bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of process errors in bed load sampling using a Dune Model
Authors: Gomez Basil , Troutman Brent M.
Published: February 2004

Reliable estimates of the streamwide bed load discharge obtained using sampling devices are dependent upon good at‐a‐point knowledge across the full width of the channel. Using field data and information derived from a model that describes the geometric features of a dune train in terms of a spatial process observed at a fixed point in time, we show that sampling errors decrease as the number of samples collected increases, and the number of traverses of the channel over which the samples are collected increases. It also is preferable that bed load sampling be conducted at a pace which allows a number of bed forms to pass through the sampling cross section. The situations we analyze and simulate pertain to moderate transport conditions in small rivers. In such circumstances, bed load sampling schemes typically should involve four or five traverses of a river, and the collection of 20–40 samples at a rate of five or six samples per hour. By ensuring that spatial and temporal variability in the transport process is accounted for, such a sampling design reduces both random and systematic errors and hence minimizes the total error involved in the sampling process.
Causes of concentration/discharge hysteresis and its potential as a tool for analysis of episode hydrochemistry
Authors: Evans Christopher , Davies Trevor D.
Published: February 2004

Episodic variations in dissolved solutes are frequently complicated by a cyclical relationship between concentration and stream discharge. Established three‐component models of runoff generation are used to explain this hysteresis effect and to illustrate how different component concentrations produce different hysteresis forms. It is demonstrated that a two‐component model cannot reproduce all the hysteresis forms commonly observed. A method, based on the three‐component system, is derived by which C/Q hysteresis can be used to predict relative component concentrations. This may provide a qualitative chemical description of sources supplying runoff for locations where these have not yet been directly established or a validation test where possible components have been sampled. The method has been tested using data collected at streams in the Adirondacks, New York, and the Northern Appalachian Plateau, Pennsylvania, during the Episodic Response Project of the U.S. Environmental Protection Agency. Predicted component compositions were in good agreement with measurements made during other studies and with those subsequently obtained from one of the Pennsylvania watersheds.
Transient effective hydraulic conductivities under slowly and rapidly varying mean gradients in bounded three‐dimensional random media
Authors: Tartakovsky Daniel M. , Neuman Shlomo P.
Published: February 2004

We have shown elsewhere [Tartakovsky and Neuman, this issue (a)] that in randomly heterogeneous media, the ensemble mean transient flux is generally nonlocal in space‐time and therefore non‐Darcian. We have also shown [Tartakovsky and Neuman, this issue (b)] that there are special situations in which this flux can be localized so as to render it Darcian in real or transformed domains. Each such situation gives rise to an effective hydraulic conductivity which relates mean gradient to mean flux at any point in real or transformed space‐time. In this paper we develop first‐order analytical expressions for effective hydraulic conductivity under three‐dimensional transient flow through a boxshaped domain due to a mean hydraulic gradient that varies slowly in space and time. When the mean gradient varies rapidly in time, the Laplace transform of the mean flux is local but its real‐time equivalent includes a temporal convolution integral; we develop analytical expressions for the real‐time kernel of this convolution integral. The box is embedded within a statistically homogeneous natural log hydraulic conductivity field that is Gaussian and exhibits an anisotropic exponential spatial correlation structure. By the effective hydraulic conductivity of a finite box in such a field we imply the ensemble mean (expected value) of all random equivalent conductivities that one could associate with the box under these conditions. We explore the influence of domain size, time, and statistical anisotropy on effective conductivity and include a simple new formula for its variation with statistical anisotropy ratio in an infinite domain under steady state. Appendixes C‐E are available with entire article on microfiche.Order by mail from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009 or by phone at 800‐966‐2481; $2.50. Document W97‐002. Payment must accompany order.
Field‐scale evaluation of in situ cosolvent flushing for enhanced aquifer remediation
Authors: Rao P. Suresh C. , Annable Michael D. , Sillan Randall K. , Dai Dongping , Hatfield Kirk , Graham Wendy D. , Wood A. Lynn , Enfield Carl G.
Published: February 2004

A comprehensive, field‐scale evaluation of in situ cosolvent flushing for enhanced remediation of nonaqueous phase liquid (NAPL)‐contaminated aquifers was performed in a hydraulically isolated test cell (about 4.3 m × 3.6 m) constructed at a field site at Hill Air Force Base, Utah. This sand‐gravel‐cobble surficial aquifer, underlain by a deep clay confining unit at about 6 m below ground surface, was contaminated with a multicomponent NAPL as a result of jet fuel and chlorinated solvent disposal during the 1940s and 1950s. The water table within the test cell was raised to create a 1.5 m saturated flow zone that contained the NAPL smear zone. The cosolvent flushing test consisted of pumping about 40,000 L (approximately nine pore volumes) of a ternary cosolvent mixture (70% ethanol, 12% n‐pentanol, and 18% water) through the test cell over a period of 10 days, followed by flushing with water for another 20 days. Several methods for assessing site remediation yielded consistent results, indicating that on the average >85% mass of the several target contaminants was removed as a result of the cosolvent flushing; NAPL constituent removal effectiveness was greater (90–99+%) in the upper 1‐m zone, in comparison to about 70–80% in the bottom 0.5‐m zone near the clay confining unit. Various interacting factors that control the hydrodynamic sweep efficiency, and the NAPL removal effectiveness during cosolvent flushing in this unconfined aquifer are discussed.
The influence of wind stress, temperature, and humidity gradients on evaporation from reservoirs
Authors: Condie Scott A. , Webster Ian T.
Published: February 2004

