bluuebunny/crossref_metadata_2025 huggingface dataset:
The finite layer method for groundwater flow models
Authors: Smith Stanley S. , Allen Myron B. , Puckett Jay , Edgar Thomas
Published: February 2004

The finite layer method (FLM) is an extension of the finite strip method familiar in structural engineering. The idea behind the method is to discretize two space dimensions using truncated Fourier series, approximating variations in the third via finite elements. The eigenfunctions used in the Fourier expansions are orthogonal, and, consequently, the Galerkin integrations decouple the weighted residual equations associated with different Fourier modes. The method therefore reduces three‐dimensional problems to sets of independent matrix equations that one can solve either sequentially on a microcomputer or concurrently on a parallel processor. The latter capability makes the method suitable for such computationally intensive applications as optimization and inverse problems. Four groundwater flow applications are presented to demonstrate the effectiveness of FLM as a forward solver.
An investigation of the validity of first‐order stochastic dispersion theories in isotropie porous media
Authors: Chin David A. , Wang Tiezheng
Published: February 2004

Monte Carlo simulations are used to (1) investigate the accuracy of approximations that are implicit in first‐order stochastic dispersion theories and (2) identify the accuracy limits of first‐order dispersion theories in isotropic porous media. The Fickian theory of Gelhar and Axness (1983), as well as the Fickian and non‐Fickian theories of Dagan (1984) and Neuman and Zhang (1990) are investigated. All Monte Carlo simulations are in three dimensions. Confidence limits of ensemble‐averaged Monte Carlo results in isotropic porous media are established for 0.1 ≤ σY ≤ 1.5. These results showed that first‐order theoretical estimates of the Eulerian velocity covariance function are quite accurate for σY < 1; theoretical estimates of the non‐Fickian longitudinal dispersivity do not deviate significantly from theory for at least σY ≤ 1.5; theoretical estimation of the transverse dispersivity is limited to σY < 1; and, the Fickian longitudinal dispersivity is overestimated by the theory of Gelhar and Axness (1983). Of all first‐order dispersion theories, the theory of Dagan (1984) is most robust in estimating the dispersivity tensor.
Modeling transport and biodegradation of benzene and toluene in sandy aquifer material: Comparisons With experimental measurements
Authors: Chen Yung‐Ming , Abriola Linda M. , Alvarez Pedro J. J. , Anid Paul J. , Vogel Timothy M.
Published: February 2004

A one‐dimensional numerical model is developed for simulating the biodegradation and transport of benzene and toluene in the subsurface environment. Modeled processes include mass exchange between the constituent phases (solid, liquid, gas, and biomass), advective and dispersive transport, and biotransformation, as well as microbial biomass production. Two substrates, two electron acceptors, one trace nutrient, and two microbial populations are modeled. Resulting governing equations include five nonlinear partial differential equations describing component transport in the bulk pore fluids, five nonlinear algebraic equations governing interphase mass exchange, and two ordinary differential equations governing microbial growth. These equations are solved through application of a Galerkin finite element method and a set iterative solution scheme. The utility and validity of the modeling approach is explored through comparisons with laboratory column experiments. Model parameters were estimated independently through laboratory batch experiments, aquifer slurry studies, or from the literature. Simulations are found to provide reasonable agreement with measurements of benzene and toluene biodegradation in saturated continuous‐flow columns packed with aquifer material. Sensitivity analyses and comparisons with column data suggest that model predictions are highly dependent upon the microbial parameters, particularly the initial active biomass concentration, the maximum specific substrate utilization rate, and the half‐saturation coefficient. The importance of the accurate estimation of these microbial parameters is emphasized.
Improving detention reservoir efficiency by a parallel drain design
Authors: Diskin M. H. , Kessler A.
Published: February 2004

In a previous paper it was established that a simple relationship exists between the peak reduction ratio and the relative storage ratio of unregulated detention reservoirs. The relationship, named the efficiency function of the reservoir, depends on the physical features of the reservoir and its outlet, and also on the shape of the inflow hydrograph. For a specified type of reservoir and a given inflow hydrograph, the efficiency function predicts the storage volume needed to produce a desired peak flow reduction. In this paper, it is demonstrated that it is possible to achieve the same peak flow reduction with a smaller reservoir if a parallel drain design is used. In this design procedure the inflow hydrograph is split into two parts. One part is routed through a detention reservoir and the second part is allowed to bypass the reservoir. The two parts are recombined below the reservoir to form the outlet hydrograph which is the sum of two hydrographs with staggered peaks. By a proper choice of the proportion of flow directed through the reservoir it is possible to achieve a desired peak flow reduction with a significant reduction in active storage.
Algorithms for solving Richards' equation for variably saturated soils
Authors: Kirkland M. R. , Hills R. G. , Wierenga P. J.
Published: February 2004

