New MODFLOW’s One-Water Hydrologic Flow Model and Application to Conjunctive Use of the Rio Grande River and Transboundary Aquifers
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The Palomas, Mesilla, and Conejos-Medanos Basins in New Mexico, Texas, and Mexico comprise a geologically and hydrologically complex region. The conjunctive use of surface-water and groundwater takes place under a myriad of legal and operational constraints, including the Rio Grande Compact, an international treaty, and the Bureau of Reclamation’s Rio Grande Project. New demands are being placed on the interconnected water system, even as the region is experiencing an extended drought. To better understand the complex hydrogeologic flow system and support ongoing resource management decisions, the U.S. Geological Survey in cooperation with the Bureau of Reclamation is developing the Rio Grande Transboundary Integrated Hydrologic Model (RGTIHM). This model uses MODFLOW’s One-Water Hydrologic Flow Model (MF-OWHM) to build on previous hydrologic modeling efforts. The RGTIHM model is being developed concurrently with the new release of MF-OWHM to take advantage of new features unique to it.The One-Water Hydrologic Flow Model is a MODFLOW-based integrated hydrologic flow model that is the most complete version, to date, of the MODFLOW family of hydrologic simulators needed for the analysis of a broad range of conjunctive-use issues. MF-OWHM fully links the movement and use of groundwater, surface water, and imported water for consumption by agriculture and natural vegetation on the landscape, and for potable and other uses within a supply-and-demand framework. MF-OWHM is based on the Farm Process for MODFLOW-2005 combined with Local Grid Refinement, Streamflow Routing, Surface-water Routing Process, Seawater Intrusion, Riparian Evapotranspiration, and the Newton-Raphson solver. MF-OWHM also includes linkages for deformation-, flow-, and head-dependent flows; additional observation and parameter options for higher-order calibrations; and redesigned code for facilitation of self-updating models and faster simulation run times. The next version of MF-OWHM, currently under development, will include a new surface-water operations module that simulates dynamic reservoir operations, the conduit flow process for karst aquifers and leaky pipe networks, a new subsidence and aquifer compaction package, and additional features and enhancements to enable more integration and cross communication between traditional MODFLOW packages. By retaining and tracking the water within the hydrosphere, MF-OWHM accounts for “all of the water everywhere and all of the time.” This philosophy provides more confidence in the water accounting by the scientific community and provides the public a foundation needed to address wider classes of problems such as evaluation of conjunctive-use alternatives and sustainability analysis, including potential adaptation and mitigation strategies, and best management practices. Thus this provides a more complete understanding of the conjunctive use of surface-water and groundwater along the transboundary aquifer of the Lower Rio Grande.