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Understanding monsoon and where all the rainwater goes

Monsoon 2021
Posted at 11:35 AM, Aug 23, 2021
and last updated 2021-08-23 19:02:49-04

TUCSON, Ariz. (KGUN) — Some years it's a boom while other years it's a total bust. So far in 2021, monsoon has been one for the record books.

It is estimated that lightning strikes a half-million times during the Arizona monsoon.

Remember -- by definition, monsoon is a season. So you should never say "monsoon season" -- just "monsoon."

So what causes the Arizona Monsoon?

"Monsoon is a seasonal shift in wind that brings in a lot of moisture into the desert region which then results in showers and thunderstorms," according to KGUN 9 Meteorologist Cuyler Diggs. "Those thunderstorms then give us about over half the average annual rainfall that we usually receive in the Southwest."

Cuyler says in Tucson the average rainfall during monsoon is just under six inches.

Monsoon runs from June 15 to Sept. 30. It used to start only after three consecutive days with a dew point of 54 degrees or higher. The weather service changed it to the calendar dates in 2008.

Monsoon storms make for amazing videos and photos.

Monsoon lighting over San Xavier

It also creates flash flooding, something people new to Tucson may not expect in the desert.

"It will immediately go into these washes, streams and creeks," explained Diggs. "I think it does catch a lot of people off guard as to how quickly the water can rise around here even on the streets."

Swiftwater rescues are fairly common, especially for those who ignore warnings to stay out of washes that are flowing.

To better understand monsoon rain I met Hydrologist Thomas Meixner where the Tanque Verde and Pantano Washes converge to form the Rillito River. A professor and head of the University of Arizona's Hydrology and Atmospheric Sciences Department, Meixner explains where all the water goes.

"Quite a bit of the water falls and goes back to the skies," said Meixner. "If you've been around town you've seen the plants really green up. They're drinking that water and sending it back to the atmosphere. So, a good portion goes there."

Meixner says much of the rest of the water from monsoon rains ends up in our washes and rivers like the Rillito.

Pantano Wash and Tanque Verde Wash converge to form the Rillito River

"Because our monsoon has been so long and intense this water is now running out to the Santa Cruz, out of Pima County, through Pinal County, up into Maricopa County and the Gila River," Meixner said.

Meixner points out some of the monsoon rain in the washes and rivers does go back into recharging our groundwater in the aquifers. He says research he did with one of his graduate students revealed how much rain it takes.

"When it rains more than about five inches for the monsoon basin-wide, we do start seeing some recharge."

The U.S. Geological Survey studied the Rillito River and discovered how much water makes it down into the Tucson aquifer.

"What really matters for how much gets down is how long, how long does flow happen," explained Meixner. "Essentially, at that point there's water at the surface, it's sinking into the sands and gravels of the wash and eventually arrives at the water table."

It might surprise you to learn just where the aquifer is under the Tucson metro area.

"The aquifer underneath Tucson is big," said Meixner. "Essentially, once you get maybe a mile or two out from the mountains you have hundreds of feet sands and gravels below your feet."

That sand and gravel is the aquifer, where water is stored.

A Tucson Water map shows the aquifer extends from Avra Valley up and around the Tucson Mountains, and across the entire valley of Tucson.

Tucson Aquifer

"But it's deep," Meixner said. "At the deepest point in the basin it's as much as 5,000 feet until you hit bedrock."

Some of that monsoon rainfall sits right under your feet, to be pumped out later for you to use.

Meixner says monsoon can be important for recharging the aquifer, but it is more important for reducing water usage in Tucson. Monsoon rainfall alleviates the need to water plants and grass.