Showing papers in "Water Resources Research in 1994"

A physically based model for the topographic control on shallow landsliding

Abstract: A model for the topographic influence on shallow landslide initiation is developed by coupling digital terrain data with near-surface through flow and slope stability models. The hydrologic model TOPOG (O'Loughlin, 1986) predicts the degree of soil saturation in response to a steady state rainfall for topographic elements defined by the intersection of contours and flow tube boundaries. The slope stability component uses this relative soil saturation to analyze the stability of each topographic element for the case of cohesionless soils of spatially constant thickness and saturated conductivity. The steady state rainfall predicted to cause instability in each topographic element provides a measure of the relative potential for shallow landsliding. The spatial distribution of critical rainfall values is compared with landslide locations mapped from aerial photographs and in the field for three study basins where high-resolution digital elevation data are available: Tennessee Valley in Marin County, California; Mettman Ridge in the Oregon Coast Range; and Split Creek on the Olympic Peninsula, Washington. Model predictions in each of these areas are consistent with spatial patterns of observed landslide scars, although hydrologic complexities not accounted for in the model (e.g., spatial variability of soil properties and bedrock flow) control specific sites and timing of debris flow initiation within areas of similar topographic control.

A distributed hydrology-vegetation model for complex terrain

Abstract: A distributed hydrology-vegetation model is described that includes canopy interception, evaporation, transpiration, and snow accumulation and melt, as well as runoff generation via the saturation excess mechanisms Digital elevation data are used to model topographic controls on incoming solar radiation, air temperature, precipitation, and downslope water movement Canopy evapotranspiration is represented via a two-layer Penman-Monteith formulation that incorporates local net solar radiation, surface meteorology, soil characteristics and moisture status, and species-dependent leaf area index and stomatal resistance Snow accumulation and ablation are modeled using an energy balance approach that includes the effects of local topography and vegetation cover Saturated subsurface flow is modeled using a quasi three-dimensional routing scheme The model was applied at a 180-m scale to the Middle Fork Flathead River basin in northwestern Montana This 2900-km2, snowmelt-dominated watershed ranges in elevation from 900 to over 3000 m The model was calibrated using 2 years of recorded precipitation and streamflow The model was verified against 2 additional years of runoff and against advanced very high resolution radiometer based spatial snow cover data at the 1-km2 scale Simulated discharge showed acceptable agreement with observations The simulated areal patterns of snow cover were in general agreement with the remote sensing observations, but were lagged slightly in time

A detachment-limited model of drainage basin evolution

Abstract: A drainage basin simulation model introduced here incorporates creep and threshold slumping and both detachment- and transport-limited fluvial processes. Fluvial erosion of natural slopes and headwater channels is argued to be dominantly detachment-limited. Such slopes undergo nearly parallel retreat and replacement with alluvial surfaces under fixed base level, in contrast with gradual slope decline for transport-limited conditions. The arrangement of divides and valleys is sensitive to initial conditions, although average morphology is insensitive. Dissected, initially flat surfaces in which downstream concavity is slight exhibit nearly parallel drainage, compared to very wandering main valleys when concavity is great. Steady state is reached after a cumulative base level drop approximately 3 times the final relief. Simulated valley systems are similar to those predicted by a previous model of optimal drainage basins. A critical value of slope divergence normalized by average slope gradient is a useful criterion for defining the valley network.

Hydraulic conductivity estimation for soils with heterogeneous pore structure

Abstract: The hydraulic conductivity function, which is required to solve the Richards equation, is difficult to measure. Therefore prediction methods are frequently used where the shape of the conductivity function is estimated from the more easily measured water retention characteristic. Errors in conductivity estimations can arise either from an invalidity of the prediction model for a given soil, or from an incorrect description of the retention data. This second error source is particularly important for soils with heterogeneous pore systems that cannot be adequately described by the usually used retention functions. To describe the retention characteristics of such soils, a flexible θ(ψ) function was formed by superimposing unimodal retention curves of the van Genuchten (1980) type. By combining this retention model with the conductivity prediction model of Mualem (1976), conductivity estimations for soils with heterogeneous pore systems are obtained. Estimated conductivities by this model and the classical van Genuchten-Mualem method can differ by orders of magnitude. Thus reported disagreements between measured and estimated conductivities may in some cases be due to an inadequate description of the retention data rather than due to a failure of the prediction model.

