bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental design for estimating parameters of rate‐limited mass transfer: Analysis of stream tracer studies
Authors: Wagner Brian J. , Harvey Judson W.
Published: February 2004

Tracer experiments are valuable tools for analyzing the transport characteristics of streams and their interactions with shallow groundwater. The focus of this work is the design of tracer studies in high‐gradient stream systems subject to advection, dispersion, groundwater inflow, and exchange between the active channel and zones in surface or subsurface water where flow is stagnant or slow moving. We present a methodology for (1) evaluating and comparing alternative stream tracer experiment designs and (2) identifying those combinations of stream transport properties that pose limitations to parameter estimation and therefore a challenge to tracer test design. The methodology uses the concept of global parameter uncertainty analysis, which couples solute transport simulation with parameter uncertainty analysis in a Monte Carlo framework. Two general conclusions resulted from this work. First, the solute injection and sampling strategy has an important effect on the reliability of transport parameter estimates. We found that constant injection with sampling through concentration rise, plateau, and fall provided considerably more reliable parameter estimates than a pulse injection across the spectrum of transport scenarios likely encountered in high‐gradient streams. Second, for a given tracer test design, the uncertainties in mass transfer and storage‐zone parameter estimates are strongly dependent on the experimental Damkohler number, DaI, which is a dimensionless combination of the rates of exchange between the stream and storage zones, the stream‐water velocity, and the stream reach length of the experiment. Parameter uncertainties are lowest at DaI values on the order of 1.0. When DaI values are much less than 1.0 (owing to high velocity, long exchange timescale, and/or short reach length), parameter uncertainties are high because only a small amount of tracer interacts with storage zones in the reach. For the opposite conditions (DaI ≫ 1.0), solute exchange rates are fast relative to stream‐water velocity and all solute is exchanged with the storage zone over the experimental reach. As DaI increases, tracer dispersion caused by hyporheic exchange eventually reaches an equilibrium condition and storage‐zone exchange parameters become essentially nonidentifiable.
Spatial and temporal variability in the R‐5 infiltration data set: Déjà vu and rainfall‐runoff simulations
Authors: Loague Keith , Kyriakidis Phaedon C.
Published: February 2004

This paper is a continuation of the event‐based rainfall‐runoff model evaluation study reported by Loague and Freeze [1985[. Here we reevaluate the performance of a quasi‐physically based rainfall‐runoff model for three large events from the well‐known R‐5 catchment. Five different statistical criteria are used to quantitatively judge model performance. Temporal variability in the large R‐5 infiltration data set [Loague and Gander, 1990] is filtered by working in terms of permeability. The transformed data set is reanalyzed via geostatistical methods to model the spatial distribution of permeability across the R‐5 catchment. We present new estimates of the spatial distribution of infiltration that are in turn used in our rainfall‐runoff simulations with the Horton rainfall‐runoff model. The new rainfall‐runoff simulations, complicated by reinfiltration impacts at the smaller scales of characterization, indicate that the near‐surface hydrologic response of the R‐5 catchment is most probably dominated by a combination of the Horton and Dunne overland flow mechanisms.
Geochemical mole‐balance modeling with uncertain data
Authors: Parkhurst David L.
Published: February 2004

Geochemical mole‐balance models are sets of chemical reactions that quantitatively account for changes in the chemical and isotopic composition of water along a flow path. A revised mole‐balance formulation that includes an uncertainty term for each chemical and isotopic datum is derived. The revised formulation is comprised of mole‐balance equations for each element or element redox state, alkalinity, electrons, solvent water, and each isotope; a charge‐balance equation and an equation that relates the uncertainty terms for pH, alkalinity, and total dissolved inorganic carbon for each aqueous solution; inequality constraints on the size of the uncertainty terms; and inequality constraints on the sign of the mole transfer of reactants. The equations and inequality constraints are solved by a modification of the simplex algorithm combined with an exhaustive search for unique combinations of aqueous solutions and reactants for which the equations and inequality constraints can be solved and the uncertainty terms minimized. Additional algorithms find only the simplest mole‐balance models and determine the ranges of mixing fractions for each solution and mole transfers for each reactant that are consistent with specified limits on the uncertainty terms. The revised formulation produces simpler and more robust mole‐balance models and allows the significance of mixing fractions and mole transfers to be evaluated. In an example from the central Oklahoma aquifer, inclusion of up to 5% uncertainty in the chemical data can reduce the number of reactants in mole‐balance models from seven or more to as few as three, these being cation exchange, dolomite dissolution, and silica precipitation. In another example from the Madison aquifer, inclusion of the charge‐balance constraint requires significant increases in the mole transfers of calcite, dolomite, and organic matter, which reduce the estimated maximum carbon 14 age of the sample by about 10,000 years, from 22,700 years to 12,600 years.
Evolution of a sediment wave in an experimental channel
Authors: Lisle Thomas E. , Pizzuto James E. , Ikeda Hiroshi , Iseya Fujiko , Kodama Yoshinori
Published: February 2004

