Vincent Carroll Tidwell

System dynamics modeling for community-based water planning: Application to the Middle Rio Grande

Abstract.The watersheds in which we live are comprised of a complex set of physical and social systems that interact over a range of spatial and temporal scales. These systems are continually evolving in response to changing climatic patterns, land use practices and the increasing intervention of humans. Management of these watersheds benefits from the development and application of models that offer a comprehensive and integrated view of these complex systems and the demands placed upon them. The utility of these models is greatly enhanced if they are developed in a participatory process that incorporates the views and knowledge of relevant stakeholders. System dynamics provides a unique mathematical framework for integrating the physical and social processes important to watershed management, and for providing an interactive interface for engaging the public. We have employed system dynamics modeling to assist in community-based water planning for a three-county region in north-central New Mexico. The planning region is centered on a ~165-km reach of the Rio Grande that includes the greater Albuquerque metropolitan area. The challenge, which is common to other arid/semi-arid environments, is to balance a highly variable water supply among the demands posed by urban development, irrigated agriculture, river/reservoir evaporation and riparian/in-stream uses. A description of the model and the planning process are given along with results and perspectives drawn from both.

X ray and visible light transmission for laboratory measurement of two‐dimensional saturation fields in thin‐slab systems

Two independent techniques, X ray absorption and light transmission, are developed and demonstrated for imaging transient saturation fields in thin-slab porous systems. The techniques yield full two-dimensional saturation fields with high spatial and temporal resolution. In the time required to make a single measurement by one of the traditional methods (e.g., gravimetric or gamma densitometry) an entire image consisting of hundreds of thousands of points is acquired by either the X ray or light technique. These methods are also very sensitive, capable of resolving a hundred or more levels of saturation at each of these points. Evaluation of these techniques is accomplished by direct comparison of X ray and light data as well as comparison with gravimetric and gamma densitometry data. Results of the comparison show very close agreement between the four techniques (on average within 5% saturation). These techniques represent useful tools for investigating processes governing unsaturated flow and transport through porous media.

Laboratory method for investigating permeability upscaling

The purpose of this work is to describe, evaluate, and demonstrate a laboratory-based method for physically investigating permeability upscaling. The method makes use of a simple instrument, the gas permeameter, to acquire rapid, precise, and nondestructive permeability measurements from heterogeneous blocks of dry rock. Critical to investigating permeability upscaling is the ability to acquire data at multiple sample supports subject to consistent boundary conditions and flow geometry. Such measurements, spanning almost 4 orders of magnitude on a per volume basis, are made with the gas permeameter by simply varying the size of the permeameter tip seal. The precision and consistency of measurements made in this way were evaluated using a suite of data collected from blocks of three relatively homogeneous materials: Berea Sandstone and two synthetic rocks. Results suggest that measurement error is small (approximately ±1% of the measured permeability) and consistent, and measurements made at different sample supports are free from systematic bias. To demonstrate the ability of this method to measure and quantify upscaling processes, limited data sets were collected with four different-sized tip seals from the Berea Sandstone block. Analysis reveals distinct and consistent trends diagnostic of permeability upscaling relating the sample mean (increased), variance (decreased), and semivariogram to increasing sample support.

Effects of spatially heterogeneous porosity on matrix diffusion as investigated by X-ray absorption imaging

Abstract High-resolution X-ray absorption imaging was used to investigate the effects of spatially heterogeneous porosity on matrix diffusion. Experiments were performed on four, centimeter-scale slabs of Culebra dolomite taken from the Waste Isolation Pilot Plant (WIPP) site. These tests involved the diffusion of potassium iodide into a single edge of each brine-saturated rock slab, while X-ray absorption imaging was used to measure the two-dimensional relative concentration distribution at different times during the experiment. X-ray imaging was also used to measure the heterogeneous, two-dimensional porosity distribution of each rock slab. The resulting high-resolution data provide unique insight into the spatially varying diffusion characteristics of each heterogeneous rock sample, which traditional methods such as through-diffusion experiments cannot. In these tests, significant variations in the diffusion coefficient were calculated over the relatively small length (centimeter) and time scales (months) investigated. Results also indicated that these variations were related to the heterogeneous porosity characteristics of each rock sample. Not only were the diffusion coefficients found to depend on the magnitude of the porosity but also on its spatial distribution. Specifically, the geometry, position, and orientation of the heterogeneous porosity features populating each rock slab appeared to influence the diffusion characteristics.

