussian. We generated stationary and nonstationary multifractals and illustrated the role of the UM parameters on simulated fields. Studies that treated Log K as the variable of concern have pointed out the necessity for large data records, especially when the underlying distribution is Levy‐stable. Our investigation revealed that even larger data records are required when treating K as a multifractal, because Log K is less intermittent (or irregular) than K.
Multicontinuum description of flow in composite heterogeneous media
Authors: Shvidler Mark , Karasaki Kenzi
Published: February 2004

Adequacy of the description of flow and transport processes in subsurface depends on how well a model represents the heterogeneity. One of the simplest models to describe the heterogeneity structure is a so‐called composite system. Flow and transport simulation in composite systems can be reduced to solving equations and averaging the solutions. A different approach related to averaging the equations leads to new equations. This description is designated as monocontinuum description. If the homogeneous components of a composite system, so‐called phases, have different hydrodynamic and/or geometric parameters, it is natural to study averaging on the individual phases along with the global averaging. This approach takes into consideration the mean fields in the individual continuum phase as well as the cross flows and cross forces between continua. However, this description is nonclosed. To overcome this difficulty, the phenomenological theory usually postulates a special interaction mechanism for closing the equations. This paper presents the exact equations of mass balance and moment balance for each phase. The exact sense of exchange terms in the multicontinuum models is explained. We demonstrate that joint consideration of the monocontinual and multicontinual systems in the case of two‐phase random composite leads to a closed description and that we can find the exchange terms. For a periodic composite system the same approach leads to a closed description for any number of phases. The terms describing the interactions between continua for systems with a random and periodical structure are calculated. We examine the hypothesis customarily made in the phenomenological models.
A Modeling and experimental study of light nonaqueous phase liquid (LNAPL) accumulation in wells and LNAPL recovery from wells
Authors: Sleep Brent E. , Sehayek Lily , Chien Calvin C.
Published: February 2004

A formulation is presented for numerical modeling of light nonaqueous phase liquid (LNAPL) accumulation in observation wells and LNAPL removal from recovery wells. The formulation includes fluid gravity segregation, changing water and LNAPL levels in the well, and the corresponding changes in fluid storage in the well bore. The method was added to a three‐dimensional finite difference model for hysteretic multiphase flow. The model predictions are compared to three‐dimensional pilot scale experiments of LNAPL infiltration and recovery. Model predictions matched experimental results well, demonstrating the validity of the model formulation. Model results indicated that soil layering and lack of mechanical equilibrium had a significant influence on LNAPL thicknesses in wells. Thus empirical approaches based on assumptions of soil homogeneity and mechanical equilibrium would not have been appropriate for prediction of LNAPL volume from toluene thicknesses measured in wells.
Hack's Law: Sinuosity, convexity, elongation
Authors: Willemin James H.
Published: February 2004

Hack's law, an empirical, power law relationship between drainage basin area and the length of the main stream channel, has long been taken to imply that drainage basins become more elongate (relatively longer and narrower) with increasing basin size. A study of the geometry of 38 basins from three distinct geomorphic settings shows that this geometric interpretation of Hack's law is only occasionally true: Even though Hack's power law relationship holds between basin area and main channel length, these basins do not necessarily become more elongate with increasing size. Rather, Hack's law is an expression of a balance between changes in basin shape and changes in channel planform geometry. For the basins in this study, changes in channel sinuosity play the most important role in this balance; changes in basin shape are far less regular. Local conditions appear to determine the partitioning of importance between changes in basin shape and channel sinuosity.
New geometry factors for hydraulic property‐based soil solution electrical conductivity models
Authors: Das Bhabani S. , Wraith Jon M.
Published: February 2004

