MEMBER SPOTLIGHT – February 2019

  • by Will Holbert
  • Feb 22, 2019
  • Newsletters

Promise and Progress With Predicting Atmospheric Rivers

This NASA Earth Observatory photo shows an atmospheric river hitting California. Advances in research and weather modeling have the potential to change how reservoirs are currently managed to maximize water storage going into the dry months. Photo courtesy of the Scripps Institution of Oceanography, University of California San Diego.

In California water, 2019 is shaping up to be the year of the atmospheric river, but not just because several have soaked the state and blanketed the Sierras. A transformation now underway combines progress in forecasting ability with a willingness to re-examine how we manage reservoirs.

This transformation is called Forecast Informed Reservoir Operations, or FIRO.  It recently took one of its first steps from working in theory to proving with results. 

At Lake Mendocino, Sonoma Water and the Army Corps of Engineers agreed to deviate from standard practice that required excess water to be released to maintain reservoir space for flood control. When an atmospheric storm rolled over the Russian River watershed in mid-January, this flexibility allowed dam operators to retain an extra 6,000 acre-feet of water. Instead of a set schedule, forecasting that can provide a 15-day weather outlook will decide if that water stays in Lake Mendocino as long as possible to maximize storage capacity for summer.

Five ACWA member agencies envision a day in the near future when the FIRO experiment at Lake Mendocino can be standard practice throughout the state, one that could increase water storage while providing improved flood control and disaster preparedness. 

“The FIRO demonstration project is so exciting because it can increase our water supply at a much lower cost than other alternatives,” said ACWA Vice President Steven LaMar. “Based upon the research being done at Lake Mendocino, the same approaches can be pursued at many other reservoirs throughout California and beyond. The additionally stored water can provide benefits to both water users and environmental needs, including endangered fish migrations.”

The five-member FIRO coalition includes Sonoma Water, Yuba Water Agency, Turlock Irrigation District, Orange County Water District and the San Diego County Water Authority. In addition to the Army Corps, the Scripps Institution of Oceanography’s Center for Western Water and Weather Extremes (CW3E) is working with the water districts as part of its larger and long-term research into atmospheric rivers, which members of the scientific and water community have shortened to “ARs.” The state Department of Water Resources is also playing a key role in supporting research, along with the National Oceanic and Atmospheric Administration and the U.S. Fish and Wildlife Service.

The coalition recently sent a letter to DWR’s Strategic Water Planning Branch, writing in support of a Draft Water Plan Update calling for coordinated climate science and monitoring efforts. The efforts included advancing forecasting models and operations, identifing new ways to observe ARs, and supporting the development of tools to assess opportunities and risks, such as floods and post-wildfire debris flows, but also groundwater recharge following atmospheric river events. 

OCWD wants to utilize the FIRO concept to capture and keep of much as 20,000 additional acre-feet stormwater at its Prado Dam during a wet year, instead of releasing it into the Pacific Ocean. That could save district ratepayers $20 million annually by offsetting the cost of water currently purchased from Northern California and the Colorado River.

Stretching the ability to accurately forecast ARs ever farther into the future has its own name – S2S – for sub-seasonal to seasonal prediction. The big goal is improving forecasting beyond two weeks and possibly into months, said Jeanine Jones, DWR Interstate Resources Manager. And ultimately, it is about more than predicting weather events, but conditions leading to those weather events. That includes atmospheric ridges that can wall off California from ARs and lengthen droughts. Beyond better reservoir management, future success with S2S forecasting can also give water managers a critical edge in ramping up conservation efforts before the hot season, or taking advantage of extra time for flood preparedness.

The challenges are many, but at least one of them boils down to access to existing technology.

“Improving S2S forecasting requires use of high resolution models, which requires more supercomputing access than we have now,” Jones said. 

Lake Mendocino, where the concept of Forecast Informed Reservoir Operations – FIRO – proved itself through a demonstration in January. The demonstration allowed the total storage of more than 68,400 acre-feet for the first time since the reservoir’s creation in 1958. Photo courtesy of Sonoma Water.

Some recent progress includes changing our perspective of ARs, by measuring them and recognizing the critical role they represent in California water. Up to half of California’s total annual precipitation and 90% of its flooding are caused by sporadic, extreme AR rain events. An entire water year, and the risk of flooding and drought, may hinge on a few AR storms. 

At Scripps, CW3E Director Marty Ralph and several other weather experts developed a scale that ranks atmospheric rivers on a range from one (weak) through five (exceptional), similar to how hurricanes are categorized. Announced in early February, the CW3E scale goes beyond the functionality of the hurricane scale system by incorporating duration and indicating the balance between beneficial results and potential for flood hazards. 

“We’re on the verge of a historic transformation in understanding how atmospheric rivers can achieve multiple goals in water system operations, including drought relief, groundwater recharge and fisheries health,” said ACWA Executive Director Dave Eggerton. “This newest tool illustrates how we can better prepare for flood events, and maximize opportunities for extra storage.”

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