SwRI® Project No. 20-17344 DEVELOPMENT OF A FINITE-ELEMENT METHOD GROUNDWATER FLOW MODEL FOR THE EDWARDS AQUIFER Final Report Prepared for Edwards Aquifer Authority Prepared by S. Beth Fratesi, Ronald T. Green, F. Paul Bertetti, Ronald N. McGinnis, Nathaniel Toll, Hakan Başağaoğlu, and Leslie Gergen Geosciences and Engineering Division Southwest Research Institute® James R. Winterle Edwards Aquifer Authority Yoar Cabeza and Jesus Carrera Institute of Environmental Assessment and Water Research Spanish Council of Scientific Research in Barcelona, Spain February 2015 Executive Summary The Edwards Aquifer Authority relies heavily on groundwater flow models to characterize groundwater flow conditions in the San Antonio segment of the Edwards Aquifer and to serve as the basis for predicting impacts of water-resource management scenarios. Currently, the Edwards Aquifer Authority uses a MODFLOW finite-difference model developed in 2004 by the U.S. Geological Survey to perform these water-resource management analyses. There are recognized limitations and shortcomings in the 2004 MODFLOW model, including questions about the conceptual model on which the numerical model is based. The Edwards Aquifer Authority undertook two initiatives to reduce uncertainty in models used to perform water-resource management analyses. One initiative was undertaken to advance the 2004 MODFLOW model through a series of recalibration exercises. The second initiative was to develop a second groundwater-flow model that is conceptually independent of the 2004 MODFLOW model. The objective of a second model is not to replace the 2004 MODFLOW model; but to provide an independent numerical tool against which to compare the 2004 MODFLOW model predictions. This report documents the development of the alternative numerical groundwater model of the Edwards Aquifer for the Edwards Aquifer Authority. Attributes of the alternative model that set it conceptually independent from the 2004 MODFLOW model are: Inclusion of the Contributing Zone Recharge is calculated directly from precipitation and is not prescribed at the upgradient side of the Recharge Zone Western boundary is refined to better define the Kinney Pool The Contributing and Recharge Zones are characterized by three layers Distinct conduits are included in the Confined Zone The alternative model employs a finite-element formulation instead of the finite-difference formulation used in the 2004 MODFLOW model The fundamental difference between the two models is the manner in which recharge is input in the alternative model. By including recharge as a direct calculation from precipitation over both the Contributing and Recharge Zones, the hydraulic lag between the time of precipitation and the time at which a hydraulic signal is transmitted through the aquifer is captured in the alternative model. Conversely, recharge is a calibrated input parameter in the 2004 MODFLOW model. Although other factors set the alternative model apart from the 2004 MODFLOW model, it is the manner in which recharge is incorporated that establishes the alternative model as conceptually independent from the 2004 MODFLOW model. Development of the alternative model was considered complete when: (i) the model was tested against the conceptual model (i.e., model cross-checked for internal consistency and consistency with observations); (ii) the conceptual model was determined to be physically reasonable and ii representative of the model domain; and (iii) steady-state and transient model predictions of the calibration targets were within the model goals. The model domain was specified to allow for model boundaries to be no-flow, to the degree possible, with the exception of springs. The model domain is specified to include all groundwater and surface-water basins that contribute water to the San Antonio segment of the Edwards Aquifer. By definition, this domain includes the entire Contributing Zone of the Edwards Aquifer. Designation of no-flow boundary conditions minimizes the uncertainty inherent with determining recharge and discharge associated with specified flux and head dependent boundaries. A hydrostratigraphic model was developed to establish the boundaries, define distribution of layer thicknesses, represent layers that are offset and locally missing due to slip on the most influential faults, and provide a high-resolution, data- and observation-constrained stratigraphic framework to support the alternative model. This hydrostratigraphic framework model refines major areas of uncertainty in the existing groundwater model, such as the Recharge Zone northern boundary condition, the influence of the western portion of the Edwards Aquifer, and the effect of deformation features (faults, fractures, and layer dip) on groundwater flow. The major river basins in the Contributing Zone were characterized as hydraulically independent. By doing this, surface water and groundwater flow from each basin to the adjoining basin was minimized. This characterization honored the conceptual model developed for the Contributing Zone in which surface water and groundwater flow in each basin was mostly restricted to each basin. This conceptualization allowed the precipitation/recharge model to be calibrated for each basin (i.e., the recharge model for each basin had its own coefficients). Comparison of the model against the conceptual model in terms of internal consistency and consistency with observations is discussed in detail in the report. Similarly, the conceptual model developed as part of this numerical model was evaluated to ensure it is physically reasonable and representative of the model domain. Subdomains of the San Antonio segment of the Edwards Aquifer are individually evaluated to ensure the conceptual model is internally consistent, physically reasonable, and representative of the model (sub) domain. Subdomains included Kinney Pool, Uvalde Pool, western San Antonio Pool (Dry Frio, Frio, and Sabinal River watersheds), Medina River watershed, interbasin area between Medina River and Cibolo Creek Basins, Cibolo Creek watershed, Guadalupe River watershed, and Blanco River watershed. To the degree possible, the water budget of each subdomain is explored and evaluated. In those cases where quantitative evaluation is possible, volumetric rates of recharge/discharge of the conceptual model are evaluated and compared with physical assessment and analysis. Elsewhere, the appropriateness of the conceptual model is qualitatively evaluated. Simulated discharge at 9 springs and hydraulic head at 102 monitoring wells were compared with observations and measurements. Index wells J-17 (Bexar County), J-27 (Uvalde County), Comal Springs, and San Marcos Springs were the most prominent data sources to match in reaching the calibration goals. The calibration period was 11 years, from 2001 to 2011. This calibration period was chosen because it includes two very wet years, 2004 and 2007, and four iii very dry years, 2006, 2008, 2009, and 2011. Additional justification for this period for calibration was that pumping data and NEXRAD (Next-Generation Radar data provided by the National Weather Service) precipitation data are available, compared with the drought of the 1950s during which pumping and precipitation data would need to be estimated. In addition, water-budget assessments derived from simulations performed with the alternative model were compared with similar assessments based on the conceptual model, U.S. Geological Survey calculations, and model results using Hydrologic Simulation Program FORTRAN. The alternative model successfully replicated the general response of the Edwards Aquifer. Seven of 14 of the target goals were met by the alternative model. In comparison, the 2004 MODFLOW model met 3 of the 14 target goals. The inability of the alternative model to match high discharge at Comal and San Marcos Springs led to the greatest discrepancy between simulation results and the target goals. The alternative model was more successful in matching low discharge at the two springs. Matching low discharge is recognized to be more important than matching high discharge in model performance. Aside from this discrepancy, simulated heads and spring discharge are in general agreement with observations, as attested by model performance statistics. Agreement between simulations and observations is encouraging when compared with existing models, given that the alternative model has the additional constraint imposed by the recharge model and a decreased degree of freedom due to the fact that recharge is calculated solely on precipitation and is not a specified, calibrated input variable. This feature in the alternative model clearly sets it apart from the 2004 MODFLOW model and substantiates its status as a conceptually independent model. These attributes of the alternative model qualify it for future use to provide the Edwards Aquifer Authority with an independent numerical tool to evaluate aquifer responses to different spatial and temporal patterns of precipitation, recharge, and pumping. iv Table of Contents 1. Introduction ….……….………………………………………………………………………...1 1.1 Scope of Work....…..........….…………………………………....................................2 1.2 Conceptual Independence of the Model…....................................................................3 1.3 Previous Investigations and Research...........................................................................3 1.4 Previous Models............................................................................................................5