bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of vertical profiles of δ18O to constrain estimates of hydraulic conductivity in a thick, unfractured aquitard
Authors: Remenda V. H. , van der Kamp G. , Cherry J. A.
Published: February 2004

Vertical distributions of δ18O were used to constrain hydraulic conductivity (K) in a thick, unfractured aquitard. Previous investigations utilizing permeability tests at a variety of scales [Keller et al., 1989] indicated that bulk K at the site ranges from 3.5× 0−11 m/s to 1.2×10−10 m/s. Advective displacements calculated on the basis of these K values lead to a wide range of predicted distributions of natural tracers after 11,000 years. Comparison of δ18O distributions and simulated advection‐diffusion profiles provide a more precise constraint on advective displacement and hence on K. The simulations, cast in terms of a Peclet number (P = V2T/D*) ranging in value from 0 to 12, indicate that transport is diffusion controlled and that K is actually less than 1.2×10−11 m/s. P greater than 0.1 (K > 1.2×10−11 m/s) overestimates transport. This result indicates that conventional permeability tests in very tight media tend to overestimate K. In practical terms, advective transport at the site is negligible, and transport can be simulated with diffusion only (P = O).
Fluid flow in fault zones: Influence of hydraulic anisotropy and heterogeneity on the fluid flow and heat transfer regime
Authors: López Dina L. , Smith Leslie
Published: February 2004

Fluid circulation, heat transfer, and the development of thermal springs are examined for vertical fault zones with anisotropic permeability and internal heterogeneity. Interactions between thermally driven convective circulation in the fault zone and topographically driven groundwater flow through the surrounding country rock are mapped in permeability space (permeability of the country rock versus fault zone permeability) and compared to earlier results for homogeneous, isotropic fault zones. Simulations with a fault zone 4–10 times more permeable in the strike than in the dip direction show that the field of steady convection expands in permeability space, promoting stable convection at both higher and lower flux rates. Higher groundwater discharge temperatures (by 12–18°C) are predicted relative to an isotropic fault because this anisotropy favors the formation of a smaller number of convection cells, creating a flow pattern that is more efficient in capturing heat from the country rock and transmitting it to a reduced number of discharge sites. Simulations with a fault zone 4–10 times less permeable in the strike than the dip direction indicate the regional flow from the country rock overrides buoyancy‐driven convective circulation in the fault zone at lower values for the country‐rock permeability. Heterogeneity internal to the fault creates complex patterns of flow and variations in the geothermal gradient that reflect the connections of higher‐permeability regions interior to and along the surface trace of the fault. Channeling of the flow leads to minor differences in the maximum discharge temperature relative to the homogeneous case but creates significant enhancement in the local heat flux owing to the higher groundwater discharge rates.
Mineral nitrogen transformations in and under seasonal snow in a high‐elevation catchment in the Rocky Mountains, United States
Authors: Williams Mark W. , Brooks Paul D. , Mosier Arvin , Tonnessen Kathy A.
Published: February 2004

In an effort to understand sources of nitrate (NO3−) in surface waters of high‐elevation catchments, nitrogen (N) transformations in and under seasonal snow were investigated from 1993 to 1995 on Niwot Ridge, an alpine ecosystem at 3,500 m located in the Colorado Front Range of the Rocky Mountains. Ammonium (NH4+) and NO3− labeled with 15N applied as nonconservative tracers to the snow showed no evidence of nitrification in the snowpack. Furthermore, NH4+ movement through the amended snowpack was highly correlated with a conservative chloride tracer (r2 = 0.99). In an unamended snowpack NH4+ concentrations in meltwater before contact with the ground were highly correlated with NO3− concentrations (r2 = 0.98), which is consistent with no nitrification in the snowpack. The isotopically labeled 15NH+4 applied to the snowpack was found in underlying soils, showing that NH4+ released from snow can be rapidly immobilized. Resin bag (mixed‐bed ion‐exchange resins) measurements (n = 22) showed that 80% of the mobile inorganic N in unamended subnivial soils was NO3−. Measurements of KCl‐extractable inorganic N from surface soils showed that highest values were prior to the initiation of snowmelt and lowest values were during the growing season. The natural δ15N abundance of unamended soils was negative and ranged from −12 to −2, suggesting that atmospheric deposition of δ15N‐depleted N is an important component of N cycling in these alpine soils. These results suggest that soil mineralization under seasonal snow, rather than snowmelt release of NO3−, may control NO3− concentrations in surface waters of high‐elevation catchments.
Wetting‐induced soil structural changes: The theory of liquid phase sintering
Authors: Or Dani
Published: February 2004