Improved methods are needed to intensively measure ionic solutes in soils. The concentration Ci of ionic solutes in soils is directly proportional to soil solution electrical conductivity σw. Time domain reflectometry (TDR) measures both soil water content θ and bulk soil electrical conductivity σa using the same probes. However, physical/conceptual models are required along with TDR measurements in order to use TDR for in situ estimates of Ci. We discuss a modeling approach [Mualem and Friedman, 1991] based on assumed analogy between tortuosity of electrical and hydraulic flow paths in variably saturated soils. We review the model approach, then derive a general expression for a pore geometry factor FG considering flow of electrical current through randomly distributed capillary soil pores. Two FG are derived based on two conceptual considerations of tortuous capillary length. Four water retention models (WRM) are used to describe soil hydraulic properties in the FG. When fitted to the same measured water retention data, the four WRMs provided substantially different magnitudes for FG. The model was then compared in terms of σw estimates using the two new FG in combination with field‐measured θ and σa. One of the new FG produced smaller estimated σw than did that proposed in the original model. This is desirable based on our own and several other published comparisons that indicated the original model may overestimate σw in comparison with independent measurements.
Designing a competent simple genetic algorithm for search and optimization
Authors: Reed Patrick , Minsker Barbara , Goldberg David E.
Published: February 2004

Simple genetic algorithms have been used to solve many water resources problems, but specifying the parameters that control how adaptive search is performed can be a difficult and time‐consuming trial‐and‐error process. However, theoretical relationships for population sizing and timescale analysis have been developed that can provide pragmatic tools for vastly limiting the number of parameter combinations that must be considered. The purpose of this technical note is to summarize these relationships for the water resources community and to illustrate their practical utility in a long‐term groundwater monitoring design application. These relationships, which model the effects of the primary operators of a simple genetic algorithm (selection, recombination, and mutation), provide a highly efficient method for ensuring convergence to near‐optimal or optimal solutions. Application of the method to a monitoring design test case identified robust parameter values using only three trial runs.
Cost‐effective long‐term groundwater monitoring design using a genetic algorithm and global mass interpolation
Authors: Reed Patrick , Minsker Barbara , Valocchi Albert J.
Published: February 2004

A new methodology for sampling plan design has been developed to reduce the costs associated with long‐term monitoring of sites with groundwater contamination. The method combines a fate‐and‐transport model, plume interpolation, and a genetic algorithm to identify cost‐effective sampling plans that accurately quantify the total mass of dissolved contaminant. The plume interpolation methods considered were inverse‐distance weighting, ordinary kriging, and a hybrid method that combines the two approaches. Application of the methodology to Hill Air Force Base indicated that sampling costs could be reduced by as much as 60% without significant loss in accuracy of the global mass estimates. Inverse‐distance weighting was shown to be most effective as a screening tool for evaluating whether more comprehensive geostatistical modeling is warranted. The hybrid method was effective for implementing such a tiered approach, reducing computational time by more than 60% relative to kriging alone.
87Sr/86Sr as a tracer of groundwater discharge and precipitation recharge in the Glacial Lake Agassiz Peatlands, northern Minnesota
Authors: Hogan James F. , Blum Joel D. , Siegel Donald I. , Glaser Paul H.
Published: February 2004

A contrast in the 87Sr/86Sr ratio of atmospherically derived Sr (∼0.710) and Sr from regional bedrock (>0.730) allows quantification of the proportions of precipitation recharge and groundwater discharge into peat columns of bogs and fens in the Glacial Lake Agassiz Peatlands. Two contrasting bog‐fen pairs were studied: the Lost River peatland located over a regional groundwater discharge zone and the Red Lake peatland situated atop a hydrologic divide where recharge is expected to occur. Bog pore waters consist of 90–100% precipitation in the upper 2 m, below which they mix with groundwater. Adjacent fens consist mostly of groundwater, ranging from ∼30% near the surface to ∼100% with depth. At the Red Lake peatland, mixing relations suggest an additional Sr source that may be peat mineralization. Distinct 87Sr/86Sr and [Ca]/[Sr] ratios for groundwater discharging into bog and fen sites are indicative of distinct groundwater sources and are likely indicative of flow paths at two different length scales.
Variability of bed mobility in natural, gravel‐bed channels and adjustments to sediment load at local and reach scales
Authors: Lisle Thomas E. , Nelson Jonathan M. , Pitlick John , Madej Mary Ann , Barkett Brent L.
Published: February 2004