Susceptibility of soils to preferential flow of water : a field study

Abstract: Flow pathways of water and solutes in soils form distinct patterns, which are not a priori predictable. Macropore structure is a prime cause, but other factors, such as differing initial or boundary conditions, may also predispose a soil to produce bypassing of infiltrating water. This study was conducted to assess the flow pathways of water in different soils and to investigate the effect of initial water content on the flow pattern. Dye-tracing experiments were carried out at 14 different field sites. The sites represent a good portion of soils used for agricultural crop production in Switzerland. Each site consisted of two 1.4 by 1.4 m plots, one of which had been covered with a plastic roof for two months before the experiment to achieve different initial water contents. Forty millimeters of water containing the dye Brilliant Blue FCF (C.I. Food Blue 2) were applied within 8 hours onto the plots with a sprinkling apparatus. One day after irrigation the plots were excavated, and the stained pattern was examined on a vertical 1 by l m soil profile. The spatial structure of flow patterns showed remarkable differences. In most soils, water bypassed the soil matrix. In some soils, dye penetrated beyond l m depth, whereas in others it remained in the top 50 cm. Structured soils were more prone to produce bypass flow, deep dye penetration, and pulse splitting than nonstructured soils. The initial water content had a less pronounced effect in some soils and no effect in others.

Digital elevation model grid size, landscape representation, and hydrologic simulations

Abstract: High-resolution digital elevation data from two small catchments in the western United States are used to examine the effect of digital elevation model (DEM) grid size on the portrayal of the land surface and hydrologic simulations. Elevation data were gridded at 2-, 4-, 10-, 30-, and 90-m scales to generate a series of simulated landscapes. Frequency distributions of slope (tan B), drainage area per unit contour length (a), and the topographic index (a/tan B) were calculated for each grid size model. Frequency distributions of a/tan B were then used in O'Loughlin's (1986) criterion for predicting zones of surface saturation and in TOPMODEL (Beven and Kirkby, 1979) for simulating hydrographs. For both catchments, DEM grid size significantly affects computed topographic parameters and hydrographs. While channel routing dominates hydrograph characteristics for large catchments, grid size effects influence physically based models of runoff generation and surface processes. A 10-m grid size provides a substantial improvement over 30- and 90-m data, but 2- or 4-m data provide only marginal additional improvement for the moderately to steep gradient topography of our study areas. Our analyses suggest that for many landscapes, a 10-m grid size presents a rational compromise between increasing resolution and data volume for simulating geomorphic and hydrological processes.

Climate, soil water storage, and the average annual water balance

Abstract: This paper describes the development and testing of the hypothesis that the long-term water balance is determined only by the local interaction of fluctuating water supply (precipitation) and demand (potential evapotranspiration), mediated by water storage in the soil. Adoption of this hypothesis, together with idealized representations of relevant input variabilities in time and space, yields a simple model of the water balance of a finite area having a uniform climate. The partitioning of average annual precipitation into evapotranspiration and runoff depends on seven dimensionless numbers: the ratio of average annual potential evapotranspiration to average annual precipitation (index of dryness); the ratio of the spatial average plant-available water-holding capacity of the soil to the annual average precipitation amount; the mean number of precipitation events per year; the shape parameter of the gamma distribution describing spatial variability of storage capacity; and simple measures of the seasonality of mean precipitation intensity, storm arrival rate, and potential evapotranspiration. The hypothesis is tested in an application of the model to the United States east of the Rocky Mountains, with no calibration. Study area averages of runoff and evapotranspiration, based on observations, are 263 mm and 728 mm, respectively; the model yields corresponding estimates of 250 mm and 741 mm, respectively, and explains 88% of the geographical variance of observed runoff within the study region. The differences between modeled and observed runoff can be explained by uncertainties in the model inputs and in the observed runoff. In the humid (index of dryness 1) parts, all of the runoff is caused by variability of forcing over time. Contributions to model runoff attributable to small-scale spatial variability of storage capacity are insignificant throughout the study area. The consistency of the model with observational data is supportive of the supply-demand-storage hypothesis, which neglects infiltration excess runoff and other finite-permeability effects on the soil water balance.

