bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of creeping soil and implications for hillslope evolution
Authors: Furbish David Jon , Fagherazzi Sergio
Published: February 2004

The geomorphic behavior of a soil‐mantled hillslope undergoing diffusive creep involves a coupling between changes in land surface elevation, soil transport rates, soil production, and soil thickness. A linear stability analysis suggests that the coupled response of the soil mantle to small perturbations in soil thickness or surface topography is influenced by two factors. The diffusive‐like behavior of soil creep has a stabilizing effect wherein perturbations in land surface elevation are damped. The relation between the soil production rate and soil thickness may be either stabilizing or destabilizing. A monotonically decreasing production rate with soil thickness reinforces the stabilizing effect of diffusive land surface smoothing. An increasing production rate with soil thickness has a destabilizing effect wherein perturbations in soil thickness or the soil‐bedrock interface are amplified, despite the presence of diffusive land surface smoothing. This coupled behavior is insensitive to the transport relation, whether the soil flux is proportional to the land surface gradient or to the product of the soil thickness and land surface gradient. The latter type of relation, nonetheless, could lead to a more complex hillslope form than might otherwise be expected for purely diffusive transport. Moreover, the response to periodic (sinusoidal) variations in the rate of stream downcutting at the lower hillslope boundary involves upslope propagation of coupled (damped) waveforms in the land surface and the soil‐bedrock interface. The distance of upslope propagation goes with the square root of the product of the transport diffusion‐like coefficient and the period of the downcutting rate. The upper part of the hillslope is therefore insensitive to relatively high‐frequency variations in stream downcutting, so together with a stable behavior of the coupled soil‐mantle‐bedrock system, this part of the hillslope may exhibit a tendency toward uniform lowering, while the lower part behaves transiently. Conversely, in the presence of low‐frequency variations in stream downcutting, hillslope morphology and soil thickness variations are more likely to reflect unsteady conditions over the entirety of the hillslope.
Generalized Maximum Likelihood Pareto‐Poisson estimators for partial duration series
Authors: Martins Eduardo S. , Stedinger Jery R.
Published: February 2004

This paper considers use of the generalized Pareto ∥GP) distribution with a Poisson model for arrivals to describe peaks over a threshold. This yields a three‐parameter generalized extreme value (GEV) distribution for the annual maximum series. Maximum likelihood estimates of the GP shape parameter к can result in absurd estimates in small samples. These problems are resolved by addition of a prior distribution on к yielding a generalized maximum likelihood estimator. Results show that a three‐ parameter partial duration series (PDS) analysis yields quantile estimators with the same precision as an annual maximum series (AMS) analysis when the generalized maximum likelihood (GML) GP and GEV estimators are adopted. For к ≤ 0 the GML quantile estimators with both PDS and AMS have the best performance among the quantile estimators examined (moments,Lmoments, and GML). The precision of flood quantiles derived from a PDS analysis is insensitive to the arrival rate λ, so that a year of PDS data is generally worth about as much as a year of AMS data when estimating the 100‐year flood.
A mathematical model for biological clogging of uniform porous media
Authors: Seki Katsutoshi , Miyazaki Tsuyoshi
Published: February 2004

We develop a theory that explains the decrease in saturated hydraulic conductivity Ks due to biological clogging of porous media. Experiments show that discontinuous microcolonies in fine‐textured soils decrease Ks more severely than biofilms do. However, most existing models for biological clogging assume that bacteria cells form biofilms which uniformly cover pore walls. We propose a mathematical model for biological clogging with a quantitative evaluation of the nonuniform microbial distribution of colonies. A series of equations describing the relation between the biological clogging and the saturated hydraulic conductivity are derived. The data of previous researchers are used to validate the model.
Thirty‐five years of research data collection at the Reynolds Creek Experimental Watershed, Idaho, United States
Authors: Slaughter Charles W. , Marks Danny , Flerchinger Gerald N. , Van Vactor Steven S. , Burgess Mike
Published: February 2004

Comprehensive, long‐term hydrologic data sets for watershed systems are valuable for hydrologic process research; for interdisciplinary ecosystem analysis; for model development, calibration, and validation; and for assessment of change over time. The Reynolds Creek Experimental Watershed in southwestern Idaho, United States, was established in 1960 and provides a research facility and comprehensive long‐term database for science. Spatial data layers for terrain, soils, geology, vegetation, and basic site mapping features and databases for fundamental hydrologic parameters of precipitation, snow, climate, soil microclimate, and stream discharge and sediment concentration are now available for water years 1962–1996 and are described in the following eight data reports.
Floods in a changing climate: Does the past represent the future?
Authors: Jain Shaleen , Lall Upmanu
Published: February 2004

