bluuebunny/crossref_metadata_2025 huggingface dataset:
Rainwater storage on plant canopies
Authors: De Ridder Koen
Published: February 2004

A physically based model is presented that describes the storage of rainwater on plant canopies. Founded on a microscopic treatment of interception and drainage, it accounts for the vertical dependence of these processes and reproduces the fact that drainage starts before the canopy is saturated. On the basis of theoretical results, a new equation for canopy water storage is proposed for use within land surface schemes as an alternative to the single‐store model, combining the advantage of a physically sounder description with a negligible additional numerical cost. A sensitivity study performed with a land surface scheme hosting the two different models shows that the difference in latent heat flux simulated with the new approach and the single‐store model can amount to several tens of watts per square meter. Finally, it is shown that the theoretical model reproduces the exact functional form of the empirical canopy drainage formula proposed by Rutter et al. [1971]. Not only does this fact inspire confidence in our model, it also opens perspectives for the measurement of certain parameters used in the new interception model.
Estimation of temperature change and of gas age‐ice age difference, 108 kyr B.P., at Vostok, Antarctica
Authors: Caillon N. , Severinghaus J. P. , Barnola J.‐M. , Chappellaz J. , Jouzel J. , Parrenin F.
Published: February 2004

Air trapped in ice core bubbles provides our primary source of information about past atmospheres. Air isotopic composition (15N/14N and 40Ar/36Ar) permits an estimate of the temperature shifts associated with abrupt climate changes because of isotope fractionation occurring in response to temperature gradients in the snow layer on top of polar ice sheets. A rapid surface temperature change modifies temporarily the firn temperature gradient, which causes a detectable anomaly in the isotopic composition of nitrogen and argon. The location of this anomaly in depth characterizes the gas age‐ice age difference (Δage) during an abrupt event by correlation with the δD (or δ18O) anomaly in the ice. We focus this study on the marine isotope stage 5d/5c transition (108 kyr B.P.), a climate warming which was one of the most abrupt events in the Vostok (Antarctica) ice isotopic record [Petit et al., 1999]. A steplike decrease in δ15N and δ40Ar/4 from 0.49 to 0.47‰ (possibly a gravitational signal due to a change in firn thickness) is preceded by a small but detectable δ15N peak (possibly a thermal diffusion signal). We obtain an estimate of 5350±300 yr for Δage, close to the model estimate of 5000 years obtained using the Vostok glaciological timescale. Our results also suggest that the use of the present‐day spatial isotope‐temperature relationship slightly underestimates (but by no more than 20±15%) the Vostok temperature change from present day at that time, which is in contrast to the temperature estimate based on borehole temperature measurements in Vostok which suggests that Antarctic temperature changes are underestimated by up to 50% [Salamatin et al, 1998].
Detection of interannual variation of vegetation in middle and southern Italy during 1985–1999 with 1 km NOAA AVHRR NDVI data
Authors: Cuomo V. , Lanfredi M. , Lasaponara R. , Macchiato M. F. , Simoniello T.
Published: February 2004

The potential of NOAA‐advanced very high resolution radiometer (AVHRR) time series for environmental studies was investigated at the pixel scale. We analyzed a multitemporal set of annual maximum value composite (MVC) of normalized difference vegetation index. Local area coverage data of middle and southern Italy were processed from measurements taken between 1985 and 1999 of the afternoon viewing of NOAA 9, 11, and 14. Significant artificial anomalies were found due to satellite switch, short‐wave calibration instability and illumination effects. We removed such systematic errors and achieved a strong reduction of standard deviation values (around 50%). Interesting results were obtained from a change detection analysis performed at the pixel level. Outcomes from satellite‐based analysis were compared with independent time series data, such as ancillary meteorological data, forest fire archives, and results from field surveys. Results showed that the geographical areas where MVC data indicated a decrease in vegetation activity match well with areas affected by forest fires, intense human activity, or rapid decline of coniferous forests. In contrast, an increase in MVC was found in regions recently involved in a growth of intensive farming or invaded by alien plants which are now recognized as a threat to native species. Our exploratory results indicate that high‐quality AVHRR data can profitably support studies on interannual dynamics of surface parameters. In particular, our parameterization of illumination and calibration adjustments can be directly applied to Mediterranean‐like ecosystems.
Bayesian confidence intervals for true fractional coverage from finite transect measurements: Implications for cloud studies from space
Authors: Astin Ivan , Di Girolamo Larry , van de Poll Helena Martine
Published: February 2004