How water moves in a water repellent sandy soil: 1. Potential and actual water repellency

Abstract: Water repellency is an important property of many soils. It causes rainwater to penetrate into the soil as preferential flow paths, and solutes can reach the groundwater more rapidly than in the case of a homogeneous wetting. Water repellency depends on several factors which are principally related to the characteristics of the organic matter of the soil. A distinction between “potential” and “actual” water repellency and the assessment of the “critical soil water content” are introduced and highlighted in this paper. Persistence and degree of potential water repellency of dried samples were examined from 10 trenches in a dune sand with grass cover using the water drop penetration time and the alcohol percentage tests. The spatial variability of water repellency and, therefore, soil wetting was extremely high. The actual water repellency was measured on field-moist samples to obtain critical soil water contents. The soil is wettable above and water repellent below these values. The critical soil water content varies between 4.75 vol % at 5–10 cm and 1.75 vol % at 45–50 cm depth in this sandy soil.

Multiscale modeling of spatially variable water and energy balance processes

Abstract: This paper presents the model development component of a body of research which addresses aggregation and scaling in multiscale hydrological modeling. Water and energy balance models are developed at the local and catchment scales and at the macroscale by aggregating a simple soil-vegetation-atmosphere transfer scheme (SVATS) across scales in a topographic framework. A spatially distributed approach is followed to aggregate the SVATS to the catchment scale. A statistical-dynamical approach is utilized to simplify the large-scale modeling problem and to aggregate the SVATS to the macroscale. The resulting macroscale hydrological model is proposed for use as a land surface parameterization in atmospheric models. It differs greatly from the current generation of land surface parameterizations owing to its simplified representation of vertical process physics and its statistical representation of horizontally heterogeneous runoff and energy balance processes. The spatially distributed model formulation is explored to understand the role of spatial variability in determining areal-average fluxes and the dynamics of hydrological processes. The simpler macroscale formulation is analyzed to determine how it represents these important dynamics, with implications for the parameterization of runoff and energy balance processes in atmospheric models.

How permeable are clays and shales

Abstract: The permeability of argillaceous formations, although rarely measured and poorly understood, is commonly a critical parameter in analyses of subsurface flow. Data now available suggest a regular relation between permeability and porosity in clays and shales and permeabilities that, even at large scales, are significantly lower than usually assumed. Permeabilities between 10 -23 and 10 -17 m 2 have been obtained at porosities between 0.1 and 0.4 in both laboratory and regional studies. Although it is clear that transmissive fractures or other heterogeneities control the large-scale hydraulic behavior of certain argillaceous units, the permeability of many others is apparently scale independent. These results have significant implications for understanding fluid transport rates and abnormal pressure generation in basins, and could prove important for waste isolation efforts.

Digital Elevation Model Networks (DEMON): A model of flow over hillslopes for computation of contributing and dispersal areas

Abstract: Current algorithms for computing contributing areas from a rectangular grid digital elevation model (DEM) use the flow-routing model of O'Callaghan and Mark (1984), which has two major restrictions: (1) flow which originates over a two-dimensional pixel is treated as a point source (nondimensional) and is projected downslope by a line (one dimensional) (Moore and Grayson, 1991), and (2) the flow direction in each pixel is restricted to eight possibilities. We show that large errors in the computed contributing areas result for any terrain topography: divergent, convergent, or planar. We present a new model, called digital elevation model networks (DEMON), which avoids the above problems by representing flow in two dimensions and directed by aspect. DEMON allows computation of both contributing and dispersal areas. DEMON offers the main advantage of contour-based models (e.g., Moore et al., 1988), the representation of varying flow width over nonplanar topography, while having the convenience of using rectangular grid DEMs.