Hydrologists have traditionally assumed that the annual maximum flood process at a location is independent and identically distributed. While nonstationarities in the flood process due to land use changes have long been recognized, it is only recently becoming clear that structured interannual, interdecadal, and longer time variations in planetary climate impart the temporal structure to the flood frequency process at flood control system design and operation timescales. The influence of anthropogenic climate change on the nature of floods is also an issue of societal concern. Here we focus on (1) a diagnosis of variations in the frequency of floods that are synchronous with low‐frequency climate state and (2) an exploration of limiting flood probability distributions implied by a long simulation of a model of the El Niño/Southern Oscillation. Implications for flood risk analysis are discussed.
A simplified technique for measuring diffusion coefficients in rock blocks
Authors: Ibaraki Motomu
Published: February 2004

This study demonstrates a simple laboratory method to measure diffusion coefficients of reactive and nonreactive solutes in rock blocks. A demonstrative diffusion measurement device consists of a small plastic cell with magnetic stirrers, the porous rock block to be measured, and nonreactive tracer solution. Concentrations of the tracer (as specific conductance) are measured in the cell continuously with data recorded using a PC. The specific conductance versus time data are fitted to an analytical solution that is numerically inverted to estimate the diffusion coefficient. Mathematical development is demonstrated, and a sensitivity analysis shows that diffusion coefficient as low as 10−9cm2/s can be measured over a test period of 12 days.
Uncertainty analysis using corrected first‐order approximation method
Authors: Tyagi Aditya , Haan C. T.
Published: February 2004

Application of uncertainty and reliability analysis is an essential part of many problems related to modeling and decision making in the area of environmental and water resources engineering. Computational efficiency, understandability, and easier application have made the first‐order approximation (FOA) method a favored tool for uncertainty analysis. In many instances, situations may arise where the accuracy of FOA estimates becomes questionable. Often the FOA application is considered acceptable if the coefficient of variation of the uncertain parameter(s) is <0.2, but this criterion is not correct in all the situations. Analytical as well as graphical relations for relative error are developed and presented for a generic power function that can be used as a guide for judging the suitability of the FOA for a specified acceptable error of estimation. Further, these analytical and graphical relations enable FOA estimates for means and variances of model components to be corrected to their true values. Using these corrected values of means and variances for model components, one can determine the exact values of the mean and variance of an output random variable. This technique is applicable when an output variable is a function of several independent random variables in multiplicative, additive, or in combined (multiplicative and additive) forms. Two examples are given to demonstrate the application of the technique.
Forward modeling of ground‐penetrating radar data using digitized outcrop images and multiple scenarios of water saturation
Authors: Kowalsky M. B. , Dietrich P. , Teutsch G. , Rubin Y.
Published: February 2004

Simple petrophysical models and a sedimentologically interpreted outcrop photograph corresponding to the plane of a ground‐penetrating radar (GPR) survey are combined to create models for the simulation of GPR. This makes possible the comparison of GPR field data, synthetic GPR sections, and a lithology image. On the basis of this comparison the usefulness of the method for identifying hydrologically significant lithofacies and the sensitivity of the results to different subsurface conditions may be investigated. In particular, GPR simulations are performed for an outcrop model at three states of water saturation: uniformly drained (uniform residual saturation), nonuniformly saturated, and fully saturated. As predicted by reflection coefficient calculations, comparison among the synthetic simulations highlights the importance of the existing pore water distribution in determining the “visibility” of lithologic elements in GPR sections. Comparisons of the synthetic GPR sections with the field data show overall agreement, though the occurrence of various observed reflections depends on the presence and distribution of pore water. Conclusions are also drawn about extending outcrop analog‐derived results to investigations of real (fully saturated) aquifers.
Inverse modeling of the hydrological and the hydrochemical behavior of hydrosystems: Characterization of Karst System Functioning
Authors: Pinault J.‐L. , Plagnes V. , Aquilina L. , Bakalowicz M.
Published: February 2004