Previous work has shown that a one‐dimensional water content‐based finite difference algorithm can offer improved CPU efficiency in modeling infiltration into very dry heterogeneous soil when compared to similar pressure‐based algorithms. The usefulness of the water content‐based method is limited, however, since it cannot be applied to saturated soils. In this paper we develop two new methods which retain the advantages of water content‐based methods while still permitting fully saturated conditions. In the first method we define a new variable for the transformed Richards equation which has the characteristics of water content when soil is unsaturated and of pressure when soil is at or near saturation. In addition to the transformed Richards' equation method, an improved pressure‐based method which uses flux updating is presented. Both methods are implemented in algorithms using a preconditioned conjugate gradient method equation solver. The performance of the algorithms is compared to that of an algorithm based on a mixed form of Richards' equation using modified Picard iteration. Two test cases are examined. The first test case examines two‐dimensional infiltration into very dry heterogeneous soil which remains unsaturated throughout the simulation period. The second test case examines a two‐dimensional developing perched water table. Results indicate that the new methods retain the advantages of the water content‐based method for fully unsaturated heterogeneous problems while allowing for fully saturated conditions. For the unsaturated problem the new algorithms were from 16 to 59 times faster than the algorithm based on the mixed form of Richards' equation using modified Picard iteration. For the saturated problem the new algorithms were from 6 to 25 times faster than the mixed base algorithm. Results also indicate that the performance of the mixed base algorithm could be significantly improved if a more effective adaptive time step were developed.
Fat fractal scaling of drainage networks from a random spatial network model
Authors: Karlinger Michael R. , Troutman Brent M.
Published: February 2004

An alternative quantification of the scaling properties of river channel networks is explored using a spatial network model. Whereas scaling descriptions of drainage networks previously have been presented using a fractal analysis primarily of the channel lengths, we illustrate the scaling of the surface area of the channels defining the network pattern with an exponent which is independent of the fractal dimension but not of the fractal nature of the network. The methodology presented is a fat fractal analysis in which the drainage basin minus the channel area is considered the fat fractal. Random channel networks within a fixed basin area are generated on grids of different scales. The sample channel networks generated by the model have a common outlet of fixed width and a rule of upstream channel narrowing specified by a diameter branching exponent using hydraulic and geomorphologic principles. Scaling exponents are computed for each sample network on a given grid size and are regressed against network magnitude. Results indicate that the size of the exponents are related to magnitude of the networks and generally decrease as network magnitude increases. Cases showing differences in scaling exponents with like magnitudes suggest a direction of future work regarding other topologic basin characteristics as potential explanatory variables.
Estimation of finite difference interblock conductivities for simulation of infiltration into initially dry soils
Authors: Zaidel Jacob , Russo David
Published: February 2004

Simulations of transient, unsaturated vertical flow in different initially dry homogeneous soils using Richards' equation suggest that, for a relatively coarse numerical grid, there is no universal weighting scheme to estimate interblock conductivities capable of giving an accurate resolution of the resulting steep wetting fronts, except for the scheme associated with the Kirchhoff transformation (KT). The principal drawback of KT is that it can hardly be applied to heterogeneous soils. A new asymptotic weighting (AW) scheme, based on the asymptotic behavior of the hydraulic conductivity function in the vicinity of residual saturation, is proposed. Numerical tests demonstrate that the AW scheme describes wetting fronts in homogeneous soils nearly as accurately as a scheme based on KT. Applicability and robustness of the AW scheme for simulating water infiltration into heterogeneous soils having substantial vertical variation of hydraulic properties are also demonstrated. Extension of the proposed scheme to higher dimensions is straightforward.
The role of nitrate in the acidification of streams in the Catskill Mountains of New York
Authors: Murdoch Peter S. , Stoddard John L.
Published: February 2004

Research on the effects of acidic deposition in the United States has focused largely on the role of sulfur deposition in the acidification of surface waters. Results from both long‐term (up to 70 years) and recent monitoring of stream chemistry in the Catskill Mountains of New York indicate, however, that nitric acid has a significant and increasing role in surface water acidification that, during high‐flow periods, rivals the role of sulfuric acid. Nitrate increases with increased streamflow throughout the year except during the late summer, when biological activity and its attendant nitrogen uptake are greatest; peak concentrations as high as 128 μeq/L have been recorded during spring snowmelt. In contrast, SO42− concentrations decrease with increased flow. The ratio of NO3−:(NO3− + SO42−) in the seven U.S. Geological Survey/U.S. Environmental Protection Agency long‐term monitoring (LTM) streams ranged from 0.36 to 0.54 from August 1983 through February 1990. Seasonal Kendall tau tests for trends suggest a general decrease in SO42− concentrations and increase in NO3− concentrations in these streams during that period. Trend analysis of 19 large streams with periods of record from 23 to 70 years indicates that NO3− concentrations have been increasing in Catskill streams since 1970. The increasing trend appears to be primarily a result of increasing peak flow concentrations; base flow concentrations in the same large streams show only small changes over the period of record. Increases in base flow NO3− concentrations in headwater LTM streams have been larger, however. Trend information and seasonal patterns suggest that Catskill watersheds may be approaching nitrogen saturation.
Incorporating freeze‐thaw calculations into a water balance model
Authors: Fox John D.
Published: February 2004