Rapid wetting of structurally unstable soils causes aggregate disintegration and settlement near the soil surface, thereby reducing soil porosity and intake properties. Beneath the slaked surface layer where unsaturated flow conditions develop, structural changes driven mostly by capillary forces take place at slower rates. The collapse of structural voids in the sublayer is due to coalescence of aggregates into larger units. We propose to adopt a model of liquid phase sintering of glass compacts to describe wetting induced densification of aggregated soil. The analogy between these processes is based on the observations that (1) the reduction in soil strength upon wetting is similar to the effect of high temperatures on glass powders and (2) in both systems capillary forces induce viscous flow. A sintering model which assumes a porous medium with tractable geometry was modified to accommodate soil conditions. The model considers the reduction in the water‐coated solid surface areas (i.e., water‐air interfaces) as the energy source for the soil viscous flow. Data from wetting experiments of aggregated silt loam were used to test the plausibility of the theory. The main unknown in the system was the viscosity of wet soil which was estimated to be in the range of 1.37×106 to 1.9×106 Pa s−1. The use of solid matrix with tractable geometry and energy considerations should prove useful for other studies of the dynamics of soil structure.
Stochastic analysis of mass flux of sorptive solutes in unsaturated heterogeneous anisotropic porous formations
Authors: Russo David
Published: February 2004

Transport of sorptive solutes that undergo a reversible equilibrium sorption reaction with a retardation factor that is negatively correlated with log conductivity was considered. First‐order analysis, based on a stochastic continuum presentation of the Eulerian retarded velocity in a partially saturated porous formation and a general Lagrangian description of the transport, was used to investigate the effects of a few length scales, characteristics of the heterogeneous transport domain, on the expected breakthrough of the sorptive solute and the uncertainty in its prediction, under steady, unsaturated flow. The length scales are the vertical distance L to the horizontal control plane (CP), the length scale of the planar source, Ξ; the log conductivity correlation scales in the vertical (Iyυ) and the horizontal (Iyh) directions; and the macroscopic capillary length scale λ. It was demonstrated in this study that as in transport of inert solutes, for a given mean water saturation and gravity‐dominated unsaturated flow, both the skewness of the mean breakthrough curve (BTC) of the sorptive solute and the uncertainty in its prediction increase in formations of small ρ = Iyh/Iyυ and large η = λ/Iyυ and relatively shallow CP (small υ = L/Iyυ) and that the uncertainty in the prediction of the mean BTC decreases with increasing size of the planar source (large ℓ = Ξ/Iyh). A new finding of the present study suggests that the relative impact of the chemical heterogeneity of the formation on the skewness of the mean BTC of the sorptive solute and the uncertainty in its prediction increases with increasing ρ (larger stratification) and decreasing η (coarser‐textured soil material). This result stems from the fact that when the mean flow is vertical, both large ρ and small η diminish the impact of the heterogeneity in the physical properties of the formation on the variability of the flow in the longitudinal direction.
Simultaneous Estimation of Transmissivity Values and Zonation
Authors: Eppstein Margaret J. , Dougherty David E.
Published: February 2004

The extended Kalman filter (EKF) has long been recognized as a powerful, yet computationally intensive, methodology for stochastic parameter estimation. Three improvements to traditional algorithms are presented and applied to heterogeneous transmissivity estimation. First, the costly EKF covariance updates are replaced by more efficient approximations. Second, the zonation structure of the distributed parameter field being estimated is dynamically determined and refined using a partitional clustering algorithm. Third, a new method of merging first and second moments of random fields that have heterogeneous statistics is introduced. We apply this method, called random field union, as an alternative to conventional random field averaging for the systematic shrinking of covariance matrices as the dimensionality of the parameter space is reduced. The effects of these three improvements are examined. In applications to steady state groundwater flow test problems, we show that the first and second improvements reduce the computational time requirements dramatically, while the second and third can improve the accuracy and stability of the results. The resulting integrated method is successfully applied to a larger, more realistic calibration test case under steady and cyclostationary flow conditions (similar to regular seasonal fluctuations). When flow is steady, the method can be viewed as iterative; when flow is transient, the method is fully recursive.
Large‐Scale Dispersion in a Sandy Aquifer: Simulation of Subsurface Transport of Environmental Tritium
Authors: Engesgaard Peter , Jensen Karsten Høgh , Molson John , Frind Emil O. , Olsen Henrik
Published: February 2004

Large‐scale dispersion in a sandy unconfined aquifer in Denmark was studied by simulating subsurface transport of environmental tritium. Subsurface transport included transport in a moderately deep unsaturated zone and in a relatively long cross section of the aquifer. The tritium data from the site enabled a four‐step modeling analysis comprising (1) estimation of tritium content in the infiltration water, (2) transport in the unsaturated zone, (3) estimation of flux‐averaged tritium concentration in the recharge water, and (4) transport in the groundwater zone. The groundwater model simulations were sensitive to the longitudinal and transverse dispersivity parameters, αL and αr, as a set of parameters, but a model sensitivity analysis showed that it was not possible to identify a unique set of parameter values. A likely range of variation for the two parameters could be identified: (αL, αT); ∈ [(1 m, 0.005 m); (10 m, 0.0 m)] the two parameters being interdependent in that an increase in αL results in a decrease in αT and vice versa. The reported dispersivities represent a scale of 1000 m, the approximate travel distance from the water table to the observation wells. If the estimated αL can be regarded as being of intermediate reliability following earlier defined criteria, the range or the representative set of values then represent the largest scale of earlier reported values. Including our range of αL in the set of reported dispersivities suggests that αL does not increase indefinitely with scale.
Regulation of Nitrate‐N Release from Temperate Forests: A Test of the N Flushing Hypothesis
Authors: Creed I. F. , Band L. E. , Foster N. W. , Morrison I. K. , Nicolson J. A. , Semkin R. S. , Jeffries D. S.
Published: February 2004