Inverse modeling of mass transfer characterizes the dynamic processes affecting the function of karst systems and can be used to identify karst properties. An inverse model is proposed to calculate unit hydrographs as well as impulse response of fluxes from rainfall‐runoff or rainfall‐flux data, the purpose of which is hydrograph separation. Contrary to what hydrologists have been doing for years, hydrograph separation is carried out by using transfer functions in their entirety, which enables accurate separation of fluxes, as was explained in the companion paper [Pinault et al., this issue]. The unit hydrograph as well as impulse response of fluxes is decomposed into a quick and a slow component, and, consequently, the effective rainfall is decomposed into two parts, one contributing to the quick flow (or flux) and the other contributing to the slow flow generation. This approach is applied to seven French karstic aquifers located on the Larzac plateau in the Grands Causses area (in the south of France). Both hydrodynamical and hydrogeochemical data have been recorded from these springs over several hydrological cycles. For modeling purposes, karst properties can be represented by the impulse responses of flow and flux of dissolved species. The heterogeneity of aquifers is translated to time‐modulated flow and transport at the outlet. Monitoring these fluxes enables the evaluation of slow and quick components in the hydrograph. The quick component refers to the “flush flow” effect and results from fast infiltration in the karst conduit network when connection is established between the infiltration and phreatic zones, inducing an increase in water head. This component reflects flood events where flow behavior is nonlinear and is described by a very short transfer function, which increases and decreases according to water head. The slow component consists of slow and fast infiltration, underground runoff, storage in annex‐to‐drain systems, and discharge from the saturated zone. These components can be further subdivided by measuring chemical responses at the karst outlet. Using such natural tracers enables the slow component of the unit hydrograph to be separated into preevent water, i.e., water of the reservoir and event water, i.e., water whose origin can be related to a particular rainfall event. These measurements can be used to determine the rate of water renewal. Since the preevent water hydrograph is produced by stored water when pushed by a rainfall event and the event water hydrograph reflects rainwater transfer, separating the two components can yield insights into the characteristics of karst aquifers, the modes of infiltration, and the mechanisms involved in karstification, as well as the degree of organization of the aquifer.
Frequency of record‐breaking floods in the United States
Authors: Vogel Richard M. , Zafirakou‐Koulouris Antigoni , Matalas Nicholas C.
Published: February 2004

The theory of record‐breaking processes offers a framework for understanding extreme events which is nearly independent of the theory of extremes. The mathematical theory of record‐breaking processes is applied to the problem of identifying nonstationarity in hydrological records. A record flood event is simply an event which exceeds all previous events. The probability distribution and first four moments of the number of record events in ann‐year period,R, are derived for a serially independent process. The variance of estimates of the mean, standard deviation, and coefficient of variationRis also derived. In addition, approximate confidence intervals are derived for the mean number of record‐breaking events in a region with spatially correlated flood series. Using these results, in combination with 1571 flood records in the United States, we document that the average number of record breaking flood events overn‐year periods ranging from [10, 80] behaved as if the annual flood series were serially independent for all regions of the United States. However, when spatial correlation of the flood records is ignored, as is the case in many previous studies, it appears as if flood records are not serially independent in the western and Midwestern regions of the United States. These results emphasize the importance of accounting for the spatial correlation structure of hydrologic records when performing regional hypothesis tests.
An examination of elevated frequency propagation over a transpolar path
Authors: Goodman John M. , Ballard John W.
Published: February 2004

A Chirpsounder® system has been installed to evaluate the communication path between Svalbard (Spitsbergen) and Barrow, Alaska. This system of measurements is used to update operational ionospheric propagation models we use to nowcast the effectiveness of high‐frequency (HF) communications over polar paths. Operations have been virtually continuous since December 2000, during which time Radio Propagation Services Inc. has made measurements of the propagation effects that have been encountered. Oblique sounder patterns have been diverse, ranging from standard (i.e., classical) ionograms with a sharp “nose” near the anticipated maximum useable frequency to rather diffuse patterns. However, the most interesting patterns consist of nonclimatological patterns of signals associated with EMOFs. These were first observed to occur during the winter, but they are in evidence throughout the year. The strong signal strengths associated with these EMOF patterns are transient and consistent with independent ionospheric layers or modulations of existing layers. The EMOF signals have been analyzed statistically, and the paper will describe the diurnal and seasonal behaviors. We will also investigate the association of these observations with polar patches and blobs, which have been observed to convect across the polar cap, especially in the winter. These observations are not simply scientific curiosities. The character and persistence of these signals have a significant impact on communication effectiveness for over‐the‐pole air transport operations. Issues of radio spectrum, service provider location, and dynamic frequency management are directly related to the phenomena observed.
The interstellar hydrogen shadow: Observations of interstellar pickup ions beyond Jupiter
Authors: McComas D. J. , Schwadron N. A. , Crary F. J. , Elliott H. A. , Young D. T. , Gosling J. T. , Thomsen M. F. , Sittler E. , Berthelier J.‐J. , Szego K. , Coates A. J.
Published: February 2004

This study analyzes the first direct, mass‐resolved observations of heliospheric pickup ions beyond the orbit of Jupiter. The Cassini Plasma Spectrometer observes H+, He+, He++, and O+ pickup ions of interstellar origin between 6.4 and 8.2 AU. Cassini's trajectory carries it through the downstream direction where we observe enhancements in the pickup He consistent with gravitational focusing by the Sun. We also show the first in situ observations of an “interstellar hydrogen shadow” where pickup H is depleted in the region behind the Sun relative to the local interstellar flow. Most H atoms cannot penetrate into this downstream shadow region both because the outward force due to radiation pressure exceeds gravitational attraction at this time and because H atoms trying to enter the shadow must pass close by the Sun where they have a high probability of being ionized and swept out with the solar wind.
Ozone perturbations in the Arctic summer lower stratosphere as a reflection of NOX chemistry and planetary scale wave activity
Authors: Akiyoshi H. , Sugita T. , Kanzawa H. , Kawamoto N.
Published: February 2004