The general probability distribution for the fractional amount of a geophysical parameter contained within a finite transect has been published previously. On the basis of this information confidence intervals were placed on the observed fraction prior to measurement from knowledge of the underlying distributions for the length of geophysical regions and of the gaps between such regions. In this form, hypothesis testing of models for these length distributions could be made given the observed fraction. However, what is also required (for change detection, for example) is the confidence interval for the true fraction given the observed fraction. Such a reversal of distribution may be provided by Bayes' theorem, as is demonstrated here for the case of underlying exponential distributions for these lengths. As an example, this is applied to transects across cloud fields observed by GMS‐5, revealing that confidence intervals for the true cloud fraction, given the observed fraction, can be rather broad over typical climate model grid scales. This is an important result given the current number of proposed satellite‐borne missions that are to make transect measurements of cloud parameters, in part to enhance such models.
A model for mineral dust emission
Authors: Shao Yaping
Published: February 2004

In the new dust emission model presented in this paper, the two major dust emission mechanisms are considered, namely, saltation bombardment and aggregates disintegration. The emission of dust generated by the first mechanism is modeled from the perspective of volume removal caused by saltating particles and that arising from the second mechanism is related to the streamwise saltation flux. The dust emission rate is constrained by using the information of minimally and fully dispersed particle size distributions. The performance of the dust emission model is compared with the available field measurements. Despite large uncertainties both in measurements and model parameters, the comparison shows that the model is promising. The accuracy of the model depends on the availability of high‐quality particle size data. It is proposed to establish a detailed database of particle size distributions for various soil types for the purpose of modeling dust emission over large areas.
Parameterization of shortwave radiation fluxes for nonuniform vegetation canopies in land surface models
Authors: Yang Rongqian , Friedl Mark A. , Ni Wenge
Published: February 2004

Net solar radiation (Q⋆) and the partitioning of Q⋆ between vegetation (Q⋆veg) and the substrate (Q⋆soil) are important quantities computed by soil‐vegetation‐atmosphere‐transfer (SVAT) models. Commonly, two‐stream models of canopy radiative transfer are used for this purpose. We examine the validity of this approach for nonuniform canopies and compare estimates of Q⋆, Q⋆veg, and Q⋆soil computed using a two‐stream model with estimates computed using a model that accounts for the effects of three‐dimensional (3‐D) structure in vegetation on radiative transfer. To accomplish these goals, a sensitivity analysis is conducted for the key input parameters to two‐stream canopy radiative transfer models. The parameters examined include leaf area index (L), leaf optical properties, solar zenith angle, leaf orientation, and background albedo. Sensitivity analyses are also conducted using a Geometric‐Optical Radiative Transfer (GORT) model. The GORT model treats vegetation canopies as being composed of 3‐D crowns and allows for both vertical variation in leaf area and horizontal variation in stem density. Results show that Q⋆ computed by the two‐stream model varies by up to 10% relative to the GORT model. Further, the partitioning of Q⋆ between the vegetation canopy and substrate computed by the two‐stream model can vary by up to 30% relative to the more realistic GORT model, even for relatively dense canopies (L = 6). These differences arise because two‐stream models use parameterizations for gap probabilities that are not realistic in discontinuous canopies. Based on this conclusion, a parameterization is proposed to include the effects of canopy heterogeneity in two‐stream models. Results from the original two‐stream model, a parameterized two‐stream model, and the GORT model are compared using data from the Boreal Ecosystem‐Atmosphere Study. These results show that the proposed parameterization captures the effect of 3‐D structure in vegetation on radiation regimes and is therefore suitable for inclusion in SVAT models. Inclusion of improved treatment for radiative fluxes in land surface parameterizations should improve modeled estimates for other flux quantities computed by SVAT models.
Overview of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) summer 1998 measurements intensive
Authors: Carroll Mary Anne , Bertman Steven B. , Shepson Paul B.
Published: February 2004

In this paper we introduce results obtained from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) program that is being conducted at the University of Michigan Biological Station in northern Michigan. PROPHET is an independent consortium of individually funded scientists whose mutual interests and varied experiences have created a synergistic collaboration focused on studies of atmospheric chemical and meteorological processes linked to tropospheric ozone. Since 1997, the PROPHET science team has combined expertise to characterize the important atmospheric issues in this region and to begin to push the limits of our knowledge of the links between the biosphere and the atmosphere. The opportunity to conduct research in the physical context of the Biological Station enables this team to interact with a tremendous range of activities related to forest and ecosystem health and uniquely positions PROPHET to make contributions to the emerging field of biosphere‐atmosphere interactions.
Evaluation of uncertainties in regional climate change simulations
Authors: Pan Z. , Christensen J. H. , Arritt R. W. , Gutowski W. J. , Takle E. S. , Otieno F.
Published: February 2004