Equations commonly used to estimate soil freeze‐thaw depths in nonpermafrost and permafrost terrain (Stefan/St. Paul equations) are combined with a daily snowpack/soil water balance model to investigate the effects of soil freezing and thawing on water balance components and the effects of climate, soil, vegetation, and topography on freeze‐thaw regimes. Sensitivity analysis indicates that soil moisture has a dominant influence on soil freezing and thawing, with distinctive effects offall soil moisture contents versus spring and summer inputs after partial soil thawing. The combined models displayed reasonable but sometimes surprising responses to specific sensitivity experiments involving temperature change, vegetation change, and topographic change. Attempts to reproduce measured freeze‐thaw regimes as reported in the literature were generally successful but limited by the short data record on most sites. An improved understanding and modeling of surface/air temperature relationships for unmodified and modified landscapes is identified as a critical need.
An experimental investigation of nonaqueous phase liquid dissolution in saturated subsurface systems: Steady state mass transfer rates
Authors: Powers Susan E. , Abriola Linda M. , Weber Walter J.
Published: February 2004

Results of an experimental investigation into steady state dissolution of nonaqueous phase liquids (NAPLs) entrapped within water saturated porous media are presented. The influence of porous media characteristics, NAPL type, and aqueous phase velocity on NAPL dissolution rates is explored through evaluation of a series of laboratory column studies. For many of the conditions tested, measured organic solute concentrations in the column effluents were below solubility limits, indicating nonequilibrium conditions. Entrapped NAPL distributions are examined and shown to depend upon porous media grading and mean grain size. Experimental results reveal a dependence of dissolution rate on the distribution pattern of entrapped NAPL, as well as upon aqueous phase velocity. A phenomenological model for the mass transfer process is developed which incorporates grain size parameters as surrogate measures of NAPL distribution. An additional set of column experiments, employing another porous medium and NAPL type, confirm the usefulness of the model as a predictor of steady state mass transfer rates in similar systems.
Inversion of the Borden Tracer Experiment data: Investigation of stochastic moment models
Authors: Woodbury Allan D. , Sudicky E. A.
Published: February 2004

Inversion of Dagan's two‐ and three‐dimensional stochastic models using Freyberg's (1986), Rajaram and Gelhar's (1988, 1991), and Barry and Sposito's (1990) moment data from the Borden experiment is carried out to examine (1) the validity of the two‐dimensional (Dagan, 1982) and three‐dimensional (Dagan, 1988) models and (2) the reduction in uncertainty of the spatial moments over nonconditioned estimates. A direct application of Bayesian statistical inference, in conjunction with Monte Carlo integration, is used to produce posterior probability density functions for the parameters. The parameter ranges from all methods show horizontal integral scales λ between 1.85 and 4.04 m, vertical integral scales λz between 0.144 and 0.459 m, and log hydraulic conductivities In (K) between 0.120 and 0.197. These results compare well to the earlier estimates of Woodbury and Sudicky (1991) and Robin et al. (1991). We show that moment prediction uncertainty is substantially reduced when both tracer moment data and prior estimates of the In (K) geostatistical parameters are incorporated into theoretical formulae based on stochastic dispersion theory.
Peak and off‐peak residential water demand
Authors: Lyman R. Ashley
Published: February 2004

Residential water demand is studied with microdata and with allowance for (1) seasonal differences in price elasticity, (2) a dynamic adjustment process, (3) a marginal price specification, (4) cross‐price effects between peak and off‐peak demand, and (5) the inclusion of a detailed set of household demographic variables including accurate measures of age distribution and household income. The findings are that the peak (summer) price elasticity of demand is more than twice the off‐peak elasticity; cross price effects are important at the 5% level of significance; variables measuring household income, property value, related property features, and age distribution are simultaneously significant; and finally, peak period adjustment rates are found to be less than off‐peak rates.
Hysteretic storativity concept for aquifer simulation
Authors: Stauffer Fritz , Franke Hans‐Joerg , Dracos Themistocles
Published: February 2004

The influence of the capillary zone on groundwater flow is usually taken into account by a constant storativity parameter in vertically integrated models of extended phreatic aquifers. Hysteresis of the water content characteristics may, however, significantly influence the response of phreatic aquifers in time‐dependent flow processes. For this reason a variable storativity concept is introduced. It is based on the hysteresis model of Mualem and is restricted to primary wetting or drying curves. Assuming a static pressure distribution in the vertical, storativity is a function of the position of the phreatic surface. The unsteady vertically integrated groundwater model thus becomes nonlinear. The model is checked with data from sandbox experiments. It accurately reproduces the time‐dependent flow influenced by hysteresis in the water content characteristics. Applications to unidirectional flow are presented in dimensionless form.
Well catchments and time‐of‐travel zones in aquifers with recharge
Authors: Lerner David N.
Published: February 2004

