The Groundwater Flow Model of the Willcox Basin, released July 2018, documents the development of the regional-scale Willcox Basin Model. The model includes the major aquifers in the Willcox Basin and extends into portions of the Aravaipa and Douglas Basins. The model was calibrated using non-linear regression (inverse modeling) techniques from 1940 to 2015, and includes more than 2,500 head targets and four regional-scale groundwater discharge (flow) targets. An important part of the model development process included testing alternative conceptual models (ACMs). Several of the most plausible ACMs are presented in the report. To provide transparency, inverse model statistics are presented in the report; in addition, the Department had the model independently peer-review by SSPA (2016). The model was also used to simulated future groundwater flow conditions from 2016 to 2115, using pumping rates assigned during the last year of the calibration (2015). Based on assumptions described in the report, results of six ACM’s are presented and provide plausible ranges of groundwater flow model solutions.