The routing of bed material through channels is poorly understood. We approach the problem by observing and modeling the fate of a low‐amplitude sediment wave of poorly sorted sand that we introduced into an experimental channel transporting sediment identical to that of the introduced wave. The wave essentially dispersed upstream and downstream without translation, although there was inconclusive evidence of translation late in the experiment when the wave was only 10–20 grain diameters high. Alternate bars migrated through zones of differing bed load transport rate without varying systematically in volume, celerity, or transport rate. Sediment that overpassed migrating bars was apparently responsible for dispersion of the wave. The evolution of the wave was well predicted by a one‐dimensional model that contains no adjusted empirical constants. Numerical experiments demonstrate, however, that the theory does not predict sediment waves that migrate long distances downstream. Such waves can only be explained by the following processes not represented by the theory: selective bed load transport, spatial variations in bar and other form roughness, the mechanics of mobile armor, and perhaps other mechanisms.
A lattice gas study of retardation and dispersion in fractures: Assessment of errors from desorption kinetics and buoyancy
Authors: Stockman Harlan W.
Published: February 2004

Lattice gas automata (LGA) were used to estimate errors in transport coefficients, as measured in laboratory experiments with Peclet numbers from 0 to 27.6 (defined relative to channel width), Damköhler numbers from 0.18 to ∞, Grashof numbers of 0 or 75, and length/width up to 180. Low Damköhler numbers yield long, low‐amplitude elution tails, which contain much of the total solute. As a consequence, at Da ≈ 0.18 and KD = 8, the solute peak travels at the same speed as the carrier fluid, yielding an apparent KD ≈ 0 after five characteristic diffusion times. Such conditions correspond, for example, to a meter‐long path through a 0.5‐cm‐wide, gas‐filled fracture. Buoyancyenhanced dispersion, found in experiments with horizontal tubes, is confirmed by the LGA analysis; however, a different mechanism is suggested for the enhancement in horizontal fractures. Both kinetic and buoyancy errors can be greatly reduced, or experiments made much smaller, if the first and second moments of a tracer pulse can be measured as functions of time.
Generalization of Ross' tilted capillary barrier diversion formula for different two‐phase characteristic curves
Authors: Webb Stephen W.
Published: February 2004

Capillary barriers, consisting of tilted fine over coarse layers under unsaturated conditions, have been suggested as a means to divert water infiltration away from sensitive underground regions. Ross [1990] has developed a relationship for the steady‐state diversion length of tilted capillary barriers assuming the quasi‐linear set of two‐phase characteristic curves. Due to its simplicity, this equation has been used to evaluate potential tilted capillary barrier performance even in cases where the layer characteristic curves are different than the quasi‐linear equation set. In the present study, Ross' capillary barrier diversion relationship is extended to other two‐phase characteristic curves. This generalization allows for the use of any set of two‐phase curves in evaluating tilted capillary barrier performance. This generalized Ross' equation compares well with detailed numerical simulations.
Experimental evidence of dynamic scaling and indications of self‐organized criticality in braided rivers
Authors: Sapozhnikov Victor B. , Foufoula‐Georgiou Efi
Published: February 2004

The evolution of an experimental braided river produced in our laboratory has been monitored and analyzed. It has been shown that in addition to the spatial scaling revealed by Sapozhnikov and Foufoula‐Georgiou [1996a], braided rivers also exhibit dynamic scaling. This implies that a smaller part of a braided river evolves identically (in the statistical sense) to a larger one provided the time is renormalized by a factor depending only on the ratio of the spatial scales of those parts. The small value of the estimated dynamic exponent z is interpreted as an indication that the evolution of small channels in a braided river system is to a large extent forced by the evolution of bigger channels. The presence of dynamic scaling is further interpreted as indicating that braided rivers may be in a critical state and behave as self‐organized critical systems.
Field study of spatial variability in unsaturated flow beneath and adjacent to playas
Authors: Scanlon Bridget R. , Goldsmith Richard S.
Published: February 2004

We quantified unsaturated flow beneath playa and adjacent interplaya settings at a site in the Southern High Plains (United States) to resolve issues related to where and how water moves through the unsaturated zone. This is the first study in which the data density (39 boreholes) and the variety of techniques used (physical, chemical, and isotopic) were sufficient to quantify spatial variability in unsaturated flow. Water contents, water potentials, and tritium eoneentrations were much higher and chloride eoneentrations were much lower beneath playas than in interplaya settings, which indicated that playas focus recharge. These results refute previous hypotheses that playas act as evaporation pans or that recharge is restricted to the annular region around playas. Water fluxes estimated from environmental tracers ranged from 60 to 120 mm yr−1 beneath playas and were ≤0.1 mm yr−1 during the past 2000–5000 years beneath natural interplaya areas not subjected to ponding. To evaluate the apparent inconsistency between high recharge rates and thick clay layers beneath playas, we applied bromide and FD&C blue dye to evaluate flow processes. These applied tracer experiments showed preferential flow along roots and desiccation cracks through structured clays in the shallow subsurface in playas.
Soil‐water movement under natural‐site and waste‐site conditions: A multiple‐year field study in the Mojave Desert, Nevada
Authors: Andraski B. J.
Published: February 2004