The classical analytical method used to map catchments and time‐of‐travel zones of pumping wells was originally derived for another purpose and takes no account of recharge (Bear and Jacobs, 1965). Generalized catchment shapes are presented here for aquifers with recharge. They can be scaled to field situations through two or three nondimensional parameters. A semianalytical path line tracing model was used to generate the shapes. It contains new expressions for pore water velocities in two infinite strip, recharged aquifers, one with two fixed head boundaries, the other with one impermeable boundary. The resulting catchments differ significantly in both shape and size from catchments estimated by the no‐recharge method. Generalized results are also presented for time‐of‐travel zones which are noticeably smaller than those calculated by the no‐recharge method. Comparison with an equivalent recently released model from the U.S. Environmental Protection Agency shows the latter to give poor estimates of catchments and time‐of‐travel zones.
Experimental investigation of dense solute plumes in an unconfined aquifer model
Authors: Oostrom M. , Dane J. H. , Güven O. , Hayworth J. S.
Published: February 2004

Unstable dense aqueous phase contaminant plumes may contaminate larger regions of an aquifer than stable plumes. To learn more about the behavior of variable density contaminant plumes, experiments were conducted in a flow container packed with homogeneous sand. Salt (NaI) solutions with different densities were introduced at different rates from a source, located on top of the sand, into the unconfined aquifer model where horizontal ambient groundwater streams were established with different horizontal velocities. Salt concentrations were measured with an automated, nonintrusive gamma radiation system. Results show that dense plumes were either stable or unstable. Important factors determining the stability of the dense plumes were the magnitude of the horizontal Darcy velocity and the relative density difference between the dense plume and the ambient groundwater. Longitudinal and transverse dispersivities, estimated from breakthrough curves and concentration profiles determined during the development of stable dense plumes, were small, indicating advection as the main mode of transport. Concentration profiles of stable dense plumes, taken along vertical transects, showed a rapid drop in relative concentration at the bottom of the plumes. When dense plumes became unstable, salt concentrations fluctuated considerably over time and concentration profiles of unstable plumes showed large mixing zones.
Impact of overland flow plane characterization on event simulations with a quasi‐physically based rainfall‐runoff model
Authors: Loague Keith
Published: February 2004

The performance of a quasi‐physically based rainfall‐runoff model is evaluated for a set of 72 events from a small upland catchment based upon two levels of characterization for overland flow plane geometry. In the first level of characterization there are 22 overland flow planes with average estimates of steady state infiltration rates based upon soil type. In the second level of characterization there are 145 overland flow planes centered upon the locations of steady state infiltration rate measurement sites. Model performance is also evaluated for four different estimates of antecedent soil water content for a subset of 39 events for the more detailed overland flow plane characterization. The results show only slight improvements in model performance for the more detailed characterization of overland flow plane geometry and soil hydraulic property information.
Partitioning solute transport between infiltration and overland flow under rainfall
Authors: Havis R. N. , Smith R. E. , Adrian D. D.
Published: February 2004

Solute transport from soil to overland flow is an important source of nonpoint pollution and was investigated through tracer studies in the laboratory and at an outdoor laboratory catchment. The depth of surface water interaction with soil, defined as the mixing zone is a useful value for approximate estimation of potential solute transport into surface water under rainfall. It was measured in the laboratory for a noninfiltration case (0.90 to 1.0 cm) and estimated through mass balance modeling for an infiltration case (0.52 and 0.73 cm). At an outdoor laboratory catchment, mixing zones were calculated through calibration of a numerical model that describes unsteady, uniform, infiltration and chemical transport. Overland flow was simulated using kinematic wave theory. Mixing zone depths ranged from 0.47 to 1.02 cm and were a linear function of rainfall intensity. Also, the fraction of solute present in the mixing zone at the time of ponding which was extracted into overland flow was a linear function of the initial soil moisture content. A steady state analytical approximation of the solute transport model was also developed which overpredicted solute transport into overland flow by 1 to 60%.
Climate and energy exchange at the snow surface in the Alpine Region of the Sierra Nevada: 1. Meteorological measurements and monitoring
Authors: Marks Danny , Dozier Jeff , Davis Robert E.
Published: February 2004

A detailed evaluation of climate conditions 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 are used to characterize the climate at four locations in the watershed during that snow season. Data from these locations are then combined into two representative sites for the watershed. Measurement approaches and methodologies and the effectiveness of instrumentation used in the study are discussed, and an estimate of the uncertainty of the monitored meteorological parameters is made. The data are integrated into a continuous hourly time series of solar and thermal radiation, air, snow and soil temperature, humidity, and wind at the two representative sites in this remote alpine watershed for an entire snow season. Snow deposition and snow cover depth and density are measured manually at regular intervals throughout the snow season. While problems were encountered monitoring air and snow surface temperature, humidity, and wind, because of the extreme conditions which are likely to occur in an alpine environment, radiation is easily monitored, and the estimated uncertainty of all measured parameters was acceptably low. This effort was required to develop a high quality time series of integrated climate data to evaluate the components of the energy balance of the snow cover during both deposition and ablation conditions.
Error analyses of simplified unsaturated flow models under large uncertainty in hydraulic properties
Authors: Govindaraju R. S. , Or D. , Kavvas M. L. , Rolston D. E. , Biggar J.
Published: February 2004