Evaporation from fetch‐limited water bodies has been investigated for the first time using a coupled atmospheric boundary layer–water body model. The model incorporates a simplified atmospheric boundary layer in which heat and moisture are advected horizontally and diffused vertically. The wind field evolves over the water body through the formation of an internal boundary layer, which is initiated by the change in roughness from the land to the water surface. The wind also responds to local stability through the inclusion of Monin‐Obukhov similarity functions. This system is coupled to a dynamically active water body based on primitive equations with full thermodynamics. This is achieved through continuity of stress and heat flux through the air‐water interface. The model results reveal that along‐wind gradients in wind stress, humidity, and temperature can all significantly influence evaporation. The most important effects are growing wind stress and increasing humidity as we move downwind across the water body. However, there is a tendency for these effects to cancel, so that the behavior can range from areally averaged evaporation weakly decreasing with fetch for very smooth land surfaces to weakly increasing with fetch for relatively rough land terrain. For most situations of interest, such as typical agricultural settings, evaporation is essentially independent of fetch. All the model results have been summarized in a simple empirical expression for evaporation based exclusively on meteorological data from over the upwind land surface. This is in good agreement with detailed measurements from a small lake in southeastern Australia.
Modeling and field evidence of finger formation and finger recurrence in a water repellent sandy soil
Authors: Ritsema Coen J. , Dekker Louis W. , Nieber John L. , Steenhuis Tammo S.
Published: February 2004

With prolonged rainfall, infiltrating wetting fronts in water repellent soils may become unstable, leading to the formation of high‐velocity flow paths, the so‐called fingers. Finger formation is generally regarded as a potential cause for the rapid transport of water and contaminants through the unsaturated zone of soils. For the first time, field evidence of the process of finger formation and finger recurrence is given for a water repellent sandy soil. Theoretical analysis and model simulations indicate that finger formation results from hysteresis in the water retention function, and the character of the formation depends on the shape of the main wetting and main drainage branches of that function. Once fingers are established, hysteresis causes fingers to recur along the same pathways during following rain events. Leaching of hydrophobic substances from these fingered pathways makes the soil within the pathways more wettable than the surrounding soil. Thus, in the long‐term, instability‐driven fingers might become heterogeneity‐driven fingers.
Quantification of microporosity by nuclear magnetic resonance relaxation of water imbibed in porous media
Authors: Hinedi Z. R. , Chang A. C. , Anderson M. A. , Borchardt D. B.
Published: February 2004

On the basis of the nuclear magnetic resonance (NMR) relaxation of imbibed water, we evaluated the interparticle and intraparticle pore sizes in packed beds of silica materials of known particle sizes and microporous structure. The NMR relaxation distribution is scaled by the surface relaxivity parameter ρ, which incorporates a surface area to volume ratio ((So/Vo) term, to yield a corresponding pore size distribution. The NMR‐derived pore sizes of nonporous silica sand agreed with the expected interparticle pore sizes estimated from the morphology of a packed bed of spheres of comparably sized particles. The NMR‐derived intraparticle pore size for porous silica was also in good agreement with reported values for the silica materials studied. Scaling of the NMR relaxation corresponding to interparticle water by the same surface interaction parameter to yield interparticle pore size in high‐surface area porous silica material, however, grossly underestimated interparticle pore size. In these high‐surface area materials the intraparticle micropores provide a higher contribution to the N2 measured surface area relative to the contribution from interparticle macropores. When the NMR relaxation method was used to evaluate the pore space in the Borden Aquifer material, the NMR‐derived pore sizes agreed with those observed in scanning electron micrographs as well as pore sizes estimated from the morphology of packed beds of comparably sized particles. For soils and aquifer materials of low to moderate surface area the NMR‐derived porosity determination may be used to adequately evaluate both solute transporting and sorbing pore sizes.
Disaggregation procedures for stochastic hydrology based on nonparametric density estimation
Authors: Tarboton David G. , Sharma Ashish , Lall Upmanu
Published: February 2004

Synthetic simulation of streamflow sequences is important for the analysis of water supply reliability. Disaggregation models are an important component of the stochastic streamflow generation methodology. They provide the ability to simulate multiseason and multisite streamflow sequences that preserve statistical properties at multiple timescales or space scales. In recent papers we have suggested the use of nonparametric methods for streamflow simulation. These methods provide the capability to model time series dependence without a priori assumptions as to the probability distribution of streamflow. They remain faithful to the data and can approximate linear or nonlinear dependence. In this paper we extend the use of nonparametric methods to disaggregation models. We show how a kernel density estimate of the joint distribution of disaggregate flow variables can form the basis for conditional simulation based on an input aggregate flow variable. This methodology preserves summability of the disaggregate flows to the input aggregate flow. We show through applications to synthetic data and streamflow from the San Juan River in New Mexico how this conditional simulation procedure preserves a variety of statistical attributes.
Subsurface flow paths in a steep, unchanneled catchment
Authors: Anderson Suzanne Prestrud , Dietrich William E. , Montgomery David R. , Torres Raymond , Conrad Mark E. , Loague Keith
Published: February 2004

