Water Resources Research

Cunderlik, Juraj M.; Ouarda, Taha B. M. J.; Bobée, Bernard

doi: 10.1029/2003WR002295pmid: N/A

The determination of seasons of high and low probability of flood occurrence is a task with many practical applications in contemporary hydrology and water resources management. Flood seasons are generally identified subjectively by visually assessing the temporal distribution of flood occurrences and, then at a regional scale, verified by comparing the temporal distribution with distributions obtained at hydrologically similar neighboring sites. This approach is subjective, time consuming, and potentially unreliable. The main objective of this study is therefore to introduce a new, objective, and systematic method for the identification of flood seasons. The proposed method tests the significance of flood seasons by comparing the observed variability of flood occurrences with the theoretical flood variability in a nonseasonal model. The method also addresses the uncertainty resulting from sampling variability by quantifying the probability associated with the identified flood seasons. The performance of the method was tested on an extensive number of samples with different record lengths generated from several theoretical models of flood seasonality. The proposed approach was then applied on real data from a large set of sites with different flood regimes across Great Britain. The results show that the method can efficiently identify flood seasons from both theoretical and observed distributions of flood occurrence. The results were used for the determination of the main flood seasonality types in Great Britain.

Daly, Edoardo; Porporato, Amilcare

doi: 10.1029/2003WR002438pmid: N/A

Particular cases of groundwater flow along a hillslope are studied using the groundwater hydraulic theory (e.g., Dupuit approximation) described by the Boussinesq equation. Analytical solutions are found as simple transformations of known similarity solutions. In particular, the expressions for phreatic surface level and volume flux and the corresponding loop‐rating curve for the flow of a groundwater mound as well as the travelling wave solutions of the Boussinesq equation are described in detail.

Ryan, Robert J.; Packman, Aaron I.; Welty, Claire

doi: 10.1029/2003WR002458pmid: N/A

We used an anthropogenic tracer signal to evaluate downstream solute transport and storage in Valley Creek, a 60 km2 watershed stream near Philadelphia, Pennsylvania, that is subject to urbanization and anthropogenically induced unsteady flow. Bromide‐bearing groundwater from an abandoned mineral processing plant enters the upstream reach of Valley Creek through a series of well‐defined seeps and springs, producing a steady and significant concentration of bromide in Valley Creek. In addition, at the time of the study, a quarry located near the center of the watershed discharged accumulated groundwater seepage to the stream on a cyclical basis. The quarry discharge reduced the bromide concentration in the main stream and produced a periodicity in the in‐stream bromide concentration downstream of the quarry. We used these variations in the ambient bromide concentration to assess solute mixing and transport in Valley Creek. We applied the USGS code OTIS to analyze solute advection, dispersion, transient storage, and groundwater inflow over a 7.5 km stream reach. To apply OTIS for unsteady flow conditions, we independently modeled the variation of stream conditions during a flow cycle and then simulated solute transport using temporal and reach average values of the longitudinal dispersion coefficient (D), transient storage exchange rate (α), and transient storage area (As/A). Calibrated values of D ranged from 0.5 m2 s−1 to 1.4 m2 s−1, As/A ranged from 0.03 to 1.3, and α ranged from 8.7 × 10−6 s−1 to 1.0 × 10−2 s−1. Observed temporal variability in the in‐stream bromide concentration was simulated well. The observed transient storage parameters reflect differences in the structure of the three test reaches. Solute storage in the middle reach was dominated by in‐stream storage produced by relict hydraulic structures, while solute transport in the other two reaches reflected differences in bed sediment characteristics between upstream and downstream reaches. However, overparameterization of the model and high sensitivity to the dispersion coefficient made it difficult to assess the magnitude of transient storage or hyporheic exchange.