Optimization of groundwater remediation using artificial neural networks with parallel solute transport modeling

Abstract: A new approach to nonlinear groundwater management methodology is presented which optimizes aquifer remediation with the aid of artificial neural networks (ANNs). The methodology allows solute transport simulations, usually the main computational component of management models, to be run in parallel. The ANN technology, inspired by neurobiological theories of massive interconnection and parallelism, has been successfully applied to a variety of optimization problems. In this new approach, optimal management solutions are found by (1) first training an ANN to predict the outcome of the flow and transport code, and (2) then using the trained ANN to search through many pumping realizations to find an optimal one for successful remediation. The behavior of complex groundwater scenarios with spatially variable transport parameters and multiple contaminant plumes is simulated with a two-dimensional hybrid finite-difference/finite-element flow and transport code. The flow and transport code develops the set of examples upon which the network is trained. The input of the ANN characterizes the different realizations of pumping, with each input indicating the pumping level of a well. The output is capable of characterizing the objectives and constraints of the optimization, such as attainment of regulatory goals, value of cost functions and cleanup time, and mass of contaminant removal. The supervised learning algorithm of back propagation was used to train the network. The conjugate gradient method and weight elimination procedures are used to speed convergence and improve performance, respectively. Once trained, the ANN begins a search through various realizations of pumping patterns to determine whether or not they will be successful. The search is directed by a simple genetic algorithm. The resulting management solutions are consistent with those resulting from a more conventional optimization technique, which combines solute transport modeling and nonlinear programming with a quasi-Newton search. The results suggest that the ANN approach has the following advantages over the conventional technique for the test remediations: more independence of the flow and transport code from the optimization, greater influence of hydrogeologic insight, and less computational burden due to the potential for parallel processing of the flow and transport simulations and the ability to “recycle” these simulations. The ANN performance was observed upon variation of the problem formulation, network architecture, and learning algorithm.

Using genetic algorithms to solve a multiple objective groundwater pollution containment problem

Abstract: The genetic algorithm (GA), a new search technique, is applied to a multiple objective groundwater pollution containment problem. This problem involves finding the set of optimal solutions on the trade-off curve between the reliability and cost of a hydraulic containment system. The decision variables are how many wells to install, where to install them, and how much to pump from each. The GA is an optimization technique patterned after the biological processes of natural selection and evolution. A GA operates on a population of decision variable sets. Through the application of three specialized genetic operators: selection, crossover, and mutation, a GA population “evolves” toward an optimal solution. In the paper, simple GAs and GAs that can solve multiple objective problems are described. Two variations of a multiple objective GA are formulated: a vector-evaluated GA (VEGA) and a Pareto GA. For the zero-fixed cost situation, the Pareto GA is shown to be superior to the VEGA and is shown to produce a trade-off curve similar to that obtained via another optimization technique, mixed integer chance constrained programming (MICCP). The effect on the VEGA and Pareto GA of parameter variation is shown. The Pareto GA is shown to be capable of incorporating the fixed costs associated with installing a system of wells. Results for several levels of fixed cost are presented. A comparison of computer resources required by the GAs and the MICCP method is given. Future research plans are discussed, including the incorporation of the objective of pump-out time into the model and the development of parallelized GAs.

Genetic algorithm solution of groundwater management models

Abstract: Groundwater simulation models have been incorporated into a genetic algorithm to solve three groundwater management problems: maximum pumping from an aquifer; minimum cost water supply development; and minimum cost aquifer remediation. The results show that genetic algorithms can effectively and efficiently be used to obtain globally (or, at least near globally) optimal solutions to these groundwater management problems. The formulation of the method is straightforward and provides solutions which are as good as or better than those obtained by linear and nonlinear programming. Constraints can be incorporated into the formulation and do not require derivatives with respect to decision variables as in nonlinear programming. More complicated problems, such as transient pumping and multiphase remediation, can be formulated and solved using this method. The computational time required for the solution of genetic algorithm groundwater management models increases with the complexity of the problem. The speedup attainable by solving genetic algorithm problems on massively parallel computers is significant for problems where the simulation time required to complete each generation is high. 47 refs., 9 figs., 5 tabs.