Local variations in boundary shear stress acting on bed‐surface particles control patterns of bed load transport and channel evolution during varying stream discharges. At the reach scale a channel adjusts to imposed water and sediment supply through mutual interactions among channel form, local grain size, and local flow dynamics that govern bed mobility. In order to explore these adjustments, we used a numerical flow model to examine relations between model‐predicted local boundary shear stress (тj( and measured surface particle size (D50) at bank‐full discharge in six gravel‐bed, alternate‐bar channels with widely differing annual sediment yields. Values of тj and D50 were poorly correlated such that small areas conveyed large proportions of the total bed load, especially in sediment‐poor channels with low mobility. Sediment‐rich channels had greater areas of full mobility; sediment‐poor channels had greater areas of partial mobility; and both types had significant areas that were essentially immobile. Two reach‐mean mobility parameters (Shields stress and Q*) correlated reasonably well with sediment supply. Values which can be practicably obtained from carefully measured mean hydraulic variables and particle size would provide first‐order assessments of bed mobility that would broadly distinguish the channels in this study according to their sediment yield and bed mobility.
Modes of seawater intrusion during transgressions
Authors: Kooi H. , Groen J. , Leijnse A.
Published: February 2004

Analytical methods and numerical experiments are used to study salinization of groundwater in response to sea level rise. The system that is studied involves a saturated porous medium with an inclined upper surface. The upper surface is progressively inundated during sea level rise to simulate transgression, the landward migration of the shoreline. Four “modes” of seawater intrusion are distinguished: (1) horizontal intrusion for slow transgression and a relatively high‐permeability (sand/silt) substrate, (2) vertical intrusion by seawater fingering for fast transgression and a sand/silt substrate, (3) vertical intrusion by diffusion for fast transgression and a low‐permeability (clay) substrate, (4) vertical intrusion by combined diffusion and low‐salinity fingering for fast transgression and a clay layer at the seafloor overlying an aquifer. These four modes are characterized by the development of very distinctive transition zones between the fresh and salt groundwater domains. An analytical expression is derived for the critical transgression rate which separates horizontal (mode 1) from dominantly vertical (modes 2–4) intrusion. For modes 3 and 4, salinization significantly lags behind sea level rise. The results are consistent with observations of fossil fresh/brackish groundwater beneath many continental shelves and shallow seas.
Cooling and ventilating the Abyssal Ocean
Authors: Orsi Alejandro H. , Jacobs Stanley S. , Gordon Arnold L. , Visbeck Martin
Published: February 2004

The abyssal ocean is filled with cold, dense waters that sink along the Antarctic continental slope and overflow sills that lie south of the Nordic Seas. Recent integrations of chlorofluorocarbon‐11 (CFC) measurements are similar in Antarctic Bottom Water (AABW) and in lower North Atlantic Deep Water (NADW), but Antarctic inputs are ≈ 2°C colder than their northern counterparts. This indicates comparable ventilation rates from both polar regions, and accounts for the Southern Ocean dominance over abyssal cooling. The decadal CFC‐based estimates of recent ventilation are consistent with other hydrographic observations and with longer‐term radiocarbon data, but not with hypotheses of a 20th‐century slowdown in the rate of AABW formation. Significant variability is not precluded by the available ocean measurements, however, and interannual to decadal changes are increasingly evident at high latitudes.
Adrift in the Beaufort Gyre: A model intercomparison
Authors: Steele M. , Ermold W. , Häkkinen S. , Holland D. , Holloway G. , Karcher M. , Kauker F. , Maslowski W. , Steiner N. , Zhang J.
Published: February 2004

Output from six regional sea ice‐ocean climate model simulations of the arctic seas is compared to investigate the models' ability to accurately reproduce the observed late winter mean sea surface salinity. The results indicate general agreement within the Nordic seas, strong differences on the arctic continental shelves, and the presence of a climate drift that leads to a high salinity bias in most models within the Beaufort Gyre. The latter is highly sensitive to the wind forcing and to the simulation of freshwater sources on the shelves and elsewhere.
The characteristic oscillation induced by coupled processes between oceanic vertical modes and atmospheric modes in the tropical Pacific
Authors: Yeh Sang‐Wook , Dewitte Boris , Jhun Jong‐Ghap , Kang In‐Sik
Published: February 2004