During the past decade, significant spatial and temporal variability in the release of nitrate‐nitrogen (N) from catchments in a sugar maple forest in central Ontario was observed. To explain this variability, we tested the flushing hypothesis [Hornberger et al., 1994], where, when the soil saturation deficit is high, N accumulates in the upper layers of the soil and, as the soil saturation deficit decreases, the formation of a saturated subsurface layer flushes N from the upper layers of the soil into the stream. We used the Regional Hydro‐Ecological Simulation System to simulate water, carbon, and N dynamics. A N flushing index was modeled as S/S30, the ratio of the current day saturation deficit to the previous 30‐day average saturation deficit. A N source index was modeled as the ratio of N supply/demand. The relationship between the simulated N indices and the observed release of N indicated two mechanisms for the release of N from catchments: (1) a N flushing mechanism, where the N‐enriched upper layer of the soil is flushed, after a period of low demand for N by the forest (e.g., during spring snowmelt and autumn stormflow, the water table rising into previously unsaturated parts of a N‐enriched soil profile) or after a period of high demand for N by the forest (e.g., during summer droughts, the water table rising into previously saturated parts of a N‐impoverished soil profile following a period of enhanced rates of nitrification); and (2) a N draining mechanism, where spring snowmelt recharge of the groundwater translocates N from the upper layer of the soil into deeper hydrological flow pathways that are released slowly over the year.
Interaction of a Rectangular Fracture with the Host Rock
Authors: Wangen Magnus , Halvorsen Gotskalk
Published: February 2004

The fluid flow interaction between a two‐dimensional rectangular fracture and its surrounding isotropic permeable and homogeneous rock is studied where the surrounding rock is of infinite extent. The interaction is found to depend on two numbers: the aspect ratio of the fracture and the ratio of the fracture permeability and the rock permeability. Furthermore, the interaction is one of two types, one where the fracture is charged and discharged from its sides and another where the charging/discharging takes place through its ends. Approximate solutions that describe these regimes are given. The squared‐width fracture permeability is considered, and we show that the interaction of the fracture and the rock is then characterized by a certain aspect ratio. Fractures with aspect ratios smaller than the characteristic aspect ratio are charged/discharged through their short ends, and fractures with aspect ratios larger than the characteristic aspect ratio are charged/discharged through their long sides. A fracture is charged/discharged equally through the short ends and the long sides at characteristic aspect ratio, and the velocity at the center of the fracture is largest for this particular aspect ratio.
Effects of slope vegetation removal on the diurnal variations of a small mountain stream
Authors: Bren Leon J.
Published: February 2004

The effect on removal of lower, mid, and upper slope vegetation on the diurnal variation in streamflow from a 46‐ha catchment was observed. The diurnal variation in streamflow of the small stream was measurable during the late‐spring‐to‐late‐autumn period. The amplitude in streamflow variation reached a maximum in early summer and declined during autumn. Observation of diurnal variations during the periods of higher flow in winter and spring showed that they may occur but were masked by much larger variations associated with storm runoff. Simulation of the characteristics of the flow measurement system showed that diurnal variations can only be studied using V‐notch weirs and float recorders during periods of low flow. No effect of the clearing of slope vegetation on the phase of the outflow could be found. However, there was evidence of a significant increase in amplitude, probably due to increased groundwater outflow from the slopes. It was concluded that the diurnal variation is due to transpiration by the riparian and near‐riparian vegetation only, and that the lower to mid slope vegetation plays little role in this variation. Simulations suggested that increased amplitude is associated with increased flow rates, and that the amplitude is not directly affected by water use of vegetation on the catchment slopes. It was concluded that the amplitude of the variation is insensitive to changes in slope hydrology and cannot be used to provide insight into deep slope processes.
Efficient subspace probabilistic parameter optimization for catchment models
Authors: Kuczera George
Published: February 2004

