Environmental Flows in a Human-Dominated System: Integrated Water Management Strategies for the Rio Grande/Bravo Basin

The transboundary Rio Grande/Bravo (RGB) Basin, shared by the United States and Mexico, has been heavily regulated to provide for human water supply and flood control, and water management is complicated by extreme hydrologic variability, over-allocation of water rights, and international treaty obligations. Dam-induced hydrogeomorphic alteration has degraded the bi-nationally protected Chihuahuan desert riverine ecosystem along the Big Bend Reach of the RGB. This thesis addresses the need for integrated water resources management in the Big Bend by exploring the performance of alternative water management policies and developing an operational reservoir rule curve to improve human and environmental water management trade-offs. A reach-scale water planning model was used to represent current water allocation and reservoir operations, operating on a monthly time-step under repetition of the historical hydrology (1955-2009). Key water management objectives (agricultural and municipal water demands, flood control, and international treaty obligations) were quantified, and a water allocation algorithm was developed to represent transboundary water management and regulations in the basin. The environment was considered by developing (1) spatially-distributed average monthly environmental flow recommendations and (2) an alternative reservoir rule curve to release water for both environmental flows and human objectives based on hydrologic conditions. The model was used to simulate business-as-usual water management (baseline) in the Big Bend and compare water system performance under baseline and environmental flow policies. An iterative simulation and evaluation process was used to adjust monthly reservoir storage zone thresholds and evaluate policy performance with respect to each water management objective based on a suite of water system performance criteria. Finally, a single reservoir operation policy was identified capable of minimizing alterations from spatially-distributed environmental flows while maintaining human water management objectives. Results from the proposed policy show that, by changing the timing but not the average annual volume of releases, re-operating Luis L. Leon reservoir has the potential to sustain key ecological and geomorphic functions in the Big Bend without significantly impacting current water management objectives. The policy proposed here increased water supply reliability and resilience from baseline water management while reducing system vulnerability in both countries. It also reduced average annual flood risk from the historic 18.2% to 14.5% and maintained the historic average annual outflow distribution from the Rio Conchos to meet Mexico's treaty obligations to the U.S. On a larger scale, this study introduces a novel, interdisciplinary methodology for integrating environmental flows into human-dominated water systems.