In a case of climate whiplash, Tucsonβs water use soared during the record-setting hot and dry periods of 2020 and early 2021, only to plunge in the much wetter second half of last year.
Use by Tucson Waterβs customers in fiscal year 2020-21 β July through June β rose 9% over the previous fiscal year. That came on top of a 2% increase in fiscal 2019-20, which included several hot, dry months in 2020.
The spikes were extremely rare for conservation-minded Tucsonans. Since 1989, total annual water use hadnβt increased β even though Tucson Waterβs service area has added at least 200,000 people.
And then, in the first six months of fiscal 2021-22, water use dropped 12.3% compared to the first half of the previous fiscal year.
This wide swing illustrates how closely water demand in hot, arid Tucson matches the weather.
It also raises questions about whether water demand here and in the Southwest in general will rise more as climate change continues to boost temperatures, putting pressures on the regionβs dwindling water supplies.
Or, will Tucsonans be able to counteract that by continuing to opt for more water-saving landscapes and plumbing fixtures, as theyβve done in recent decades?
Pandemic factor
Calendar year 2020 was Tucsonβs driest and second-hottest year since record-keeping started in 1895. The first half of 2021 continued the dry spell, making the period from January 2020 through June 2021 Tucsonβs driest 18 months on record.
By contrast, 2021 was the 14th wettest on record, says the National Weather Service. Tucson received 11.02 inches more rain last year β 15.19 inches total β than in 2020.
Similar to the swing in water use in Tucson, Metro Water, which serves many suburban customers here, reported a 10.1% increase for fiscal 2020-21 and a 13% decline in the first half of fiscal β21-22.
Phoenixβs total water use jumped 10% in calendar year 2020 and rose another 6% in 2021.
But experts caution that weather may not have been the only factor driving water use. This period also coincided with the pandemic that kept many people at home for work, education and entertainment, in generally greater numbers in 2020 than 2021. That probably led them to use more water at home.
Cases of COVID-19 along with hospitalizations and deaths remained very high at times last year, but as more people were vaccinated, many ventured from their homes more often for work and play.
βYou canβt separate COVID-19 and climate fluctuations. We donβt have a good understanding of what COVID did to commercial and residential demands,β said Gary Woodard, a private water consultant and a retired University of Arizona water researcher.
βSome office buildings shut down and they turned off the thermostats. Some couldnβt do that. So many more people were at home β more toilets were flushed at home. We may never sort it out,β Woodard said.
But Metro Water and Phoenix officials said they believe their recent water use increases were due more to hot and/or dry weather than to COVID.
Environmental warning?
Some environmentalists are concerned the spike in water use is a red flag, offering a warning that as temperatures keep rising, as most climate experts say they will, more water will be used, reversing decades of decline in per-capita use in Tucson.
That view is backed by a 2013 national study from the nonprofit Water Research Foundation. It forecast significant water use increases by 2090 as temperatures rise in three Western cities: Colorado Springs, San Diego and Las Vegas.
βMy instincts are that we would have to up our game, and tighten up what we do. So investments we make in dollars and staff time in conservation programs would have to be increased,β said Val Little, a longtime Tucson water conservation activist and a member of the city of Tucsonβs Citizens Water Advisory Committee.
βAs it is now, weβre sort of holding pretty steady with conservation savings, but weβre being hit by increased heat, increased soil drying, more erratic rainfall, and the Colorado River is getting more precarious,β Little said.
Also, a recent federal study warned that warming weather in the Tucson area could reduce replenishment of the aquifer by rainfall, thus reducing groundwater supplies.
But Woodard said water use increases caused by heat or drought may be offset by continued increases in efficiency in fixtures such as shower heads and washing machines, replacement of swamp coolers with less thirsty refrigerated air conditioners, and voluntary changes to less thirsty landscaping.
βTo just say we assume the world will remain static, and people will keep the technology they have and assume the demand will go up, thatβs just wrong,β said Woodard, who consults for municipal water providers, the Arizona Department of Water Resources and private sector clients.
Just a blip?