The estimation of catchment model parameters has proven to be a difficult task for several reasons, which include ill‐posedness and the existence of multiple local optima. Recent work on global probabilistic search methods has developed robust techniques for locating the global optimum. However, these methods can be computationally intensive when the search is conducted over a large hypercube. Moreover, specification of the hypercube may be problematic, particularly if there is strong parameter interaction. This study seeks to reduce the computational effort by confining the search to a subspace within which the global optimum is likely to be found. The approach involves locating a local optimum using a local gradient‐based search. It is assumed that the local optimum belongs to a set of optima which cluster about the global optimum. A probabilistic search is then conducted within a hyperellipsoid defined by the second‐order approximation to the response surface around the local optimum. A case study involving a five‐parameter conceptual rainfall‐runoff model is presented. The response surface is shown to be riddled with local optima, yet the second‐order approximation provides a not unreasonable description of parameter uncertainty. The subspace search strategy provides a rational means for defining the search space and is shown to be more efficient (typically twice, but up to 5 times more efficient) than a search over a hypercube. Four probabilistic search algorithms are compared: shuffled complex evolution (SCE), genetic algorithm using traditional crossover, and multiple random start using either simplex or quasi‐Newton local searches. In the case study the SCE algorithm was found to be robust and the most efficient. The genetic algorithm, although displaying initial convergence rates superior to the SCE algorithm, tended to flounder near the optimum and could not be relied upon to locate the global optimum.
Partial transport of a sand/gravel sediment
Authors: Wilcock Peter R. , McArdell Brian W.
Published: February 2004

Grains of a single size within a mixed‐size bed are entrained over a range of flows. Within this range some grains exposed on the bed surface are active (entrained at least once over the duration of a transport event), while the remaining surface grains are immobile, a condition we define as partial transport. We demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand/gravel mixture. The active proportion of the bed surface increases with bed shear stress τ0. At a given τ0, 90% of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed. Mobilization of grains in a size fraction increases from 10% to 90% over a range of τ0 of a factor of 2. The bounds of this range increase with grain size Di so that at a given τ0, sizes over a range of a factor of 4 are in a state of partial transport. Fractional transport rates are independent of Di for fully mobilized fractions and decrease rapidly with Di for partially mobile fractions. Partial transport is associated with substantial transport rates of finer, fully mobile sizes, limits both the rate and size distribution of grain exchange with the bed subsurface, and may be the dominant transport condition in many gravel‐bed rivers.
Direct Sample Estimators ofLMoments
Authors: Wang Q. J.
Published: February 2004

Direct estimators ofLmoments are derived by following closely the definition ofLmoments. Using the derived direct estimators eliminates the need for using probability‐weighted moments when estimatingLmoments and thus the need for bringing in the subject of probability‐weighted moments when describing the method ofLmoments. Source code of a FORTRAN subroutine for calculatingLmoments using the direct estimators is listed.
Rapid exchange effects on isotope ratios in groundwater systems: 1. Development of a transport‐dissolution‐exchange model
Authors: Johnson Thomas M. , DePaolo Donald J.
Published: February 2004

A transport‐reaction model calculates isotope ratios (e.g., 87Sr/86Sr) in groundwater systems where isotope ratios in the fluid are affected by two distinct types of water‐rock interaction: rapid exchange reactions and slower dissolution reactions. Rapid exchange reactions buffer the groundwater isotope ratios, and a two‐stage evolution is observed. After a perturbation in the system an exchange front, which can be described by a retardation equation, propagates through the system at a velocity equal to the groundwater velocity divided by a retardation factor. If steady boundary conditions are maintained, the exchange front is advected out of the system and a quasi‐steady state develops in which dissolution reactions influence the isotope ratios but the exchange reactions have no effect. Because clay minerals which readily exchange ions or water molecules with groundwater are found in most hydrologic systems, exchange reactions may significantly affect interpretation of isotopic data.
Transfer of nonsorbing solutes to a streambed with bed forms: Theory
Authors: Elliott Alexander H. , Brooks Norman H.
Published: February 2004

A theoretical analysis of the exchange of nonsorbing solutes between a permeable streambed and the overlying water of a stream or river is presented in this paper. In a companion paper [Elliott and Brooks, this issue] the results of experimental studies of such exchange are presented. The analysis focuses on water movements into, within, and out of the bed which result from the presence of bed forms (ripples and dunes) and highlights the mechanisms of “pumping” and “turnover.” Pumping is the movement of fluid through the bed induced by steady dynamic pressure variations over bed forms. Turnover occurs as moving bed forms trap and release interstitial fluid. The detailed analysis was used to generate the distribution of the residence time of solute within the bed, which can be used to calculate the net mass exchange due to pumping and turnover.
Performance of conceptual rainfall‐runoff models in low‐yielding ephemeral catchments
Authors: Ye W. , Bates B. C. , Viney N. R. , Sivapalan M. , Jakeman A. J.
Published: February 2004

