A REVIEW OF REGIONAL GROUNDWATER FLOW MODELING IN HEAVILY IRRIGATED BASINS OF THE WESTERN UNITED STATES

Water resources in agriculturally dominated basins of the western US are stressed due to long-term impacts from irrigation and aridity. We reviewed 83 major regional groundwater flow models in the contiguous western US, having active modeled areas greater than 1,000 km². Our review provides modelers, water managers, and decision makers with insights about the current status and trends in modeling studies to aid future model development.

Models were selected from seven of the top irrigated western states (AZ, CA, CO, ID, KS, NE and TX) based on three criteria of intensity of irrigation and groundwater use. Reviewed groundwater models were classified into three types: (1) Resource Evaluation Models (33, 40%), used to quantify water budget components, typically considered as preliminary models intended to be updated later, or used to re-calibrate models; (2) Management/Planning Models (44, 53%), used to explore and identify management plans based on the simulated response of the groundwater system to various future water development or climate scenarios, sometimes with water use constraints; and (3) Water Rights Models (6, 7%), used by regulatory organizations to make water administration decisions based on model output quantifying water shortages incurred by water users or climate variability.

We observed an increasing trend of ~0.5 models/yr along with the more recent use of integrated groundwater-surface water modeling codes; state-level water management organizations fund the largest number of models (45, 40%), followed by federal agencies (34, 30%) and then district/subbasin-level organizations (21, 19%), and university, private companies, and municipalities (13, 12%); the majority of models were created and used to assess groundwater-surface water interactions, namely streamflow depletions/accretions caused by pumping/artificial recharge; and 25 (30%) of the reviewed models provide results of a recovery period with predictive simulations of either constant, decreased, or no pumping for extended periods of time. Sustainable water use and system resilience have become major concerns for water managers and decision makers who use models to scrutinize and choose best practices and policies while weighing several physical, economic, and environmental factors.