Leconte, Robert; Brissette, François; Galarneau, Martine; Rousselle, Jean

doi: 10.1029/2003WR002312pmid: N/A

An approach for mapping near‐surface soil moisture at the watershed scale from RADARSAT‐1 synthetic aperture radar (SAR) data was developed and tested on seven RADARSAT‐1 SAR images acquired over the northern portion of the Châteauguay River Basin in southwestern Quebec, Canada, dominated by agricultural and herbaceous fields. A soil surface roughness map was first retrieved from a SAR image by inverting an empirical backscatter model with known (or assumed) soil moisture. The resulting map was then used with the backscatter model to recover near‐surface soil moisture for the remaining SAR images. Field campaigns were conducted concurrent to SAR image acquisitions to measure soil moisture and surface roughness in 24 fields. Good agreement was observed between watershed‐scale soil moisture values and measurements averaged for all sampled fields, with a correlation coefficient of 0.96 and an RMS error of 2.2%. However, considerable scatter was found between observed and SAR‐derived soil moisture estimates at the field scale. Although the generated maps reveal reasonable small‐scale soil moisture variability, no definitive conclusions could be drawn as to whether or not the proposed approach can quantify soil moisture at the field or within field scale due to insufficient ground measurements. Furrows and herbaceous and crop vegetation, which are known to affect the radar signal, appeared to have little influence on the ability to retrieve soil moisture at the watershed scale for the images analyzed in this study.

Thyer, Mark; Beckers, Jos; Spittlehouse, Dave; Alila, Younes; Winkler, Rita

doi: 10.1029/2003WR002414pmid: N/A

This study evaluates the performance and internal structure of the distributed hydrology soil vegetation model (DHSVM) using 1998–2001 data collected at Upper Penticton Creek, British Columbia, Canada. It is shown that clear‐cut snowmelt rates calculated using data‐derived snow albedo curves are in agreement with observed lysimeter outflow. Measurements in a forest stand with 50% air crown closure suggest that the fraction of shortwave radiation transmitted through the canopy is 0.18–0.28 while the hemispherical canopy view factor controlling longwave radiation fluxes to the forest snowpack is estimated at 0.81 ± 0.07. DHSVM overestimates shortwave transmittance (0.50) and underestimates the view factor (0.50). An alternative forest radiation balance is formulated that is consistent with the measurements. This new formulation improves model efficiency in simulating streamflow from 0.84 to 0.91 due to greater early season melt that results from the enhanced importance of longwave radiation below the canopy. The model captures differences in canopy rainfall interception between small and large storms, tree transpiration measured over a 6‐day summer period, and differences in soil moisture between a dry and a wet summer. While the model was calibrated to 1999 snow water equivalent (SWE) and hydrograph data for the untreated control basin, it successfully simulates forest and clear‐cut SWE and streamflow for the 3 other years and 4 years of preharvesting and postharvesting streamflow for the second basin. Comparison of model states with the large array of observations suggests that the modified model provides a reliable tool for assessing forest management impacts in the region.

Sukop, Michael C.; Or, Dani

doi: 10.1029/2003WR002333pmid: N/A

The lattice Boltzmann method (LBM) has emerged as a powerful tool for simulating the behavior of multiphase fluid systems in complex pore networks. Specifically, the single component multiphase LBM can simulate the interfacial phenomena of surface tension and adsorption and thus be used for modeling fluids such as water and its vapor in porous media. This paper provides an introduction to LBM applications to interface configurations in partially saturated porous media. Key elements of this LBM application are fluid‐fluid and fluid‐solid interactions that successfully mimic the Young‐Laplace equation and liquid film adsorption. LBM simulations of liquid behavior in simple pore geometry considering capillarity and adsorption are in good agreement with analytical solutions and serve as critical first steps toward validating this approach. We demonstrate the usefulness of LBM in constructing virtual liquid retention measurements based on porous media imagery. Results of this study provide a basis for application of LBM to understanding liquid configurations in more complex geometries and clear a path for applications involving interface migration, flow, and transport in partially saturated porous media.

Tronicke, Jens; Holliger, Klaus; Barrash, Warren; Knoll, Michael D.

doi: 10.1029/2003WR002031pmid: N/A

We have investigated the potential of combining cross‐hole georadar velocity and attenuation tomography as a method for characterizing heterogeneous alluvial aquifers. A multivariate statistical technique, known as k‐means cluster analysis, is used to correlate and integrate information contained in velocity and attenuation tomograms. Cluster analysis allows us to identify objectively the major common trends in the tomographic data and thus to “reduce” the information to a limited number of characteristic parameter combinations. The application of this procedure to two synthetic data sets indicates that it is a powerful tool for converting the complex relationships between the tomographically derived velocity and attenuation structures into a lithologically and hydrologically meaningful zonation of the probed region. In addition, these synthetic examples allow us to evaluate the reliability of further petrophysical parameter estimates. We find that although absolute values of the tomographically inferred petrophysical parameters often differ significantly from the actual parameters, the clustering approach enables us to reliably identify the major trends in the petrophysical properties. Finally, we have applied the approach to a cross‐hole georadar data set collected in a well‐studied alluvial aquifer. A comparison of the clustered tomographic section with well‐log data demonstrates that our approach delineates the hydrostratigraphic zonation.