Low‐yielding catchments with ephemeral streams involve highly nonlinear relationships between rainfall and runoff, and there is much less documentation and appreciation of the ability to predict streamflow in these veiy difficult cases than in humid catchments. The predictions of three conceptual rainfall‐runoff models are assessed in three low‐yielding, emphemeral streams over a 10‐year period. The models are a simple conceptual model, Generalized Surface inFiltration Baseflow (GSFB; eight parameters), a hybrid metric/conceptual model, Identification of Hydrographs and Components from Rainfall, Evaporation and Streamflow data (IHACRES; six parameters), and a complex conceptual model, the Large Scale Catchment Model (LASCAM; 22 parameters). The Salmon (0.82 km2), Stones (15 km2), and Canning (517 km2) catchments in Western Australia were selected for their range of sizes and low runoff yields (1.6–12.2% of rainfall). Their behavior is representative of a large part of Australia and semiarid regions, where antecedent conditions are critical determinants of streamflow response to rainfall. Such catchments provide a stern test of the capability of conceptual models. Five‐year calibration and validation performances were assessed with a range of statistics. The models were run daily but performance was assessed on both a daily and monthly basis by aggregating daily model streamflows and observations up to monthly. The models performed well, particularly in the monthly case where often more than 90% of the variance of observed streamflow was explained in simulation on independent periods. However, while the simple conceptual model is adequate for monthly time periods, the daily simulation results indicate that a slightly more complex model (the hybrid model or the complex conceptual model) is required for daily predictions in these dry catchments. The model simulation results extend the following notion of Jakeman and Hornberger [1993] from humid to semiarid ephemeral catchments: that a model of about six parameters, albeit in an appropriate model structure, is sufficient to characterize the information in rainfall‐discharge time series over a wide range of catchment sizes. Models of such modest complexity also predict runoff with good accuracy outside calibration periods, even in ephemeral, low‐yielding catchments. The simulation results highlight the critical importance of the deep groundwater and antecedent moisture conditions on stream yields in ephemeral catchments and point to the desirability of accounting for these factors in arid‐zone modeling.
Prediction of relative permeabilities for unconsolidated soils using pore‐scale network models
Authors: Rajaram Harihar , Ferrand Lin A. , Celia Michael A.
Published: February 2004

Pore‐scale network models of two‐phase capillary displacement in porous media may be used to predict constitutive relationships between the capillary pressure, saturation, and relative permeabilities at the continuum scale. Since these constitutive relationships reflect ensembles of pore‐scale events, spatial correlation in the pore structure is expected to be an important consideration. Correlations in the pore structure are also evident in recent experimental studies. Pore‐scale network models are used here in a quantitative investigation of the influence of correlations on the capillary pressure‐saturation‐relative permeability relationships. The pore space is modeled using a cubic lattice larger than 12 times the spatial correlation scale in each direction. The following two types of correlations are incorporated: spatial correlation between the radii of bonds oriented along the same direction and cross correlation between the radii of bonds emanating from the same site in all directions. Results demonstrate that the saturation‐relative permeability relationship is not very sensitive to unidirectional spatial correlation but is sensitive to cross correlation. Pore‐scale model parameters are fit to capillary pressure‐saturation data from several real soils, followed by a prediction of the relative permeability curves for the same soils. The predicted relative permeabilities are compared to the measured values and predictions using the traditional van Genuchten relationships. The pore‐scale model is shown to fit the capillary‐pressure saturation curves and predict the saturation‐relative permeability curves with a degree of accuracy comparable to the van Genuchten relationships. While these computations have been restricted to unconsolidated soils, they demonstrate the potential of pore‐scale models for predicting other transport properties such as dispersivities and mass transfer coefficients.
Sand Deposition in the Colorado River in the Grand Canyon from Flooding of the Little Colorado River
Authors: Wiele S. M. , Graf J. B. , Smith J. D.
Published: February 2004

Methods for computing the volume of sand deposited in the Colorado River in Grand Canyon National Park by floods in major tributaries and for determining redistribution of that sand by main‐channel flows are required for successful management of sand‐dependent riparian resources. We have derived flow, sediment transport, and bed evolution models based on a gridded topography developed from measured channel topography and used these models to compute deposition in a short reach of the river just downstream from the Little Colorado River, the largest tributary in the park. Model computations of deposition from a Little Colorado River flood in January 1993 were compared to bed changes measured at 15 cross sections. The total difference between changes in cross‐sectional area due to deposition computed by the model and the measured changes was 6%. A wide reach with large areas of recirculating flow and large depressions in the main channel accumulated the most sand, whereas a reach with similar planimetric area but a long, narrow shape and relatively small areas of recirculating flow and small depressions in the main channel accumulated only about a seventh as much sand. About 32% of the total deposition was in recirculation zones, 65% was in the main channel, and 3% was deposited along the channel margin away from the recirculation zone. Overall, about 15% of the total input of sand from this Little Colorado River flood was deposited in the first 3 km below the confluence, suggesting that deposition of the flood‐derived material extended for only several tens of kilometers downstream from the confluence.
An aggregate stochastic dynamic programming model of multireservoir systems
Authors: Archibald T. W. , McKinnon K. I. M. , Thomas L. C.
Published: February 2004

We present a new method of determining an operating policy for a multireservoir system in which the operating policy for a reservoir is determined by solving a stochastic dynamic programming model consisting of that reservoir and a two‐dimensional representation of the rest of the system. The method is practical for systems with many reservoirs because the time required to determine an operating policy only increases quadratically with the number of reservoirs in the system and because the operating policy for a reservoir is a function of few variables. We apply the method to examples of multireservoir systems with between 3 and 17 reservoirs and show that the operating policies determined are very close to optimal.
The Use of Streambed Temperature Profiles to Estimate the Depth, Duration, and Rate of Percolation Beneath Arroyos
Authors: Constantz Jim , Thomas Carole L.
Published: February 2004