The functional relationships between soil hydraulic properties (K, θ and ψ) are needed for analyzing unsaturated water flow in soils and are generally of a nonlinear form. These relationships, estimated from field‐measured data, usually show large variability because of measurement errors and natural spatial heterogeneity exhibited by many field soils. A probabilistic approach is often employed to account for this uncertainty in parameters whereby the coefficients in the resulting partial differential equation are treated as random quantities (usually characterized by their moments). The objective of this analysis is to evaluate the errors resulting when simplified solutions based on simple forms of the hydraulic properties are used rather than nonlinear and numerically complicated models. As an example of a parametric functional relationship for K‐θ‐ψ, we adopt the Brooks‐Corey formulation in which a single parameter λ is considered as random to emulate the spread encountered in practice. For data that show a large enough spread and when one is interested in the mean behavior of K, θ or ψ, simple flow models such as the Green‐Ampt model and the linear model are proposed as alternatives to the more accurate (but numerically demanding) Richards' equation. To compare models, scaled error definitions are introduced and are expressed as a percentage of the saturated hydraulic conductivity of the soil. These time dependent errors are then computed under various boundary conditions. In general, the Green‐Ampt solution is found to yield smaller errors than the linear model when compared with solutions from Richards' equation. It is concluded that the use of simplified models is justified when field data exhibit sufficient spread, and a tight bound cannot be placed on the parameter estimates.
The analysis of tracer experiments conducted in divergent radial flow fields
Authors: Novakowski Kent S.
Published: February 2004

Analytical models are developed to interpret tracer experiments conducted in divergent flow fields. The models account for the mixing of tracer in both the source and observation wells and for the diffusion of tracer into adjacent aquitards. A solution for the particular case of an observation well of infinitesimally small volume is also presented. Solutions are derived for both resident and flux concentrations and for macroscopically discontinuous conditions at both the inlet and outlet boundaries. The solutions for resident and flux concentration are identical when the mixing volume in the observation well is of finite volume. Two tracer experiments were conducted in a horizontally fractured shale to demonstrate the application of the new analytical models. Experiments were conducted with observation wells of finite volume and using a new sampling packer which eliminates the volume of the observation well. The results show that for experiments conducted in fractured media, the analytical model developed in this paper provides an improved fit to the shape of the concentration curve relative to models which do not account for mixing in the source and observation wells. Alternatively, by modifying the field method such that the volume of the observation well is eliminated using the sampling packer, either conventional models or the new model for concentration in the formation will provide equally suitable fits to the concentration data.
Irrigation management and investment under saline, limited drainage conditions: 2. Characterization of optimal decision rules
Authors: Knapp Keith C.
Published: February 2004

Optimal decision rules and dynamics are characterized for the irrigation management model in the previous paper (Knapp, this issue (a)). For a constant crop and irrigation system, the decision rules are concave in E[s] and SD[s]. That is, seasonal water applications first increase as these variables increase, and then possibly decrease. Optimal water applications are observed to be decreasing in the correlation coefficient, although the effect is small. Optimal water applications and soil salinity follow a cyclical time path with a multicrop rotation. The decision rules for applied water are also found to vary for the same crop depending on place within the crop rotation. A range of irrigation systems are efficient depending on prices and other parameters. Investment dynamics are generally a constant type of irrigation system with possibly an initial transition period of another system type. More complex investment behavior is observed with constrained water supplies.
The unstable surface layer above forest: Regional evaporation and heat flux
Authors: Brutsaert Wilfried , Parlance Marc B.
Published: February 2004

Radiosonde measurements above the heterogeneous forest of the Landes region in southwestern France provided vertical profiles of potential temperature and specific humidity in the atmospheric boundary layer. For all of the 62 profiles analyzed under unstable atmospheric conditions a surface sublayer could be identified within which the Monin‐Obukhov similarity was consistent with the regional surface fluxes of sensible heat and latent heat (evaporation). For the potential temperature the vertical extent of this sublayer was found to be 41 (±30) ≤ (z − d0)/Z0 ≤ 130 (±49), where Z0 = 1.2 m is the roughness height and d0 = 6.0 m is the displacement height; for the specific humidity it was 48 (±36) ≤ (z − d0)/z0 ≤ 153 (±63). These results show that irregular forest surfaces are not anomalous, as regards the lower limit of the surface layer in comparison with surfaces with smaller roughness. The surface fluxes derived from the profile measurements were compared with flux measurements obtained by means of an eddy correlation system atop a 29 m mast, some 9 m above a mature section of forest some 4.5 km away from the launching site of the radiosondes; these independent measurements were made by a team from the Institute of Hydrology (Wallingford, England). On average, these two types of estimates were in good agreement. For the sensible heat flux the correlation coefficient was r = 0.75. For the evaporation rate it was r = 0.66. For the evaporation obtained by means of the energy budget from the sensible heat flux it improved to r = 0.82. The profile‐derived fluxes did not compare as favorably with corresponding flux values measured above agricultural crops in clearings. This confirms that the forest was the dominant surface at the regional scale.
Effects of hysteresis on groundwater recharge from ephemeral flows
Authors: Parissopoulos G. A. , Wheater H. S.
Published: February 2004