Lemke, Lawrence D.; Abriola, Linda M.; Goovaerts, Pierre

doi: 10.1029/2003WR001980pmid: N/A

The influence of aquifer property correlation on multiphase fluid migration and entrapment was explored through the use of correlated and uncorrelated porosity, permeability, and capillary pressure‐saturation (Pc‐Sat) parameter fields in a cross‐sectional numerical multiphase flow model. Data collected from core samples in a nonuniform sandy aquifer were used to generate three‐dimensional aquifer parameter fields. Porosity was assumed to be uniform or simulated using sequential Gaussian simulation (SGS). Permeability (k) was modeled independently of porosity using SGS as well as simulated geostatistical indicator classes derived from measured grain size distribution curves. Retention characteristics were assigned employing Leverett scaling of a representative Pc‐Sat curve to the geostatistical k fields or, alternatively, on the basis of simulated indicator classes and porosity values. Ensemble dense nonaqueous phase liquid (DNAPL) infiltration and entrapment behavior for a hypothetical tetrachloroethylene (PCE) spill was simulated in four sets of two‐dimensional profiles extracted from these realizations. Comparisons of saturation profiles and spatial moments from point source DNAPL infiltration simulations suggest that choices involving the geostatistical algorithm used to model k and the incorporation of variable versus uniform porosity have a smaller influence than choices involving the scaling of capillary retention properties to k. From these simulations it is apparent that the degree of spatial correlation in Pc‐Sat parameters exerts a controlling influence on predicted DNAPL spreading and redistribution in saturated aquifers. The resultant distribution of mass within a DNAPL source zone will have implications for DNAPL recovery and subsequent mass fluxes in remediation operations.

Patriarche, Delphine; Michelot, Jean‐Luc; Ledoux, Emmanuel; Savoye, Sébastien

doi: 10.1029/2003WR002600pmid: N/A

Argillites are one of the rock types studied by French authorities for their confining properties for the isolation of radioactive wastes. One of the main objectives of such study is the better understanding of water transport through rocks with very low water content and hydraulic conductivity, using modeling of tracer profiles. This article presents the protocol developed and applied for acquiring data on chloride in interstitial water of the Toarcian argillites in Tournemire (southern France). This protocol is based on laboratory experiments involving diffusion process and on modeling. Experimental data obtained during transient and steady parts of diffusion allow for the assessment of the diffusion coefficient and initial concentration in pore water, respectively. Profiles for both have been acquired along the geological sequence; they are used in part 2 of this article for proposing a hydrogeological model where diffusion appears to be the main process for mass transport through the argillites and for comparing deuterium and chloride transport.

Tiedeman, Claire R.; Hsieh, Paul A.

doi: 10.1029/2003WR002401pmid: N/A

We simulate three types of forced‐gradient tracer tests (converging radial flow, unequal strength two well, and equal strength two well) and natural‐gradient tracer tests in multiple realizations of heterogeneous two‐dimensional aquifers with a hydraulic conductivity distribution characterized by a spherical variogram. We determine longitudinal dispersivities (αL) by analysis of forced‐gradient test breakthrough curves at the pumped well and by spatial moment analysis of tracer concentrations during the natural‐gradient tests. Results show that among the forced‐gradient tests, a converging radial‐flow test tends to yield the smallest αL, an equal strength two‐well test tends to yield the largest αL, and an unequal strength two‐well test tends to yield an intermediate value. This finding is qualitatively explained by considering the aquifer area sampled by a particular test. A converging radial‐flow test samples a small area, and thus the tracer undergoes a low degree of spreading and mixing. An equal strength two‐well test samples a much larger area, so the tracer is spread and mixed to a greater degree. Results also suggest that if the distance between the tracer source well and the pumped well is short relative to the lengths over which velocity is correlated, then the αL estimate can be highly dependent on local heterogeneities in the vicinity of the wells. Finally, results indicate that αL estimated from forced‐gradient tracer tests can significantly underestimate the αL needed to characterize solute dispersion under natural‐gradient flow. Only a two‐well tracer test with a large well separation in an aquifer with a low degree of heterogeneity can yield a value of αL that characterizes natural‐gradient tracer spreading. This suggests that a two‐well test with a large well separation is the preferred forced‐gradient test for characterizing solute dispersion under natural‐gradient flow.