Simulation of groundwater-level changes from projected groundwater withdrawals in the Truxton basin, northwestern Arizona

A three-dimensional, numerical groundwater flow model of the Hualapai Plateau and Truxton basin was developed to assist water-resource managers in understanding the potential effects of projected groundwater withdrawals on groundwater levels and storage in the basin. The Truxton Basin Hydrologic Model (TBHM) is a transient model that simulates the hydrologic system for the years 1976 through 2139, including hypothetical low-, medium-, and high-groundwater withdrawal scenarios beginning in 2020. The simulated effects of these withdrawal scenarios are presented as groundwater-level changes from the year 2020 to 2070, and from 2020 to 2140. Hydrologic properties in the TBHM are derived from calibration of a steady-state model of the predevelopment (before 1976) groundwater system. The future pumping scenarios are each simulated with three different interpretations of basin depth supported by geophysical data. For each of the resulting nine transient models, a Monte Carlo approach is used to produce a range of possible and probable groundwater-level changes at points throughout the basin given probabilistic ranges of hydrologically reasonable aquifer property values supported by the model calibration results. The ensemble of models that simulate the future pumping scenarios include pumping from the existing well field (three wells) plus additional pumping from a proposed new well. Simulated high future pumping increases progressively to 1,840 acre-feet per year in 2120 and produces a range of drawdowns between 20 and 39 feet (ft) near the pumping center, with a median drawdown of 28 ft. The low future pumping scenario, which increases progressively to 650 acre-ft per year in 2120, produces a range of drawdowns between 5 and 15 ft, with a median drawdown of 10 ft at the same location over the same period of time.