A recent report from Arizona’s water agency shows a steep decline in groundwater levels in every large basin tested, mainly due to agriculture.

A state law passed in 2022 requires the Department of Water resources to assess Arizona’s groundwater basin supply and demand each year at the beginning of December. According to the department’s first annual report, all but the smallest basin are seeing rapid water decline. 

Sarah Porter, director of ASU’s Kyl Center for Water Policy, says the report’s findings are unsurprising. 

“The groundwater in aquifers in Arizona is mostly what's called fossil groundwater. It took thousands of years for the water to get there and humans have figured out ways of withdrawing the water a lot faster than it replenishes,” Porter said.

In three of the seven basins studied by ADWR, agriculture accounts for at least 98% of the basin’s water usage.

For example, groundwater levels in the Willcox basin in southeastern Arizona have rapidly declined since 2005 — from more than 1,639,100 acre feet of water to negative 624,300 acre feet in 2022.  

The negative amount means the water table is drawn to below the depth of the average well. So water still exists, but it's deeper in the ground than most wells can access.

All the “stored water” — water accessible by most wells — was withdrawn from the Willcox basin as of 2018.

“This is something that’s pretty significant,” said Natalie Mast, ADWR’s director of active management areas. In past reports, water was only measured as low as 1,000 feet underground — Mast said the department used a customized analysis for each basin. 

“In many cases they go much deeper, what we’re really looking at is how challenging it’s going to be to get to that water,” Mast said.

According to the report, “despite significant volumes of mountain-front recharge replenishing groundwater supplies, groundwater in storage is decreasing yearly in the Willcox basin. Incidental recharge from agriculture contributes to replenishing groundwater supplies, but this is far from enough to balance demand.”

Before now, data on groundwater levels was pretty limited to Active Management Areas and Irrigation Non-Expansion Areas where groundwater is monitored. This report includes data from both basins in AMAs and INAs and outside of them. 

EDITOR’S NOTE: This story has been modified to clarify the depth of the new measurements.