We have run two regional climate models (RCMs) forced by three sets of initial and boundary conditions to form a 2×3 suite of 10‐year climate simulations for the continental United States at approximately 50 km horizontal resolution. The three sets of driving boundary conditions are a reanalysis, an atmosphere‐ocean coupled general circulation model (GCM) current climate, and a future scenario of transient climate change. Common precipitation climatology features simulated by both models included realistic orographic precipitation, east‐west transcontinental gradients, and reasonable annual cycles over different geographic locations. However, both models missed heavy cool‐season precipitation in the lower Mississippi River basin, a seemingly common model defect. Various simulation biases (differences) produced by the RCMs are evaluated based on the 2×3 experiment set in addition to comparisons with the GCM simulation. The RCM performance bias is smallest, whereas the GCM‐RCM downscaling bias (difference between GCM and RCM) is largest. The boundary forcing bias (difference between GCM current climate driven run and reanalysis‐driven run) and intermodel bias are both largest in summer, possibly due to different subgrid scale processes in individual models. The ratio of climate change to biases, which we use as one measure of confidence in projected climate changes, is substantially larger than 1 in several seasons and regions while the ratios are always less than 1 in summer. The largest ratios among all regions are in California. Spatial correlation coefficients of precipitation were computed between simulation pairs in the 2×3 set. The climate change correlation is highest and the RCM performance correlation is lowest while boundary forcing and intermodel correlations are intermediate. The high spatial correlation for climate change suggests that even though future precipitation is projected to increase, its overall continental‐scale spatial pattern is expected to remain relatively constant. The low RCM performance correlation shows a modeling challenge to reproduce observed spatial precipitation patterns.
Supercritical channel flow in the coastal atmospheric boundary layer: Idealized numerical simulations
Authors: Söderberg Stefan , Tjernström Michael
Published: February 2004

Numerical simulations of supercritical coastal flows were performed with a three‐dimensional, nonlinear, hydrostatic meso‐γ‐scale model with a higher‐order turbulence closure. The purpose was to resolve questions from previous studies related to terrain forcing. Idealized terrain, representing northern California, was used; this facilitated testing of hypotheses in a simple fashion. It was found that the angle by which the coast turns away from the flow regulates the acceleration in the expansion fan and determines how far downstream the maximum in the wind speed is located. Simulations with curved coastlines confirmed that the gradual curvature of the coastal mountains north of Cape Mendocino is sufficient to excite an expansion fan. A decreasing height of the terrain along the coast lead to an increased acceleration of the flow, largest when the slope of the terrain was confined to the change in coastline orientation. A cape perpendicular to the main coastal mountains significantly blocked the flow, even when as low as half the upstream marine atmospheric boundary layer (MBL) depth. Moreover, a realistic cape caused gravity‐wave breaking. These simulations confirmed that the local terrain at Cape Mendocino is responsible for the collapse of the MBL in Shelter Cove. One striking feature in this study was that the flow started to accelerate far upstream of the change in coastline orientation, even in supercritical conditions. This phenomenon was most pronounced in the cases with the highest wind speeds. Using normalized wind speed and the Bernoulli function from calculated back‐ trajectories, it was concluded that this feature is not due to deviations from linearity or from the shallow‐water concept.
Internal layer tracing and age‐depth‐accumulation relationships for the northern Greenland ice sheet
Authors: Fahnestock M. , Abdalati W. , Luo S. , Gogineni S.
Published: February 2004

Clues to previous ice sheet structure and long‐term glaciological processes are preserved in the internal layering configuration of the Greenland ice sheet. Information about these internal layers has been retrieved over many parts of the ice sheet with the University of Kansas ice‐penetrating radar. We report on the coherence of these layers over very large distances, describe a method of tracing these layers along thousands of kilometers of flight line, and do so for one flight during the 1999 Program for Arctic Regional Climate Assessment (PARCA) aircraft campaign. We determine the ages of these layers, based on information at the GRIP ice core site, and extend these ages along the flight line to Camp Century, where they are compared to modeled‐derived age estimates. These ages agree with each other to between 2 and 15%, differences that can be substantially reduced with minor changes to the model parameters (accumulation rate and shear layer depth). Finally, we are able to derive estimates of accumulation rates along the flight line by fitting the age‐depth data from layer tracing to a Dansgaard‐Johnsen model with a minimization technique, providing estimates that match recent accumulation patterns within a few centimeters per year.
South Pole submillimeter sky opacity and correlations with radiosonde observations
Authors: Chamberlin Richard A.
Published: February 2004

