As a result of continuing observations of groundwater level declines in the Willcox Basin, the Arizona Department of Water Resources initiated development of a numerical groundwater flow model in late 2015.

The Willcox Basin covers an area of approximately 1,911 square miles in southeastern Arizona and is essentially a closed basin.

That modelling has been completed. Major findings of the modelling include:

• The three-year ADWR modelling project finds high rates of groundwater pumping in Willcox Basin altering the groundwater flow system “to a significant extent.”
• Evidence of the amount of groundwater removed from storage between 1940 and 2015 ranged from 4.9 million to 6.2 million acre-feet (an acre-foot equals 325,851 gallons of water, or the amount of water needed to cover an acre of land a foot deep).
• Projecting forward to simulate a period of time between 1940 and 2115, the modelling concluded a net “change in storage” – meaning, in this case, a reduction in storage – ranging from 19.8 million acre-feet to 24 million acre-feet.
• Going forward to 2115, the modelling simulated long-term “draw down” in various portions of the aquifer range from a minimum of 354 feet (in the aquifer system north of the Willcox Playa) to as much as 917 feet (in the aquifer system near Kansas Settlement).
• Both data and modelling indicate that significant declines in regional groundwater levels continue to occur.
• Based on the Willcox Model results, pre-development estimates of groundwater in storage circa 1940 ranged from 80 to 97 million acre-feet. Recent estimates of groundwater in storage (2015) range from 73 to 92 million acre-feet. Projection estimates of remaining groundwater in storage range from 57 to 77 million acre-feet. However it must be noted that a significant portion of the remaining groundwater in storage is found at considerable depth and may not practical to remove.

The Groundwater Flow Model of the Willcox Basin

An Executive Summary of the results of the Flow Model