Release or store? Agencies manage reservoir flows
By Christine Souza
On Jan. 25, 2016, Lake Oroville on the Feather River stood at 39 percent of capacity, or 60 percent of the historical average, according to the California Department of Water Resources.
Photo/California Department of Water Resources
In late January 2017, Lake Oroville has risen to 81 percent capacity or 124 percent of the historical average, according to DWR. Lake Oroville is a key facility of the State Water Project and its largest reservoir, with a capacity of 3.5 million acre-feet.
Following years of drought and with 2017 shaping up to be what the California Department of Water Resources calls "one of the wettest years ever," some observers have wondered why water agencies are not storing more water in reservoirs for future use.
Because preventing loss of life and reducing property damage from flooding becomes a top priority during strong storms such as those that reached the state in January, agencies say they must release water from reservoirs to make room for anticipated runoff from subsequent storms. At the same time, the agencies say, they work to fill reservoirs, serve municipal and agricultural water customers, create hydroelectric power and help fish.
California Farm Bureau Federation Managing Counsel Chris Scheuring said it is important to ensure all California water users are efficient with this valuable resource, from farmers and urban residents to the water managers and officials who make daily decisions about how water should be captured or diverted.
"It's a difficult tradeoff. California's hydrology is often feast or famine," Scheuring said. "While flood protection is foremost in everybody's mind during a huge storm, it is also critically important for us to capture every drop of the water when it comes—if that can be done without endangering life or property. That's because we know the dry times are coming, and water released is water gone."
How do agencies decide how much water they can store, and how much they must release, during storms?
The U.S. Army Corps of Engineers handles and monitors flood control for about 33 reservoirs in California. It makes decisions about when and how much water is released, based on flood-control manuals written by the Corps at the time each facility was constructed.
Joe Forbis, chief of the Corps Water Management Section, said flood-control manuals are updated periodically.
"Water control manuals have information on why the dam was built, what purpose it has, whether it is just for flood control, or if it is for water supply, or water supply and irrigation, or water conservation," Forbis said. "Each includes the water control plan that describes how to operate the project for flood control, such as how full it should be and, depending on the time of year, how quickly you should release water, what's the most that you should release based on downstream constraints."
The Corps considers annually whether a manual should be updated, Forbis said, but the ability to actually carry out an update depends on funding.
He said the Corps can deviate from the flood manual, such as in a case where districts submit a request to increase the amount of water stored behind a dam.
"Deviations can happen as often as they are requested, but there is an extensive analysis to make sure that if you are reducing flood-control space, that that deviation isn't significantly increasing the risk of flood damages downstream," Forbis said. "If an agency is requesting to store more water year after year and it is approved each year, that is one way to initiate a possible update to the flood manual."
Before the big January storms, California State Climatologist Mike Anderson said, a number of reservoirs had storage levels below what officials call "top of conservation," or the amount of water that can be stored before the Corps becomes involved for flood releases.
"The local flood operator can fill that (top of conservation) space as they wish. Once you hit top of conservation, then you enter what they call a flood pool, or space reserved in the wintertime to do exactly what they did: catch the floodwaters," Anderson said. "Once the threat downstream has passed, the Corps will evacuate that space to get ready for the next storm."
DWR, which manages the State Water Project, relies on water from its key reservoir, Lake Oroville. Anderson pointed out that a big facility such as Oroville has a very large space in its conservation pool—750,000 acre-feet—and "we were able to catch all of that in a week. It is very rare to have that scenario."
That allowed Lake Oroville to mitigate the flood threat at the same time as it recouped storage that dwindled during the drought, he said.
"We saw a lot of the reservoirs make that transition from trying to recover from spent storage, into then operating in the flood pool where the Army Corps of Engineers gets involved and manages that space to protect for floods," Anderson said.
The state's water managers rely on cooperation and communications with other agencies, and on data and historical information, to guide when and how much water should be released downstream.
"It is the same concepts (that are used by each facility), just depending on the size of the facility and the amount of water that can come out of the watershed behind it, will dictate how deep that flood reserve space has to be," Anderson said.
DWR looks to data collected during past storms to determine how aggressively to operate the flood space, he said, and considers information such as satellite data that may not have been available in the past.
"We're trying to be more big picture and getting more eyes and coordination involved, trying to do a better job of managing the system and keeping everybody safe," Anderson said.
The U.S. Bureau of Reclamation, operator of the Central Valley Project, enters each water year with a carryover, or amount of water from the previous year that is stored in its key reservoirs—Shasta, Folsom and Friant.
Louis Moore, a spokesman for the bureau's Mid-Pacific Region, said moving water "is a collaborative decision that requires us to look at what deliveries we need to make and what impacts are occurring in the ocean that could affect our resources in the delta."
"If you don't put enough water in the river system, sometimes the ocean, because of storm surge, will put saltwater upstream, which causes problems and damages farmland," Moore said. "We're also looking at endangered species, delivering water to customers south of the delta and flood effects that we need to manage."
To assure flood protection at this time of year, he said, the bureau must maintain Folsom Lake at about 60 percent of capacity.
"We want to capture as much water as possible, but if a storm is forecast to produce significant runoff, then we have to make sure that space is open again," Moore said. "It's a lot of considerations—federal, state and local laws, biological opinions, water delivery contract agreements—there are numerous folks that discuss how, when and what volumes that should be. It's a team effort that folks try and meet demand while making those releases."
Along with new or expanded facilities, Farm Bureau's Scheuring said, reservoir operating manuals may ultimately need to be revised in a more systematic and integrated way to account for hydrologic change—including the possibility of longer droughts, bigger storms, more rain earlier in the season and less natural recharge.
"Part of balancing flood protection with storage needs is ensuring reservoirs are operated with the best possible flow and forecast data," he said.
(Christine Souza is an assistant editor of Ag Alert. She may be contacted at email@example.com.)
Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.