The effects of hysteresis on the movement of the saturated and unsaturated soil water phase due to infiltration from ephemeral surface water flows are investigated for different scenarios of flood events in homogeneous and heterogeneous media with the use of a two‐dimensional model based on Richards' equation and the dependent domain hysteresis model of Mualem (1984). Hysteresis effects were found in general to be small, but sensitive to water ponding depth, hydraulic contact between surface and groundwater and initial moisture distribution. In all cases tested, hysteresis resulted in higher rise of the toe of the water mound formed beneath the wadi despite a decrease of cumulative infiltration.
Variations of marine plankton δ13C with latitude, temperature, and dissolved CO2 in the world ocean
Authors: Goericke Ralf , Fry Brian
Published: February 2004

Variations of the 13C content of marine participate organic carbon (δ13CPOC) in the modern ocean were studied using literature data to test the assumptions underlying the calculation of atmospheric pCO2 through geological time from the δ13C of sedimentary organic matter. These assumptions are that (1) concentrations of CO2 in the atmosphere and the surface ocean are at equilibrium at all times and latitudes and that (2) carbon isotopic fractionation of phytoplankton (ϵp) covaries primarily with concentrations of dissolved molecular CO2 ([CO2]aq). Previous studies and compilations have shown that the first assumption does not strictly hold, although [CO2]aq may be predicted with a reasonable degree of accuracy from sea surface temperature for specific regions of the world ocean. The second assumption is shown to be questionable due to the weak covariation of ϵp and [CO2]aq in the modern ocean. The large residual variance for regressions of ϵp against [CO2]aq suggests that factors other than [CO2]aq strongly affect carbon isotopic fractionation in phytoplankton. It is concluded that the relationship between ϵp and [CO2]aq cannot be easily calibrated using δ13CPOC data from the modern ocean.
The lifetime of excess atmospheric carbon dioxide
Authors: Moore Berrien , Braswell B. H.
Published: February 2004

We explore the effects of a changing terrestrial biosphere on the atmospheric residence time of CO2 using three simple ocean carbon cycle models and a model of global terrestrial carbon cycling. We find differences in model behavior associated with the assumption of an active terrestrial biosphere (forest regrowth) and significant differences if we assume a donor‐dependent flux from the atmosphere to the terrestrial component (e.g., a hypothetical terrestrial fertilization flux). To avoid numerical difficulties associated with treating the atmospheric CO2 decay (relaxation) curve as being well approximated by a weighted sum of exponential functions, we define the single half‐life as the time it takes for a model atmosphere to relax from its present‐day value half way to its equilibrium pCO2 value. This scenario‐based approach also avoids the use of unit pulse (Dirac Delta) functions which can prove troublesome or unrealistic in the context of a terrestrial fertilization assumption. We also discuss some of the numerical problems associated with a conventional lifetime calculation which is based on an exponential model. We connect our analysis of the residence time of CO2 and the concept of single half‐life to the residence time calculations which are based on using weighted sums of exponentials. We note that the single half‐life concept focuses upon the early decline of CO2 under a cutoff/decay scenario. If one assumes a terrestrial biosphere with a fertilization flux, then our best estimate is that the single half‐life for excess CO2 lies within the range of 19 to 49 years, with a reasonable average being 31 years. If we assume only regrowth, then the average value for the single half‐life for excess CO2 increases to 72 years, and if we remove the terrestrial component completely, then it increases further to 92 years.
Perturbation of stratospheric nitrogen dioxide by volcanic aerosol in the Arctic
Authors: Lateltin E. , Pommereau J.‐P. , Le Texier H. , Pirre M. , Ramaroson R. A.
Published: February 2004

NO2 vertical profiles observed from balloon borne UV‐visible spectrometers during EASOE were simulated in order to quantify the NOx depletion by volcanic aerosol. Calculations were performed with a box photochemical model integrated along isentropic trajectories at 6 potential temperature levels (350, 380, 400, 475, 550 and 700 K), and initialized as far as possible from experimental data and from 2D model zonal averages, when not available. NO2 vertical profiles are well captured by the model including current gas phase and heterogeneous chemistry. The very low NO2 concentrations reported during the coldest and darkest part of the winter might be reproduced by gas phase chemistry alone. They are largely due to the lack of sunlight. Otherwise, in fall and spring at warmer temperature, NO2 concentrations are significantly lower than that simulated in pure gas phase chemistry. The depletion is due to the presence of volcanic aerosols. The responsible reaction is the heterogeneous conversion of N2O5 into nitric acid on sulfuric acid droplets. Sunlight attenuation by the aerosol layer contributes for a minor part only. The role of the conversion of ClONO2 is negligible. However, NO2 cannot be totally removed by the heterogeneous conversion of N2O5 on aerosol. There is a saturation effect which originates in the slow gas phase conversion of NO2 into N2O5. The NO2 concentration at saturation is then mostly controlled by temperature.
An overview of millimeter‐wave spectroscopic measurements of chlorine monoxide at Thule, Greenland, February–March, 1992: Vertical profiles, diurnal variation, and longer‐term trends
Authors: de Zafra R. L. , Emmons L. K. , Reeves J. M. , Shindell D. T.
Published: February 2004