Submillimeter wave‐length observations of the South Pole wintertime atmospheric opacity were performed on a regular basis starting in the year 1992. The absorption spectrum of pressure broadened water vapor lines is known to dominate submillimeter atmospheric transparency, and changes in opacity should be correlated with changes in the precipitable water vapor column (PWV). Indications of PWV were derived from radiosonde observations and the expected correlation with opacity was observed. Thus submillimeter radiometry can be used to predict radiosonde derived PWV and vice versa. These comparisons can be used to help interpret radiosonde PWVs. For example, we show that the absolute calibration of one instrument type, the A.I.R. Model 5A, in use at the South Pole since February 22, 1997, is doubtful and indicates wintertime PWVs about a factor of 2 smaller than expected when compared to opacity and PWV relationships established previously. Comparisons were also made to PWVsat, the precipitable water vapor column derived by assuming saturation over ice. Radiosonde data going back to 1961 were available, and we examined long‐term trends in the wintertime PWV and PWVsat. The primary emphasis here was to reveal the expected long‐term site quality of the South Pole for performing ground based submillimeter astronomy. Thus we focused on wintertime conditions. Our comparisons of submillimeter opacity to PWV also provide robust determinations of the magnitude of dry air opacity, important for improving submillimeter radiative transfer models.
Variability of the polar night jet in the northern and southern hemispheres
Authors: Kuroda Yuhji , Kodera Kunihiko
Published: February 2004

The spatial and temporal characteristics of the month‐to‐month variability of the polar night jet and its relationship with tropospheric circulation is investigated for both the Northern and Southern Hemispheres. The variabilities of the hemispheres have many common characteristics of the Polar Night Jet Oscillation (PJO). These common characteristics include the following: (1) the anomalous zonal‐mean zonal winds shift poleward and downward; (2) the anomalous polar temperatures propagate downward from the stratopause to the upper troposphere; and (3) they are made through a wave‐mean flow interaction with mainly the planetary wave of zonal wave number one. Annular modes associated with the PJOs appear in both hemispheres when the zones of maximum polar temperature anomaly descend to the lowermost stratosphere and upper troposphere. The major difference in the PJOs of the two hemispheres is found in their temporal characteristics. In the Southern Hemisphere, the phase of the PJO is closely locked to the annual cycle, while in the Northern Hemisphere it exhibits quasiperiodic variability with its envelope controlled by the annual cycle. The origin of the differences between the PJOs is discussed based on the theory of the wave‐mean flow interaction.
Investigation of the interaction between sources and sinks of atmospheric ammonia in an upland landscape using a simplified dispersion‐exchange model
Authors: Loubet Benjamin , Milford Celia , Sutton Mark A. , Cellier Pierre
Published: February 2004

Exchange of atmospheric ammonia (NH3) with vegetation is characterized by the juxtaposition of sources and sinks at a landscape level. Such situations lead to a large fraction of the landscape being exposed to local advection effects that if not accounted for, introduce errors in standard micrometeorological measurements of NH3 exchange with the surface. In this study, a simplified dispersion ‐ exchange model for NH3 (Flux Interpretation by Dispersion and Exchange over Short Range, FIDES) is evaluated and used to assess the advection fluxes at 260 m downwind of an isolated pasture, grazed with sheep, using the measurements of a classical three‐point NH3 gradient system located on adjacent moorland. The method consists of fitting the measured and modeled concentration profile by adjusting at the same time the emission strength of the local source and the exchange rate of NH3 to the moorland area downwind. A local dispersion and surface exchange model such as FIDES has proved to be a valuable tool to estimate advection corrections, given sound estimates of background NH3 concentrations, source location, and standard meteorological parameters. According to the model results the advection fluxes at the moorland measurement site, at 1.0 m height and 260 m downwind of the grazed pasture, were positive. For 80% of the situations they ranged between 30% and 60% of the vertical flaxes. In stable conditions the advection fluxes were large and more sensitive to the surface exchange parameters. These results demonstrate that if not accounted for, advection fluxes may lead to a severe underestimate of the NH3 deposition to seminatural ecosystems, such as moorland, in the vicinity of ground level agricultural sources.
Dynamic modeling of the transition from passive to active rifting, application to the Pannonian Basin
Authors: Huismans Ritske S. , Podladchikov Yuri Y. , Cloetingh Sierd
Published: February 2004

We examine a number of first‐order features of Pannonian basin evolution in terms of the feedback relation between passive far‐field‐induced extension and active Raleigh Taylor instable upwelling of the asthenosphere. We show that active mantle upwelling following a phase of passive extension are viable mechanisms explaining the Pannonian basin formation. The dynamic interplay between far‐field‐driven passive extension and active thinning of the mantle lithosphere by convective upwelling beneath the rift zone is modeled using thermomechanical finite element methods. Our modeling results predict a first phase of passive lithospheric thinning which is followed by a second phase of late synrift to postrift active mantle lithosphere thinning due to buoyancy‐induced flow beneath the rift zone. We argue that the pattern of coeval extension in the thinning region and compression in the flanking regions may be explained by the buoyancy forces due to lithosphere thinning. It is demonstrated that timescales of and stresses generated by both processes are comparable. The model appears also to explain the occurrence of late shallow mantle‐related decompression melts in the Pannonian region and late regional doming.
Mesozoic‐Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes
Authors: Thomson Stuart N. , Hervé Francisco , Stöckhert Bernhard
Published: February 2004

