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Both the seasonal monsoons and winter storms that have been so sparse this year are critical for groundwater recharge in the Tucson basin, but the winter precipitation contributes most to yearly recharge, says Wally Wilson, water resources manager for Metro Water District.

“There is episodic recharge that is measurable along the major rivers when there’s a good (monsoon) storm event, but it dissipates quickly,” Wilson said. Winter rain and snow in the mountains is “much more significant with regard to the water table.”

In comparison to winter storms, monsoon events are more sporadic and dispersed, and Wilson said they typically don’t generate the kind of slow, sustained rainfall that a winter storm front brings in. Saturation of rainfall is crucial to water recharge.

“Snowmelt in the rain in the mountains moves a lot slower and has opportunities to infiltrate more, and that makes it into the groundwater system more efficiently,” Wilson said. “Whereas, a one-square-mile downpour on a part of the city, a lot of that makes it into washes, but it’s not a long-lasting event where that water is in that wash channel for a long period of time and having more opportunity to infiltrate.”

In addition to volume and saturation of water, there are several other factors that prevent monsoon rains from permeating the water table as effectively, said Jennifer McIntosh, associate head of hydrology and atmospheric sciences at the University of Arizona.

“Much of our precipitation goes back up to the atmosphere through transpiration by plants and evaporation at the surface,” she said.

Without as many growing plants, winter rains are more easily absorbed into the soil, and into the water table.

“If it’s been really dry over the winter, then that means the soils, come summertime, are really dry. So it takes several big monsoon storms in order to get recharge,” McIntosh said.

But that’s not to say monsoon rains don’t still play an important role in water recharge.

“A strong monsoon season definitely contributes to natural recharge,” Wilson said. “With a really good rain event where the Rillito was running for several days, you’ll see a bump in the water level that would rise and fall, but it’s a very localized little ribbon of recharge.”

As McIntosh explained, the effects of the annual monsoon rainfall, and even precipitation from winter storms, on natural water recharge are seen most clearly along the city’s washes.

“The only places that we see recharge in the Tucson basin are along our major washes,” she said. “Once you get away from the washes, even just adjacent to them, the ground water is decades to hundreds to thousands of years old, so that water wasn’t recharged anytime recently and it’s not being affected by what’s happening seasonally.”

In short, no monsoon storm or record-breaking winter snowpack is going to fill up Tucson’s aquifer entirely, but specific areas still feel the direct impacts of yearly weather patterns and rainfall. (The city does artificially recharge its Colorado River allotment, brought to Tucson via the Central Arizona Project canal, through basins that let it seep into the ground. It is then pumped to provide drinking water.)

Localized areas along the washes are also where the impacts of Arizona’s ongoing megadrought are most obvious, McIntosh said.

“One example is Sabino and Tanque Verde; they flow into the Rillito. That’s an area of localized recharge where we see that recent water reaching the water table, and it’s in those locations that we see the effects of the megadrought,” McIntosh said. “We’ve seen this decline in water tables in wells in shallow alluvial aquifers in washes. That’s because of the megadrought.”

In 20 years, McIntyre said, the water level in one of the wells monitored by Pima County has dropped some 10 feet.

“And it’s been in a steady decline since the early 2000s,” she said.

According to McIntosh, the only time the water table of the well has risen recently was in 2021 after extreme monsoon storms.

Monsoon rainfall is more important than winter rain from the perspective of the ecosystem, another expert said.

“Because the ecosystem is under so much stress in the summer, the additional rainfall makes an enormous difference,” said Kathy Jacobs, director of the Center for Climate Adaptation Science and Solutions, and professor of environmental science at the University of Arizona. “Many plants and animals are also having a hard time. And in fact, if we lose the riparian areas or the areas along streams and washes, you lose the vast majority of the habitat for birds and fishes and so forth.”

Life is a delicate balance in the desert, and whether it’s winter storms or summer monsoons, every drop of water counts.