Measurements of chlorine monoxide in the stratosphere over Thule, Greenland (76.3N, 68.4W) were made quasi‐continuously during the period February 8 to March 24, 1992, using a high‐sensitivity ground based mm‐wave spectrometer. These observations give diurnal, short term, and long term changes in the mixing ratio and vertical distribution of ClO. At an equivalent time after the Antarctic winter solstice, very large concentrations (up to ∼1.5 ppbv) occur in lower stratospheric ClO, resulting in massive ozone destruction. We saw no evidence for large (∼1 to 1.5 ppbv) amounts of ClO in the 16–25 km range over Thule in February or March, in agreement with UARS (satellite) observations by the MLS mm‐wave spectrometer for this period, and in marked contrast to UARS/MLS and ER‐2 aircraft measurements over northern Europe and eastern Canada, respectively, during January, 1992. We have evidence for smaller enhancements (∼0.2 to 0.5 ppbv) in the 18–30 km range during late February‐early March, which could result from transport of residual low NO2 air following earlier PSC processing (the last of which occurred at least one month earlier, however) or the result of chemical processing by Pinatubo aerosols. Direct influence of Pinatubo aerosols on Arctic ozone during the spring of 1992 has been difficult to assess, and this enhancement of low‐altitude ClO might be a significant indicator of aerosol effects.
Comparisons of seismic heterogeneity models and convective flow calculations
Authors: Woodward Robert L. , Dziewonski Adam M. , Peltier W. Richard
Published: February 2004

By comparing models of the Earth's mantle obtained from seismic tomography with the predictions of convective flow calculations we may interpret the seismic results in terms of mantle flow scenarios. The results of such comparisons are consistent with a picture of convective flow in the Earth's mantle in which whole mantle style flow is dominant at present but in which phase transition induced localized layering also exists. In addition, direct examination of the tomographic images show regions where the boundary between upper and lower mantle marks a significant discontinuity in heterogeneity while in other regions heterogeneities appear to be continuous across it.
Visualizing the evolution of the stratospheric polar vortex in January 1992
Authors: Norton W. A. , Carver G. D.
Published: February 2004

From a GCM simulation of a period in EASOE, the evolution of the northern hemisphere polar vortex is examined by means of a sequence of three dimensional visualizations. Use is made of two key dynamical features, the edge of the polar vortex and the height of the tropopause. This technique clearly shows how the vortex is disturbed by an anticyclonic tropospheric anomaly and how this disturbance vertically propagates up the vortex. An objective method for finding the edge of the vortex is presented.
A gravity model for the Sudbury Structure along the Lithoprobe seismic line
Authors: McGrath P. H. , Broome H. J.
Published: February 2004

Previous gravity models of the Sudbury Structure (1850 Ma) were constrained by surface geology, and by density measurements of surface and borehole rock samples. Recent high‐resolution seismic reflection data provide additional constraints for modeling new gravity observations made along the Sudbury Lithoprobe transect. Results indicate, 1) density distributions constrained by the seismic data yield calculated gravity values matching the Bouguer gravity data, 2) the main sources of gravitational disturbance are external to the Sudbury Structure, 3) the positive gravity anomaly trend south of the Sudbury Structure is associated with mafic rocks of Proterozoic age, and 4) the large, ramplike, gravity anomaly paralleling the northwest margin of the Sudbury Structure is an expression of a northward dipping boundary within the Archean basement. The presence of a hidden mafic layer beneath the Sudbury Structure is not required to model the Bouguer gravity data. An enigma is an 8 mGal, positive, gravity anomaly over the south central Sudbury Structure.
Temperature dependence of the ozone absorption spectrum over the wavelength range 410 to 760 nm
Authors: Burkholder James B. , Talukdar Ranajit K.
Published: February 2004

The ozone, O3, absorption cross sections between 410 and 760 nm, the Chappuis band, were measured at 220, 240, 260, and 280 K relative to that at room temperature using a diode array spectrometer. The measured cross sections varied very slightly, <1%, with decreasing temperature between 550 and 660 nm, near the peak of the Chappuis band. At wavelengths away from the peak, the absorption cross sections decreased with decreasing temperature; e.g., ∼40% at 420 nm between 298 and 220 K. These results are compared with previous measurements and the impact on atmospheric measurements are discussed.
Gasdynamic models of the solar wind/interstellar medium interaction
Authors: Steinolfson R. S. , Pizzo V. J. , Holzer T.
Published: February 2004