An intermediate coupled model (ICM) based on an ocean model with three vertical modes and a statistical atmosphere model was used to investigate the relationship between the dominant modes of variability in the ocean and the atmosphere in the Tropical Pacific. Tests on sensitivity to the oceanic vertical structure of the ocean show that the time scale of variability in the sea surface temperature anomaly (SSTA) mainly depends on the relative contribution of the baroclinic modes. The first two atmospheric modes, i.e. the central and ‘off‐equatorial‧ modes, play different roles in modulating the impact of oceanic baroclinic modes on SST changes through coupled processes. In the model, the central mode excites the second oceanic mode which drives the observations‐like interannual variability in the model. The atmospheric off‐equatorial mode, on the other hand, favors the gravest baroclinic mode through the forcing of Rossby waves and consequently shortens the frequency of SSTA variability in the tropical Pacific. This suggests a possible mechanism governing time modulation of the El Niño events characteristics in the tropical Pacific.
First seismic receiver functions on the Moon
Authors: Vinnik Lev , Chenet Hugues , Gagnepain‐Beyneix Jeannine , Lognonne Philippe
Published: February 2004

We applied the S receiver function technique [Farra and Vinnik, 2000] to the recordings of deep moon‐quakes at seismograph station Apollo 12 in order to detect phases converted (Sp) and reflected beneath the station. We detected Sp phases from the base of the surficial low‐velocity zone and from the mantle‐crust boundary. The average P velocity in the surficial layer 1 km thick should be a few times higher than in reference model [Toksoz et al., 1974]. The observed time, amplitude and waveform of Sp phase from the mantle‐crust boundary are close to those predicted by the reference model but with a modified surficial layer. The S wavetrains within the first 10 s may contain waves scattered in the mantle. This scattering is stronger than in the Earth at comparable depths. The polarized component in the coda waves that we observe is another previously unknown phenomenon.
Sea surface height changes in the North Atlantic Ocean related to the North Atlantic Oscillation
Authors: Esselborn Saskia , Eden Carsten
Published: February 2004

Interannual sea surface height (SSH) variability as measured by the Topex/Poseidon satellite altimeters is investigated for the North Atlantic Ocean between 1992 and 1998. The SSH variability exhibits a basin‐wide coherent dipole structure between the subtropical and the subpolar North Atlantic. The SSH dipole pattern changed sign between 1995 and 1996, coinciding with a change of sign of the North Atlantic Oscillation (NAO). The large‐scale SSH pattern is reproduced with an ocean general circulation model, and can be traced back to changes in the atmospheric forcing related to the NAO. The model reveals that the interannual SSH anomalies are mainly caused by changes in the oceanic heat transport which are connected with the response of the large‐scale ocean circulation to changes in the wind stress curl. Variations in the local heat flux reinforce these SSH anomalies but are of minor importance.
Effect of solar soft X‐rays on the lower ionosphere
Authors: Solomon Stanley C. , Bailey Scott M. , Woods Thomas N.
Published: February 2004

New measurements of solar irradiance in the soft X‐ray region of the spectrum from the SNOE satellite [Bailey et al., 2000] show that the full‐disk solar irradiance in the ∼2–20 nm region is higher than a standard model [ Hinteregger et al., 1981] by a factor of ∼4 at all levels of solar activity. This confirms contentions developed from several lines of evidence, [e.g., Richards and Torr, 1984; Richards et al., 1994] but to a larger degree than previously suspected. This finding has many implications for the thermosphere and ionosphere, two of which are examined here. Greatly improved agreement between measurement and model of the photoelectron spectrum is obtained when the solar soft X‐ray input is adjusted to the SNOE measurements. We also found good agreement between electron density profiles measured by the incoherent scatter radar at Millstone Hill [ Buonsanto et al., 1992] and our model, solving a fundamental problem identified in that work.
Iron in East Antarctic snow: Implications for atmospheric iron deposition and algal production in Antarctic waters
Authors: Edwards Ross , Sedwick Peter
Published: February 2004