Tracer studies during catchment‐scale sprinkler experiments illuminate the pathways of subsurface flow in a small, steep catchment in the Oregon Coast Range. Bromide point injections into saturated materials showed rapid flow in bedrock to the catchment outlet. Bedrock flow returned to the colluvium, sustaining shallow subsurface flow there. The bromide peak velocity of ∼10−3 m s−1 exceeded the saturated hydraulic conductivity of intact bedrock. This, and the peak shapes, verify that fractures provide important avenues for saturated flow in the catchment. Deuterium added to the sprinkler water moved through the vadose zone as plug flow controlled by rainfall rate and water content. Ninety‐two percent of the labeled water remained in the vadose zone after 3 days (∼140mm) of sprinkling. Preferential flow of new water was not observed during either low‐intensity irrigation or natural storms; however, labeled preevent water was mobile in shallow colluvium during a storm following our spiking experiment. In response to rainfall, waters from the deeper bedrock pathway, which have traveled through the catchment, exfiltrate into the colluvium mantle and mix with relatively young vadose zone water, derived locally, creating an area of subsurface saturation near the channel head. This effectively becomes a subsurface variable source area, which, depending on its size and the delivery of water from the vadose zone, dictates the apportioning of old and new water in the runoff and, correspondingly, the runoff chemistry. The slow movement of water through the vadose zone allows for chemical modification and limits the amount of new water in the runoff. Moreover, it suggests that travel time of new rain water does not control the timing of runoff generation.
Regional hydrologic analysis: Ordinary and generalized least squares revisited
Authors: Kroll Charles N. , Stedinger Jery R.
Published: February 2004

Generalized least squares (GLS) regional regression procedures have been developed for estimating river flow quantiles. A widely used GLS procedure employs a simplified model error structure and average covariances when constructing an approximate residual error covariance matrix. This paper compares that GLS estimator (
), an idealized GLS estimator (
) based on the simplifying assumptions of
with true underlying statistics in a region, the best possible GLS estimator (
) obtained using the true residual error covariarice matrix, and the ordinary least squares estimator (
). Useful analytic expressions are developed for the variance of
,
and
For previously examined cases the average sampling mean square error (mses) of
was the same as the mses of
and the mses of
usually was larger than the mses of both
and
The loss in efficiency of
was mostly due to estimating streamflow statistics employed in the construction of the residual error covariance matrix rather than the simplifying assumptions in presently employed GLS estimators. The new analytic expressions were used to compare the performance of the OLS and GLS estimators for new cases representing greater model variability across sites as well as the effect return period has on the estimators' relative performance. For a more heteroscedastic model error variance and larger return periods, some increase in the mses of
relative to the mses of
was observed.
Nitrogen transport and transformations in a shallow aquifer receiving wastewater discharge: A mass balance approach
Authors: DeSimone Leslie A. , Howes Brian L.
Published: February 2004

Nitrogen transport and transformations were followed over the initial 3 years of development of a plume of wastewater‐contaminated groundwater in Cape Cod, Massachusetts. Ammonification and nitrification in the unsaturated zone and ammonium sorption in the saturated zone were predominant, while loss of fixed nitrogen through denitrification was minor. The major effect of transport was the oxidation of discharged organic and inorganic forms to nitrate, which was the dominant nitrogen form in transit to receiving systems. Ammonification and nitrification in the unsaturated zone transformed 16–19% and 50–70%, respectively, of the total nitrogen mass discharged to the land surface during the study but did not attenuate the nitrogen loading. Nitrification in the unsaturated zone also contributed to pH decrease of 2 standard units and to an N2O increase (46–660 µg N/L in the plume). Other processes in the unsaturated zone had little net effect: Ammonium sorption removed <1% of the total discharged nitrogen mass; filtering of particulate organic nitrogen was less than 3%; ammonium and nitrate assimilation was less than 6%; and ammonia volatilization was less than 0.25%. In the saturated zone a central zone of anoxic groundwater (DO ≤ 0.05 mg/L) was first detected 17 months after effluent discharge to the aquifer began, which expanded at about the groundwater‐flow velocity. Although nitrate was dominant at the water table, the low, carbon‐limited rates of denitrification in the anoxic zone (3.0–9.6 (ng N/cm3)/d) reduced only about 2% of the recharged nitrogen mass to N2. In contrast, ammonium sorption in the saturated zone removed about 16% of the recharged nitrogen mass from the groundwater. Ammonium sorption was primarily limited to anoxic zone, where nitrification was prevented, and was best described by a Langmuir isotherm in which effluent ionic concentrations were simulated. The initial nitrogen load discharged from the groundwater system may depend largely on the growth and stability of the sorbed ammonium pool, which in turn depends on effluent‐loading practices, subsurface microbial processes, and saturation of available exchange sites.
Basin‐scale geohydrologic drought flow features of riparian aquifers in the Southern Great Plains
Authors: Brutsaert Wilfried , Lopez James P.
Published: February 2004