Soil‐water movement under natural‐site and simulated waste‐site conditions were compared by monitoring four experimental sites in the Mojave Desert, Nevada, during a 5‐year period: one vegetated soil profile, one soil profile where vegetation was removed, and two nonvegetated test trenches. Precipitation ranged from 14 to 162 mm/yr. Temporal changes in water content measured by neutron probe were limited to the upper 0.5–1 m; values ranged from 0.01 to 0.19 m3/m3. Water potential and temperature were measured by thermocouple psychrometers; 77% remained operable for ≥4.5 years. For vegetated soil, precipitation that accumulated in the upper 0.75 m of soil was removed by evapotranspiration: water potentials decreased seasonally by 4 to >8 MPa. During 2 years with below‐average precipitation, water potentials below the app arent root zone decreased by 2.3 (1.2‐m depth) to 0.4 MPa (5‐m depth), and the gradients became predominantly upward. Water potentials then rebounded during 2 years with near‐ and above‐average precipitation, and seasonally variant water potential gradients were reestablished above the 4.2‐m depth. Under nonvegetated waste‐site conditions, data indicated the long‐term accumulation and shallow, but continued, penetration of precipitation: water potentials showed moisture penetration to depths of 0.75−1.85 m. The method of simulated‐waste drum placement (stacked versus random) and the associated differences in subsidence showed no measurable influence on the water balance of the trenches: subsidence totaled ≤13 mm during the study. Water potentials below the trenches and below the 2‐m depth for the nonvegetated soil remained low (≈−5.5 to −7.5 MPa) and indicated the persistence of typically upward driving forces for isothermal water flow. Water fluxes estimated from water potential and temperature data suggested that isothermal liquid, isothermal vapor, and nonisothermal vapor flow need to be considered in the conceptualization of unsaturated flow at the field sites. Below the depth of temporal water content change, the estimated liquid fluxes ranged from 10−10 to 10−15 cm/s, isothermal vapor fluxes ranged from 10−10 to 10−13 cm/s, and the nonisothermal vapor fluxes ranged from 10−8 to 10−10cm/s.
Examination of two methods for estimating regional evaporation using a coupled mixed layer and land surface model
Authors: Kim Cornelis P. , Entekhabi Dara
Published: February 2004

The Priestley and Taylor [1972] method (PT) and the complementary relationship (CR) introduced by Bouchet [1963] for the estimation of extended area or regional evapotranspiration are examined using a model of the coupled interactions between the mixed layer and the land surface. The model is shown to be in good agreement with observations from the First International Satellite Land Surface Climatology Project Field Experiment. It appears that the parameter α in the PT is slightly lower than the suggested constant value 1.26 and is quite insensitive to the growth of the boundary layer. We find some support for the CR but do not resolve the problem of estimating the potential evaporation under unstressed conditions.
New piecewise‐continuous hydraulic functions for modeling preferential flow in an intermittent‐flood‐irrigated field
Authors: Mohanty B. P. , Bowman R. S. , Hendrickx J. M. H. , van Genuchten M. T.
Published: February 2004

Modeling water flow in macroporous field soils near saturation has been a major challenge in vadose zone hydrology. Using in situ and laboratory measurements, we developed new piecewise‐continuous soil water retention and hydraulic conductivity functions to describe preferential flow in tile drains under a flood‐irrigated agricultural field in Las Nutrias, New Mexico. After incorporation into a two‐dimensional numerical flow code, CHAIN_2D, the performance of the new piecewise‐continuous hydraulic functions was compared with that of the unimodal van Genuchten‐Mualem model and with measured tile‐flow data at the field site during a number of irrigation events. Model parameters were collected/estimated by site characterization (e.g., soil texture, surface/subsurface saturated/unsaturated soil hydraulic property measurements), as well as by local and regional‐scale hydrologic monitoring (including the use of groundwater monitoring wells, piezometers, and different surface‐irrigation and subsurface‐drainage measurement systems). Comparison of numerical simulation results with the observed tile flow indicated that the new piecewise‐continuous hydraulic functions generally predicted preferential flow in the tile drain reasonably well following all irrigation events at the field site. Also, the new bimodal soil water retention and hydraulic conductivity functions performed better than the unimodal van Genuchten–Mualem functions in terms of describing the observed flow regime at the field site.
A connection between topographically driven runoff generation and channel network structure
Authors: Woods Ross A. , Sivapalan Murugesu
Published: February 2004

