bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial analysis of hydraulic conductivity measured using disc infiltrometers
Authors: Mohanty B. P. , Ankeny M. D. , Horton R. , Kanwar R. S.
Published: February 2004

Spatial variability of surface hydraulic properties and the extrinsic (e.g., traffic, cropping, etc.) and intrinsic (e.g., soil type, pore size distribution, etc.) factors associated with these properties are important for infiltration and runoff processes in agricultural fields. Disc infiltrometers measured infiltration at 296 sites arranged on two parallel transects. To examine and differentiate the factors contributing to spatial structure under different field conditions these measurements were made in the corn rows, no‐track interrows, and wheel track interrows of the field using four different soil water tensions Ψ (0, 30, 60, and 150 mm). Unsaturated hydraulic conductivity (K) and saturated hydraulic conductivity (Ks) were maximum in the corn rows and minimum in wheel track interrows, with no‐track interrows intermediate. Exponents (α parameters) of Ks and K relationships (K = Ks exp−αΨ) for corn rows and no‐track interrows were not significantly different from each other but were significantly different from a for the wheel track interrows at P = 0.01 level. Spatial variability of K and Ks values showed some pseudoproportional effect in nugget variance for all three field conditions. No‐track interrows clearly showed an inverse trend for semivariogram of K with changing tension (Ψ) values, whereas differences were found for corn rows and wheel traffic interrows. The spatial structure of a for all three field conditions were mostly white noise. Under corn rows, in addition to random variation, a small five‐row periodic variation at the P = 0.20 level, matching the five‐row traffic configuration, was discovered. The spatial structure of α was influenced by soil type for the no‐track interrows. Spatial structure was absent in wheel track interrows, indicating the destruction of pore structure due to compaction.
Effect of rainfall‐sampling errors on simulations of desert flash floods
Authors: Michaud Jene Diane , Sorooshian Soroosh
Published: February 2004

The effect of rainfall‐sampling errors on distributed hydrologic simulations was evaluated in a study conducted with localized thunderstorms and a midsized (150 km2) semiarid watershed. Rainfall fields based on observations from a very dense rain gage network were compared to rainfall fields based on observations from a subset of the original gages. The rain gage density of the “sparse” network (1 gage per 20 km2) was selected to represent the typical gage density of a local evaluation in real time (ALERT) type flash flood warning system. Inadequate rain gage densities in the case of the sparse network produced errors in simulated peaks that, on the average, represented 58% of the observed peak flow. Approximately half of the difference between observed and simulated peaks was due to rainfall‐sampling errors. Simulations were also conducted with rainfall that is similar to the next generation weather radar (NEXRAD) digital precipitation estimates in that it represents areal averages within 4 km × 4 km pixels. Spatial averaging of rainfall over 4 km × 4 km pixels led to consistent reductions in simulated peaks that, on the average, represented 50% of the observed peak flow. Hence it appears that the current spatial resolution of ALERT‐type precipitation measurements and 4 km × 4 km radar precipitation estimates may not be sufficient to produce reliable rainfall‐runoff simulations/forecasts in midsized watersheds of the southwestern United States subject to localized thunderstorms and large infiltration losses.
The pricing of water in a university town: An economic analysis of draining a cash cow
Authors: Joyce B. Patrick , Merz Thomas E.
Published: February 2004

This paper analyzes some economic issues involved with the common practice of using metered water rate revenue to fund debt retirement associated with the provision of municipal water and wastewater services. We conclude that rather than simply raising the metered rate, city officials should seriously consider increasing the tax rate levied under the local property tax. There is an important trade‐off in the choice of a price policy. An increased property tax rate can result in tax savings to some home owners, which lowers their net expenditure for water. However, a corresponding decrease in the metered rate may increase water consumption, which in turn raises operating cost. In order to do what is best for home owners, it might make sense to give other customers (e.g., a university) an easy ride, even if the latter, because of its low (inelastic) price elasticity of demand for water, is viewed by the municipality as a cash cow.
The unit response of groundwater outflow from a hillslope
Authors: Brutsaert Wilfried
Published: February 2004

Subsurface flow from a hillslope can be described by hydraulic groundwater theory, as formulated by the Boussinesq equation. Solution of its linear version brings out some of the essential features of this phenomenon. Arbitrary inputs can be accommodated by simple convolution of the sudden drawdown problem. The relative magnitude of the driving mechanisms, namely the streamwise pressure gradient, resulting in diffusion and gravity (i.e., slope), resulting in advection, can be determined by a simple dimensionless parameter.
Multiscale modeling of spatially variable water and energy balance processes
Authors: Famiglietti J. S. , Wood E. F.
Published: February 2004