To evaluate the deposition and solubility of aerosol iron in the Antarctic seasonal sea ice zone (SSIZ), iron was measured in snow samples collected from three areas in the SSIZ (Prydz Bay, Dumont d'Urville Sea and Ross Sea) and one continental area (Princess Elizabeth Land) of East Antarctica. Concentrations of total‐dissolvable iron (that soluble at pH ∼2) ranged from 20–2950 pg g−1, with the lowest concentrations measured in snow from the Dumont d'Urville Sea. Using estimates of snow accumulation rates, we calculate atmospheric iron deposition fluxes of 0.017–0.11 mg m−2 yr−1 (0.30–2.0 µmol m−2 yr−1), which are generally lower than previously published estimates. Measurements of iron in filtered meltwaters of snow samples from Prydz Bay and Princess Elizabeth Land suggest that ∼10–90% of the total atmospheric iron is readily soluble. Assuming our results to be broadly representative of atmospheric deposition over seasonally ice‐covered, high‐nutrient Antarctic waters, we use our mean estimates of atmospheric iron deposition (1.1 µmol m−2 yr−1) and solubility (32%) to calculate that atmospheric iron potentially supports annual phytoplankton production of 1.1 × 1012 mole C in the Antarctic SSIZ, which is less than 5% of the estimated total annual primary production in this ocean region.
Grape harvest dates for checking NAO paleoreconstructions
Authors: Souriau Annie , Yiou Pascal
Published: February 2004

Dates of the grape harvests in northeastern France and Switzerland exhibit significant correlations with the North Atlantic Oscillation (NAO), a large‐scale mode of climate variability over North Atlantic. These correlations, established for the monthly values of the NAO index over the last 175 years, reveal characteristic signatures as a function of frequency. Conversely, these signatures may be used to test the robustness of existing NAO paleoreconstructions, thanks to the availability of harvest dates for northwestern Europe since the end of the fifteenth century. Climatic signal appears prevailing over possible human factors in these series. The results demonstrate the importance of including old instrumental data and of providing monthly values rather than annual means in the NAO reconstructions. They suggest that the available long harvest dates series could constitute an interesting proxy to constrain such reconstructions.
Interaction between normal faults and fractures and fault scarp morphology
Authors: Hilley George E. , Arrowsmith J. Ramón , Amoroso Lee
Published: February 2004

Fault slip and geomorphic surface processes create and modify bedrock normal fault scarps. Our field studies and numerical models show that mechanical interaction between near‐surface fractures and active faults may diminish scarp slopes and broaden deformation near the surface. In our models, increasing fracture density and depth reduces and complicates the scarp's topographic expression. Fracture depth and density, and orientation of the structures control the location and magnitude of zones of positive and negative shear displacements along the fractures. Our field studies of the interaction of active graben bounding normal faults and fractures in Northern Arizona illustrate the mechanical analysis. Reduction in scarp slopes and broadening of the deformation field may cause incorrect interpretations of shallow bedrock scarp morphologies as old, rather than as the result of slip along fractures around the fault.
Severe chemical ozone loss inside the Arctic Polar Vortex during winter 1999–2000 Inferred from in situ airborne measurements
Authors: Richard E. C. , Aikin K. C. , Andrews A. E. , Daube B. C. , Gerbig C. , Wofsy S. C. , Romashkin P. A. , Hurst D. F. , Ray E. A. , Moore F. L. , Elkins J. W. , Deshler T. , Toon G. C.
Published: February 2004

Lower stratospheric in situ observations are used to quantify both the accumulated ozone loss and the ozone chemical loss rates in the Arctic polar vortex during the 1999–2000 winter. Multiple long‐lived trace gas correlations are used to identify parcels in the inner Arctic vortex whose chemical loss rates are unaffected by extra‐vortex intrusions. Ozone‐tracer correlations are then used to calculate ozone chemical loss rates. During the late winter the ozone chemical loss rate is found to be −46±6 (1σ) ppbv/day. By mid‐March 2000, the accumulated ozone chemical loss is 58±4% in the lower stratosphere near 450 K potential temperature (∼19 km altitude).
Historical and future trends of the Sahara Desert
Authors: Liu Ping , Washington Warren M. , Meehl Gerald A. , Wu Guoxiong , Potter Gerald L.
Published: February 2004