Challenging models for flow in unsaturated, fractured rock through exploration of small scale processes

Fluid flow in unsaturated, fractured rock is studied with respect to applied environmental problems ranging from remediation of existing contaminated sites to evaluation of potential sites for isolation of hazardous or radioactive wastes. Spatial scales for such problems vary from meters to kilometers with temporal scales from months to tens of thousands of years. Because such scales often preclude direct physical exploration of system response and detailed site characterization, the authors are regularly forced to use their understanding (or misunderstanding) of the underlying physical processes to predict large scale behavior. It is essential that conceptual models used as the basis for prediction be firmly grounded in physical reality. In this paper, they provide examples of how recent advances in understanding of small-scale processes within discrete fractures may influence the behavior of fluid flow in fracture networks and ensembles of matrix blocks sufficiently to impact the formulation of intermediate-scale effective media properties. The authors also explore, by means of a thought experiment, how these same small-scale processes could couple to produce a large-scale system response inconsistent with current conceptual models of flow through unsaturated, fractured rock. 20 refs.

Heterogeneity, permeability patterns, and permeability upscaling: Physical characterization of a block of Massillon sandstone exhibiting nested scales of heterogeneity

Over 75,000 permeability measurements were collected from a meter-scale block of Massillon sandstone, characterized by conspicuous cross bedding that forms two distinct nested-scales of heterogeneity. With the aid of a gas minipermeameter, spatially exhaustive fields of permeability data were acquired at each of five different sample supports (i.e. sample volumes) from each block face. These data provide a unique opportunity to physically investigate the relationship between the multi-scale cross-stratified attributes of the sandstone and the corresponding statistical characteristics of the permeability. These data also provide quantitative physical information concerning the permeability upscaling of a complex heterogeneous medium. Here, a portion of the data taken from a single block face cut normal to stratification is analyzed. Results indicate a strong relationship between the calculated summary statistics and the cross-stratified structural features visible evident in the sandstone sample. Specifically, the permeability fields and semivariograms are characterized by two nested scales of heterogeneity, including a large-scale structure defined by the cross-stratified sets (delineated by distinct bounding surfaces) and a small-scale structure defined by the low-angle cross-stratification within each set. The permeability data also provide clear evidence of upscaling. That is, each calculated summary statistic exhibits distinct and consistent trends with increasing sample support. Among these trends are an increasing mean, decreasing variance, and an increasing semivariogram range. Results also clearly indicate that the different scales of heterogeneity upscale differently, with the small-scale structure being preferentially filtered from the data while the large-scale structure is preserved. Finally, the statistical and upscaling characteristics of individual cross-stratified sets were found to be very similar owing to their shared depositional environment; however, some differences were noted that are likely the result of minor variations in the sediment load and/or flow conditions between depositional events.

Permeability Upscaling Measured on a Block of Berea Sandstone: Results and Interpretation

To physically investigate permeability upscaling, over 13,000 permeability values were measured with four different sample supports (i.e., sample volumes) on a block of Berea Sandstone. At each sample support, spatially exhaustive permeability datasets were measured, subject to consistent flow geometry and boundary conditions, with a specially adapted minipermeameter test system. Here, we present and analyze a subset of the data consisting of 2304 permeability values collected from a single block face oriented normal to stratification. Results reveal a number of distinct and consistent trends (i.e., upscaling) relating changes in key summary statistics to an increasing sample support. Examples include the sample mean and semivariogram range that increase with increasing sample support and the sample variance that decreases. To help interpret the measured mean upscaling, we compared it to theoretical models that are only available for somewhat different flow geometries. The comparison suggests that the nonuniform flow imposed by the minipermeameter coupled with permeability anisotropy at the scale of the local support (i.e., smallest sample support for which data is available) are the primary controls on the measured upscaling. This work demonstrates, experimentally, that it is not always appropriate to treat the local-support permeability as an intrinsic feature of the porous medium, that is, independent of its conditions of measurement.