This paper presents the model development component of a body of research which addresses aggregation and scaling in multiscale hydrological modeling. Water and energy balance models are developed at the local and catchment scales and at the macroscale by aggregating a simple soil‐vegetation‐atmosphere transfer scheme (SVATS) across scales in a topographic framework. A spatially distributed approach is followed to aggregate the SVATS to the catchment scale. A statistical‐dynamical approach is utilized to simplify the large‐scale modeling problem and to aggregate the SVATS to the macroscale. The resulting macroscale hydrological model is proposed for use as a land surface parameterization in atmospheric models. It differs greatly from the current generation of land surface parameterizations owing to its simplified representation of vertical process physics and its statistical representation of horizontally heterogeneous runoff and energy balance processes. The spatially distributed model formulation is explored to understand the role of spatial variability in determining areal‐average fluxes and the dynamics of hydrological processes. The simpler macroscale formulation is analyzed to determine how it represents these important dynamics, with implications for the parameterization of runoff and energy balance processes in atmospheric models.
Sensible and latent heat flux predictions using conditional sampling methods
Authors: Katul Gabriel , Albertson John , Chu Chia‐Ren , Parlange Marc , Stricker Han , Tyler Scott
Published: February 2004

The conditional sampling formulation typically used in eddy accumulation flux measurements was tested at two sites using velocity, temperature, and specific humidity time series measurements. The first site was at Owen's Lake in southern California, and the second site was an irrigated bare soil field at the Campbell Tract facility in Davis, California. The constant β relating the turbulent flux to the accumulated concentration difference between updrafts and downdrafts was related to the statistics of the vertical velocity. It was found that this formulation for β reproduces the extreme fluxes (both sensible and latent heat) better than previous β formulations. The overall comparison between eddy correlation measured fluxes and conditional sampling predicted fluxes for a wide range of atmospheric stability conditions was excellent (R2 = 0.97). On average, the mean β obtained from our experiments for temperature and water vapor is in good agreement (±4%) with other reported β values for carbon dioxide, water vapor, ozone, and temperature. These results suggest that the conditional sampling formulation used in eddy accumulation methods is useful for routine flux measurements of passive greenhouse gases.
Morphological Estimation of the Time‐Integrated Bed Load Transport Rate
Authors: Lane S. N. , Richards K. S. , Chandler J. H.
Published: February 2004

Recent research has suggested that given the problems associated with more conventional methods of estimating the bed load transport rate in dynamic rivers, measurement of the transfer of bed material, as indicated by changes in river channel form, may be an appropriate alternative. Thus far, this methodology has been successfully employed to estimate medium‐term bed load transport rates at the reach scale. This paper illustrates the potential of such a technique and extends it further through application to the within‐reach scale and estimation of spatially distributed patterns of bed load transport. It uses a data collection technique that allows the rapid field acquisition of digital terrain models of river channel form. Intercomparison of digital terrain models from separate time periods allows calculation of both the total volumes and the distributed volumes of erosion and deposition. The total volumes can be combined with upstream information on sediment supply to assess the contribution of a particular reach to the total bed load transport rate. The distributed information can be combined with a likely direction of movement of material in transport to estimate the distributed pattern of the time‐averaged bed load transport rate. Both of these approaches show a good correspondence with observed patterns of the bed load transport rate.
The role of stress release fracturing in the development of cavernous porosity in carbonate aquifers
Authors: Sasowsky Ira D. , White William B.
Published: February 2004

Stress relief fracturing provides the large mechanical apertures needed for the development of conduit porosity in carbonate aquifers. Examination of valleys incised into Mississippian age limestones throughout the Appalachian plateaus has demonstrated a consistent pattern taken in the development of conduits in these settings. Master conduits (caves) are found beneath the valley walls in general, not directly beneath the actual surface channel. Common features of the studied “Cumberland style” caves are valley‐parallel master conduit segments, exceptional concordance of conduit orientation with minor surface topographic variations, development on distinct “levels,” and development in a downdip position from master surface streams. The recognition that these conduits form along geologically young fracture systems of increased aperture is a departure from previous theories of cave origin. Prior theories have either explicitly or implicitly assumed that caves develop along joints and fractures of significant geologic age, related more to regional tectonic forces than to present‐day topographic influences.
An analytical solution to equations describing rate‐limited soil vapor extraction of contaminants in the vadose zone
Authors: Goltz Mark N. , Oxley Mark E.
Published: February 2004