Ozone concentration perturbations in the high‐latitude lower stratosphere in the Northern Hemisphere were observed by Improved Limb Atmospheric Spectrometer (ILAS) after the polar vortex breakdown at the beginning of May 1997 and until the end of June of that same year. Simulations and a passive tracer experiment using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) nudging chemical transport model (CTM) show that the low‐ozone perturbations observed in May were caused by the Arctic polar vortex debris, while those after the end of May resulted from a dynamical elongation due to zonal wave number 2 planetary waves of the low‐ozone region in the summer polar stratosphere, which had been developed by the catalytic ozone destruction cycle of NOX. These low‐O3 air masses of different origin were advected or elongated from the polar region to the ILAS measurement points. An episodic event of a dynamical O3 perturbation in June 1997 on a chemically induced meridional O3 gradient is described. These results show that a timing of the polar vortex breakdown and activity of planetary waves after the breakdown may affect the O3 background gradient in the summer lower stratosphere at middle and high latitudes.
Spatial and temporal variability of meteorological variables at Haut Glacier d'Arolla (Switzerland) during the ablation season 2001: Measurements and simulations
Authors: Strasser Ulrich , Corripio Javier , Pellicciotti Francesca , Burlando Paolo , Brock Ben , Funk Martin
Published: February 2004

During the ablation period 2001 a glaciometeorological experiment was carried out on Haut Glacier d'Arolla, Switzerland. Five meteorological stations were installed on the glacier, and one permanent automatic weather station in the glacier foreland. The altitudes of the stations ranged between 2500 and 3000 m a.s.l., and they were in operation from end of May to beginning of September 2001. The spatial arrangement of the stations and temporal duration of the measurements generated a unique data set enabling the analysis of the spatial and temporal variability of the meteorological variables across an alpine glacier. All measurements were taken at a nominal height of 2 m, and hourly averages were derived for the analysis. The wind regime was dominated by the glacier wind (mean value 2.8 m s−1) but due to erosion by the synoptic gradient wind, occasionally the wind would blow up the valley. A slight decrease in mean 2 m air temperatures with altitude was found, however the 2 m air temperature gradient varied greatly and frequently changed its sign. Mean relative humidity was 71% and exhibited limited spatial variation. Mean incoming shortwave radiation and albedo both generally increased with elevation. The different components of shortwave radiation are quantified with a parameterization scheme. Resulting spatial variations are mainly due to horizon obstruction and reflections from surrounding slopes, i.e., topography. The effect of clouds accounts for a loss of 30% of the extraterrestrial flux. Albedos derived from a Landsat TM image of 30 July show remarkably constant values, in the range 0.49 to 0.50, across snow covered parts of the glacier, while albedo is highly spatially variable below the zone of continuous snow cover. These results are verified with ground measurements and compared with parameterized albedo. Mean longwave radiative fluxes decreased with elevation due to lower air temperatures and the effect of upper hemisphere slopes. It is shown through parameterization that this effect would even be more pronounced without the effect of clouds. Results are discussed with respect to a similar study which has been carried out on Pasterze Glacier (Austria). The presented algorithms for interpolating, parameterizing and simulating variables and parameters in alpine regions are integrated in the software package AMUNDSEN which is freely available to be adapted and further developed by the community.
Transfer function models of suspended sediment concentration
Authors: Lemke Karen A.
Published: February 2004

The majority of past suspended sediment concentration modeling efforts have relied on simple regression analysis. Problems with these models, and in particular, sediment rating curves, have been gaining attention recently. It is argued here that many of the problems result from the fact that simple regression models are inappropriate for modeling fluvial systems because they fail to adequately represent the dynamic nature of fluvial processes. Transfer function models are system models which can account for the dynamic nature of fluvial systems. Single input‐single output and multiple input‐single output transfer function models for daily suspended sediment concentration are developed for two drainage basins in Iowa. Interpretation of the models with respect to hydrologic theory indicates that model form and parameter estimates can be related to drainage basin size, land use, and physiographic characteristics.
Growth of alternate bars in unsteady flow
Authors: Tubino Marco
Published: February 2004

A theoretical model is formulated to investigate the development of the amplitude of alternate bars in unsteady flows. The problem is tackled by means of a weakly nonlinear analysis developed in a neighborhood of the threshold conditions for bar formation. Bar response to unsteady flow is found to depend on a parameter
that is a measure of the ratio between the time scale of the basic flow and the time scale of bar growth. The present theory shows that if
is O(1), as often occurs in nature, flow unsteadiness affects the instantaneous growth rate and phase of bar perturbations and controls the final amplitude reached by the bed configuration. A procedure for determining the final amplitude for a given flood event is proposed. Flume experiments were performed to test the main theoretical results. The bed response to unsteady flow was measured for different values of the period of the flood. The observed temporal behavior of the bar amplitude proves to be strongly affected by the unsteady character of the flow for
of O(1), as predicted by the theory.
Hydraulic response of highly compressible aquitards during consolidation
Authors: Rudolph D. L. , Frind E. O.
Published: February 2004