Temporal variations in a streambed temperature profile between 30 and 300 cm beneath Tijeras Arroyo, New Mexico, were analyzed at 30‐min intervals for 1990 to estimate the depth, duration, and rate of percolation during streamflows. The depth of percolation was clearly documented by the rapid response of the streambed temperature profile to streamflows. Results indicate that the streambed possessed small thermal gradients with significant diurnal variations from late November to late May, indicating that ephemeral streamflows created continuous, advection‐dominated heat transport to depths below 300 cm during this period. Timing and duration of percolation suggested by temporal variations in the temperature profile were verified by comparison with measured streamflow records for the study reach over 1990. Percolation rates were estimated using a technique based on the travel time of the daily maximum temperature into the streambed. Percolation rates were compared with streambed seepage rates determined from measurements of streamflow loss, stream surface area, and stream evaporative loss for the entire study reach. Travel time estimates of streambed percolation rates ranged from 9 to 40 cm/hr, while streamflow estimates of streambed seepage rates ranged from 6 to 26 cm/hr during the study period. Discrepancies between streambed percolation and seepage rates may be caused by differences in the areal extent of measurements for percolation versus seepages rates. In summary, the depth, timing, and duration of streamflow‐induced percolation were well documented by temporal variations in a single streambed temperature profile, while rates of percolation based on the temperature profile were about double the seepage rates based on streamflow records for the entire study reach.
A new method for the determination of flow directions and upslope areas in grid digital elevation models
Authors: Tarboton David G.
Published: February 2004

A new procedure for the representation of flow directions and calculation of upslope areas using rectangular grid digital elevation models is presented. The procedure is based on representing flow direction as a single angle taken as the steepest downward slope on the eight triangular facets centered at each grid point. Upslope area is then calculated by proportioning flow between two downslope pixels according to how close this flow direction is to the direct angle to the downslope pixel. This procedure offers improvements over prior procedures that have restricted flow to eight possible directions (introducing grid bia) or proportioned flow according to slope (introducing unrealistic dispersion). The new procedure is more robust than prior procedures based on fitting local planes while retaining a simple grid based structure. Detailed algorithms are presented and results are demonstrated through test examples and application to digital elevation data sets.
Design of interface shape for protective capillary barriers
Authors: Selker John
Published: February 2004

Sloping interfaces of fine over coarse porous material have been considered for use as barriers to infiltration for many years. Previous literature has developed analytical solutions for flow over such interfaces, numerical simulation of such flow, and the effects of anisotropy on the diversion capacity of such a system. In all of these analyses, however, the interface was assumed to consist of a constant slope, facilitating certain calculations. In this note it is shown that both from a performance perspective (as measured by the ability to divert fluid) and a design perspective (ease of computing diversion capacity) the shape of the interface should be curved, typically in a parabolic shape. Use of this shape is predicted to double barrier diversion capacity. Interfaces made up of easy to construct segmented approximations to the parabola are predicted to provide significantly enhanced performance as well.
Permeability reductions induced by sorption of surfactant
Authors: Renshaw Carl E. , Zynda Gregory D. , Fountain John C.
Published: February 2004

Surfactant enhanced aquifer remediation (SEAR) is an attractive alternative to traditional pump and treat methods for remediating aquifers contaminated with nonaqueous phase liquids. However, initial studies indicate that the application of surfactant can reduce aquifer permeability by more than an order of magnitude, limiting the efficiency of SEAR. A series of column experiments using mixtures of medium sand and montmorillonite clay demonstrate that existing permeability reduction models for biofouling and deep‐bed filtration poorly predict surfactant induced permeability reductions. An alternative permeability reduction model is proposed which is based on the assumption that the sorbed surfactant effectively increases the volume fraction of the clay. The model is shown to reasonably predict observed permeability reductions, particularly for clay fractions less than 20%. A numerical simulation of surfactant transport that incorporates the effective clay fraction model demonstrates that induced permeability reductions significantly influence the transport of surfactant through an aquifer.
Convection in groundwater below an evaporating Salt Lake: 2. Evolution of fingers or plumes
Authors: Wooding R. A. , Tyler Scott W. , White Ian , Anderson P. A.
Published: February 2004