Effects of digital elevation model map scale and data resolution on a topography-based watershed model

Abstract: The effects of digital elevation model (DEM) map scale and data resolution on watershed model predictions of hydrologic characteristics were determined for TOPMODEL, a topography-based watershed model. The effects of topography on watershed hydrology are represented in TOPMODEL as the distribution of ln (a/tan B), where ln is the Napierian logarithm, a is the upslope area per unit contour length, and tan B is the gravitational gradient. The minimum, maximum, mean, variance, and skew values of the ln (a/tan B) distribution were computed from 1:24,000-scale (24K) DEMs at 30- and 90-m resolutions and from 1:250,000-scale (250K) DEMs at 90-m resolution for 71 areas in Pennsylvania, New York, and New Jersey. An analysis of TOPMODEL showed that model predictions of the depth to the water table, the ratio of overland flow to total flow, peak flow, and variance and skew of predicted streamflow were affected by both the DEM map scale and data resolution. Further TOPMODEL analyses showed that the effects of DEM map scale and data resolution on model predictions were due to the sensitivity of the predictions to the mean of the ln (a/tan B) distribution, which was affected by both DEM map scale and data resolution. DEM map scale affected the mean of the ln (a/tan B) distribution through its influence on the mean of the ln (a) distribution, which characterizes land-surface shape, and the mean of ln (1/tan B) distribution, which characterizes land-surface slope. DEM resolution, in contrast, affected the mean of the ln (a/tan B) distribution primarily by its influence on the mean of the ln (a) distribution.

Three-parameter lognormal distribution model for soil water retention

Abstract: Many models for soil water retention have been proposed. However, most of these models are curve-fitting equations and do not emphasize the physical significance of their empirical parameters. A new retention model that exhibits increased flexibility was developed by applying three-parameter lognormal distribution laws to the pore radius distribution function ƒ(r) and to the water capacity function, which was taken to be the pore capillary pressure distribution function ƒ(ψ). This model contains three parameters that are closely related to the statistics of ƒ(ψ): the bubbling pressure ψc, the mode ψ0 of ƒ(ψ) and the standard deviation σ of transformed ƒ(ψ). By comparison of this model with three existing models (the van Genuchten model, the Brooks-Corey model, and the modified Tani model), it was shown that ψc, ψ0, and σ are all essential for a general retention model.

An experimental study of complete dissolution of a nonaqueous phase liquid in saturated porous media

Abstract: The attenuation of gamma radiation was utilized to measure changing residual trichloroethylene (TCE) saturation in an otherwise water-saturated porous medium as clean water was flushed through the medium. A front over which dissolution actively occurred was observed. Once developed, this front varied in length from ≈11 mm to ≈21 mm, lengthening as it moved through the porous medium. Gamma attenuation measurements and analyses of effluent water samples indicate that there was minimal if any transport of TCE as colloidal droplets. Even as trapped TCE ganglia decreased in size due to dissolution, there is no evidence that they became mobile and advected downgradient. An extraction of the porous medium at the completion of one experiment indicated that less than 0.002% of the original TCE mass remained, suggesting that minimal amounts of separate phase TCE remained trapped within the medium after flushing with 290 pore volumes. Mass transfer rate coefficients were computed and are shown to be a function of Darcy flux, TCE volumetric content, and distance into the region of residual TCE.

The concept of the Dilution Index

Abstract: In many applications, it is important to make the distinction between spreading and dilution of a plume in groundwater. Spreading is associated with the stretching and deformation of a contaminant plume, whereas dilution is associated with the increase in volume of the fluid occupied by the solute. The dilution and spreading of a Gaussian plume in a homogeneous porous medium with constant velocity are related in a simple fashion and are both characterized by the same parameters, the dispersion coefficients. However, the geological formations of interest in field applications are heterogeneous, and the plumes are irregular in shape. The dispersion coefficients that are deduced from tracer tests usually measure an overall rate at which a tracer plume spreads about its centroid and depend critically on the heterogeneity of the formation. These macroscopic dispersion coefficients are not reliable measures of the rate at which the maximum concentration is reduced because in heterogeneous formations the rates of dilution and spreading can be quite different. The main objective of this work is to introduce a new macroscopic measure of dilution, the dilution index E. Examples serve to demonstrate the usefulness of the measure. A general expression for the rate of dilution of a tracer plume is derived. The exact rate of increase of the dilution index under the idealized conditions of constant dispersion coefficients and a Gaussian plume is computed, and a lower bound is found to the same quantity for non-Gaussian plumes. For the general heterogeneous case the analysis demonstrates that the instantaneous rate of increase of ln E is proportional to the small-scale dispersion coefficients, everything else being the same. The rate of increase of ln E depends also on the degree of irregularity in the shape of the plume. Thus, in the long term, geologic heterogeneity should increase the rate of dilution because spatial variability in the flow velocity tends to deform plumes and make them less regular.