Woodard said the past two yearsβ water use flip-flop amounts to a blip, βso anomalous that you canβt say thereβs a trend.β
As many Tucsonans like to boast, Tucson Waterβs customers typically use less or at least no more water in most years than they did 30 or more years ago, when the utilityβs customer base was far smaller than its current 739,000 people.
In calendar year 2020, total water use rose about 6%, to 98,279 acre-feet, from calendar year 2019. That was Tucson Waterβs highest total annual demand since the early 1990s.
Water use per person also rose in 2020, by 6 gallons, to 82 gallons per person, compared to 76 in 2019.
But the 2019 total water use of 91,920 acre-feet was the utilityβs lowest annual water use since the 1980s, said Candice Rupprecht, Tucson Waterβs water conservation programs manager. An acre-foot will cover a football field about 1 foot deep with water.
The utility doesnβt yet have a total water use figure for all 2021 but βwe know it came down from 2020,β Rupprecht said.
Soil moisture
In a hot, dry year, there is less rainfall to nourish plants, requiring more water from garden hoses.
But, less obviously, the plants also need more water because they and the neighboring soil release more moisture into the air through evapotranspiration. It causes soil to evaporate and plants to release water into the atmosphere as vapor via stomata β tiny, pore-like structures on leaf surfaces.
The lack of soil moisture during protracted hot, dry spells also is a big factor driving up water use, Rupprecht said. Drier soils require more saturation before plants can take up water from them, she said.
Water demand for cooling towers used by large structures such as office buildings, high schools and middle schools, hotels and apartment complexes as part of their air conditioning systems also rises in hot and dry weather, Rupprecht said.
Overall, Rupprecht said itβs hard to discern a single pattern driving higher water use in 2020-21 and the lower water use in 2021-22, when COVID and wild weather swings were happening at once.
But Phoenix water officials definitely attributed their spike in water use, particularly in July and August 2020, to the intense heat and drought, said Cynthia Campbell, Phoenixβs water resources management adviser.
She added, however, that at times, city water officials βkind of laughed to ourselves, that it may also be COVID, because if people are sitting at home watching their landscape die, they might turn on their hose sooner.β
More warming is projected
Where officials, environmentalists and outside experts agree is that temperatures in Tucson and across the Southwest will keep rising for the foreseeable future, due to climate change triggered by continued greenhouse gas emissions.
The federal governmentβs most recent National Climate Assessment from 2017 predicts that average high temperatures for all of Pima County will rise to a range of 87 to 88 degrees by the 2050s and to a range of 88 to 92 degrees by the 2090s.
Those projected figures are up to 12% hotter than the average daily maximum temperature for 1961 to 1990 in all of Pima County.
The 2013 Water Research Federation study did not look at Arizona cities, but projected annual water demand could rise in 2090 by up to nearly 45% in Colorado Springs, 23% in San Diego County, and 15% in Las Vegas compared to the period 1971 to 2000, under what forecasters called a hot-dry scenario. The report projected lesser water demand increases by 2055.
That study didnβt take into account possible impacts on water use from more water-efficient technologies, municipal conservation programs or land use pattern changes, in which higher densities would require less landscaping and water use. But the study did say those and other behavioral changes could moderate the influence of hotter and drier weather.
βWould it moderate it significantly? I donβt think I can say. You need to do additional research on that,β said Jack Kiefer, the studyβs lead author and a senior associate for an Illinois engineering firm. βWhat we were doing there was basically saying βthis is what would change if climate changed and nothing else is changingβ.β
In Tucson, researcher and consultant Woodard said one form of climate change β the urban heat island effect that boosts nighttime temperatures β wonβt affect water use for landscaping because plants donβt need much water at night.
But higher nighttime temperatures could increase water use by swimming pools because the warmer weather will increase even nighttime evaporation. Cooling towers could also need more water since air conditioning often runs at night, he said.
Hotter summer days ushered in by climate change will also increase cooling towersβ and swamp coolersβ water use, Woodard said. Itβs likely to trigger a bigger water demand increase in early spring and late fall, he said, because growing and irrigation seasons will be extended.
But he said the warmer weather could also accelerate existing practices by many Tucsonans to change cooling technologies and reducing water-sucking landscaping.
βThe net impact on water demand is uncertain,β he said.