Fission track (FT) analysis is applied to assess the Mesozoic and Cenozoic thermal and denudational history of the Patagonian Andes between 44° and 51°S and the geologic and geomorphic response of late Cenozoic subduction of the active Chile rise mid‐oceanic spreading center on the overriding plate. Seventy‐two FT ages from 43 samples are presented. Zircon FT ages indicate fast post intrusion cooling of Cretaceous parts of the Patagonian batholith and previously unreported Miocene magmatic rocks south of 48°S. Metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early Carboniferous and Late Permian. Apatite FT data reveal initiation of accelerated cooling and denudation at ca. 30 Ma at the western margin of southern continental South America followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca. 12–8 Ma at the position of the present‐day main topographic divide. This migration is proposed to be related to either coeval eastward migration of the retroarc deformation, the effects of subduction erosion in the overriding plate at the Peru‐Chile trench or less likely, shallowing of the angle of subduction. East of the divide, <3 km of denudation has occurred since the Late Cretaceous. Enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca. 28–26 Ma that triggered orographically enhanced precipitation on the west side of the Patagonian Andes allowing increased erosion by fluvial incision and mass transport processes. The actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation.
Bed load texture in an Alpine gravel bed river
Authors: Habersack Helmut M. , Laronne Jonathan B.
Published: February 2004

The spatial and temporal variations in bed load texture measured in a gravel bed river with a Helley‐Smith sampler are compared with its bed material texture. Most measurements were undertaken at low excess shear stress, with low bed load discharge over an armored bed. Cross‐sectionally averaged fractional bed load discharge increases with water discharge, but the explanation of the variance in fractional discharge is low (20–40%). Cross‐sectional distribution of shear velocity explains in part the spatial variation in bed load texture. Because local hydraulic parameters are weak predictors of fractional bed load discharge for the granule‐pebble range at nonequilibrium conditions, it reinforces the hypothesis that this size range, which comprises the bulk of bed load, derives from upstream sources. Temporal variations in the cross‐sectional distribution of bed load texture are shown to depend on variations in bed topography and local shear stress. We demonstrate that the dominance of granule‐pebble fractions is in part related to the hiding behavior of smaller fractions.
A comparison of metric and conceptual approaches in rainfall‐runoff modeling and its implications
Authors: Kokkonen Teemu S. , Jakeman Anthony J.
Published: February 2004

The aim of the present paper is to compare metric and conceptual approaches to rainfall‐runoff modeling in terms of calibration and simulation performances and parameter invariance. This is investigated by applying two models of equal complexity (i.e., possessing the same number of parameters), but with different levels of “conceptualization,” to two catchments with different climatology. Level of conceptualization is understood as the degree to which the model structure and its parameters can be related to catchment‐scale hydrological processes. The results suggest that the model with less conceptualization provides, in general, a more accurate reproduction of streamflow, even on independent data sets, but this difference only becomes clear when models are applied to the drier catchment. The paper corroborates that the more process complexity one wants to include in the model structure, the more types of data and higher information content are required to estimate the process parameters and to test the model performance. When only rainfall‐runoff data are available, it is difficult to justify substantial conceptualization of complex processes.
Simulations of two‐dimensional modeling of biomass aggregate growth in network models
Authors: Dupin Hubert J. , Kitanidis Peter K. , McCarty Perry L.
Published: February 2004

We investigate the mechanisms by which microorganism that grow in the form of aggregates impact the permeability and the transport properties of porous media modeled as two‐dimensional networks. In a companion paper [Dupin et al., this issue] we present a model of processes in a single channel. In this paper, we describe how to assemble channels into networks. Simple networks are investigated to identify phenomena of interest: four channels of different width operating in parallel to study the effect of local heterogeneity; a periodic network to quantify the effects of distance from the injection point on clogging and substrate utilization; and square lattice 5×5 random width networks. Although square lattice random width networks are deemed better approximations of porous media, the simpler networks exhibit all the phenomena of interest, with the added advantage of these phenomena being decoupled. Results of numerical simulations for different network types under various boundary conditions show that aggregates have a far greater potential than biofilms to clog a porous medium.
Comparison of bed load transport in torrents and gravel bed streams
Authors: Rickenmann Dieter
Published: February 2004