Runoff generation can be modelled using a topographic wetness index; the fraction of the catchment where the wetness index exceeds some threshold is assumed to be saturated. We show that distributions of a topographic index can share a common form across many catchments, particularly for sufficiently large catchments. This leads us to speculate that the topographic structure of drainage networks provides a unifying theme for runoff generation. We identify those catchments which share the common distribution of topographic index, and label them “organized.” Visual examination of maps of this index and of subcatchment area indicates that the organized catchments are those which have a “proper” network: The branching structure provides an orderly increase in subcatchment area as one proceeds downstream. We consider that the presence of a “properly” branched network is the essence of “organization.” It has previously been observed that catchments can have a power law distribution of subcatchment area: our data show that the distributions of wetness index and subcatchment area are closely connected for catchments with a well‐developed channel network. Since the distribution of subcatchment area is a signature of the channel network, this permits the reparameterization of runoff generation models using channel network properties. As a result, the runoff generation model is a truly a catchment‐scale model and can no longer be disaggregated to the point scale. This view of catchment response is closely related to the representative elementary area concept, where catchment response is “simple” for sufficiently large catchments and depends only on catchment‐scale phenomena.
Is the drainage network a fractal Sierpinski space?
Authors: Moussa Roger
Published: February 2004

The most common approach to extract the channel network from digital elevation models is to specify a threshold area, S, which is the minimum area required to drain to a point within which a channel forms. This threshold area is usually specified arbitrarily, although it is recognized that different threshold areas will result in substantially different channel networks for the same basin. This paper studies the effects of S, which is also the scale of observation, on scaling and morphometric properties such as the number of sources and the areas of source basins and lateral basins. The Gardon basin, located in southern France, was extensively studied. The results indicate that morphometric properties vary considerably with S and that the spatial distribution of source basins can be considered as a fractal Sierpinski space. A simple model based on the procedure similar to the Sierpinski carpet construction is proposed to explain empirical relations between the number of source areas and the threshold, S. The model presents a new approach to estimate basin fractal properties and defines new indices to characterize the spatial distribution of first‐order basins. This methodology is useful for hydrologists and geomorphologists dealing with river networks and spatial patterns of various basin properties such as vegetation, soil, soil moisture, and human activities.
The quest for more powerful validation of conceptual catchment models
Authors: Mroczkowski Marek , Raper Paul G. , Kuczera George
Published: February 2004

The power of a validation strategy (that is, its ability to discriminate between good and bad model hypotheses) depends on what kind of data are available and how the data are used to challenge the hypothesis. Several validation strategies are examined from the perspective of power and practical applicability. It is argued that validation using multiresponse data in a catchment experiencing a shift in hydrologie regime due to disturbance or extreme climatic inputs is a considerably more powerful strategy than traditional split‐sample testing using streamflow data alone in undisturbed catchments. A case study testing two model hypotheses is presented using paired catchments for which multiple‐response data in the form of streamflow, stream chloride, and groundwater levels were available. The first catchment, Salmon, was maintained as an established forest, while the second, Wights, was clear‐felled and converted to pasture about 3 years after monitoring started. The hypotheses consider the same lumped hydrosalinity model with the first (H1) excluding a groundwater discharge zone and the second (H2) including it. It was found that even with three concurrent responses from the undisturbed Salmon catchment, H1 could not be rejected, leaving an important part of the model conceptualization unidentified. Moreover, a streamflow split‐sample test for the disturbed Wights catchment failed to conclusively reject H1; parameters could be found which accurately tracked the streamflow changes following forest clearing yet produced erroneous simulations of responses such as stream chloride and groundwater storage. It was only when H1 was subjected to the scrutiny of three catchment responses from the disturbed Wights catchment that it could be rejected. This highlights the importance of challenging model hypotheses under the most demanding of tests, which, in this study, coincided with multiple‐response validation in a disturbed catchment.
Constant‐concentration boundary condition: Lessons from the HYDROCOIN variable‐density groundwater benchmark problem
Authors: Konikow L. F. , Sanford W. E. , Campbell P. J.
Published: February 2004

In a solute‐transport model, if a constant‐concentration boundary condition is applied at a node in an active flow field, a solute flux can occur by both advective and dispersive processes. The potential for advective release is demonstrated by reexamining the Hydrologic Code Intercomparison (HYDROCOIN) project case 5 problem, which represents a salt dome overlain by a shallow groundwater system. The resulting flow field includes significant salinity and fluid density variations. Several independent teams simulated this problem using finite difference or finite element numerical models. We applied a method‐of‐characteristics model (MOCDENSE). The previous numerical implementations by HYDROCOIN teams of a constant‐concentration boundary to represent salt release by lateral dispersion only (as stipulated in the original problem definition) was flawed because this boundary condition allows the release of salt into the flow field by both dispersion and advection. When the constant‐concentration boundary is modified to allow salt release by dispersion only, significantly less salt is released into the flow field. The calculated brine distribution for case 5 depends very little on which numerical model is used, as long as the selected model is solving the proper equations. Instead, the accuracy of the solution depends strongly on the proper conceptualization of the problem, including the detailed design of the constant‐concentration boundary condition. The importance and sensitivity to the manner of specification of this boundary does not appear to have been recognized previously in the analysis of this problem.
Benefit‐cost analysis of best management practices implemented to control nitrate contamination of groundwater
Authors: Yadav Satya N. , Wall David B.
Published: February 2004

