Groundwater model of the Harney Basin, southeastern Oregon

Groundwater development, mainly for large-scale irrigation, has increased substantially in the Harney Basin of southeastern Oregon since 2010. Concurrently, some areas of the basin experienced groundwater-level declines of more than 100 feet, and some shallow wells have gone dry. The Oregon Water Resources Department has limited new groundwater development in the basin until an improved understanding of the groundwater-flow system is available. The groundwater resources report by Gingerich and others (2022, U.S. Geological Survey Scientific Investigations Report 2021–5103, https://doi.org/10.3133/sir20215103) provides that understanding. This report describes the development of a numerical groundwater-flow model that can be used as a tool to help improve that understanding. The Harney Basin Groundwater Model was developed using the finite-difference groundwater-modeling software U.S. Geological Survey modular finite-difference groundwater-flow model (MODFLOW 6) and associated Python pre- and post-processing routines. The groundwater model encompasses the entire 5,240-square-mile Harney Basin and adjacent areas and is calibrated to the hydrologic conditions from 1930 to 2018. The model has a uniform grid consisting of 78,064 nearly square cells, each covering 2,005 by 2,007 feet (about 92 acres) and has 10 layers (780,640 total cells) representing the vertical distribution of hydrogeologic units. The results from the calibrated model simulations indicate that groundwater pumpage exceeded recharge since about the mid-1980s, resulting in an estimated net cumulative depletion of groundwater storage (discharge minus recharge) of about 840,000 acre-feet and also indicated declines in groundwater evapotranspiration and spring and stream discharge. Model simulations show as much as 100 feet of groundwater-level decline in some areas and more than 40 feet of decline in widespread areas in recent decades. Model simulations are consistent with field observations of groundwater levels through time.