Low‐streamflow hydrographs from 22 subbasins in the U.S. Department of Agriculture‐Agricultural Research Service (USDA‐ARS) Washita River Experimental Watershed complex in central Oklahoma were subjected to recession slope analysis; this method, after that of Brutsaert and Nieber [1977], was derived from a Dupuit‐Boussinesq formulation for the groundwater outflows from the adjoining phreatic aquifers. The longtime aquifer response characteristics were generally found to be close to linear, and the short‐time response characteristics were consistent with Boltzmann similarity. Representative values of the resulting basin‐scale effective groundwater parameters were (35 days)−1 for the low‐flow extinction coefficient (i.e., a storage half‐life of 25 days); 0.021 m2 s−1 for the hydraulic diffusivity, Dh; 0.0035 m2s−½ for the hydraulic desorptivity, Deh; 8 × 10−4 ms−1 for the hydraulic conductivity k; and 0.018 for the drainable porosity (or specific yield), ƒ. The variabilities of Dh, Deh, and k from basin to basin could be better represented by the log‐normal than by the normal distribution; ƒ could be described nearly equally well by both. The storage half‐life is moderately and positively correlated with basin size; in the case of k the correlation is negative but weaker. Any scale dependence of Dh, Deh, and ƒ appears to be negligible.
Geostatistical analysis of ground‐penetrating radar data: A means of describing spatial variation in the subsurface
Authors: Rea Jane , Knight Rosemary
Published: February 2004

We have investigated the use of ground‐penetrating radar (GFR) as a means of characterizing the heterogeneity of the subsurface. Radar data were collected at several sites in southwestern British Columbia underlain by glaciodeltaic sediments. A cliff face study was conducted in which geostatistical analysis of a digitized photograph of the face and the radar image of the face showed excellent agreement in the maximum correlation direction and the correlation length determined from these two data sets. Other two‐dimensional (2‐D) sections of radar data were divided into sedimentary architectural elements on the basis of the distinct radar appearance of these sedimentary units. Examples of four sedimentary units were used to obtain semivariograms from the radar data and resulted in maximum correlation lengths between 0.5 and 4.8 m. A 3‐D radar survey, collected over a package of gravel and sand foresets, was analyzed to determine the paleoflow direction; a correlation length of 4 m was found in that direction.
Early development of Karst aquifers on percolation networks of fractures in limestone
Authors: Siemers Jörg , Dreybrodt Wolfgang
Published: February 2004

We have modeled flow and dissolution processes within percolation networks representing stochastic primary fracture systems in limestone. At the beginning of karstification, flow is evenly distributed on all fractures available. As the system develops by dissolutional widening of the fractures, preferred flow pathways evolve, which attract more and more flow, until at breakthrough the total flow rate increases dramatically. These breakthrough times have been investigated with regard to their dependence on the initial fracture aperture width, the dimension of the evolving karst aquifer, hydraulic gradients, connectivity of the percolation network, and the chemical parameters, such as higher‐order rate constants and exponents, and equilibrium concentration. They exhibit a behavior similar to that of breakthrough times for one‐dimensional conduits [Dreybrodt, 1996]. Our results show that the structure of the evolving karst aquifer is determined by the initial geological setting. However, the final conduits are selected from competing pathways with potential breakthrough times close to each other, whereby the details of the distribution of undersaturation with respect to calcite play an important role.
Distributed soil erosion simulation for effective erosion prevention
Authors: Mitas Lubos , Mitasova Helena
Published: February 2004

We present a bivariate model of erosion, sediment transport, and deposition by overland flow, designed for complex terrain, soil, and cover conditions. We use a Green's function Monte Carlo method to solve the underlying continuity equations, leading to improved robustness and implementation efficiency. By deriving the relationship between the terrain shape and erosion/deposition pattern, we clarify the physical interpretation of terrain curvatures and overall importance of the bivariate formulation. We explain the impact of various soil and cover properties by simulating the detachment and transport capacity limited erosion for uniform land use and by predicting the erosion/deposition distribution for a conventional, spatially variable land use at an experimental farm. We compare the results with the observed colluvial deposits and linear erosion features and illustrate the application of the model for improving the effectiveness of erosion prevention measures.
Parameter estimation using groundwater age and head data, Cape Cod, Massachusetts
Authors: Portniaguine O. , Solomon D. K.
Published: February 2004