The interaction between the solar wind and the interstellar medium is modeled self‐consistently using numerical solutions of the time‐dependent gasdynamic equations in spherical and cylindrical coordinates. For the results presented here it is assumed that the solar system moves through the surrounding medium with a supersonic velocity. After an initial (nonequilibrium) state has been specified, the numerical solution follows the evolution in time until the interaction relaxes to a dynamic equilibrium. As would be expected, the solutions show the formation of a bow shock upstream of the traveling solar system to deflect the interstellar plasma around the cavity created by the solar wind. A termination shock also forms to slow and compress the solar wind plasma. For the simulation in spherical coordinates, the downstream portion of the termination shock reaches equilibrium more than three times further from the Sun than the equilibrium distance to the termination shock on the upstream side.
Stratospheric aerosol size distributions from multispectral lidar measurements at Sodankylä during EASOE
Authors: Stein B. , Del Guasta M. , Kolenda J. , Morandi M. , Rairoux P. , Stefanutti L. , Wolf J. P. , Wöste L.
Published: February 2004

Multispectral lidar data measured at Sodankylä (67°N, 26°E), have been used to characterize stratospheric aerosols. Using depolarization measurements, particles have been classified into PSC and volcanic aerosols. While Pinatubo aerosol could be observed inside the polar vortex from the beginning of the campaign, only few PSC events have been observed. For Pinatubo aerosol lognormal size distribution parameters have been retrieved, indicating particle distributions with 0.1 µm to 0.3 µm median‐radius between 17 and 22 km and up to 0.7 µm below. Height resolved size distribution parameters and aerosol surface areas are given.
Evolution of southern hemisphere spring air masses observed by HALOE
Authors: Pierce R. Bradley , Grose William L. , Russell James M. , Tuck Adrian F.
Published: February 2004

The evolution of Southern Hemisphere air masses observed by the Halogen Occultation Experiment (HALOE) during September 21 through October 15, 1992, is investigated using isentropic trajectories computed from United Kingdom Meteorological Office (UKMO) assimilated winds and temperatures. Maps of constituent concentrations are obtained by accumulation of air masses from previous HALOE occultations. Lagged correlations between initial and subsequent HALOE observations of the same air mass are used to validate the air mass trajectories. High correlations are found for lag times as large as 10 days. Frequency distributions of the air mass constituent concentrations are used to examine constituent distributions in and around the Southern Hemisphere polar vortex.
Millimeter‐wave measurements of chlorine monoxide at the Jungfraujoch Alpine Station
Authors: Gerber Louis , Kämpfer Niklaus
Published: February 2004

During the European Arctic Stratospheric Ozone Experiment (EASOE) in winter 91/92, first measurements with a new 204GHz radiometer were performed at the Jungfraujoch Alpine Station (7°59′E 46°33′N) in the Swiss Alps. The altitude of this site (3570m) allows detection of weak spectral lines like the ones of the chlorine monoxide J 11/2→9/2 rotational transition. At the end of January 1992, a large amount of ClO in the lower stratosphere was measured, indicating that highly perturbed airmasses had been transported towards mid latitudes. The maximum value of the ClO volume mixing ratio was about 1ppbv +/−0.4 ppbv in heights of 15–25km.
The dynamics of the Arctic Polar Vortex during the EASOE Campaign
Authors: Farman J. C. , O'Neill A. , Swinbank R.
Published: February 2004

A blocking ridge over the Atlantic and N.W.Europe was the most persistent feature of the upper troposphere during the winter of 1991/1992. With brief intermissions, blocking lasted for almost three months. It produced large anomalies in the mean ozone fields for December 1991 and for January 1992; record low amounts of total ozone were reported over Europe. The influence of this blocking system on the structure of the polar vortex is discussed. In the lower stratosphere there was a persistent collocation of the core of the jet stream with temperatures low enough to promote heterogeneous processes, subjecting a large amount of air to chemical processing. However the minor warmings of mid‐January and February make it difficult to identify chemical ozone loss against the background of the large anomalies present in 1991/92.
Total reactive nitrogen, N2O, and ozone in the winter Arctic stratosphere
Authors: Kondo Y. , Schmidt U. , Sugita T. , Aimedieu P. , Koike M. , Ziereis H. , Iwasaka Y.
Published: February 2004

Total reactive nitrogen (NOy) and ozone were measured between 14 and 31 km during ascent on board a balloon launched from Kiruna, Sweden on January 31, 1992. Measurements of N2O were also made from the same gondola but during the fast parachute descent. Kiruna was located outside the polar vortex below 20 km and inside it above 23 km. The observed strong positive correlation between NOy and ozone as well as the negative correlation between NOy and N2O up to 22 km suggests the absence of denitrification at these altitudes. Very low N2O mixing ratios of about 13 ppbv were observed at altitudes between 23 and 27 km. In this region, the NOy mixing ratio was 13.6±0.5 ppbv, deviating from the above linear relationship by 6 ppbv. This deficit in NOy is interpreted in terms of downward transport of air from the upper stratosphere where photochemical NOy loss occurs.