The transient hydraulic behavior of highly compressible aquitards is investigated through numerical analysis and field studies. Variations in the hydraulic parameters of an aquitard during consolidation are accounted for by incorporating empirical relationships derived from standard consolidation tests into the one‐dimensional flow equation. The resulting equation is highly nonlinear. The hydraulic response of an aquitard with physical properties typical of the compressible clays of Mexico City is evaluated with both standard linear analysis and with a nonlinear approach where the hydraulic parameters are stress‐dependent. The results indicate that decreases in the hydraulic diffusivity of the aquitard due to consolidation lead to diminished leakage flux, slower groundwater velocities and solute transport rates, longer transient response periods and less land subsidence than would be predicted by classical linear theory. Evaluation of field data from a site near Mexico City indicates that significant interpretive errors may arise if the stress dependence of the hydraulic parameters is ignored in these types of systems.
A numerical investigation of free surface—Seepage face relationship under steady state flow conditions
Authors: Shamsai Abolfazl , Narasimhan T. N.
Published: February 2004

The relationship between free surface and seepage face under steady conditions of flow has been analyzed for radial and planar flow configurations. The numerical studies, carried out with a saturated‐unsaturated flow model, included the simulation of a series of experimental observations documented by Hall (1955). The numerical analysis took into consideration effects of capillary fringe, model geometry, as well as converging and diverging patterns of flow. For the control series of three cases studied, discharge rates, free surface location, seepage face height and the spatial distribution of potentials closely matched the experimental observations. Additional parametric studies showed that converging patterns of flow favor more pronounced development of seepage face than divergent flows. In two‐dimensional planar flow, larger drawdowns tend to favor relatively more pronounced development of seepage face. Comparison of the detailed simulation results (taking unsaturated flow into consideration) with results generated using Dupuit‐Forchheimer assumptions suggest that the latter may provide discharge estimates that are in error by 12 to 20% both for radial and for two‐dimensional planar flows.
A Monte Carlo Study of rainfall sampling effect on a distributed catchment model
Authors: Krajewski Witold F. , Lakshmi Venkataraman , Georgakakos Konstantine P. , Jain Subhash C.
Published: February 2004

A Monte Carlo study of a physically based distributed‐parameter hydrologic model is described. The catchment model simulates overland flow and streamflow, and it is based on the kinematic wave concept. Soil Conservation Service curves are used to model rainfall excess within the basin. The model was applied to the Ralston Creek watershed, a small (7.5 km2) rural catchment in eastern Iowa. Sensitivity of the model response with respect to rainfall‐input spatial and temporal sampling density was investigated. The input data were generated by a space‐time stochastic model of rainfall. The generated rainfall fields were sampled by the varied‐density synthetic rain gauge networks. The basin response, based on 5‐min increment input data from a network of high density with about 1 gauge per 0.1 km2, was assumed to be the “ground truth,” and other results were compared against it. Included in the study was also a simple lumped parameter model based on the unit hydrograph concept. Results were interpreted in terms of hydrograph characteristics such as peak magnitude, time‐to‐peak, and total runoff volume. The results indicate higher sensitivity of basin response with respect to the temporal resolution than to the spatial resolution of the rainfall data. Also, the frequency analysis of the flood peaks shows severe underestimation by the lumped model. This may have implications for the design of hydraulic structures.
Convex interpolation for gradient dynamic programming
Authors: Foufoula‐Georgiou Efi
Published: February 2004

Local approximation of functions based on values and derivatives at the nodes of a discretized grid are often used in solving problems numerically for which analytical solutions do not exist. In gradient dynamic programming (Foufoula‐Georgiou and Kitanidis, 1988) the use of such functions for the approximation of the cost‐to‐go function alleviates the “curse of dimensionality” by reducing the number of discretization nodes per state while obtaining high‐accuracy solutions. Also, efficient Newton‐type schemes can be used for the stage‐wise optimization, since now the approximation functions have continuous first derivatives. Our interest is in the case where the cost‐to‐go function is convex. However, the interpolants may not always be convex, introducing numerical problems. In this paper we address the problem of interpolating nodal values and derivatives of a one‐dimensional convex function with a convex interpolant so that potential computational difficulties due to approximation‐induced nonconvexity are avoided, and an efficient convergence to global instead of local optimal controls is guaranteed at every single‐stage optimization.
Trends in stream quality in the continental United States, 1978–1987
Authors: Lettenmaier Dennis P. , Hooper Eric R. , Wagoner Colin , Faris Kathleen B.
Published: February 2004