A practical inversion procedure is used to parametrize an unconfined aquifer on Cape Cod, Massachusetts, using groundwater age and hydraulic head data. The inversion resulted in estimates of the recharge flux, homogeneous but anisotropic hydraulic conductivity, porosity, boundary hydraulic heads, and the aquifer thickness. The range of estimated values agreed well with independent measurements at the site. By themselves hydraulic heads are sensitive to the ratio of recharge flux to hydraulic conductivity. The age data are sensitive to the ratio of recharge to porosity. Together, age and head data provide a constraint on both boundary values and material properties. However, a sensitivity analysis shows that porosity, hydraulic conductivity, and recharge are all correlated, and thus a unique inverse solution requires an independent constraint on one of these parameters. Simulations also show that groundwater ages at a given point on a streamline are sensitive to the recharge flux at the point where the streamline begins at the water table. This implies that a vertical profile of age data will be sensitive to the upstream recharge flux, suggesting that spatial patterns in recharge can be estimated using vertical profiles from multilevel monitoring wells. Because measurements of groundwater age can be made in many shallow aquifers, this approach may be applicable to other sites.
Toward improved calibration of hydrologic models: Multiple and noncommensurable measures of information
Authors: Gupta Hoshin Vijai , Sorooshian Soroosh , Yapo Patrice Ogou
Published: February 2004

Several contributions to the hydrological literature have brought into question the continued usefulness of the classical paradigm for hydrologic model calibration. With the growing popularity of sophisticated “physically based” watershed models (e.g., land‐surface hydrology and hydrochemical models) the complexity of the calibration problem has been multiplied many fold. We disagree with the seemingly widespread conviction that the model calibration problem will simply disappear with the availability of more and better field measurements. This paper suggests that the emergence of a new and more powerful model calibration paradigm must include recognition of the inherent multiobjective nature of the problem and must explicitly recognize the role of model error. The results of our preliminary studies are presented. Through an illustrative case study we show that the multiobjective approach is not only practical and relatively simple to implement but can also provide useful information about the limitations of a model.
A Soil Moisture–Rainfall Feedback Mechanism: 2. Numerical experiments
Authors: Zheng Xinyu , Eltahir Elfatih A. B.
Published: February 2004

Here we develop a numerical model to investigate the hypothesis proposed by a companion paper [Eltahir, this issue], which describes a soil moisture–rainfall feedback mechanism. The model is designed to describe the seasonal evolution of the West African monsoon rainfall and is used to perform numerical experiments that elucidate the mechanisms of the response of rainfall to soil moisture anomalies. A significant rainfall anomaly is simulated by the model in response to a hypothetical soil moisture anomaly that has been imposed during early summer. However, the magnitude of this anomaly almost vanishes when the net radiation at the surface is not allowed to respond to the soil moisture anomaly. Hence the results of the numerical experiments support the proposed hypothesis and highlight the crucial importance of the radiative and dynamical feedbacks in regulating the rainfall anomalies that result from the soil moisture anomalies.
Temperature variability in lake sediments
Authors: Fang Xing , Stefan Heinz G.
Published: February 2004

Sediment temperatures were simulated from the sediment‐water interface down to 10 m for circular lakes with surface areas of 0.2–10 km2 and maximum lake depths of 4–24 m. The calculations were made using daily weather conditions measured over 19 years (1961–1979) at three geographic locations, representing climate conditions from north to south latitudes in the central United States. A one‐dimensional sediment temperature model coupled with a year‐round lake water temperature model (MINLAKE96) was used. The models were tested against extensive water temperature and some sediment temperature data. Lake sediment equilibrium temperatures at 10 m below the water‐sediment interface were found almost equal to the annual mean temperature of the overlaying water and hence dependent on the lake temperature regime, which in turn depends on the local climate and a lake's physical characteristics (e.g., surface area As, maximum depth HMAX, and water transparency as measured by Secchi depth). The lake geometry ratio As0.25/HMAX(m−0.5) gives a good indication whether a lake will permanently stratify in summer or not, and that makes a large difference for the sediment temperature regime. Sediment temperatures at 10 m below the littoral waters of a lake are almost equal to subsurface ground (terrestrial) temperatures, regardless of the type of lake, whereas sediment temperatures at 10 m below the deepest portion of a stratified lake have a strong linear relationship with the logarithm of the lake geometry ratio. Two generalized 10‐m sediment temperature profiles were developed, one for ice‐covered lakes and the other for year‐round open water lakes. Sediment temperatures at 10 m below the sediment‐water interface at different depths of a lake can be reconstructed from the generalized profiles using Secchi depth, surface area, maximum depth, and annual mean air temperature as input. Sediment temperature at several meters depth (e.g., 10 m) below a lake bed is a useful and necessary initial condition to predict water temperature dynamics in lakes of the temperate zone and hence other water quality parameters whose kinetics are linked to water temperature.
L moment diagrams for censored observations
Authors: Zafirakou‐Koulouris Antigoni , Vogel Richard M. , Craig Scott M. , Habermeier Joerg
Published: February 2004

Observed data sets containing values above or below the analytical threshold of measuring equipment are referred to as censored. Such data are frequently encountered in quality and quantity monitoring applications of water, soil, and air samples. Most of the previous literature on the statistical analysis of censored data relates to the problems of moment, parameter, and quantile estimation methods. Such estimation methods usually assume an underlying probability distribution. Few goodness‐of‐fit methods exist for censored data. We introduce L moment diagrams for the evaluation of the goodness of fit of alternative distributional hypotheses for left‐censored data. Experiments with artificial censored data sets document the conditions under which L moment diagrams should be useful. Our approach, likeHosking's [1995] approach for right censoring, derived L moment diagrams for left‐censored observations from partial probability‐weighted moments.
Estimating the spatial distribution of snow in mountain basins using remote sensing and energy balance modeling
Authors: Cline Donald W. , Bales Roger C. , Dozier Jeff
Published: February 2004