Data are compiled on bed load transport and discharge in gravel bed rivers and torrents with bed slopes up to 0.17. The transport characteristics of 19 streams are compared with simple bed load transport equations and with measurements from flume experiments. In simple form the bed load transport is a function of an “effective” discharge times a bed slope factor. The efficiency of the streams in transporting bed load is defined as the deviation from the transport function. It varies over several orders of magnitude, particularly for smaller and steeper streams, whereas flume experiments and larger streams are in better agreement with the simple bed load transport relation. The large variation and the strong decrease in efficiency in smaller streams appears to be associated with substantial form losses for relative flow depths (defined by h/d90) smaller than ∼4–6.
Bacterial transport in heterogeneous porous media: Observations from laboratory experiments
Authors: Silliman S. E. , Dunlap R. , Fletcher M. , Schneegurt M. A.
Published: February 2004

Transport of bacteria through heterogeneous porous media was investigated in small‐scale columns packed with sand and in a tank designed to allow the hydraulic conductivity to vary as a two‐dimensional, lognormally distributed, second‐order stationary, exponentially correlated random field. The bacteria were Pseudomonas ftuorescens R8, a strain demonstrating appreciable attachment to surfaces, and strain Ml, a transposon mutant of strain R8 with reduced attachment ability. In bench top, sand‐filled columns, transport was determined by measuring intensity of fluorescence of stained cells in the effluent or by measuring radiolabeled cells that were retained in the sand columns. Results demonstrated that strain Ml was transported more efficiently than strain R8 through columns packed with either a homogeneous silica sand or a more heterogeneous sand with iron oxide coatings. Two experiments conducted in the tank involved monitoring transport of bacteria to wells via sampling from wells and sample ports in the tank. Bacterial numbers were determined by direct plate count. At the end of the first experiment, the distribution of the bacteria in the sediment was determined by destructive sampling and plating. The two experiments produced bacterial breakthrough curves that were quite similar even though the similarity between the two porous media was limited to first‐ and second‐order statistical moments. This result appears consistent with the concept of large‐scale, average behavior such as has been observed for the transport of conservative chemical tracers. The transported bacteria arrived simultaneously with a conservative chemical tracer (although at significantly lower normalized concentration than the tracer). However, the bacterial breakthrough curves showed significant late time tailing. The concentrations of bacteria attached to the sediment surfaces showed considerably more spatial variation than did the concentrations of bacteria in the fluid phase. This contrast between behavior in the fluid phase and on the solids is consistent with field observations by other authors and initial modeling of these heterogeneous media.
Measurement of the temporal progression of scour in a pool‐riffle sequence in a gravel bed stream using an electronic scour monitor
Authors: DeVries Paul , Burges Stephen J. , Daigneau Julie , Stearns Daniel
Published: February 2004

A relatively inexpensive prototype monitor was designed and developed to record temporal variation in scour depth and was field‐tested in a gravel bed stream. The device consists of plastic practice golf balls that are fitted internally with ring magnets and strung on a two‐conductor cable enclosing a small reed switch. The balls are installed and oriented near‐vertically in the streambed. As each ball is disturbed and released, it slides along the cable past the reed switch, and the time of circuit closure caused by passage of the magnet is recorded by a data logger. The device can be applied in arrays that span large areas of the streambed, including in wide channels that are inaccessible during a flood. Data obtained from 19 devices installed in an aggrading site described scouring processes in a pool‐riffle interface during a bed load transport event. Substantial bed excavation occurred in the region of the pool edge during the rising stage, indicating existence of a local, temporally varying imbalance in bed load transport rate. Bed disturbance in the rest of the site prior to aggradation was limited to the surface and immediate subpavement layer.
Analytical solution to a sharp interface problem in a vortex‐generated flow
Authors: Kacimov A. R.
Published: February 2004

Conformal mappings and integral representations of the Dirichlet boundary value problem for analytic functions are employed to solve explicitly the problem of steady, two‐dimensional, Darcian seepage from a reservoir with fresh water to a sea in a confined aquifer of a finite thickness. A sharp interface between moving fresh and stagnant saline water forming a wedge is determined depending on one dimensionless parameter, which includes the difference in water elevations between the reservoir and the sea, the contrast in water densities, and the aquifer thickness. If the acting head reaches some critical (minimal) value, saline water will always stay at some depth; that is, the wedge will have an infinite width. In this case the interface coincides with the Saffman‐Taylor shape of a finger in a Hele‐Shaw apparatus.
Comparative rainfall observations from pit and aboveground rain gauges with and without wind shields
Authors: Duchon Claude E. , Essenberg Gavin R.
Published: February 2004