The nonparametric seasonal Kendall's test, and a related multivariate test were used to analyze 403 National Stream Quality Accounting Network (NASQAN) stations for trend for the period 1978–1987. Nominal sampling frequencies were quarterly or bimonthly. For all groups and individual constituents, trends were detected for only a minority of stations at the relatively liberal 10% significance level. The groups with the greatest percentage of trends were common ions (mostly upward) and nutrients and suspended sediment (mostly upward for total nitrogen and mostly downward for total phosphorus). In the pH and alkalinity group, the trends were dominantly upward; while this might be the result of reductions in atmospheric deposition, this hypothesis is clouded by the existence of uptrends in sulfate at many of the same stations. The trace metal analyses showed that the constituents with the greatest number of trends (dominantly downward) were arsenic and cadmium; most of the stations with downtrends were in the major population areas of the east and midwest. Additional exploratory analyses of possible relationships between trends and land use and population did not give strong evidence of possible causation.
Worth of radar data in the real‐time prediction of mean areal rainfall by nonadvective physically based models
Authors: Georgakakos Konstantine P. , Krajewski Witold F.
Published: February 2004

Covariance analysis was used to determine the reduction in rainfall forecast and estimation variances offered by radar reflectivity data. Covariance analysis of a particular nonadvective linear physically based model indicated that the utility of the radar reflectivity data of various elevation angles is limited in mean areal rainfall predictions, even when a very small density of rain gauges exists over the region of interest and good quality radar data are used. This applies to both raw reflectivity and radar rainfall data converted through a Z‐R relationship. The ratio of mean areal rainfall prediction variances, defined as variance with radar data divided by variance without radar data, was found to be greater than 0.8 in most cases. On the other hand, the radar data reduced the estimated variance of the vertically integrated liquid water content considerably, even when high‐density rain gauge data were present.
Geomorphological dispersion
Authors: Rinaldo Andrea , Marani Alessandro , Rigon Riccardo
Published: February 2004

The hydrologic response of a channel network is defined by decomposing the process of runoff formation into two distinct contributions, one accounting for the mechanisms of travel time within individual reaches (hydrodynamic dispersion), and the other accounting for the morphology of the network structure (geomorphological dispersion). Exact Laplace transforms of first passage time distributions at the outlet of a network are obtained by a consistent approximation of travel time distributions through individual reaches. The moments of such distributions are obtained analytically in the general case. Closed‐form first‐passage distributions are obtained in the particular case of basin‐constant hydrodynamic dispersion. The variance of the resulting travel time distributions is shown in this paper to be made up of two additive contributions corresponding to the two dispersion mechanisms considered. The geomorphologic dispersion coefficient is shown to depend on the ratios of bifurcation, length and area of the network suggesting that, at the scale of an organized network, heterogeneities other than those related to the convection field shape the dispersive character of transport. In particular, a significant application of the general solution to Hortonian channel networks suggests that models based on accurate specification of the geometry and the topology of the network and a simplified dynamics capture the foremost features of the travel time distributions in a broad range of dispersivities within individual reaches.
Measurement and spatial variation of thermal infrared surface emissivity in a savanna environment
Authors: Van de Griend A. A. , Owe M. , Groen M. , Stoll M. P.
Published: February 2004

Knowledge of the surface thermal emissivity is necessary for the application of thermal infrared remote sensing, which forms the basis for water and surface energy balance monitoring from space. Thermal emissivity was measured in the wavelength band 8–14 μm for a series of representative surfaces within a natural bush‐savanna environment intermixed with cultivated fields in Botswana. The measurements were performed with an emissivity box, which may be constructed with different dimensions to meet the general requirements of the user. The device is well‐suited for bare soil, grass, and other types of soil and low‐vegetation cover combinations. The measured emissivity varied between 0.914 for bare soil (loamy sand) and 0.986 for a surface completely covered with savanna shrub (Euclea undulata). Calculations are based on the measurements of the radiative surface temperature of three different reflecting configurations of the box. Repetitive estimates of the emissivity were found to be very reproducible for the same surface types and showed the large spatial variability of surface emissivity within the study area. This paper describes the emissivity box together with other versions of the box concept found in the literature, and it describes the operation and calibration procedure as well as the results and consequences of the emissivity measurements for the application of thermal infrared remote sensing.
Stream chemistry in the eastern United States: 1. Synoptic survey design, acid‐base status, and regional patterns
Authors: Kaufmann Philip R. , Herlihy Alan T. , Mitch Mark E. , Messer Jay J. , Overton W. Scott
Published: February 2004