Soil vapor extraction (SVE) is a technique that is commonly used to remove volatile organic compounds from the vadose zone. Recent research has demonstrated that rate‐limited sorption and desorption of these compounds can have a profound impact on the concentration reductions achievable by SVE. In this note, one‐dimensional equations presented by Brusseau (1991), which describe rate‐limited transport of sorbing organic compounds in the vadose zone, are modified to model a SVE remediation at an idealized site and analytically solved. The analytical model presented herein describes transport of a sorbing organic contaminant in a converging radial flow field in the vadose zone, with sorption rate limitations described by first‐order rate expressions. The model equations are solved in the Laplace domain and numerically inverted to simulate contaminant concentrations at an extraction well. A Laplace domain solution for the total contaminant mass remaining in the vadose zone is also derived. It is shown that under certain conditions, rate‐limited sorption can have a significant impact upon SVE remediation in the vadose zone. The solutions presented in this note may be useful in verifying numerical codes which are being developed to model organic transport in the vadose zone under conditions of rate‐limited sorption.
Release of metals from a Cd‐contaminated streambed in response to experimental acidification and neutralization
Authors: Curtis P. Jefferson , Walker Paddy
Published: February 2004

Short‐term (hours) experimental acidification of a Cd‐contaminated Precambrian Shield stream with low buffering capacity caused the release of Ca, Mg, Cd, Mn, Fe, and Al from the streambed. Release of metals was consistent with pH‐dependent desorption and dissolution of amorphous hydroxides. Partial neutralization of the stream with MgCO3 inhibited metal release for most metals; however, excess Mg appeared to stimulate Cd release. Thus addition of Mg (and probably Ca) compounds to neutralize acidified water may be ineffective for reducing Cd concentrations in solution because added base cations apparently compete with Cd for adsorption sites.
The significance of heterogeneity of evolving scales to transport in porous formations
Authors: Dagan Gedeon
Published: February 2004

Flow takes place in a heterogeneous formation of spatially variable conductivity, which is modeled as a stationary space random function. To model the variability at the regional scale, the formation is viewed as one of a two‐dimensional, horizontal structure. A constant head gradient is applied on the formation boundary such that the flow is uniform in the mean. A plume of inert solute is injected at t = 0 in a volume V0. Under ergodic conditions the plume centroid moves with the constant, mean flow velocity U, and a longitudinal macrodispersion coefficient dL may be defined as half of the time rate of change of the plume second spatial moment with respect to the centroid. For a log‐conductivity covariance CY of finite integral scale I, at first order in the variance σY2 and for a travel distance L = Ut ≫ I, dL → σY2UI and transport is coined as Fickian. Ergodicity of the moments is ensured if l ≫ I, where l is the initial plume scale. Some field observations have suggested that heterogeneity may be of evolving scales and that the macrodispersion coefficient may grow with L without reaching a constant limit (anomalous diffusion). To model such a behavior, previous studies have assumed that CY is stationary but of unbounded integral scale with CY ∼ arβ (−1 < β < 0) for large lag r. Under ergodic conditions, it was found that asymptotically dL ∼ aUL1+β, i.e., non‐Fickian behavior and anomalous dispersion. The present study claims that an ergodic behavior is not possible for a given finite plume of initial size l, since the basic requirement that l ≫ I cannot be satisfied for CY of unbounded scale. For instance, the centroid does not move any more with U but is random (Figure 1), owing to the large‐scale heterogeneity. In such a situation the actual effective dispersion coefficient DL is defined as half the rate of change of the mean second spatial moment with respect to the plume centroid in each realization. This is the accessible entity in a given experiment. We show that in contrast with dL, the behavior of DL is controlled by l and it has the Fickian limit DL ∼ aUl1+β (Figure 3). We also discuss the case in which Y is of stationary increments and is characterized by its variogram γy. Then U and dL can be defined only if γY is truncated (equivalently, an “infrared cutoff” is carried out in the spectrum of Y). However, for a bounded U it is shown that DL depends only on γY. Furthermore, for γY = arβ, DL ∼ aUl2Lβ−1; i.e., dispersion is Fickian for 0 < β < 1, whereas for 1 < β < 2, transport is non‐Fickian. Since β < 2, DL cannot grow faster than L = Ut. This is in contrast with a recently proposed model (Neuman, 1990) in which the dispersion coefficient is independent of the plume size and it grows approximately like L1.5.
Data and analysis note: A new type of article for Water Resources Research
Authors: Hornberger George M.
Published: February 2004

Water Resources Research (WRR) is pleased to announce a new type of article, the data and analysis note, which will fill a gap in current AGU publications. The purpose of this new type of manuscript is to describe data sets, analysis procedures, and algorithms that are of interest and significance to a segment of the profession but are too specialized and extensive to publish in toto in WRR.