A study of rain gauge observations was initiated in 1995 to assess the magnitude of undercatch due to wind and the effect of reducing undercatch using an Alter‐type wind shield. The observation site was near the Norman, Oklahoma, airport with good exposure in all directions. The experimental setup comprised three tipping‐bucket rain gauges, three weighing‐bucket rain gauges, and anemometers at 1 and 2 m. For each type of gauge, one was placed in a pit with its orifice at ground level, and the other two were placed above ground, one with an Alter shield and one without. One‐minute rainfall accumulations and wind speed averages were obtained from 101 rainfall events over 30 months. The results show that for typical rainfall events, the undercatch of the unshielded tipping‐bucket rain gauges was 4% relative to the tipping‐bucket pit gauge. The comparable figure for the unshielded weighing‐bucket gauge was 5%. Both Alter‐shielded gauges showed less than a 1% reduction in undercatch relative to the unshielded gauges for typical rainfall events. We discuss the sources of errors that can be expected in similar rain gauge measurements when the gauges are properly maintained.
Experimental study of the transport of mixed sand and gravel
Authors: Wilcock Peter R. , Kenworthy Stephen T. , Crowe Joanna C.
Published: February 2004

We measured a wide range of transport rates for five different sand/gravel mixtures in a laboratory flume. Each mixture used the same gravel, and sand was added to produce mixtures containing 6, 15, 21, 27, and 34% sand. Control of other variables allows us to isolate the effect of bed sand content on transport. As sand content increases, gravel transport rates increase by orders of magnitude, even though the proportion of gravel in the bed decreases. The increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27%. The increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size, indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content. Bed surface grain size was measured at the end of each run. Surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate. This casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate.
Stochastic analysis of early tracer arrival in a segmented fracture pathway
Authors: Painter Scott , Cvetkovic Vladimir
Published: February 2004

A stochastic model for transport and retention in fractured rock is proposed as an alternative to more elaborate numerical simulations. Contaminants are transported by advection through a series of n rock fractures. The microscopic processes of matrix diffusion and sorption act together to retain contaminants in the host rock. The transport pathway is characterized by large‐scale fluctuations in the advection velocity caused by spatial variability in the fracture attributes along the pathway. The moments of the time of arrival of an initial solute pulse are related in a generic way to the joint distribution of fracture aperture and length. These moments are controlled primarily by a dimensionless retention parameter that incorporates all the deterministic model parameters and by correlation between fracture length and aperture. The results show that a systematic bias in the predicted contaminant transport can be introduced by neglecting spatial variability in fracture attributes. The relative contributions of fracture‐to‐fracture variability and within‐fracture variability are also explored. The proposed stochastic methodology can be merged with relatively simple numerical simulations to broaden the applicability..
Concentration variance and spatial covariance in second‐order stationary heterogeneous conductivity fields
Authors: Vanderborght Jan
Published: February 2004

The concentration variance and spatial covariance resulting from convective‐dispersive transport driven by a uniform mean flow in second‐order stationary conductivity fields was derived in a Lagrangian framework based on a first‐order approximation of solute particle trajectories. The approximation of the concentration (co)variances for large injection volumes is considerably simplified by defining the concentration at a certain location x and time t in terms of the “backward” solute trajectory probability distribution. This is the probability that the trajectory of a fictitious microscopic and indivisible solute particle, which is in a volume Δx centered around x at time t, was at the time of solute injection, t0, in the injection volume, V0. The approximated concentration (co)variances were validated against concentration covariances derived from transport simulations in generated second‐order stationary conductivity fields. The approximate solutions reproduced fairly well the effects of local scale dispersion and of the spatial variability of the hydraulic conductivity on the concentration (co)variance. The effect of the spatial structure of the hydraulic conductivity field on the spatial covariance of the concentrations was investigated in order to identify parameters that can be unequivocally determined from the structure of the concentration field. For a given spreading of the solute plume in the mean flow direction, X11(t), a given spatial correlation length of the loge transformed hydraulic conductivity in the transverse to flow direction, Iƒ2, and a given local scale dispersion Dd the concentration covariance was nearly invariant and hardly influenced by the anisotropy of the covariance function, e, and the variance, σf2, of the loge transformed conductivity. As a result, e and σf2 cannot be unequivocally determined from the spatial structure of the concentration field. The concentration variance and spatial covariance in the transverse to mean flow direction are predominantly determined by the lateral component of the local scale dispersion, Dd22 and by the spatial correlation length of the loge transformed conductivity in the transverse to flow direction, Iƒ2. These two parameters might be unequivocally derived from the concentration (co)variance.
Evaluation of the filter paper technique for in situ sampling of solute transport in unsaturated soils and tuffs
Authors: Tseng Peng‐Hsiang , Bussod Gilles Y.
Published: February 2004