To assess the regional acid‐base status of streams in the mid‐Atlantic and southeastern United States, spring base flow chemistry was surveyed in a probability sample of 500 stream reaches representing a population of 64,300 reaches (224,000 km). Approximately half of the streams had acid‐neutralizing capacity (ANC) ≤ 200 μeq L−1. Acidic (ANC ≤ 0) streams were located in the highlands of the Mid‐Atlantic region (southern New York to southern Virginia, 2330 km), in coastal lowlands of the Mid‐Atlantic (2600 km), and in Florida (462 km). Acidic streams were rare (less than 1%) in the highlands of the Southeast. Inorganic monomeric aluminum (Alim) concentrations were highest in acidic streams of the Mid‐Atlantic Highlands where over 70% of the acidic streams had Alim greater than 100 μg L−l, a concentration above which deleterious biological effects have frequently been reported. Dissolved organic carbon concentrations were much higher in lowland coastal streams, compared with inland streams. Our data support a hypothesis that atmospheric sources and watershed retention control regional patterns in streamwater sulfate concentrations. Most stream watersheds retain the vast majority of the total nitrogen loading from wet deposition. The data suggest, however, that some deposition nitrogen may be reaching streams in the Northern Appalachians. These results show that acidic surface waters are found outside the glaciated northeastern portions of the United States and that watershed sulfate retention is not sufficient to prevent acidic conditions in some Mid‐Atlantic Highlands streams.
Hydrologic characteristics and water balance of an Alpine Basin in the Sierra Nevada
Authors: Kattelmann Richard , Elder Kelly
Published: February 2004

A water balance prepared for the Emerald Lake basin illustrates the absolute and relative magnitudes of the main water transfers in this alpine catchment in the Sierra Nevada over two water years. The water balance over both years, expressed as equivalent water depths averaged over the catchment area, was total precipitation (3670 mm)— total losses in the atmosphere (800 mm) = total streamflow (2830 mm) + residual (40 mm). Snow dominated the water balance during the study period, accounting for 95% of the precipitation. The spring snowmelt periods accounted for more than 85% of the annual streamflow and were followed by long recessions through summer, autumn, and winter. Estimated total evaporation from snow, water surfaces, soil, and vegetation at Emerald Lake was 22% of the estimated precipitation. Evaporation from snow was the principal water loss to the atmosphere, accounting for about 80% of the total evaporation. Actual evapotranspiration estimated over 15 non winter months was less than 4% of the precipitation during the two water years. Groundwater storage and release accounted for only a small portion of the total quantity of water in the annual water balance of this largely impermeable basin.
Decision support system for water transfer evaluation
Authors: Stansbury J. , Woldt W. , Bogardi I. , Bleed A.
Published: February 2004

A decision support system (DSS) is developed to help decision makers analyze the economic, social, and ecological ramifications of water transfers. Such a DSS is needed because the U.S. is evolving from a development water economy to a more complex one that emphasizes conservation and reallocation (i.e., transfers). The DSS is composed of three main modules: (1) a conjunctive surface‐ground water model, (2) the impact analysis segment which uses a geographical information system that integrates model output with information from the study area to estimate economic, social, and ecological impacts, and (3) a multicriteria decision making algorithm that ranks the transfer schemes based on trade‐offs of indicators which are assembled into a hierarchical structure. A case study in which 10 different water transfer alternatives are examined is provided to demonstrate an application of the proposed DSS.
Theoretical study of the significance of nonequilibrium dissolution of nonaqueous phase liquids in subsurface systems
Authors: Powers Susan E. , Loureiro Celso O. , Abriola Linda M. , Weber Walter J.
Published: February 2004

The objective of this work is to assess the potential significance of deviations from local equilibrium for the exchange of mass between residual nonaqueous phase liquids and the aqueous phase in the saturated groundwater zone. A one‐dimensional convection‐dispersion mass balance equation incorporating a first‐order interphase mass transfer rate relationship and temporal changes in blob configuration is used to model this system. Analytical and numerical methods are employed to examine the steady state and transient behavior of the system under a variety of hypothetical aquifer conditions and pumping remediation schemes. Sensitivity of the model to several parameters including mass transfer coefficient, blob size and shape, and Darcy velocity is explored. Results of the theoretical assessment indicate that nonequilibrium effects could play a significant role in some contamination scenarios, primarily for large blob sizes and relatively high velocities. Design of soil flushing techniques will be impacted by these conclusions. Uncertainty in several parameter values used in this analysis indicate the need for further experimental investigation of this process.
Hillslope infiltration: Divergent and convergent slopes
Authors: Philip J. R.
Published: February 2004

This perturbation analysis of divergence/convergence effects on hillslope infiltration and downslope unsaturated flow indicates that, in general, these effects are unimportant for usual hillslope topography. Only where the radius of the contour is less than about 10 times the characteristic infiltration length lgrav (or 10 times the sorptive length) need the perturbation be taken into account. This result opens up a considerable extension of applicability of the analysis of infiltration on planar hillslopes to embrace slopes with curved contours, such as ridge noses and hollows.
A biophysical surface energy budget analysis of soil temperature in the boreal forests of interior Alaska
Authors: Bonan Gordon B.
Published: February 2004