Implementing best management practices (BMPs) can reduce nitrate concentration in groundwater, but does it pay to invest in programs that reduce nitrate by encouraging increased adoption of BMPs? In this paper we evaluate water quality improvement by benefit‐cost analysis of adopting BMPs under such a program. The analysis shows that under current levels of contamination, costs of the program to foster BMP implementation will be equal to annually accrued benefits over a period of 6 years. However, under the worsening scenarios of increased nitrate‐N concentrations, the same costs will be equal to the benefits in a 4‐ to 5‐year period. If water quality improves to acceptable levels through adoption of BMPs, the results reveal that in the long run, investing in a BMP program will be more cost effective to reduce contamination than to seek alternative sources of safe drinking water supplies.
Multiscaling properties of rainfall and bounded random cascades
Authors: Menabde Merab , Harris Daniel , Seed Alan , Austin Geoff , Stow David
Published: February 2004

The connection among different types of exponents characterizing multiscaling properties of rainfall and a criterion for stationarity of random fields are discussed, and a new phenomenological model for rainfall time series simulation is proposed. The bounded random cascade model presented here is a generalization of the well‐known α model with the multiplicative weights of the generator converging to unity as the cascade proceeds to smaller scales. This allows one to directly simulate a multiscaling random field with an energy spectrum exponent, β>1, which is typical for rainfall time series data but which cannot be produced by a standard α model. A procedure is proposed for estimating the cascade parameters of this new bounded α model from observed data. Parameters are estimated from two data sets with different degrees of intermittency and different spectral exponents. The bounded α model simulations using these parameters produced realistic rainfall time series with spectral exponents similar to their observed counterparts.
Transient flow in bounded randomly heterogeneous domains: 2. Localization of conditional mean equations and temporal nonlocality effects
Authors: Tartakovsky Daniel M. , Neuman Shlomo P.
Published: February 2004

In randomly heterogeneous porous media one cannot predict flow behavior with certainty. One can, however, render optimum unbiased predictions of such behavior by means of conditional ensemble mean hydraulic heads and fluxes. We have shown in paper 1 [Tartakovsky and Neuman, this issue] that under transient flow, these optimum predictors are governed by nonlocal equations. In particular, the conditional mean flux is generally nonlocal in space‐time and therefore non‐Darcian. As such, it cannot be associated with an effective hydraulic conductivity except in special cases. Here we explore analytically situations under which localization is possible so that Darcy's law applies in real, Laplace, and/or infinite Fourier transformed spaces, approximately or exactly, with or without conditioning. We show that the corresponding conditional effective hydraulic conductivity tensor is generally nonsymmetric. An alternative to Darcy's law in each case, valid under mean no‐flow conditions along Neumann boundaries, is a quasi‐Darcian form that includes only a symmetric tensor which, however, does not constitute a bona fide effective hydraulic conductivity. Both lack of symmetry and differences between Darcian and quasi‐Darcian forms disappear to first (but not necessarily higher) order of approximation in the (conditional) variance of natural log hydraulic conductivity. We adopt such an approximation to investigate analytically the effect of temporal nonlocality on one‐and three‐dimensional mean flows in infinite, statistically homogeneous media. Our results show that temporal nonlocality may manifest itself under either monotonic or oscillatory time variations in the mean hydraulic gradient. The effect of temporal nonlocality increases with the variance of log hydraulic conductivity and is more pronounced in one dimension than in three.
Regional interpretation of water‐quality monitoring data
Authors: Smith Richard A. , Schwarz Gregory E. , Alexander Richard B.
Published: February 2004