Exploring the Water-Thermoelectric Power Nexus

AbstractIn 2005, thermoelectric power accounted for 41% of all freshwater withdrawals and roughly 3% of all consumptive use in the United States. With the demand for electricity projected to increase by 24% by 2035 concerns have been raised as to the availability of water for this growing industry; particularly, as the siting of several new thermoelectric facilities have been challenged on the basis of water supply. To address this concern we estimate the potential impact of water availability on future expansion of the thermoelectric power industry. Specifically, both the extent and location of thermoelectric developments at risk due to limited fresh water supply is estimated for a variety of alternative energy futures that differ according to the assumed mix of fuels utilized in new plant construction. According to the analyzed scenarios water consumption for thermoelectric power generation is projected to increase by 36–43% between 1995 and 2035, with much of this development expected to occur in basin...

Upscaling experiments conducted on a block of volcanic tuff: Results for a bimodal permeability distribution

Permeability upscaling is physically investigated by making over 31,000 permeability measurements on a meter-scale block of volcanic tuff. The experiments are made possible by a specially adapted minipermeameter test system. Here we present and analyze 5185 permeability values, corresponding to five different sample supports (i.e., sample volumes) collected from one of the six block faces. The results show that the measured spatial permeability patterns, bimodal permeability distribution, and semivariogram structure/length scales are closely related to the strong textural contrast characterizing the tuff sample (i.e., highly porous pumice fragments embedded in a tight rock matrix). Each of the summary statistics shows distinct and consistent trends with increasing sample support (i.e., upscaling). As the sample support increases, the mean and variance decrease according to a power law relation, and the semivariogram range increases linearly, while the general structure of the semivariogram (isotropic, spherical model) remains unchanged. Interpretation of these results is pursued from two very different points of view; one addresses upscaling of the ensemble statistics, while the second examines upscaling from a local or pointwise perspective. We find the general upscaling trends exhibited by the ensemble statistics (given above) to be consistent with the basic concepts of volume averaging, albeit nonlinear volume averaging. The bimodal characteristics of the tuff sample and the nonuniform flow conditions imparted by the minipermeameter contribute to the nonlinearity. The local analysis reveals strong variability in permeability upscaling from point to point throughout the sampling domain. Specifically, the permeability upscaling exhibited by zones rich in pumice is very different from zones dominated by matrix, unless the averaging volume is significantly larger than the spatial correlation scale.

Mapping water availability, projected use and cost in the western United States

New demands for water can be satisfied through a variety of source options. In some basins surface and/or groundwater may be available through permitting with the state water management agency (termed unappropriated water), alternatively water might be purchased and transferred out of its current use to another (termed appropriated water), or non-traditional water sources can be captured and treated (e.g., wastewater). The relative availability and cost of each source are key factors in the development decision. Unfortunately, these measures are location dependent with no consistent or comparable set of data available for evaluating competing water sources. With the help of western water managers, water availability was mapped for over 1200 watersheds throughout the western US. Five water sources were individually examined, including unappropriated surface water, unappropriated groundwater, appropriated water, municipal wastewater and brackish groundwater. Also mapped was projected change in consumptive water use from 2010 to 2030. Associated costs to acquire, convey and treat the water, as necessary, for each of the five sources were estimated. These metrics were developed to support regional water planning and policy analysis with initial application to electric transmission planning in the western US.