We present a modeling approach that couples information about snow cover duration from remote sensing with a distributed energy balance model to calculate the spatial distribution of snow water equivalence (SWE) in a 1.2 km2mountain basin at the peak of the accumulation season. In situ measurements of incident solar radiation, incident longwave radiation, air temperature, relative humidity, and wind speed were distributed around the basin on the basis of topography. Snow surface albedo was assumed to be spatially constant and to decrease with time. Distributed snow surface temperature was estimated as a function of modeled air temperature. We computed the energy balance for each pixel at hourly intervals using the estimated radiative fluxes and bulk‐aerodynamic turbulent‐energy flux algorithms from a snowpack energy and mass balance model. Fractional snow cover within each pixel was estimated from three multispectral images (Landsat thematic mapper), one at peak accumulation and two during snowmelt, using decision trees and a spectral mixture model; from these we computed snow cover duration at subpixel resolution. The total cumulative energy for snowmelt at each remote sensing date was weighted by the fraction of each pixel's area that lost its snow cover by that date to determine an initial SWE for each pixel. We tested the modeling approach in the well‐studied Emerald Lake basin in the southern Sierra Nevada. With no parameter fitting the modeled spatial pattern of SWE and the mean basin SWE agreed with intensive field survey data. As the modeling approach requires only a remote sensing time series and an ability to estimate the energy balance over the model domain, it should prove useful for computing SWE distributions at peak accumulation over larger areas, where extensive field measurements of SWE are not practical.
Soil respiration and georespiration distinguished by transport analyses of vadose CO2, 13CO2, and 14CO2
Authors: Keller C. Kent , Bacon Diana Holford
Published: February 2004

Georespiration and soil respiration operate on organic carbon pools of vastly different sizes and mean residence times (MRT). Both processes occur in the shallow subsurface at the Dalmeny site in southern Saskatchewan, Canada. Steady and transient, heterogeneous simulations of vadose CO2,13CO2, and 14CO2 show that at least 98% of all subsurface respiration occurs in the solum where the MRT of labile soil carbon is about 10 years. Root respiration dominates the total during the growing season. Remaining CO2 generation occurs near the capillary surface at 6.5–7.5 m depth, where δ14C of respired CO2 indicates an MRT of about 22,000 years. This value is consistent with a respiration substrate dominated by Cretaceous‐age kerogen in the till. The simulated oxidation/georespiration rate at this depth is also consistent with observed depletion of kerogen C from the vadose zone during the Holocene. Field relations in this setting indicate that georespiration is controlled hydrogeologically by the development of aerobic vadose zones; we speculate that this may be more generally true on a global basis. Where soil parent materials contain ancient carbon, georespiration should be considered as a possible factor complicating studies of soil carbon turnover.
A carbon budget for the Arctic Ocean
Authors: Anderson Leif G. , Olsson Kristina , Chierici Melissa
Published: February 2004

We present a carbon budget for the Arctic Ocean that is based on estimates of water mass transformation and transport. The budget is constrained by conservation of mass and salt. In the model, the Eurasian and Canadian basins have been divided into five and six boxes, respectively, based on the prevailing water masses. In addition, there are three boxes representing the shelf areas. Total dissolved inorganic (CT) and organic (TOC) carbon concentrations for the different water masses and different regions are used, together with the volume flows, to calculate the carbon transports. The carbon budget calculation shows that at present the oceanic transport into the Arctic Ocean is larger than out, that is, 3.296±0.008 Gt C yr−1 of CT transported in and 3.287±0.004 Gt C yr−1 transported out with the corresponding values for TOC being 0.134±0.009 Gt C yr−1 and 0.122±0.006 Gt C yr−1, respectively. However, the outflowing waters are older than the inflowing waters and had thus been exposed to an atmosphere with lower concentration of anthropogenic carbon dioxide when entering the Arctic Ocean. When recalculating the budget to the preindustrial scenario, assuming steady state, the CT transport changes to 3.243 Gt C yr−1 in and 3.266 Gt C yr−1 out. To balance the preindustrial transports, assuming no change in the TOC fluxes, a direct input of atmospheric carbon dioxide of 0.011±0.014 Gt C yr−1 is required. Added to this is the burial of organic matter which is calculated as 0.013±0.010 Gt C yr−1 using a recycling efficiency of 80 % [Hulth, 1995] and a new production of 0.05 Gt C yr−1 [Anderson et al., 1994]. An indirect contribution of atmospheric carbon dioxide via runoff adds 0.017±0.004 Gt C yr−1, resulting in a preindustrial total atmospheric input of 0.041 ±0.018 Gt C yr−1.
Budgets of fixed nitrogen in the Orinoco Savannah Region: Role of pyrodenitrification
Authors: Sanhueza Eugenio , Crutzen Paul J.
Published: February 2004