A comparison of Picard and Newton iteration in the numerical solution of multidimensional variably saturated flow problems

Abstract: Picard iteration is a widely used procedure for solving the nonlinear equation governing flow in variably saturated porous media. The method is simple to code and computationally cheap, but has been known to fail or converge slowly. The Newton method is more complex and expensive (on a per-iteration basis) than Picard, and as such has not received very much attention. Its robustness and higher rate of convergence, however, make it an attractive alternative to the Picard method, particularly for strongly nonlinear problems. In this paper the Picard and Newton schemes are implemented and compared in one-, two-, and three-dimensional finite element simulations involving both steady state and transient flow. The eight test cases presented highlight different aspects of the performance of the two iterative methods and the different factors that can affect their convergence and efficiency, including problem size, spatial and temporal discretization, initial solution estimates, convergence error norm, mass lumping, time weighting, conductivity and moisture content characteristics, boundary conditions, seepage faces, and the extent of fully saturated zones in the soil. Previous strategies for enhancing the performance of the Picard and Newton schemes are revisited, and new ones are suggested. The strategies include chord slope approximations for the derivatives of the characteristic equations, relaxing convergence requirements along seepage faces, dynamic time step control, nonlinear relaxation, and a mixed Picard-Newton approach. The tests show that the Picard or relaxed Picard schemes are often adequate for solving Richards' equation, but that in cases where these fail to converge or converge slowly, the Newton method should be used. The mixed Picard-Newton approach can effectively overcome the Newton scheme's sensitivity to initial solution estimates, while comparatively poor performance is reported for the various chord slope approximations. Finally, given the reliability and efficiency of current conjugate gradient-like methods for solving linear nonsymmetric systems, the only real drawback of using Newton rather than Picard iteration is the algebraic complexity and computational cost of assembling the derivative terms of the Jacobian matrix, and it is suggested that both methods can be effectively implemented and used in numerical models of Richards' equation.

Three‐dimensional analysis of infiltration from the disc infiltrometer: 2. Physically based infiltration equation

Abstract: In situ measurement of soil hydraulic properties may be achieved by analyzing the unconfined efflux from disc tension infiltrometers, once consistent infiltration equations can be derived. In this paper an analytical, three-dimensional infiltration equation is developed, based on the use of parameters with sound physical meaning and adjustable for varying initial and boundary conditions. The equation is valid over the entire time range. For practical purposes, a simplified solution is also derived. The full and simplified equations give excellent agreement with published experimental results and are particularly useful for determining soil hydraulic properties through application of inverse procedures.

Numerical simulation of downstream fining by selective transport in gravel bed rivers: Model development and illustration

Abstract: A numerical model has been developed for the routing of gravel-sized sediment along a river channel which is free to adjust both its long profile and surface texture. Hydraulic calculations use a step-backwater approach, and sediment transport is predicted with the method of Parker (1990a), which uses a low degree of size selectivity. Exchange of sediment between the surface and subsurface is described using the modified Exner equation of Parker and Sutherland (1990). The model is applied to an idealized channel based on the highly concave Allt Dubhaig, Scotland, in which fining by particle wear is minor. The rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to <102 years under the present flow regime of the river. The evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described. It is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective. This run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions, which are summarized here and will be described in a subsequent paper.