Observed soil degree‐days (SDD) for 20 forest stands in the discontinuous permafrost zone of interior Alaska range from 483 to 2217. These stands differ in terms of forest structure, topography, and soils. A biophysical model that solves the surface energy budget of a multilayer forest canopy was used to examine which site factors were most important in controlling the observed soil temperature gradient. Simulated soil temperature averaged 851 SDD for the 20 sites. Sensitivity analyses indicated that this average could vary by 0–88 SDD (0–10% of the mean) because of possible parameter error. Removing the forest canopy and the moss cover caused the soil to warm, on average, by 408 and 345 SDD, respectively. Elevation and soil drainage differences among sites were of secondary importance, causing SDD to deviate by 71 and 66 SDD, respectively. Slope and aspect had little effect on soil temperature.
A hierarchical approach to regional flood frequency analysis
Authors: Gabriele Salvatore , Arnell Nigel
Published: February 2004

A hierarchical approach to regional flood frequency estimation is described, where higher‐order parameters controlling the skewness of annual maximum floods are determined from larger regions than are used for determining parameters controlling the coefficient of variation. Procedures based on two‐component extreme value and generalized extreme value distributions are developed, and it is shown that dividing a region into subregions to estimate some parameters can lead to more precise quantile estimates. The practical problem of defining the optimum number of subregions is discussed.
Large‐scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 1. Experimental design and observed tracer movement
Authors: LeBlanc Denis R. , Garabedian Stephen P. , Hess Kathryn M. , Gelhar Lynn W. , Quadri Richard D. , Stollenwerk Kenneth G. , Wood Warren W.
Published: February 2004

A large‐scale natural gradient tracer experiment was conducted on Cape Cod, Massachusetts, to examine the transport and dispersion of solutes in a sand and gravel aquifer. The nonreactive tracer, bromide, and the reactive tracers, lithium and molybdate, were injected as a pulse in July 1985 and monitored in three dimensions as they moved as far as 280 m down‐gradient through an array of multilevel samplers. The bromide cloud moved horizontally at a rate of 0.42 m per day. It also moved downward about 4 m because of density‐induced sinking early in the test and accretion of areal recharge from precipitation. After 200 m of transport, the bromide cloud had spread more than 80 m in the direction of flow, but was only 14 m wide and 4–6 m thick. The lithium and molybdate clouds followed the same path as the bromide cloud, but their rates of movement were retarded about 50% relative to bromide movement because of sorption onto the sediments.
Groundwater contamination at the Kesterson Reservoir, California: 1. Hydrogeologic setting and conservative solute transport
Authors: Benson Sally M. , White Art F. , Halfman Susan , Flexser Steven , Alavi Moshen
Published: February 2004

Shallow groundwater under Kesterson Reservoir, Merced County, California, was contaminated over a period of 6 years while the reservoir was used for disposal of brackish subsurface agricultural drainage water from farmlands in the San Joaquin Valley. Drainage water contained an average of 10,000 mg/L total dissolved solids (TDS), high concentrations of selenium (300 μg/L), nitrate (37 mg/L as nitrogen), boron (15 mg/L), and lesser amounts of mercury, chromium, and molybdenum. Infiltration and lateral migration of this water created a 20‐m‐deep plume of high‐TDS, boron‐rich water covering an area of about 5 km2. Transport of selenium and nitrate was inhibited by biogeochemical reactions in the pond bottom sediments, so that migration of these two constituents into the aquifer was limited. This paper focuses on physical parameters affecting seepage and lateral migration of nonreactive solutes and on hydrogeological factors associated with selenium break‐through into the shallow aquifer. An accompanying paper describes biogeochemical processes that limit nitrate and selenium migration (White et al., this issue). The regional hydrogeologic system at Kesterson Reservoir is dominated by the presence of four lithostratigraphic units. These are, in order of increasing depth, a fine‐grained surfical layer extending to depths of as much as 8 m, a semiconfined sandy aquifer extending to 60 m, a widespread lacustrine clay confining layer, and a confined sandy aquifer. Within each of these units, order of magnitude variations in the hydrologic properties of the sediments occur ubiquitously over lateral distances of tens of meters and vertical distances of a fraction of a meter. Seepage rates from the ponds into the underlying aquifer are dominated by local hydrogeologic properties and thickness of the fine‐grained surface layer. Selenium migrated into the aquifer in an area where the fine‐grained surface layer was thin or absent. Once in the aquifer, local rates of migration are controlled by the small‐scale structure and hydrogeologic properties of the sediments. The aquifer is composed of a heterogeneous mixture of sands, silts, and clays. Individual sand beds, which average about 0.3 m thick, do not appear to extend over lateral distances of more than 30 m. The large range of velocities combined with a limited lateral extent of individual layers are expected to create a broadly dispersed plume of solutes migrating downgradient from Kesterson. A noninvasive electromagnetic survey of electrical conductivity distributions in the shallow aquifer appears to confirm this conceptual model for transport in the shallow aquifer.