The downward convection of salt fingers or plumes developed from the unstable boundary layer of an evaporating “dry” salt lake is examined using a numerical model and Hele‐Shaw cell experiments. In the convecting layer the early small waves evolve into fingerlike or plumelike formations, the number of fingers or plumes decreasing with time owing to differential growth and/or coalescence. Comparison of intermediate formational stages of this pattern with the pattern generated by a two‐dimensional numerical simulation shows good qualitative agreement. However, there is a significant mismatch of the growth rates at long times. In the computer simulation the plume length develops approximately twice as rapidly as it does in the experimental case. A simple numerical experiment independent of the salt‐lake boundary conditions is compared to previously published laboratory‐scale measures of plume development in Hele‐Shaw cells which confirms the retardation of Hele‐Shaw plumes by a factor of approximately 50%. This departure is attributed to the differences in dimensionality between the Hele‐Shaw flow domain and the model domain. The data indicated that leading plumes develop isolated behavior at long times and may not be adequately represented in Hele‐Shaw analog models, and numerical simulation provides a more accurate simulation of field‐scale behavior.
Channel incision on the Lower American River, California, from streamflow gage records
Authors: James L. Allan
Published: February 2004

Channel incision along the lower American River from 1905 to 1995 is investigated using channel cross‐section plots and statistical analysis of stage‐discharge data from two streamflow gages located at three sites. Channel incision lowered thalweg elevations at rates of up to 8.2 cm yr−1, and flow stages decreased at rates of up to 4.3 cm yr−1 for periods lasting several decades. At a critical flood risk location in Sacramento, flow stages lowered 2 m from 1924 to 1970. Channel incision was the result of channel recovery from aggradation due to hydraulic gold‐mining sediment and was exacerbated by sediment storage behind dams. Prolonged erosion and transport of historical alluvium in this river suggest that G. K. Gilbert's symmetrical sediment wave model is inappropriate for the lower American River and may not adequately allow for the importance of sediment storage and remobilization in fluvial systems.
An analysis of the soil moisture‐rainfall feedback, based on direct observations from Illinois
Authors: Findell Kirsten L. , Eltahir Elfatih A. B.
Published: February 2004

Many global and regional climate modeling studies have demonstrated the importance of the initial soil water condition in their simulations of regional rainfall distribution. However, none of these modeling studies has been tested against directly observed data. This study tests the hypothesis that soil saturation is positively correlated with subsequent precipitation by analyzing a 14‐year soil moisture data set from the state of Illinois. The linear correlation between an initial soil saturation condition and subsequent rainfall is significant during the summer months, reaching a peak of r2 > 0.4 in mid‐June. This result is consistent with the hypothesis that knowledge of late spring/early summer soil moisture conditions can aid in the prediction of drought or flood years, but it does not necessarily prove that feedback from anomalous soil moisture reservoirs is the cause of anomalous summer conditions. Further analyses indicate that from early June to mid‐August, persistence in rainfall cannot fully account for the observed correlations, suggesting the likelihood of a physical feedback mechanism linking early summer soil saturation with subsequent precipitation. However, spatial and temporal data limitations restrict the potential for drawing strong new conclusions from the Illinois study.
Beach water table fluctuations due to wave run‐up: Capillarity effects
Authors: Li L. , Barry D. A. , Parlange J.‐Y. , Pattiaratchi C. B.
Published: February 2004

High‐frequency beach water table fluctuations due to wave run‐up and run‐down have been observed in the field [Waddell, 1976]. Such fluctuations affect the infiltration/exfiltration process across the beach face and the interstitial oxygenation process in the beach ecosystem. Accurate representation of high‐frequency water table fluctuations is of importance in the modeling of (1) the interaction between seawater and groundwater, more important, the effects on swash sediment transport and (2) the biological activities in the beach ecosystem. Capillarity effects provide a mechanism for high‐frequency water table fluctuations. Previous modeling approaches adopted the assumption of saturated flow only and failed to predict the propagation of high‐frequency fluctuations in the aquifer. In this paper we develop a modified kinematic boundary condition (kbc) for the water table which incorporates capillarity effects. The application of this kbc in a boundary element model enables the simulation of high‐frequency water table fluctuations due to wave run‐up. Numerical tests were carried out for a rectangular domain with small‐amplitude oscillations; the behavior of water table responses was found to be similar to that predicted by an analytical solution based on the one‐dimensional Boussinesq equation. The model was also applied to simulate the water table response to wave run‐up on a sloping beach. The results showed similar features of water table fluctuations observed in the field. In particular, these fluctuations are standing wave‐like with the amplitude becoming increasingly damped inland. We conclude that the modified kbc presented here is a reasonable approximation of capillarity effects on beach water table fluctuations. However, further model validation is necessary before the model can confidently be used to simulate high‐frequency water table fluctuations due to wave run‐up.
Field‐scale water transport in unsaturated crystalline rock
Authors: Gimmi T. , Schneebeli M. , Flühler H. , Wydler H. , Baer T.
Published: February 2004