Comparison of simple versus complex distributed runoff models on a midsized semiarid watershed

Abstract: The increasing availability of distributed rainfall data and computational resources is providing the opportunity to use distributed models for rainfall-runoff forecasting or other applications. This paper compares the accuracy of simulations from a complex distributed model (KINEROS), a simple distributed model (based on the Soil Conservation Service (SCS) method), and a simple lumped model (SCS method). The 150 km2, semiarid Walnut Gulch experimental watershed was the test site; models were validated using 24 severe thunderstorms and rain gauge densities similar to those found at flash flood warning sites (one gauge per 20 km2). Under these circumstances, none of the models were able to accurately simulate peak flows or runoff volumes from individual events. Models showed somewhat more skill in predicting time to peak and the ratio of peak flow to volume. When calibration was performed, the accuracy of the complex distributed model was similar to that of the simple distributed model. Without calibration, the complex distributed model was more accurate than the simple distributed model. The spatially lumped model performed very poorly. The complex distributed model was validated under real-time forecasting conditions; forecasts based on observed rainfall had lead times of 30—75 min.

Road surface drainage, channel initiation, and slope instability

Abstract: Field surveys of road drainage concentration at three sites in the western United States are used to test simple models relating channel initiation and shallow landsliding to ground slope and contributing area thresholds. The form of boundaries between data for locations where road drainage concentration is associated with either shallow landsliding, channel initiation by overland flow, or no observable geomorphic effect is consistent with theoretically derived drainage area-slope relations. Comparison of survey data with results of previous studies in these areas indicates that the drainage area required to support a channel head is smaller for road-related runoff than for undisturbed slopes. Contrary to current land management paradigms in the Pacific Northwest, drainage concentration from ridgetop roads may cause both landsliding and integration of the channel and road networks. Road drainage concentration increases the effective length of the channel network and strongly influences the distribution of erosional processes in each of the study areas. The approach of using field reconnaissance to establish thresholds for erosion associated with road drainage provides a useful method to define regional criteria for road design that should reduce impacts on downstream channel systems.

How water moves in a water repellent sandy soil: 2. Dynamics of fingered flow

Abstract: The dynamics of fingered flow in a water repellent sandy soil was studied in the field by sampling 10 vertical trenches in a 1-year cycle. In dry soil, fingers were formed in those places in the top layer which have the lowest degree of potential water repellency. The finger diameters varied roughly between 10 and 50 cm depending on the sequence of weather conditions. The fingers were wet in the topsoil and increasingly drier with depth. In none of the trenches sampled were there any serious indications of finger merger. The actual water repellent soil volumes between fingers were excluded from the transport of water and solutes for at least several hours. The temporal and spatial variability of these actual water repellent soil volumes is illustrated and evaluated with respect to simulation model development.

Cation and proton exchange, pH variations, and carbonate reactions in a freshening aquifer

Abstract: Freshening of aquifers is accompanied by sequential elution of the saltwater (seawater) cations from the sediment's exchange complex. The resulting Chromatographic patterns are modeled with a one-dimensional geochemical transport model that can handle the complex interplay of transport and mineral and ion exchange equilibria. The transport part is based on the mixing cell approach, with different time steps for advective and diffusive transport used when required by small grid size. The chemical reactions are calculated explicitly after each time step with the geochemical model PHREEQE. Ion exchange is included in the form of association half reactions, which allows simulation of the dynamic nature of the exchange process. The variation in the constant for proton association is obviated with an activity coefficient for H-X that is derived from the constant capacitance model. All coefficients for the exchange model are obtained by fitting to literature data to be able to perform the modeling as realistically as possible. The code is applied to a laboratory column experiment, and subsequently used to demonstrate Chromatographic development of solute profiles in a freshening aquifer. Sequential peaks of Mg2+, K+, and Na+ along a flow path in the Aquia aquifer in Maryland are modeled, and the results confirm that the variation of water qualities in this aquifer has basically a Chromatographic origin. Proton exchange acts here as a source of acid in NaHCO3 water in which calcite dissolves. This explains the Na+ to HCO3− ratio and high δ13C observed in these waters.