We describe a method for using spatially referenced regressions of contaminant transport on watershed attributes (SPARROW) in regional water‐quality assessment. The method is designed to reduce the problems of data interpretation caused by sparse sampling, network bias, and basin heterogeneity. The regression equation relates measured transport rates in streams to spatially referenced descriptors of pollution sources and land‐surface and stream‐channel characteristics. Regression models of total phosphorus (TP) and total nitrogen (TN) transport are constructed for a region defined as the nontidal conterminous United States. Observed TN and TP transport rates are derived from water‐quality records for 414 stations in the National Stream Quality Accounting Network. Nutrient sources identified in the equations include point sources, applied fertilizer, livestock waste, nonagricultural land, and atmospheric deposition (TN only). Surface characteristics found to be significant predictors of land‐water delivery include soil permeability, stream density, and temperature (TN only). Estimated instream decay coefficients for the two contaminants decrease monotonically with increasing stream size. TP transport is found to be significantly reduced by reservoir retention. Spatial referencing of basin attributes in relation to the stream channel network greatly increases their statistical significance and model accuracy. The method is used to estimate the proportion of watersheds in the conterminous United States (i.e., hydrologic cataloging units) with outflow TP concentrations less than the criterion of 0.1 mg/L, and to classify cataloging units according to local TN yield (kg/km2/yr).
Preferred states in spatial soil moisture patterns: Local and nonlocal controls
Authors: Grayson Rodger B. , Western Andrew W. , Chiew Francis H. S. , Blöschl Günter
Published: February 2004

In this paper we develop a conceptual and observational case in which soil water patterns in temperate regions of Australia switch between two preferred states. The wet state is dominated by lateral water movement through both surface and subsurface paths, with catchment terrain leading to organization of wet areas along drainage lines. We denote this as nonlocal control. The dry state is dominated by vertical fluxes, with soil properties and only local terrain (areas of high convergence) influencing spatial patterns. We denote this as local control. The switch is described in terms of the dominance of lateral over vertical water fluxes and vice versa. When evapotranspiration exceeds rainfall, the soil dries to the point where hydraulic conductivity is low and any rainfall that occurs essentially wets up the soil uniformly and is evapotranspired before any significant lateral redistribution takes place. As evapotranspiration decreases and/or rainfall increases, areas of high local convergence become wet, and runoff that is generated moves downslope, rapidly wetting up the drainage lines. In the wet to dry transitional period a rapid increase in potential evapotranspiration (and possibly a decrease in rainfall) causes drying of the soil and “shutting down” of lateral flow. Vertical fluxes dominate and the “dry” pattern is established. Three data sets from two catchments are presented to support the notion of preferred states in soil moisture, and the results of a modeling exercise on catchments from a range of climatic conditions illustrate that the conclusions from the field studies may apply to other areas. The implications for hydrological modeling are discussed in relation to methods for establishing antecedent moisture conditions for event models, for distribution models, and for spatially distributing bulk estimates of catchment soil moisture using indices.
Effects of grain‐scale mass transfer on the transport of volatile organics through sediments: 1. Model development
Authors: Cunningham Jeffrey A. , Werth Charles J. , Reinhard Martin , Roberts Paul V.
Published: February 2004

In the first paper of this two‐paper series, we present a new model that attributes nonequilibrium sorption of moderately hydrophobia, volatile organic compounds to intragranular diffusion. The model differs from those of previous researchers in that for the first time, it combines the following elements: (1) We account for two distinct intragranular rate‐limiting diffusion processes, occurring in series and at widely different timescales; (2) we describe the slower of the two processes with a gamma distribution of diffusion rates; and (3) we use the disparity of timescales of the two processes to approximate a boundary condition for the distributed diffusion equation, allowing it to be solved analytically. The slower diffusion process is attributed to activated diffusion through very small pores, called micropores. In paper 2 [Werth et al., this issue] we evaluate the capabilities of the model and use it to interpret experimental results.
Precipitation and snow budget over the southwestern United States during the 1994–1995 winter season in a mesoscale model simulation
Authors: Kim Jinwon
Published: February 2004

Precipitation and snow budget over the southwestern United States during the 1994–1995 winter were simulated using an interactively coupled mesoscale atmosphericland surface model. Temporal and spatial distributions of simulated precipitation agreed closely with available observations. It is found that precipitation in California is closely correlated with the moisture influx from the Pacific Ocean. The observed and simulated precipitation indicates that over 70% of the total precipitation in the area during this period was associated with less than 30% of the total winter storms passing this region. Amounts of the simulated season total precipitation generally increased with elevation, with maximum precipitation, mostly in the form of snow, occurring along the ridge of the Sierra Nevada. Simulated surface snow budget indicates that snow plays a major role in seasonal surface hydrology at high elevations where most of surface runoff originates during spring and summer seasons.
Data for Ashley River to test channel network and river basin heterogeneity concepts
Authors: McKerchar A. I. , Ibbitt R. P. , Brown S. L. R. , Duncan M. J.
Published: February 2004

To provide data to investigate hypotheses about the evolution of channel networks, specifically the optimal channel network concept, discharge and channel properties were measured at 336 sites in a 121 km2 basin over a 5‐day period of reasonably steady flows. The data are also suitable for investigating how discharge increases down river channels. The data collection was a major logistical exercise which involved 80 person‐days in remote field locations. In the expectation that the data will be of use to other researchers, this paper describes how the data were measured, checked and archived. The archive is available at http://www.niwa.cri.nz/hydrology/ashpage.htm and includes the associated time series of streamflow at the basin outlet and the channel network as plotted on 1:50,000 scale maps.
Adjustment of daily precipitation data at 10 climate stations in Alaska: Application of World Meteorological Organization intercomparison results
Authors: Yang Daqing , Goodison Barry E. , Ishida Shig , Benson Carl S.
Published: February 2004

