Sep. 18—Supply and demand models show Mesa has a sufficient water supply to cover significant increases in demand through 2040, but city officials can't guarantee that.
Mesa Water Resources Director Chris Hassert on Sept. 14 cautioned the city council that there's a lot of uncertainty in current projections of Mesa's future water demands.
In a study session on the city's future water supply versus demand, Hassert's presentation showed the city can tap into numerous buckets to meet increases in water consumption.
His presentation showed the city has breathing room to accommodate "moderate growth" in water demand through 2040, when Hassert believes Mesa will hit peak water demand close to build-out.
His department's analysis focused on the eastern and southern water zones of Mesa — regions are supplied with Colorado River water through the Central Arizona Project.
The western "city zone" served by SRP water is "more straightforward," he said.
Mayor John Giles said that seeing the graphs of water supply and demand gave him "comfort."
But he added that the data is also "a cautionary tale ... that there is a compelling argument that we all need to be making for water conservation."
Giles may have been referring to the uncertainty around expected future cuts to Mesa's allocation of Colorado River water or Hassert's warning that the city's future water demands are difficult to predict.
Hassert said Mesa's future needs are somewhat of a mystery because so much of the remaining vacant land in Mesa is slated for non-residential use.
Non-residential users vary more widely in consumption than individual households.
Just 11% of Mesa's water customers are non-residential users, but they account for 53% of the city's total water consumption.
Hassert anticipates the share of non-residential water consumption to grow in the future.
In 2029, Mesa passed an ordinance for managing large water users, defined as 500,000 gallons per day or 550 acre-feet per year, which requires they enter into a Sustainable Service Agreement with the city.
Hassert told council members he plans to bring proposed modifications to the large-user ordinance to council in the next year or two, based on the lessons the department has learned since its passage.
He said one goal of the changes would be to increase certainty around future water demands on undeveloped tracts. This might be done by integrated water use into zoning.
Another "cautionary tale" from Hassert's supply and demand graphs was the challenge posed by deep cuts in Mesa's Colorado River allocation.
Hassert said the department wanted to put the city's water portfolio through a "stress test," so it modeled supply and demand with a 50% reduction in Mesa's allocation of Colorado River water beginning in 2026.
In 2026, the Colorado Basin states will be revisiting the agreement that guides allocations, and water officials expect the total withdrawals from the river will be lowered to better match the river's actual annual flow.
Hassert doesn't consider a 50% cut likely, but said this level of cuts was mentioned last year as a possible emergency measure to keep Lakes Mead and Powell above deadpool. So, city staff wanted to see how Mesa's portfolio would perform in extraordinary conditions.
In the scenario of deep cuts to the Colorado River, Mesa would face a supply shortfall in 2036 if demand growth remained on the "moderate" trajectory.
There would even be a gap between supply and demand in the lower "average" growth projection in 2037.
To fill these shortfalls, the city would need water sources that are still in the conceptual stages today, such as raising Bartlett Dam and purifying wastewater to drinking standards — called direct potable reuse.
Hassert pointed out that the department's projections don't account for future conservation measures or technology changes that reduce water demand.
"If you look at a master plan we did 10 years ago, it showed that same crazy (demand) slope going up — because you have to be conservative," Hassert said, "but look what's happened over 10 and even 20 years."
Hassert was referring to Mesa's relatively flat total water demand since 2003.
Though the department is conservative in its planning, Hassert says he's optimistic that technological innovations and Mesa residents' efforts to conserve water will help to flatten Mesa's demand curve.
Hassert pointed to Google's announcement last week that the $600 million data center it is building in Mesa will be cooled with air instead of water.
"They've changed their technology completely, and they went from very intensive water use to very little water use," he said.