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MODFLOW-NWT model used in Simulation of Groundwater Flow, and Analysis of Projected Water Use for the Washita River Alluvial Aquifer, Western Oklahoma

July 16, 2021

In 2020 the U.S. Geological Survey, in cooperation with the Oklahoma Water Resources Board, published a calibrated numerical groundwater-flow model and associated model documentation report that evaluated the effects of potential groundwater withdrawals on groundwater flow and availability in the Washita River alluvial aquifer in western Oklahoma. The results of groundwater-availability scenarios run on the calibrated numerical groundwater-flow model could be used by the Oklahoma Water Resources Board to evaluate the maximum annual yield of groundwater from the Washita River alluvial aquifer in Oklahoma. A conceptual groundwater-flow model is a simplified description of the major inflow and outflow sources (hydrologic boundaries) of a groundwater-flow system as well as an accounting of the estimated mean flows from those sources (water budget) for a specified period of time. The conceptual model was necessary to provide constraints used in the construction and calibration of a scientifically defensible numerical groundwater-flow model that reasonably represents the groundwater-flow system. A finite-difference numerical groundwater-flow model of the Washita River alluvial aquifer was constructed by using MODFLOW-2005 with the Newton formulation solver (MODFLOW-NWT). Data inputs for each package were specified in machine-readable text files. The numerical model of the Washita River alluvial aquifer had 384 rows, 927 columns, about 30,000 active cells of 350 by 350 ft, and 2 convertible layers. The top layer (layer 1) represented the Washita River alluvial aquifer. The second layer (layer 2) represented the undifferentiated Permian-age bedrock units. The numerical model was temporally discretized into 433 monthly transient stress periods representing the period 1980-2015. An initial steady-state stress period, in which the groundwater-flow equation had no storage component, represented mean annual inflows to and outflows from the aquifer and produced a solution that was used as the initial condition for subsequent transient stress periods. The numerical model was constructed in units of meters and days. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report.