A methodology for adjusting the daily precipitation measured by the U.S. National Weather Service (NWS) 8‐inch standard gauge for wind‐induced undercatch, wetting loss, and trace amount of precipitation is provided. The application of the proposed adjustment procedures was made at 10 NWS climate stations in Alaska for 1982 and 1983. The results show the following: (1) Daily adjustment for wind‐induced undercatch, wetting loss, and trace amount of precipitation increases the gauge‐measured annual precipitation by 65–800 mm for the 2 years (about 10‐140% of the gauge‐measured yearly total) at the 10 stations in Alaska; (2) compared to wetting loss and trace amount of precipitation, wind‐induced undercatch is the source of greatest error, although wetting loss and trace amount of precipitation are also significant systematic errors in the northern Alaska regions of low precipitation; (3) in the similar climate condition, the NWS 8‐inch standard gauges with an Alter wind shield have a much lower adjustment for wind‐induced undercatch than the unshielded gauges at nearby stations, and the unshielded gauges placed on the roof of the weather office building have a higher adjustment for wind‐induced errors than those gauges mounted on the ground; (4) monthly adjustment factors (adjusted/measured precipitation) differ by station, and at an individual station by type of precipitation; (5) considerable intra‐annual variation of the magnitude of the adjustments has been found in Alaska owing to the fluctuation of wind speed, air temperature, and frequency of snowfall. Using the constant correction factors (derived at a single intercomparison site) to the archived monthly precipitation records produces significant undercorrection of the wind‐induced errors at high wind sites and overcorrecting of the errors at low wind sites. To avoid the undercorrection or overcorrection of the wind‐induced errors, a constant correction factor should not be applied to gauge‐measured snow data. Daily adjustments for systematic errors need to be applied to the archived precipitation data. It is expected that the adjustments will have an impact on climate monitoring.
Hydraulic characterization of aquifers, reservoir rocks, and soils: A history of ideas
Authors: Narasimhan T.N.
Published: February 2004

Estimation of the hydraulic properties of aquifers, petroleum reservoir rocks, and soil systems is a fundamental task in many branches of Earth sciences and engineering. The transient diffusion equation proposed by Fourier early in the 19th century for heat conduction in solids constitutes the basis for inverting hydraulic test data collected in the field to estimate the two basic parameters of interest, namely, hydraulic conductivity and hydraulic capacitance. Combining developments in fluid mechanics, heat conduction, and potential theory, the civil engineers of the 19th century, such as Darcy, Dupuit, and Forchheimer, solved many useful problems of steady state seepage of water. Interest soon shifted towards the understanding of the transient flow process. The turn of the century saw Buckingham establish the role of capillary potential in governing moisture movement in partially water‐saturated soils. The 1920s saw remarkable developments in several branches of the Earth sciences; Terzaghi's analysis of deformation of watersaturated earth materials, the invention of the tensiometer by Willard Gardner, Meinzer's work on the compressibility of elastic aquifers, and the study of the mechanics of oil and gas reservoirs by Muskat and others. In the 1930s these led to a systematic analysis of pressure transients from aquifers and petroleum reservoirs through the work of Theis and Hurst. The response of a subsurface flow system to a hydraulic perturbation is governed by its geometric attributes as well as its material properties. In inverting field data to estimate hydraulic parameters, one makes the fundamental assumption that the flow geometry is known a priori. This approach has generally served us well in matters relating to resource development primarily concerned with forecasting fluid pressure declines. Over the past two decades, Earth scientists have become increasingly concerned with environmental contamination problems. The resolution of these problems requires that hydraulic characterization be carried out at a much finer spatial scale, for which adequate information on geometric detail is not forthcoming. Traditional methods of interpretation of field data have relied heavily on analytic solutions to specific, highly idealized initial‐value problems. The availability of efficient numerical models and versatile spreadsheets of personal computers offer promising opportunities to relax many unavoidable assumptions of analytical solutions and interpret field data much more generally and with fewer assumptions. Currently, a lot of interest is being devoted to the characterization of permeability. However, all groundwater systems are transient on appropriate timescales. The dynamics of groundwater systems cannot be understood without paying attention to capacitance. Much valuable insights about the dynamic attributes of groundwater systems could be gained by long‐term passive monitoring of responses of groundwater systems to barometric changes, Earth tides, and ocean tides.
Geomorphology and endangered fish habitats of the upper Colorado River: 1. Historic changes in streamflow, sediment load, and channel morphology
Authors: Van Steeter Mark M. , Pitlick John
Published: February 2004