The casual network that historically has permitted data to be shared on an informal basis among investigators in a particular subdiscipline of water resources is no longer a sufficient method of distributing data. The many investigators spread around the world, the large quantity of scientific data, the costs of maintaining and increasing the quality of the data, and the increasing use of a variety of data by nonspecialists have made it increasingly difficult to share data adequately on an informal basis. A wide variety of international, national, and program‐oriented data centers have been organized over the past quarter century to archive data. Nonetheless, important data sets are often unknown or inaccessible to many potential users. Furthermore, many scientists expend considerable effort and resources to obtain and reduce data to usable form, but for a variety of reasons do not bring these data to a stage where the scientific results merit journal publication. Thus the scientist receives little or no recognition for producing a widely used data set. Similarly, geoscientists who develop well‐documented algorithms, analysis procedures, and instrumentation often receive limited recognition for their efforts. The products resulting from the diligent efforts of these scientists may not be widely known and disseminated. To alleviate the problems outlined above, WRR is instituting a new type of manuscript called the “data and analysis note.”
Watershed Scale Shear Stress From Tethersonde Wind Profile Measurements Under Near Neutral and Unstable Atmospheric Stability
Authors: Parlange M. B. , Katul G. G.
Published: February 2004

Mean wind speed profiles were measured in the atmospheric surface layer, using a tethersonde system, above the Ojai Valley Watershed in southern California. The valley is mainly planted with mature avocado and orange trees. The surface shear stress and latent and sensible heat fluxes were measured above the trees which are up to 9 m in height. Near‐neutral wind speed profile measurements allowed the determination of the watershed surface roughness (z0 = 1.4 m) and the momentum displacement height (d0 = 7.0 m). The wind speed measurements obtained under unstable atmospheric stability were analyzed using Monin‐Obukhov similarity theory. New stability correction functions proposed based on theory and experiments of Kader‐Yaglom as well as the now classic Businger‐Dyer type functions were tested. The watershed shear stress values calculated using the surface layer wind speed profiles with the new Monin‐Obukhov stability functions were found to be improved in comparison with the values obtained with the Businger‐Dyer functions under strongly unstable stability conditions. The Monin‐Obukhov model with the Businger‐Dyer stability correction function underpredicted the momentum flux by 25% under strongly unstable stability conditions, while the new Kader‐Yaglom formulation compared well on average (R2 = 0.77) with the surface eddy correlation measurements for all atmospheric stability conditions. The unstable 100‐m drag coefficient was found to be u*2/V1002 = 0.0182.
Influence of diurnal variations in stream temperature on streamflow loss and groundwater recharge
Authors: Constantz Jim , Thomas Carole L. , Zellweger Gary
Published: February 2004

We demonstrate that for losing reaches with significant diurnal variations in stream temperature, the effect of stream temperature on streambed seepage is a major factor contributing to reduced afternoon streamflows. An explanation is based on the effect of stream temperature on the hydraulic conductivity of the streambed, which can be expected to double in the 0° to 25°C temperature range. Results are presented for field experiments in which stream discharge and temperature were continuously measured for several days over losing reaches at St. Kevin Gulch, Colorado, and Tijeras Arroyo, New Mexico. At St. Kevin Gulch in July 1991, the diurnal stream temperature in the 160‐m study reach ranged from about 4° to 18°C, discharges ranged from 10 to 18 L/s, and streamflow loss in the study reach ranged from 2.7 to 3.7 L/s. On the basis of measured stream temperature variations, the predicted change in conductivity was about 38%; the measured change in stream loss was about 26%, suggesting that streambed temperature varied less than the stream temperature. At Tijeras Arroyo in May 1992, diurnal stream temperature in the 655‐m study reach ranged from about 10° to 25°C and discharge ranged from 25 to 55 L/s. Streamflow loss was converted to infiltration rates by factoring in the changing stream reach surface area and streamflow losses due to evaporation rates as measured in a hemispherical evaporation chamber. Infiltration rates ranged from about 0.7 to 2.0 m/d, depending on time and location. Based on measured stream temperature variations, the predicted change in conductivity was 29%; the measured change in infiltration was also about 27%. This suggests that high infiltration rates cause rapid convection of heat to the streambed. Evapotranspiration losses were estimated for the reach and adjacent flood plain within the arroyo. On the basis of these estimates, only about 5% of flow loss was consumed via stream evaporation and stream‐side evapotranspiration, indicating that 95% of the loss within the study reach represented groundwater recharge.
Simulated Transport of Three Cations Through Porous Media: Effect of Different Approaches to Modeling Cation Exchange Reactions
Authors: Grant S. A. , Mansell R. S. , Bloom S. A. , Rhue R. D.
Published: February 2004

Batch cation exchange and column experiments were conducted to evaluate selectivity coefficients which have been suggested for describing cation exchange reactions in solute transport models. Vanselow selectivity coefficients were calculated for cation exchange equilibria with a cation resin and for equilibria reported in the literature with a Yolo loam soil. Experimental column data were compared with data from simulations generated by a numerical solute transport model to evaluate Vanselow, Gaines‐Thomas, and statistical thermodynamic selectivity coefficients. With the cation resin, the statistical thermodynamic selectivity coefficient gave the most reliable estimate of column effluent cation concentrations. In a column packed with the Yolo loam soil, the Vanselow selectivity coefficient gave the most accurate prediction of column response. Use of variable (as opposed to fixed) Vanselow selectivity coefficients gave more accurate predictions of column experiments. The use of ternary cation exchange data did not improve predictions of column response.
Spatial Prediction of Soil Salinity Using Electromagnetic Induction Techniques: 2. An Efficient Spatial Sampling Algorithm Suitable for Multiple Linear Regression Model Identification and Estimation
Authors: Lesch Scott M. , Strauss David J. , Rhoades James D.
Published: February 2004