Human activities have strongly altered the amount of fixed nitrogen that cycles in many regions of the industrialized world, with serious environmental consequences. Past studies conducted at the Orinoco savannahs of Venezuela offer a unique possibility for reviewing the cycling of nitrogen species in a tropical environment. The available information for the Orinoco savannahs is critically reviewed, and, despite many uncertainties, we present a budget analysis of both the fixed N in the soil‐vegetation system and atmospheric NOy. Analysis of the data indicates that nitrogen fixation, especially by legumes, and ammonia emission from vegetation and animal wastes needs considerable attention in future research efforts. In contrast with many regions of the world, in the studied region, nonindustrial sources, foremost biomass burning, dominate the soil‐vegetation and atmospheric budgets of fixed N. In general, N cycling is mainly driven by biomass burning. The resulting pyrodenitrification in the soil‐vegetation system is the largest single process that, during the following wet season, may promote biological fixation to compensate for the N losses from fires during the burning season. However, a gradual impoverishment of the N status of the savannah ecosystems cannot be excluded. During the dry season, biomass burning is also the main source of atmospheric NOy, which is largely exported, mainly in the direction of the Amazon forest. Together with other nutrients, a “fertilization” of the Amazon forest due to biomass burning in the savannah may be the result. These issues require further scientific analysis.
Energetic electron injections into the inner magnetosphere during the Jan. 10–11, 1997 magnetic storm
Authors: Li Xinlin , Baker D. N. , Temerin M. , Cayton T. , Reeves G. D. , Araki T. , Singer H. , Larson D. , Lin R. P. , Kanekal S. G.
Published: February 2004

At the end of the main phase of the Jan. 10–11, 1997 magnetic storm, a rapid enhancement of 0.4‐1.6 MeV electrons across L=4.2–6 was measured by particle detectors on the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and three Global Positioning System (GPS) satellites. This enhancement, over two orders of magnitude at L=4.2–5, occurred around 11:00 UT on Jan. 10, when the AE index reached ∼ 2000 nT and when a solar wind pressure pulse arrived. Using data from multiple satellites and ground stations, we determine that the rapid enhancement of 0.4–1.6 MeV electrons in the magnetosphere at L=4.2–6 was due to a combination of intense substorm activity during the early part of the magnetic storm which produced a source population and the following pressure pulse which quickly energized some of these electrons by moving them into stronger magnetic field.
SOx oxidation and volatile aerosol in aircraft exhaust plumes depend on fuel sulfur content
Authors: Miake‐Lye R. C. , Anderson B. E. , Cofer W. R. , Wallio H. A. , Nowicki G. D. , Ballenthin J. O. , Hunton D. E. , Knighton W. B. , Miller T. M. , Seeley J. V. , Viggiano A. A.
Published: February 2004

Volatile and nonvolatile aerosols were measured in the wake of a B757 airliner in flight, in concert with measurements of gaseous SOx and CO2 emissions, while the airplane was burning fuel with a sulfur content of either 72 parts per million by mass (ppmm) or 676 ppmm. The volatile aerosol number density exceeded that of the nonvolatile for both fuels and, while the nonvolatile (soot) component was largely insensitive to the fuel sulfur content, the volatile component depleted the gas‐phase sulfur species with a condensed fraction that increased from 6% (low S) to 31% (high S). The large proportion of SOx in the aerosol phase and its nonlinear dependence on fuel sulfur content cannot be explained by known combustion mechanisms and has the potential for significant environmental effects.
Comparison of in situ and satellite‐derived cloud properties during SUCCESS
Authors: Young David F. , Minnis Patrick , Baumgardner Darrel , Gerber H.
Published: February 2004

The validation of cloud microphysical properties derived from satellite data with in situ observations is generally difficult due to the spatial and temporal variability of observed clouds. Two missions during SUCCESS concentrated on orographic wave clouds that exhibited relative spatial and temporal homogeneity. A comparison has been made of effective ice particle diameters derived from multispectral satellite data with in situ measurements from the MASP and PVM instruments aboard the DC‐8 aircraft. For both days, the satellite‐derived effective diameters fall well within the bounds of the in situ data.
Ozone depletion in and below the Arctic Vortex for 1997
Authors: Knudsen B. M. , Larsen N. , Mikkelsen I. S. , Morcrette J.‐J. , Braathen G. O. , Kyrö E. , Fast H. , Gernandt H. , Kanzawa H. , Nakane H. , Dorokhov V. , Yushkov V. , Hansen G. , Gil M. , Shearman R. J.
Published: February 2004