Safe disposal of toxic wastes in geologic formations requires minimal water and gas movement in the vicinity of storage areas. Ventilation of repository tunnels or caverns built in solid rock can desaturate the near field up to a distance of meters from the rock surface, even when the surrounding geological formation is saturated and under hydrostatic pressures. A tunnel segment at the Grimsel test site located in the Aare granite of the Bernese Alps (central Switzerland) has been subjected to a resaturation and, subsequently, to a controlled desaturation. Using thermocouple psychrometers (TP) and time domain reflectometry (TDR), the water potentials ψ and water contents θ were measured within the unsaturated granodiorite matrix near the tunnel wall at depths between 0 and 160 cm. During the resaturation the water potentials in the first 30 cm from the rock surface changed within weeks from values of less than −1.5 MPa to near saturation. They returned to the negative initial values during desaturation. The dynamics of this saturation‐desaturation regime could be monitored very sensitively using the thermocouple psychrometers. The TDR measurements indicated that water contents changed close to the surface, but at deeper installation depths the observed changes were within the experimental noise. The field‐measured data of the desaturation cycle were used to test the predictive capabilities of the hydraulic parameter functions that were derived from the water retention characteristics ψ(θ) determined in the laboratory. A depth‐invariant saturated hydraulic conductivity ks = 3.0 × 10−11m s−1 was estimated from the ψ(t) data at all measurement depths, using the one‐dimensional, unsaturated water flow and transport model HYDRUS [Vogel et al., 1996]. For individual measurement depths, the estimated ks varied between 9.8 × 10−12 and 6.1 × 10−11 m s−1. The fitted ks values fell within the range of previously estimated s for this location and led to a satisfactory description of the data, even though the model did not include transport of water vapor.
A global high‐resolution emission inventory for ammonia
Authors: Bouwman A. F. , Lee D. S. , Asman W. A. H. , Dentener F. J. , Van Der Hoek K. W. , Olivier J. G. J.
Published: February 2004

A global emissions inventory for ammonia (NH3) has been compiled for the main known sources on a 1° × 1° grid, suitable for input to global atmospheric models. The estimated global emission for 1990 is about 54 Tg N yr−1. The major sources identified include excreta from domestic animals (21.6 Tg N yr−1) and wild animals (0.1 Tg N yr−1), use of synthetic N fertilizers (9.0 Tg N yr−1), oceans (8.2 Tg N yr−1), biomass burning (5.9 Tg N yr−1), crops (3.6 Tg N yr−1), human population and pets (2.6 Tg N yr−1), soils under natural vegetation (2.4 Tg N yr−1), industrial processes (0.2 Tg N yr−1), and fossil fuels (0.1 Tg N yr−1). About half of the global emission comes from Asia, and about 70% is related to food production. The regions with highest emission rates are located in Europe, the Indian subcontinent, and China, reflecting the patterns of animal densities and type and intensity of synthetic fertilizer use. The overall uncertainty in the global emission estimate is 25%, while the uncertainty in regional emissions is much greater. As the global human population will show considerable growth in the coming decades, food production and associated NH3emissions are likely to increase as well.
Mountain glaciers: Recorders of atmospheric water vapor content?
Authors: Broecker Wallace S.
Published: February 2004

A case is made that taken together, the 0.9 km lowering of tropical snow lines and the 8‰ reduction in the δ18O for tropical mountain Huascarán snowfall point to reductions in the water vapor inventory of the glacial atmosphere large enough to account for a several degree global cooling. The major weakness in this argument is that as no means exists to estimate the change in the magnitude of the radiative cooling term in the tropical atmosphere's heat budget, the assumption must be made that during glacial time the magnitude of this term was close to its present value. Changes in the inventory of atmospheric water vapor are tantalizing for they offer not only a means of cooling the glacial Earth but also a means of producing the abrupt changes in global climate, now well documented in climate record.
Air‐borne dust fluxes to a deep water sediment trap in the Sargasso Sea
Authors: Jickells T. D. , Dorling S. , Deuser W. G. , Church T. M. , Arimoto R. , Prospero J. M.
Published: February 2004

The record of atmospheric dust deposition as recorded by a deep sea sediment trap in the Sargasso Sea is presented. The record is shown to be consistent with the limited available data on directly measured atmospheric dust loadings. The seasonality of the sediment trap dust flux is different from that of the atmospheric deposition as a result of seasonal biological cycles in the surface water. On the longer term the sediment trap dust flux undergoes quite large variations in the annual average flux from 3.6 to 9.4 mg m−2 d−1. These variations are shown to reflect changes in atmospheric transport efficiency from source regions in North Africa rather than changes in the strength of the dust source in that region. The changes in the dust inputs to this area of the Sargasso Sea appear not to have changed the flux of carbon reaching the deep water, and the implications of this are discussed.
Sensing integrated water vapor along GPS ray paths
Authors: Ware Randolph , Alber Chris , Rocken Christian , Solheim Fredrick
Published: February 2004

We demonstrate sensing of integrated slant‐path water vapor (SWV) along ray paths between Global Positioning System (GPS) satellites and receivers. We use double differencing to remove GPS receiver and satellite clock errors and 85‐cm diameter choke ring antennas to reduce ground‐reflected multipath. We compare more than 17,000 GPS and pointed radiometer double difference observations above 20° elevation and find 1.3 mm rms agreement. Potential applications for SWV data include local and regional weather modeling and prediction, correction for slant wet delay effects in GPS surveying and orbit determination, and synthetic aperture radar (SAR) imaging. The method is viable during all weather conditions.