Modeling Groundwater Flow to Understand the Water Resources of the Lower Pecos River Watershed Final Report June 30, 2016 Ronald T. Green, Ph.D., P.G., Nathaniel Toll, F. Paul Bertetti, P.G., and Nicola Hill Geosciences and Engineering Division Southwest Research Institute® Table of Contents Introduction ..................................................................................................................................... 6 Background ..................................................................................................................................... 8 Previous Groundwater Flow Models .............................................................................................. 8 River Description ............................................................................................................................ 8 Study Domain ............................................................................................................................... 12 Hydrostratigraphy ......................................................................................................................... 12 Groundwater Elevation Data ..................................................................................................... 15 Streamflow Data ........................................................................................................................ 17 Electrical Resistivity Survey ..................................................................................................... 19 Model Construction ...................................................................................................................... 24 Numerical Model Development ................................................................................................ 29 Recharge .................................................................................................................................... 32 Discharge ................................................................................................................................... 34 Model Calibration ..................................................................................................................... 35 Model Simulation.......................................................................................................................... 35 Groundwater Elevation ............................................................................................................. 36 Model Discharge ....................................................................................................................... 40 Discussion and Conclusions ......................................................................................................... 41 Future Work .................................................................................................................................. 43 Acknowledgements ....................................................................................................................... 43 References ..................................................................................................................................... 43 2 List of Figures Figure 1. Map of the lower Pecos River watershed. ....................................................................... 7 Figure 2. Locations of flow measurements taken on the Pecos River .......................................... 10 Figure 3. Discharge in the Pecos River intermittently measured between 1918 and 1968 ......... 11 Figure 4. Map of the lower Pecos River watershed. Major tributaries are illustrated by color. .. 14 Figure 5. Stratigraphy of the study area ........................................................................................ 15 Figure 6. Location of the lower Pecos River and the selected model domain. ............................. 16 Figure 7. Location of USGS river gauging stations relative to the study domain ........................ 18 Figure 8. Location of the electrical resistivity geophysical survey. ............................................. 20 Figure 9. Location of the electrical resistivity geophysical survey. ............................................. 21 Figure 10. Close in view of the location of the electrical resistivity geophysical survey. ............ 22 Figure 11. Electrical resistivity transect DR-7. ............................................................................. 23 Figure 12. Electrical resistivity transects DR-8 and DR-9. .......................................................... 24 Figure 13. Top slice of the lower Pecos model mesh. .................................................................. 25 Figure 14. Oblique view of the model mesh looking from the south to the north. ....................... 26 Figure 15. Model cross-section along two-dimensional Polyline. ................................................ 27 Figure 16. Model cross-section along two-dimensional Polyline ................................................. 28 Figure 17. Hydraulic Conductivty of Edwards Layer. .................................................................. 30 Figure 18. Hydraulic Conductivty of Trinity Layer...................................................................... 31 Figure 19. Error Plot Steady State Heads ..................................................................................... 32 Figure 20. Steady-state simulation results.. .................................................................................. 36 Figure 21. Groundwater elevation contours at steady state (ft). ................................................... 37 Figure 22. Darcy flux .................................................................................................................... 37 Figure 23. Locations of calibration target wells ........................................................................... 38 Figure 24. Time histories of calculated and observed hydraulic heads at well 4449301 ............. 39 Figure 25. Time histories of calculated and observed hydraulic heads at well 4555702 ............. 39 Figure 26. Time histories of calculated and observed hydraulic heads at well 5445502 ............. 40 Figure 27. Discharge on the Pecos River ...................................................................................... 41 3 List of Tables Table 1. Hydraulic parameters for the lower Pecos River watershed model ................................ 29 Table 2. Average lake evaporation, Ei, by month for Texas Quadrangle 807 .............................. 33 Table 3. Weighting factors, Φi, to account for antecedent moisture ............................................. 34 4 5 Modeling Groundwater Flow to Understand the Water Resources of the Lower Pecos River Watershed Introduction The groundwater resources of many arid and semi-arid regions of the United States and elsewhere are increasingly stressed due to already limited recharge being further reduced by prolonged and more frequent drought cycles as well as increased pumping to supply growing populations. Climate change that results in altered precipitation patterns can exacerbate these stresses, particularly if changes in precipitation result in reduced recharge. While characterization of water resources is desirable wherever water is used, accurate assessment of water availability is especially critical in areas where resources are limited and stressed. In addition, environments where carbonate aquifers are relied on to provide meaningful water supply warrant special considerations when characterizing and managing the water resources. Effective management of the water resources in such environments requires that the groundwater and surface-water regimes be sufficiently characterized and modeled to provide the proper tools to simulate not only groundwater and surface-water flow, but also the hydraulic interactions between the two regimes. A critical factor for effective management is specification of the sustainability of the water resources. Compounding this challenge is determining what constitutes the sustainable or safe yield of groundwater from a specified domain. This concept has been actively discussed and debated for decades (Lee, 1915; Theis, 1940; Bredehoeft et al. 1982; Sophocleous, 1997; Alley and Leake, 2004; Watson et al., 2014). On one end of the debate, development of groundwater is considered safe if the rate of groundwater extraction does not exceed the rate of recharge. The term “safe yield,” when defined as the attainment and maintenance of a long-term balance between the amount of groundwater withdrawn annually and the annual amount of recharge, reflects this perspective, but has been discredited as an indicator of sustainability (Sophocleous, 1997). Conversely, sustainable yield can be defined as a function of increased recharge and decreased discharge induced by pumping. A critical aspect to sustainable yield is the effect of pumping on surface water flow, which highlights the need to effectively characterize both surface water and groundwater in addition to their hydraulic interaction. A general quantitative method to define “sustainability” has been elusive; thus, sustainable yield is typically formulated with respect to specific water-management objectives and the constraints associated with the targeted resources. The Rio Grande is sourced by water from Colorado, New Mexico, Texas, and Mexico. Approximately 1,600,000 acre-ft/yr flows into the Amistad Reservoir. The