The performance of a filter paper sampling system for monitoring solute transport in unsaturated soils and tuffs was evaluated using numerical simulations. The sampling system consisted of a collection borehole and a sampling assembly designed specifically for the easy removal and replacement of a filter paper pad used to collect solution samples at locations throughout the borehole. Simulations were conducted to approximate an experimental setup at Busted Butte, Nevada, for two hypothetical soil formations: a Calico Hills tuff and a loamy sand. The purpose of the simulations was to evaluate any significant effects the sampling system might have on the prevailing flow and transport processes. The simulation results revealed that the overall disturbance of the system is a superposition of three major components caused by the installation and operation of the sampling device. Two of these components delayed the observed solute breakthroughs and hence increased the mean and variance of solute travel times, whereas the third disturbed the system by increasing the mean solute velocity. The analytical solution of the one‐dimensional convective‐dispersive equation in the fluid coordinate frame was introduced to approximate the system response at different levels of disturbance for a better understanding of the system dynamics. The general difficulties in interpreting flow and transport properties using field‐measured data are illustrated. To better interpret field data, it is important to understand the limitations a measuring device may impose on data analysis.
A method for determining intracloud lightning and ionospheric heights from VLF/LF electric field records
Authors: Smith D. A. , Heavner M. J. , Jacobson A. R. , Shao X. M. , Massey R. S. , Sheldon R. J. , Wiens K. C.
Published: February 2004

The Los Alamos Sferic Array (LASA) recorded VLF/LF electric‐field‐change signals from over ten million lightning discharges during the period from 1998 to 2001. Using the differential‐times‐of‐arrival of lightning sferics recorded by three or more stations, the latitudes and longitudes of the source discharges were determined. Under conditions of favorable geometry and ionospheric propagation, sensors obtained ionospherically reflected skywave signals from the lightning discharges in addition to the standard groundwave sferics. In approximately 1% of all waveforms, automated processing identified two 1‐hop skywave reflection paths with delays indicative of an intracloud (height greater than 5 km) lightning source origin. For these events it was possible to determine both the height of the source above ground and the virtual reflection height of the ionosphere. Ionosphere heights agreed well with published values of 60 to 95 km with an expected diurnal variation. Source height determinations for 100,000+ intracloud lightning events ranged from 7 to 20 km AGL with negative‐polarity events occurring above ∼15 km and positive‐polarity events occurring below ∼15 km. The negative‐polarity events are at a suprisingly high altitude and may be associated with discharges between the upper charge layer of a storm and a screening layer of charge above the storm. Approximately 100 of the intracloud events with LASA height determinations were also recorded by VHF receivers on the FORTE satellite. Independent FORTE source height estimates based on delays between direct and ground‐reflected radio emissions showed excellent correlation with the VLF/LF estimates, but with a +1 km bias for the VLF/LF height determinations.
On the observed variability of the cross–polar cap potential
Authors: Bristow W. A. , Greenwald R. A. , Shepherd S. G. , Hughes J. M.
Published: February 2004

A study of the cross–polar cap potential estimated by the HF Super Dual Auroral Radar Network (SuperDARN) during periods of steady interplanetary magnetic field (IMF) and solar wind velocity is presented. The potential estimates were examined as time series along with time series of the IMF and solar wind. In addition, the data were examined statistically to obtain a best fit linear expression for the potential as a function of the solar wind and IMF, which was used for comparison with observed potential values. It was found that the observed potential was significantly more variable than would have been predicted by the observed solar wind and IMF. The variability was found to be on the order of 20 kV, while the predicted variability would have been only 5 kV. Implications of these findings with regard to magnetospheric physics, ionospheric simulations, and space weather are discussed.
A study of tidal and planetary wave periodicities present in midlatitude sporadic E layers
Authors: Haldoupis Christos , Pancheva Dora , Mitchell N. J.
Published: February 2004

The diurnal and semidiurnal atmospheric tides are known to be of fundamental importance in the formation of midlatitude sporadic E layers, acting through their vertical windshear forcing of the long‐living metallic ions in the lower thermosphere. Also, recent studies suggested that planetary waves play a role on sporadic E generation as well, a fact that went unnoticed in the long‐going research of sporadic layers. In this paper a methodology is employed to investigate the tidal and planetary wave periodicities imprinted onto sporadic E critical frequencies foEs. In this approach, standard analysis techniques used in neutral atmospheric dynamics are applied on foEs time series obtained during summertime when sporadic E occurrence is nearly continuous. It is shown that besides the dominant and known 24‐hour and 12‐hour tidal periodicities in foEs, there is often a weaker terdiurnal (8‐hour) oscillation present as well. In addition, there are planetary wave periodicites in foEs with periods near the normal Rossby modes, that is, 2, 5, 10, and 16 days. It is also found that the tidal oscillations in foEs undergo a strong amplitude modulation with periods comparable to the dominant planetary wave periodicities present in the data. Our results are in line with recent findings based on a single event study which suggested that sporadic E layers are affected indirectly by planetary waves through their nonlinear interaction and modulation of the atmospheric tides at lower altitudes. The close relationship between neutral wave dynamics and midlatitude sporadic E periodicities suggests that the ionosonde data can be used as an alternative means of studying tidal and planetary wave characteristics and their climatology in the lower thermosphere.