As California experiences more extreme swings between wet and dry periods, new strategies are needed to adapt. One strategy showing great promise is “Forecast Informed Reservoir Operations.”
Rules for operating many of the nation’s dams were created decades before forecasting was good enough to influence how they are managed. However, forecasting for atmospheric rivers has dramatically improved in recent years. Forecast Informed Reservoir Operations (FIRO) builds on this enhanced weather forecasting ability by leveraging forecast improvements to boost water supplies without the need for additional infrastructure.
Why is FIRO needed?
Reservoirs are often operated for multiple purposes, such as water supply, flood control, hydropower, and recreation, which can sometimes be a challenging balancing act between competing needs.
Resources
The Army Corps of Engineers is responsible for managing dams constructed with federal funds that have been authorized for flood control. The Corps develops a Water Control Manual that guides water release decisions for the dam. These manuals are generally created within a year of the completion of the dam. The manuals are meant to be updated if conditions or physical attributes of the project change, but in reality, most manuals have not been meaningfully updated due to a lack of appropriations.
A key element of the manual is the guide curve, which is the maximum allowable reservoir storage to meet flood control objectives. Since the risk of flood events can vary seasonally, many guide curves vary seasonally as well. In the 1960s, when dam construction was at its peak, skill in weather forecasting was limited, so the procedures for developing guide curves depended on observations (water on the ground) and the likelihood of future weather. The manual codifies these procedures, and reservoir operators are compelled to use them.
Weather and streamflow forecasting skills have improved dramatically over the last 50 years through investments in research, modeling, monitoring, and technology. On the West Coast, the Center for Western Weather and Water Extremes (CW3E) is leading research into understanding and forecasting extreme weather events, including atmospheric rivers. Atmospheric rivers are narrow corridors of concentrated moisture in the atmosphere that can bring heavy rainfall and cause significant flooding, especially along the West Coast.
Atmospheric rivers have a dual nature, bringing both benefits and challenges. They range in strength from mild to extreme, with the strongest ARs often responsible for significant winter floods along the West Coast. Conversely, smaller ARs play a vital role in supplying much-needed water.
How it works
The reservoir is divided into “pools” of water: The flood control pool sits on top of the water supply pool; the line between these pools is fixed is a hard boundary set by the water control manual. FIRO doesn’t move the line but rather creates a buffer that allows more water to be stored for water supply. The key is knowing with near certainty that a storm is either on the way or not, which allows operators to retain or release water accordingly.
In practice, it’s not easy as multiple stakeholders have claims to the water supply pool, and every reservoir has a unique set of social, geographic, and hydrologic variables. Thus, FIRO projects are collaborative efforts between public agencies, scientists, and stakeholders to address all opportunities and concerns.
Testing the waters: FIRO Projects
Pilot projects have been underway in California, Washington, and Texas. Here are three of the projects in California.
Project: Russian River/Lake Mendocino
In 2013, the Sonoma region had two large storms a week apart early in the season; using the operating rules in the manual, reservoir operators released water to free up space. No one knew it would become one of the driest years on record, and the reservoir levels were very low at the end of the season. So the question was, could some of the water have been saved if it was known there were no large storms in the forecast?
This led to the first FIRO project at Lake Mendocino. The goal was to update the Water Control Manual by applying forecasting advancements to increase water supply without reducing Lake Mendocino’s existing flood protection capacity or downstream flows for fish habitat.
The Center for Western Weather and Water Extremes (CW3E) and Sonoma Water Agency led the multiple-agency effort that included a Steering Committee of key decision-makers in the watershed.
Analyses of weather forecasts and hydrology determined a five-day forecast would be enough time for reservoir operators to release water before a major storm. In its first two years of using forecasts in 2019 and 2020, Lake Mendocino increased its water storage by 19 percent, enough to supply approximately 30,000 households for an entire year.
Lake Mendocino was the first reservoir in the country authorized to use weather forecasts to decide when to keep and when to release water. The successful project at Lake Mendocino led to projects at other reservoirs in California which will test the use of FIRO at reservoirs more affected by snowmelt or with greater flood risk downstream.
Project: Prado Dam
To increase the efficiency of stormwater capture at Prado Dam, CW3E is collaborating with the Orange County Water District and U.S. Army Corps of Engineers, Los Angeles District to see if the lessons learned from implementing FIRO at Lake Mendocino can be transferred and applied to the Santa Ana River Watershed.
The Prado Dam project explored the FIRO approach within an urban environment with high flood risks. By utilizing atmospheric river forecasting tools, operators could temporarily retain more stormwater behind the dam and release it strategically to facilitate recharge into the Orange County groundwater basins. The project found that FIRO increased groundwater recharge from 4,000 to 6,000 acre-feet; in a wet year, FIRO could provide up to 23,000 acre-feet of additional groundwater recharge.
Project: Yuba Feather Rivers
DWR and Yuba Water Agency are coordinating with the Center for Western Weather and Water Extremes and the U.S. Army Corps of Engineers on assessing the viability of FIRO at Lake Oroville on the Feather River and New Bullards Bar on the Yuba River.
The Yuba-Feather Rivers system has a long history of catastrophic floods, with five significant floods since 1950. Implementing FIRO at these two reservoirs would allow for coordinated, early releases of water in advance of strong atmospheric rivers, creating additional reservoir capacity to manage incoming inflows.
Atmospheric rivers are responsible for most of the flood risk here; snowpack and runoff are also significant variables. Flooding in the watersheds also depends on how soggy the ground is when a storm hits, the elevation where rain turns to snow, and whether rain falls on top of snow. So, the instrumentation for this project includes multiple atmospheric river monitoring stations, soil moisture measurements, and radar that points upward to determine the altitude of where rain turns to snow and ice.
A preliminary viability assessment was completed in December of 2022. The Yuba-Feather FIRO assessment is occurring at the same time as the Corps’ Water Control Manuals updates, which are expected to be completed by the end of 2024.
Partnership is key
The pilot projects have demonstrated significant value for the Army Corps to develop a program to evaluate the use of FIRO in all new water control manual updates, thus making it possible for California to explore implementing FIRO broadly as an adaptation strategy to climate change. The Center for Western Weather and Water Extremes (CW3E) at UC San Diego’s Scripps Institution of Oceanography is an integral partner in the effort by providing state-of-the-art models and data gathered from satellites, radar, and aircraft.
At the heart of the FIRO program is a partnership model that brings together public agencies, scientists, engineers, forecasters, and dam operators to consider all the factors that could affect reservoir operations and build trust in a more flexible approach. This approach nurtures knowledge-sharing and improvements in tools and technologies.
“It’s why we’re succeeding,” said Marty Ralph, director of the Center for Western Weather and Water Extremes, in a PPIC interview. “I think this is a very promising approach for addressing weather extremes in California.”
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Page last updated September 4, 2024.