A quasi‐dynamic wetness index for characterizing the spatial distribution of zones of surface saturation and soil water content

Abstract: A quasi-dynamic wetness index that accounts for variable drainage times since a prior rainfall event is derived from simple subsurface flow theory. The method is tested through a series of field observations and numerical experiments using a spatially distributed, dynamic hydrologic model. The quasi-dynamic wetness index is shown to be a useful extension of previously developed static indices for predicting the location of zones of soil saturation and the distribution of soil water (i.e., the soil water content overlying a shallow impermeable or semiimpermeable layer). The new index is not constrained by the steady state assumption that forms the basis of existing indices.

Modeling of soil water retention from saturation to oven dryness

Abstract: Most analytical formulas used to model moisture retention in unsaturated porous media have been developed for the wet range and are unsuitable for applications in which low water contents are important. We have developed two models that fit the entire range from saturation to oven dryness in a practical and physically realistic way with smooth, continuous functions that have few parameters. Both models incorporate a power law and a logarithmic dependence of water content on suction, differing in how these two components are combined. In one model, functions are added together (model “sum”); in the other they are joined smoothly together at a discrete point (model “junction”). Both models also incorporate recent developments that assure a continuous derivative and force the function to reach zero water content at a finite value of suction that corresponds to oven dryness. The models have been tested with seven sets of water retention data that each cover nearly the entire range. The three-parameter sum model fits all data well and is useful for extrapolation into the dry range when data for it are unavailable. The two-parameter junction model fits most data sets almost as well as the sum model and has the advantage of being analytically integrable for convenient use with capillary-bundle models to obtain the unsaturated hydraulic conductivity.

Bare soil surface resistance to evaporation by vapor diffusion under semiarid conditions

Abstract: Based on Kohsiek's fast air circulation chamber, a method has been developed to measure the surface resistance to vapor diffusion in a drying topsoil. This resistance is important to estimate evaporation from bare soils using an aerodynamic resistance formulation. Measurements were done for a fine sandy loam during a dry down after artificial wetting. Surface resistance started to increase at a moisture content of 15% by volume in the 1-cm top layer, which is 50% of its moisture content at field capacity. Calculations of the aerodynamic resistance were corrected for stability and were used to isolate the real surface resistance from the bulk resistance. Resistances could be modeled as a function of the top 1 cm soil moisture and varied between approximately 10 s/m for a wet and several thousand seconds per meter for a dry top layer. The measurements demonstrated a very pronounced diurnal course due to drying of the very top layer during the day and recovery of the moisture profile during nighttime hours.

Optimal design of water distribution networks

Abstract: Optimal design of a water distribution network is formulated as a two-stage decomposition model. The master (outer) problem is nonsmooth and nonconvex, while the inner problem is linear. A semi-infinite linear dual problem is presented, and an equivalent finite linear problem is developed. The overall design problem is solved globally by a branch and bound algorithm, using nonsmooth optimization and duality theory. The algorithm stops with a solution and a global bound, such that the difference between this bound and the true global optimum is within a prescribed tolerance. The algorithm has been programmed and applied to a number of examples from the literature. The results demonstrate its superiority over previous methods.

A class of stochastic models for relating synoptic atmospheric patterns to regional hydrologic phenomena

Abstract: A model for multistation precipitation, conditional on synoptic atmospheric patterns, is presented. The model, which we call the nonhomogeneous hidden Markov model (NHMM), postulates the existence of an unobserved weather state, which serves as a link between the large-scale atmospheric measures and the small-scale spatially discontinuous precipitation field. The weather state effectively acts as an automatic classifier of atmospheric patterns. The weather state process is assumed to be conditionally Markov, given the atmospheric data. The rainfall process is then assumed to be conditionally independent given the weather state. Various parameterizations for the weather state process and the rainfall process are discussed, and a likelihood-based estimation procedure is described. Model-based estimates of the storm duration distribution and first and second moments of the rainfall process are derived. As an example the model is fit to a four-station network of rain gauge stations in Washington state. The observed first and second moments are reproduced very closely. The fitted duration distributions are somewhat lighter tailed than the observed distribution at two of the four stations but provide a good fit at the other two. We conclude that the NHMM has promise as a method of relating synoptic atmospheric data to rainfall and other regional or local hydrologic processes.