The winter 1996/97 was quite unusual with late vortex formation and polar stratospheric cloud (PSC) development and subsequent record low temperatures in March. Ozone depletion in the Arctic vortex is determined using ozonesondes. The diabatic cooling is calculated with PV‐theta mapped ozone mixing ratios and the large ozone depletions, especially at the center of the vortex where most PSC existence was predicted, enhances the diabatic cooling by up to 80%. The average vortex chemical ozone depletion from January 6 to April 6 is 33, 46, 46, 43, 35, 33, 32 and 21 % in air masses ending at 375, 400, 425, 450, 475, 500, 525, and 550 K (about 14–22 km). This depletion is corrected for transport of ozone across the vortex edge calculated with reverse domain‐filling trajectories. 375 K is in fact below the vortex, but the calculation method is applicable at this level with small changes. The column integrated chemical ozone depletion amounts to about 92 DU (21%), which is comparable to the depletions observed during the previous four winters.
FAST/TEAMS observations of charge exchange signatures in ions mirroring at low altitudes
Authors: Kistler L. M. , Möbius E. , Klumpar D. M. , Popecki M. A. , Tang L. , Jordanova V. , Klecker B. , Peterson W. K. , Shelley E. G. , Hovestadt D. , Temerin M. , Ergun R. E. , McFadden J. P. , Carlson C. W. , Mozer F. S. , Elphic R. C. , Strangeway R. J. , Cattell C. A. , Pfaff R. F.
Published: February 2004

Using the TEAMS instrument on FAST, we have observed a case in which signatures of the inner edge of the plasma sheet are clearly observed in the energy spectra of ions mirroring at the FAST altitude. That inner edge is dominated by He+. We show that this is the natural composition expected from charge exchange as the ions drift in from the plasma sheet.
Ballooning instability of a thin current sheet in the high‐Lundquist‐number magnetotail
Authors: Bhattacharjee A. , Ma Z. W. , Wang Xiaogang
Published: February 2004

Two‐dimensional simulations of the magnetotail in the high‐Lundquist‐number regime indicate the slow growth of thin current sheets and an impulsive intensification of the cross‐tail current density at near‐Earth distances during a short interval just before the onset of the expansion phase, consistent with multi‐satellite observations. Such a two‐dimensional magnetotail, symmetric along y and containing a thin current sheet, is found to be unstable to a symmetry‐breaking, ideal compressible ballooning instability with high wave number along y. The linear instability is demonstrated by numerical solutions of the ideal ballooning eigenmode equation for a sequence of two‐dimensional thin current sheet configurations in the impulsive growth phase. Line‐tied boundary conditions at the ionosphere are imposed, and shown to play a crucial role in the stability analysis. It is suggested that the ideal ballooning instability, which has strong spatial variation along y, provides a possible mechanism for disrupting the cross‐tail current at onset.
The effect of surface geometry on fracture permeability: A case study using a sinusoidal fracture
Authors: Waite Miles E. , Ge Shemin , Spetzler Hartmut , Bahr David B.
Published: February 2004

Measurements of water flow through a sinusoidal fracture were undertaken to compare sinusoidal flow with parallel plate flow. Flow rates were measured through a fracture with an amplitude of 1.02 mm and a wavelength of 5.08 mm, with vertical separation ranging from 0.21 mm to 0.71 mm, and for Reynolds numbers (Re) ranging from 0.05 to 58. In addition, a series of lattice gas automata flow simulations of water flow through a sinusoidal fracture with the same dimensions were performed. Results from both the experiments and the numerical simulations reflect that, compared to a parallel plate channel, a sinusoidal channel has a significantly lower ability to transmit fluid. This is proposed to be due to the variation in the aperture measured normal to the flow path, the tortuosity of the flow path, and potential instabilities within the flow field.
Ion sputtering and surface erosion at Europa
Authors: Ip W.‐H. , Williams D. J. , McEntire R. W. , Mauk B. H.
Published: February 2004

New results from the energetic particle detector (EPD) experiment on the Galileo mission are used to update the ion sputtering rate at Europa. It is found that the resultant gas emission rates are 5.7 × 1027 O2 s−1 and 9.5× 1027 H2O s−1 in total if the maximum self‐sputtering effect of the exospheric pickup ions is taken into consideration. This means that the surface number density of O2 molecules could be as much as 1.8 × 109 cm−3 and the average column density is 3.4 × 1015 O2 molecules cm−2. The corresponding ion production rate is on the order of 1.3 × 1027 ions s−1 probably leading to a plasma density of about 2000 ions cm−3 in the ionospheric wake region. The magnetospheric particle bombardment effect is also effective in redistributed mass across Europa's surface. The maximum mass redistribution rate can be estimated to be ≈ 80 m per 100 Myrs with the associated net erosion rate ≈ 20 m/100 Myrs. At the same time, mass resurfacing rate by liquid water or out‐gassing could reach similar magnitude.
Changes in upper stratospheric CH4 and NO2 as measured by HALOE and implications for changes in transport
Authors: Nedoluha G. E. , Siskind D. E. , Bacmeister J. T. , Bevilacqua R. M. , Russell J. M.
Published: February 2004

Measurements from HALOE indicate that there has been a significant decrease in CH4 in the middle to upper stratosphere from 1991–1997. Two dimensional model calculations suggest that a decrease in the tropical upwelling is the most plausible explanation for the CH4 decrease. The changes in CH4 resulting from solar cycle variations or changes in tropospheric CH4 and CFC emission rates are shown to be much smaller than the measured variations from ∼35–65 km. The HALOE measurements of NO2 in the lower stratosphere near the equator also show an increase which is consistent with a reduction in the upward transport rate. Above ∼40 km the NO2 measurements appear to be consistent with the net effect of opposing solar and dynamical trends.