After more than 25 years of mostly below-normal runoff winters, the Salt River Project water system’s resiliency continues to be tested.

But, as it has for 115 years, Salt River Project’s system is doing exactly what it was designed to do: capture runoff from snowmelt during wet years such as 2017 and store it for dry periods such as this year.

The reality is that central Arizona is in the midst of one of the driest runoff seasons on record. The good news, however, is that SRP’s reservoirs are still a combined 59 percent full. Theodore Roosevelt Lake, which holds about two-thirds of the combined water SRP stores, is currently at 56 percent full after maxing out last April at 76 percent. The real-time status of the Salt and Verde reservoir system is available on the SRP home page at www.srpnet.com and at the new SRP Watershed Connection website at watershedconnection.com.

“One year after a very productive 2017 runoff season, one of our most beneficial winters in the last 20 years when nearly 1 million acre-feet of rain and snowmelt made their way into our water-storage system, the Salt and Verde system has carry-over storage that enables us to meet our water demand,” said Charlie Ester, SRP’s manager of Surface Water Resources.

“Looking back to 1950, our current storage is greater than roughly two-thirds of the years. It is only natural to connect the very dry fall we had to a potential issue with current water supplies, but we are not there yet. In fact, we are already making decisions for next winter just in case we have another dry runoff season.”

When necessary, SRP supplements its surface water from the Salt and Verde reservoirs with groundwater pumped from 280 wells in the Valley. Combined, these wells can provide just under 50 percent of the water needed in the Valley during a year.

This year, because of the severely dry conditions, SRP has increased pumping from 150,000 acre-feet to 200,000 acre-feet. That decision was made not because of the need for the additional water this year, said Bruce Hallin, SRP’s director of Water Supply, but as insurance should next winter remain in severe drought conditions.

Hallin said the decision to increase pumping will not affect SRP water resource management practices, as SRP is maintaining a normal water allocation of 3.3 acre-feet/acre to its water customers. In fact, SRP has only reduced its water allocation four times since 1950 – the last two years being in the depths of the current drought in the winters of 2003 and 2004 when total reservoir storage fell to 25 percent and a reduction in the allocation to 2.0 acre-feet/acre was enacted.

SRP has cooperatively worked over the years with the Arizona Water Banking Authority, Gila River Indian Community and Valley cities to proactively store underground more than 2 million acre-feet of water for use during drought emergencies, Hallin said.

“While SRP is not directly able to recover and use this water for our own demand, we are working together for the good of the state and our customers to maintain water-supply resilience and reliability,” he said. “This significant supply of banked water will prove essential to maintaining Arizona’s vibrant economy during times of shortage.”

Prior to each runoff season, SRP hydrologists prepare models that assume that the Salt and Verde watersheds are about to start the next 11-year drought – and plan accordingly for lower-quartile runoff when developing reservoir operating plans. This is a statistical value of runoff that one would expect to exceed 75 percent of the time, Ester said. The 30-year median runoff is about 534,336 acre-feet.

Even two years ago, prior to the much-advertised “Godzilla El Niño” winter event, SRP planned for lower-quartile runoff and was proved correct over the conventional wisdom that expected drought-ending rains. And, as a result of that calculated modeling, SRP reservoirs were exactly on plan at the end of that poor 2016 El Niño runoff season.

Ester also said the drier-than-usual winter of 2018 was not unexpected, tipped off by weak to moderate La Niña conditions in the tropical eastern Pacific Ocean this past fall that are highly correlated to dry fall and winter weather.

“The severity of the dry weather is what is unusual,” said Ester. “As a result, our runoff forecast calls for 105,000 acre-feet this winter, which is less than half the average of the six winters prior to the productive 2017 runoff season. Without additional late spring precipitation, this year’s runoff season total will be one of the driest on record.”

The dry winter the state experienced is not unprecedented, Ester said. The October-December 2017 period was the third-driest on the Salt and Verde watershed in the 115 years of record keeping by SRP. Extreme fall dryness has occurred on occasion throughout SRP’s history.

Ester said 20 and 30-year drought cycles are normal for this arid region. The current drought cycle started in the mid-1990s and should play itself out over the next few years. After that, if history repeats itself, he said Arizona may enter the next wetter cycle – when there are more above-median runoff years than below-median years.

“From tree ring studies that look back nearly 1,000 years, we know that these extended drought periods also include ‘spike years’ such as 2017, when our total system capacity climbed from 44 percent to 76 percent in one winter,” he said.

“SRP has been proactive in managing our surface water and groundwater supplies conjunctively, which has allowed us to ensure there is a reliable supply of water for the Valley through many dry years. We plan for an extended dry period every year with the goal of achieving water supply certainty and avoiding surprises.”

While a shortage of water on the Colorado River may occur at some point within the next few years, Ester said Arizona water managers have been planning wisely and have taken proactive steps to ensure that the state can weather the ongoing dry period thanks to decades of implementing effective water supply and demand management programs.

SRP is the largest raw water supplier in the greater Phoenix metropolitan area, delivering about 800,000 acre-feet annually to agricultural, urban and municipal water users.