In our companion paper we described a regression‐based statistical methodology for predicting field scale salinity (ECe) patterns from rapidly acquired electromagnetic induction (ECa) measurements. This technique used multiple linear regression (MLR) models to construct both point and conditional probability estimates of soil salinity from ECasurvey data. In this paper we introduce a spatial site selection algorithm designed to identify a minimal number of calibration sites for MLR model estimation. The algorithm selects sites that are spatially representative of the entire survey area and simultaneously facilitate the accurate estimation of model parameters. Additionally, we introduce two statistical criteria that are useful for selecting optimal MLR variable combinations, describe a technique for identifying faulty signal data, and explore some of the differences between our recommended model‐based sampling plan are some more commonly used design‐based ampling plans. Survey data from two of the fields analyzed in the previous paper are used to demonstrate these techniques.
Anaerobic Transformation of Chlorinated Aliphatic Hydrocarbons in a Sand Aquifer Based on Spatial Chemical Distributions
Authors: Semprini Lewis , Kitanidis Peter K. , Kampbell Don H. , Wilson John T.
Published: February 2004

We estimated the distribution of chlorinated aliphatic hydrocarbons (CAHs) from groundwater samples collected along three transects in a sand aquifer. Trichloroethylene (TCE) leaked and contaminated the aquifer probably more than a decade before we collected the measurements. The data show significant concentrations of TCE, cis‐l,2‐dichloroethylene (c‐DCE), vinyl chloride (VC), and ethene. We attributed DCE, VC, and ethene to the reductive dehalogenation of TCE. The CAH concentrations varied significantly with depth and correlate with sulfate and methane concentrations. Anoxic aquifer conditions exist with methane present at relatively high concentrations at depth. High concentrations of TCE correspond with the absence of methane or low methane concentrations, whereas products of TCE dehalogenation are associated with higher methane concentrations and low sulfate concentrations. Indications are that the dechlorination of TCE and DCE to VC and ethene is associated with sulfate reduction and active methanogenesis. TCE dechlorination to DCE is likely occurring under the less reducing conditions of sulfate reduction, with further reductions to VC and ethene occurring under methanogenic conditions. We estimated that about 20% of TCE has dechlorinated to ethene. The analysis of the data enhanced our knowledge of natural in situ transformation and transport processes of CAHs.
An Updated Model of Induced Airflow in the Unsaturated Zone
Authors: Baehr Arthur L. , Joss Craig J.
Published: February 2004

Simulation of induced movement of air in the unsaturated zone provides a method to determine permeability and to design vapor extraction remediation systems. A previously published solution to the airflow equation for the case in which the unsaturated zone is separated from the atmosphere by a layer of lower permeability (such as a clay layer) has been superseded. The new solution simulates airflow through the layer of lower permeability more rigorously by defining the leakage in terms of the upper boundary condition rather than by adding a leakage term to the governing airflow equation. This note presents the derivation of the new solution. Formulas for steady state pressure, specific discharge, and mass flow in the domain are obtained for the new model and for the case in which the unsaturated zone is in direct contact with the atmosphere.
Particle Size Variations Between Bed Load and Bed Material in Natural Gravel Bed Channels
Authors: Lisle Thomas E.
Published: February 2004

Particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled, natural gravel bed channels. The ratio (D*) of median particle size of bed material to the transport‐ and frequency‐weighted mean of median bed load size decreases to unity with increasing drainage area, bank‐full discharge, dimensionless stream power, and bed material sorting. In channels with high values of D*, significant volumes of fine bed load are transported during discharges that are less than bank‐full, which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels. This indicates transport of fine bed load over a more stable substrate of coarser bed material. The apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size. Likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities. A simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low‐order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour.
Using the Nonstationary Spectral Method to Analyze Flow Through Heterogeneous Trending Media
Authors: Li Shu‐Guang , McLaughlin Dennis
Published: February 2004