The Parallel Climate Model (PCM) Version 1.1 simulates a reasonable twentieth century climatology in the Sahara Desert. From late 1940s to the end of 1980s, the simulated Sahara Desert, bounded by the 50 mm mean annual rainfall isoline, becomes larger and shifts eastward. The model produces a decreasing rainfall trend while the surface temperature and meridional boundaries are almost stable. In the usual scenario with increasing greenhouse gases from the 1980s to the 2090s the Sahara becomes smaller, moves north and west and continues to dry. Both the size change and latitudinal shift show a century long trend. Compared to 1961–90 climatology, the average northward shift is around 1° and the surface temperature about 2.8°C warmer to the end of 21st century. The local greenhouse effect may cause such warming trend.
Mars Global Surveyor radio science electron density profiles : Neutral atmosphere implications
Authors: Bougher Stephen W. , Engel Steffi , Hinson David P. , Forbes Jeffrey M.
Published: February 2004

The Mars Global Surveyor (MGS) Radio Science (RS) experiment permits retrieval of electron density profiles versus height (∼90–200 km) from occultation measurements. An initial set of electron profiles is examined spanning high northern latitudes, early morning solar local times and high solar zenith angles (78 to 81°) near aphelion. Sampling for these 32‐profiles is well distributed over longitude. The height of the photochemically driven ionospheric peak is observed to respond to the background neutral density structure, with a mean height during this season at this location of ∼ 34.4 km. Strong wave‐3 oscillations about this mean are clearly observed as a function of longitude, and correspond to neutral density variations measured by the MGS Accelerometer (ACC) experiment. The wave‐3 tidal pattern implicated by both the RS and ACC datasets is consistent with a semi‐diurnal wave frequency. Clearly, the height of the martian dayside ionospheric peak is a sensitive indicator of the state of the underlying Mars atmosphere. This ionospheric peak height can be used as a proxy of the longitude specific non‐migrating tidal variations present in the Mars lower thermosphere.
Initial results from the IMAGE Extreme Ultraviolet Imager
Authors: Sandel Bill R. , King Robert A. , Forrester W. T. , Gallagher Dennis L. , Broadfoot A. Lyle , Curtis C. C.
Published: February 2004

The Extreme Ultraviolet Imager (EUV) of the IMAGE mission is providing the first global images of the He+ distribution in Earth's plasmasphere. The EUV instrument images the He+ resonance line emission at 30.4 nm at a time resolution of 10 minutes and a spatial resolution of 0.1 RE. Each image encompasses the entire plasmasphere in a single frame, to permit study of the structure and dynamics of the plasmasphere in its entirety. Here we survey several of the most striking features of the plasmasphere as seen by EUV. These features include convection tails, depleted regions that we call “voids,” isolated magnetic flux tubes filled to higher He+ density than their neighbors, and “shoulders” in the He+ distribution.
Chebyshev‐collocation method applied to solve ODEs in geophysics singular at the Earth center
Authors: Guo Jun‐Yi , Ning Jin‐Sheng , Zhang Fei‐Peng
Published: February 2004

The Chebyshev‐collocation method is briefly presented in a manner favorable for solving ODEs in geophysics depending on geocentric distance and singular at the Earth center. The main advantage of this method is that it is not needed to treat the ODEs particularly at the Earth center as is traditionally done when using the Runge‐Kutta method. Comparisons of spheroidal free oscillation periods and load Love numbers with earlier works show very close agreement.
Conductive crust imaged in western Turkey by MT
Authors: Bayrak M. , Nalbant S. S.
Published: February 2004

Wide‐band magnetotelluric (MT) data across the İzmir‐Ankara Suture Zone (İASZ), the Bornova Flysch Zone and the Menderes Massif of western Turkey images shallow and deep crustal structures. Inversions of the MT data reveals two subzones of the crust with varying thickness; the viscoelastic lower crust appears as a conductive zone (<75 ohm m), while the brittle upper crust is shown as a relatively high resistivity zone in the geoelectrical model. We also observe a very interesting twin‐core structure with a very high resistivity (>2000 ohm m) at a depth from 5 km to 25 km under the İASZ. A hypocentral cross‐section of earthquakes along the profile shows more dense seismic activity in and around the twin‐core structure. Due to this correspondence, we interpret that decoupling of the twin‐core structure could be responsible for the denser seismic activity around the İASZ. The depth of the lithosphere/asthenosphere boundary is rather shallow in the geoelectrical model, about 45 km at the SE and NW end of the profile, and 55 km in the middle.