Groundwater Aquifers Can Expect A Boost From March Rains

By Bret Jaspers
Published: Monday, March 23, 2020 - 4:41pm
Updated: Monday, March 23, 2020 - 4:44pm

March rain has left Salt River Project reservoirs as full as they’ve been in a decade. The company is discharging water to make room for the runoff, providing a boost to the underlying aquifers.

The utility says the Salt and Verde river systems are at a combined 94% of capacity, almost 20 points higher than last year. Theodore Roosevelt Lake holds about two-thirds of SRP’s stored water and is over 90 percent full. 

The utility is sending discharge from the reservoirs, called “spillage,” down the Salt River. 

“When these rivers flow, they basically have a direct link to the regional aquifer,” said SRP’s Charlie Ester. “And a flowing river in the desert southwest is the number one way to get water into the regional aquifer.”

A higher aquifer helps just about any user who taps into groundwater. 

“We’re helping out the aquifer, but no one in particular can lay claim to it,” Ester said.

As of March 22, Phoenix has seen 1.94 inches of rain in March, according to preliminary data from the National Weather Service. The normal accumulation for that month is less than half that. 

While it is possible the rain could reduce the strain on the Central Arizona Project, which supplies Colorado River water to users in Maricopa, Pinal, and Pima Counties, the system does not expect a change in orders this year.

“We did observe a reduction in customer orders in March, as compared to the original Annual Operating Plan schedule,” said Central Arizona Project spokesperson DeEtte Person. “We also adjusted our operations at Waddell Pump Generating Plant to accommodate additional runoff into Lake Pleasant from the Agua Fria River watershed. However, as temperatures increase, we anticipate that demand will pick up, resulting in customers likely still taking delivery of their full water order by the end of the year.”

Any period of wet weather does not mean the area is out of a long-term drought. 

Ester said that droughts have wet years and wet periods have dry years, although he is wondering if things are starting to change.

“Three of the last four years now have been wet,” he said. “Maybe the [drought] that we’ve been in since 1996 is beginning to end, and we might be in the process of transitioning to the next wet cycle.”

Whatever goes on in the Central Arizona watershed, however, does not allow conclusions to be drawn about the entire Colorado River basin. That’s partly because the Upper Colorado River basin is much more susceptible to the impacts of climate change.

“We get most of our runoff during the winter when the sun angle is very low, so there’s very little transpiration and evaporation,” Ester said. “What water we get is available to runoff. In the Upper Colorado, their runoff season is in the middle part of the summer. Plants are growing. The sun angle is high. They suffer a lot more losses during runoff because of that than we do.”