This paper describes a nonstationary spectral theory for analyzing flow in a heterogeneous porous medium with a systematic trend in log hydraulic conductivity. This theory relies on a linearization of the groundwater flow equation but does not require the stationarity assumptions used in classical spectral theories. The nonstationary theory is illustrated with a two‐dimensional analysis of a linear trend aligned with the mean flow direction. In this case, closed‐form solutions can be obtained for the effective hydraulic conductivity, head covariance, and log conductivity‐head cross covariance. The effective hydraulic conductivity decreases from the geometrical mean as the mean slope of the log conductivity increases. Trending leads to a reduction of head variance and a structural change in the head covariance and the log conductivity‐head cross covariance. Such changes have important implications for measurement conditioning (or cokriging) methods which rely on the head covariance and log conductivity‐head covariance. The nonstationary spectral analysis is also compared with classical spectral analysis. This comparison indicates that the classical spectral method correctly predicts the normalized head covariance in a linear trending media. The stationary spectral method fails to capture the qualitative influence of trends on the effective hydraulic conductivity and the log conductivity‐head cross covariance, although the magnitude of the error is relatively small for realistic values of the mean log conductivity slope. The stationary and nonstationary results are the same when there is no trend in log conductivity. The trending conductivity example illustrates that the nonstationary spectral method has all the capabilities of the classical spectral approach while not requiring as many restrictive assumptions.
Evaluation of 11 Equations for Determining Evaporation for a Small Lake in the North Central United States
Authors: Winter T. C. , Rosenberry D. O. , Sturrock A. M.
Published: February 2004

Eleven equations for calculating evaporation were compared with evaporation determined by the energy budget method for Williams Lake, Minnesota. Data were obtained from instruments on a raft, on land near the lake, and at a weather station 60 km south of the lake. The comparisons were based on monthly values for the open‐water periods of 5 years, a total of 22 months. A modified DeBruin‐Keijman, Priestley‐Taylor, and a modified Penman equation resulted in monthly evaporation values that agreed most closely with energy budget values. To use these equations, net radiation, air temperature, wind speed, and relative humidity need to be measured near the lake. In addition, thermal surveys need to be made to determine change in heat stored in the lake. If data from distant climate stations are the only data available, and they include solar radiation, the Jensen‐Haise and Makkink equations resulted in monthly evaporation values that agreed reasonably well with energy budget values.
Influence of Natural Organic Matter on Colloid Transport Through Saprolite
Authors: Kretzschmar Ruben , Robarge Wayne P. , Amoozegar Aziz
Published: February 2004

Mobile colloids in soils and their underlying strata may play an important role in the translocation of some contaminants from surface sources to groundwater. This study was conducted to evaluate the role of adsorbed natural organic matter (NOM) in the transport of submicron soil colloids through a commonly occurring type of saprolite in North Carolina. Intact saprolite columns from 4 m below the soil surface were used to study the movement of a conservative tracer (3H2O) and of soil colloids with and without adsorbed NOM. For natural (i.e., untreated) soil colloids having high colloidal stability due to adsorbed NOM, the filier efficiency of the saprolite decreased rapidly to zero as increasing amounts of colloids were deposited on the pore walls in the saprolite (blocking effect). Colloid breakthrough curves exhibited little tailing, indicating that colloid deposition was largely irreversible. The colloids were excluded from about 33% of the water‐filled pore space, resulting in faster transport of colloids as compared to 3H2O. When the colloids were treated with NaOCl to remove adsorbed NOM, colloidal stability and mobility were strongly decreased. For these suspensions the filter efficiency of the columns increased as increasing amounts of colloids were deposited in the saprolite (filter ripening). After addition of small amounts of humic acid (1 mg L−1) to the NaOCl‐treated colloids, they exhibited very similar transport behavior as the untreated soil colloids. Stabilization of colloids by NOM and the possible occurrence of the blocking effect or filter ripening must be considered in future models of subsurface colloid transport.
Upscaling Capillary Pressure‐Saturation Curves in Heterogeneous Porous Media
Authors: Desbarats A. J.
Published: February 2004

Numerical models of flow in the vadose zone require capillary pressure‐saturation relationships for the grid blocks discretizing the medium. However, the scale of these grid blocks is generally much larger than the sample scale at which capillary pressure‐saturation relationships are measured. This study investigates the upscaling of capillary curves under static conditions of capillary‐gravity equilibrium. The averaging regions are horizontal layers, parallel to the phreatic surface, as in one‐dimensional models of infiltration. At the point or sample scale it is assumed that all capillary curves can be normalized using the Leverett (1941∥ approach to a common dimensionless form described by the Corey‐Brooks model. Static water saturation at any point in the medium is then expressed as a parametric function of three regionalized variables, permeability, porosity, and irreducible water saturation. Water saturations at the layer scale are obtained by spatial averaging of point‐scale values. With certain assumptions it is shown that the nondimensional relationship between capillary pressure and saturation at the layer scale is the same as at the sample scale. In general, the nondimensional scaling of layer‐averaged capillary pressure requires exhaustive knowledge of the spatial distribution of the permeability‐porosity ratio within the averaging region. However, when permeability and porosity are jointly lognormal, this spatial distribution may be approximated by a lognormal model requiring only two parameters for each averaging region. The developments are illustrated using published field data from the Apache Leap tuff site. Exact and estimated upscaled capillary curves are compared in simulated coregionalized fields of permeability, porosity, and irreducible water saturations. Results show that the upscaled capillary pressure‐saturation curves accurately reproduce the vertical saturation profile obtained by exhaustive spatial averaging of point‐scale water saturations.
Effects of Spatial Variability and Scale on Areally Averaged Evapotranspiration
Authors: Famiglietti J. S. , Wood E. F.
Published: February 2004