The hydrologic, geomorphic, and ecologic effects of reservoir operations are thought to be key factors in the decline of native fishes in the upper Colorado River basin. The present paper examines the extent to which changes in streamflow and sediment loads have affected alluvial reaches of the Colorado River near Grand Junction, Colorado. The analysis shows that since 1950, annual peak discharges of the Colorado River and its major tributary, the Gunnison River, have decreased by 29–38%. The total volume of runoff delivered to the study area has not changed significantly over the period of record, but the annual hydrograph has been modified greatly by reductions in peak flows and augmentation of base flows. Annual suspended sediment loads of the Colorado River and Gunnison River have likewise decreased. This was particularly apparent during the period from 1964 to 1978, when annual sediment loads were 40–65% less than the long‐term average. Analysis of aerial photographs indicates that between 1937 and 1993 the main channel of the Colorado River has narrowed by an average of 20 m and about 1/4 of the area formed by side channels and backwaters has been lost.
On gravel‐bed roughness characterization
Authors: Nikora Vladimir I. , Goring Derek G. , Biggs Barry J. F.
Published: February 2004

The random field approach for gravel‐bed roughness characterization, which is based on the presentation of bed elevations as a three‐dimensional random field, is justified as an alternative to the characteristic particle size approach. We first show that the bed elevation distribution is close to Gaussian and then investigate gravel‐bed roughness using the second‐order structure function. The latter reveals two distinct regions: a scaling region at small spatial lags and a saturation region at large scales. The scaling exponent H (a form of Hurst exponent) appears to be isotropic and universal for both manually created “unworked” gravel beds (H=0.5) and natural water‐worked gravel beds (H=0.79). However, the gravel‐bed roughness, in general, is not isotropic and should be characterized by three independent characteristic scales. A simple model of gravel‐bed roughness based on the structure function parameterization is developed and compared with the characteristic particle size approach.
Model of contaminant transport in porous media in the presence of colloids applied to actinide migration in column experiments
Authors: Lührmann L. , Noseck U. , Tix C.
Published: February 2004

We simulate column experiments of europium migration in humic‐rich groundwater performed with different Darcy velocities to investigate the predictive capability of colloid‐facilitated transport models for actinides. The simulation results indicate two retention mechanisms: sorption of dissolved contaminants and filtration of colloidal contaminants by the matrix. These interaction processes, which determine the breakthrough curves and their tailing behavior, are kinetically controlled. Sorption constants are derived from batch experiments. Sorption rates are obtained by the simulation and are compared with the values of the filtration rate predicted by the colloid filtration theory.
Interannual, seasonal, and spatial patterns of meltwater and solute fluxes in a seasonal snowpack
Authors: Harrington Robert , Bales Roger C.
Published: February 2004

Meltwater discharge and electrical conductivity were measured in eight 1 × l m lysimeters, and snow accumulation and electrical conductivity of melted samples were measured in snow pits during four snowmelt seasons at Mammoth Mountain, California. The peak snow‐water equivalent ranged from 0.57 to 2.92 m over the four melt seasons. Lysimeter discharges ranged from 20% to 205% of the mean flow; however, mean lysimeter flow was representative of snow ablation observed in snow pits. The electrical conductivity in snow pit samples and meltwater averaged 2–3 μS cm−1. Peak meltwater electrical conductivity ranged from 6 to 14 times the bulk premelt snowpack electrical conductivity. Snow depth did not affect the magnitude of the ionic pulse, and ion depletion as a function of snow ablation was similar from year to year despite interannual contrasts in melt rate and snow accumulation. Diel fluctuations in electrical conductivity were more pronounced in shallower snowpacks.
A Soil Moisture–Rainfall Feedback Mechanism: 1. Theory and observations
Authors: Eltahir Elfatih A. B.
Published: February 2004

This paper presents a hypothesis regarding the fundamental role of soil moisture conditions in land‐atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associated with relatively large boundary layer moist static energy, which favors the occurrence of more rainfall. Since soil moisture conditions themselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This mechanism is based on considerations of the energy balance at the land‐atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil moisture–rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land surface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net solar radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil moisture conditions would tend to enhance net terrestrial radiation at the surface through cooling of surface temperature, reduction of upwards emissions of terrestrial radiation, and simultaneous increase in atmospheric water vapor content and downwards flux of terrestrial radiation. Thus, under wet soil moisture conditions, both components of net radiation are enhanced, resulting in a larger total flux of heat from the surface into the boundary layer. This total flux represents the sum of the corresponding sensible and latent heat fluxes. Simultaneously, cooling of surface temperature should be associated with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever these processes occur over a large enough area, the enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics of localized convective storms as well as the dynamics of large‐scale atmospheric circulations have been shown to be sensitive to the distribution of boundary layer moist static energy by several previous studies. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper [Zheng and Eltahir, this issue] using a simple numerical model.