This paper explores the effects of spatial variability and scale on areally averaged evapotranspiration. A spatially distributed water and energy balance model is employed to determine the effect of explicit patterns of land surface characteristics and atmospheric forcing on areally averaged evapotranspiration over a range of increasing spatial scales. The analysis is performed from the local scale to the catchment scale. The study area is King's Creek catchment, an 11.7 km2 watershed located on the native tallgrass prairie of Kansas. It is shown that a threshold scale, or representative elementary area (REA) exists for evapotranspiration modeling. It is shown further that the dominant controls on the scaling behavior of catchment‐average evapotranspiration, and thus the size of the REA, depend on the dominant controls on its components (bare‐soil evaporation, wet canopy evaporation, and dry canopy transpiration) and whether evapotranspiration is occurring at potential rates or soil‐ and vegetation‐controlled rates. The existence of an REA for evapotranspiration modeling suggests that in catchment areas smaller than this threshold scale, actual patterns of model parameters and inputs may be important factors governing catchment‐scale evapotranspiration rates in hydrological models. In models applied at scales greater than the REA scale, spatial patterns of dominant process controls can be represented by their statistical distribution functions. It appears that some of our findings are fairly general and will therefore provide a framework for understanding the scaling behavior of areally averaged evapotranspiration at the catchment and larger scales. Our results may have further implications for representing subgrid‐scale land surface heterogeneity in hydrological parameterizations for atmospheric models.
A Land Cover‐Based Snow Cover Representation for Distributed Hydrologic Models
Authors: Donald J. R. , Soulis E. D. , Kouwen N. , Pietroniro A.
Published: February 2004

A snow cover depletion curve (SDC) summarizes the relationship between snow cover distribution and an average snow cover property, such as depth or water equivalent, for a given area. Snow cover depletion curves have been developed for, and applied in, hydrological models on a watershed or elevation zone basis. However, land cover‐;based SDCs are not prominent in the literature. For this study the areal distribution of snow cover for dominant land cover units was measured during the winters of 1991 and 1992 in the Laurel Creek watershed in southern Ontario, Canada. On the basis of these data a general model for land cover‐;based SDCs is developed for these land cover units, namely, short grass, ploughed fields, and deciduous forests. This model is derived from the three‐parameter lognormal distribution, which is shown to characterize the areal depletion curves of the land cover units studied. The SDCs based on this new model provide a formal distributed snow cover representation that can be used in vegetation‐based distributed hydrological models requiring accurate spatial representations of snow cover attributes.
A fast and exact method for multidimensional Gaussian stochastic simulations: Extension to realizations conditioned on direct and indirect measurements
Authors: Dietrich C. R. , Newsam G. N.
Published: February 2004

Recently Dietnch and Newsam [1993] derived a fast and exact method for generating unconditional realizations of a stationary, multidimensional Gaussian random field on a rectangular sampling grid. The method is based on embedding the random field covariance matrix in a larger positive definite matrix with circulant/block circulant structure. The circulant structure of the embedding matrix means that a square root of this matrix can be efficiently computed by the fast Fourier transform; realizations are then generated by multiplying vectors of white noise by this square root. This paper extends the method to generating realizations conditioned on direct and/or indirect measurements of the field collected at an arbitrary set of scattered data points.
Transport of Chromium and Selenium in a Pristine Sand and Gravel Aquifer: Role of Adsorption Processes
Authors: Kent D. B. , Davis J. A. , Anderson L. C. D. , Rea B. A.
Published: February 2004

Field transport experiments were conducted in an oxic sand and gravel aquifer using Br (bromide ion), Cr (chromium, injected as Cr(VI)), Se (selenium, injected as Se(VI)), and other tracers. The aquifer has mildly acidic pH values and low concentrations of dissolved salts. Within analytical errors, all mobile Cr was present as Cr(VI). All mobile Se was probably present as Se(VI). Adsorption of Cr and Se onto aquifer sediments caused retardation of both tracers. Breakthrough curves for Cr and Se had extensive tails, which caused large decreases in their maximum concentrations relative to the nonreactive Br tracer after only 2.0 m of transport. A surface complexation model was applied to the results of laboratory studies of Cr(VI) adsorption on aquifer solids from the site based on adsorption onto hydrous ferric oxide. The modeling results suggested that the dominant adsorbents in the aquifer solids have lower affinities for anion adsorption than pure hydrous ferric oxide. The steep rising limbs and extensive tails observed in most of the breakthrough curves are qualitatively consistent with the equilibrium surface complexation model; however, slow rates of adsorption and desorption may have contributed to these features. Variations during transport in the concentrations of Cr, Se, and other anions competing for adsorption sites likely gave rise to variations in the extent of adsorption. Adequate description of the observed retardation of Cr and Se would require a coupled transport‐adsorption model that can account for these effects. Companion experiments in the mildly reducing zone of the aquifer (Kent et al., 1994) showed a loss of Cr mass, probably resulting from reduction to Cr(III), and little retardation of mobile Cr and Se during transport; this contrast illustrates the influence of aquifer chemistry on the transport of redox‐sensitive solutes.
Regulating Groundwater Pollution: Effects of Geophysical Response Assumptions on Economic Efficiency
Authors: Fleming R. A. , Adams R. M. , Kim C. S.
Published: February 2004

Most economic studies of groundwater pollution ignore important geophysical complexities of groundwater contamination. For example, most studies assume that nitrogen fertilizer instantaneously leaches into an underlying water aquifer. In reality, there are time lags between fertilizer application and nitrate contamination of groundwater which complicate establishment of efficient regulations. This paper uses an optimal control model to examine empirically the effects of time lags on regulatory efficiency. Results indicate that ignoring time lags can lead to regulatory actions that set suboptimal user fees, which lead to levels of damage greater than anticipated. The results confirm that transport time lags are important when setting pollution control policies. In the case of very long time lags, pollution control policies may have no effect. The impact that time lags have on policy is stable with regard to changes in key model parameters and changes in the production function.
On the solar ultraviolet spectral irradiance during the Maunder Minimum
Authors: Lean J. L. , White O. R. , Skumanich A.
Published: February 2004

In the historical record of solar activity the period from 1645 to 1715 is a singular epoch during which the number of sunspots decreased markedly for a generation. Known as the Maunder Minimum, this solar epoch coincided with the coldest part of the Little Ice Age (circa 1450 to 1850). We estimate the change at this time in the output of solar ultraviolet (UV) radiation at wavelengths from 120 to 300 nm, relative to contemporary observations. Since this portion of the solar UV spectrum determines ozone composition in the stratosphere, our results bear on the historical variability of ozone and its potential climatic effects. Between the Maunder Minimum and 1986 (the present day solar activity minimum between cycles 21 and 22) we estimate reductions of 64% in the irradiance of the Lyman α line of neutral hydrogen (at 121.6 nm), 8% at 200 nm, and 3.5% in the wavelength range from 210 to 250 nm. The reduction in the solar output from the entire spectral band between 120 and 300 nm is estimated to be 0.17 W/m2, which is approximately 6% of the change in the total solar irradiance of 2.7 W/m2 previously estimated by us (Lean et al., 1992a) over the same time span. Because of this diminished UV output due to very low solar activity the Maunder Minimum total ozone concentration may have been 4% below its 1980 level. While the climatic consequences of such a change have yet to be determined, recent work by Haigh (1994) on modulation of radiative climate forcing by stratospheric ozone emphasizes the need to understand the role of UV irradiance variability as one forcing mechanism.
Oceanic radiocarbon: Separation of the natural and bomb components
Authors: Broecker Wallace S. , Sutherland Stewart , Smethie William , Peng Tsung‐Hung , Ostlund Gote
Published: February 2004

An improved method has been developed for the separation of the natural and bomb components of the radiocarbon in the ocean. The improvement involves the use of a very strong correlation between natural radiocarbon and dissolved silica. This method is applied to radiocarbon measurements made on samples collected during the Geochemical Ocean Sections Study (GEOSECS), Transient Tracers in the Ocean (TTO) and South Atlantic Ventilation Experiment (SAVE) expeditions. On the basis of this new separation we provide not only an estimate of the global inventory of bomb 14C at the time of the GEOSECS survey but also the distribution of bomb radiocarbon along four thermocline isopycnals in each ocean. We also document the evolution of the bomb 14C inventory and penetration along thermocline isopycnals in the North Atlantic Ocean between the times of the GEOSECS (1972–1973) and TTO (1980–1982) surveys and in the South Atlantic Ocean between the times of the GEOSECS (1973) and SAVE (1987–1989) surveys. In addition, we show that the bomb tritium to bomb 14C ratio (expressed in the tritium unit (TU) 81 units/100‰) for waters entering the thermocline of the northern hemisphere is about 9 times higher than for those entering the southern hemisphere thermocline. This contrast offers long‐term potential as an indicator of inter‐hemispheric transport of upper ocean waters.