Formula for winter reservoir management more complicated than it appears

Published Nov. 21, 2016

With the rainy season upon us, the U.S. Army Corps of Engineers Sacramento District is buckling down to ensure the Central Valley’s reservoirs are ready for whatever the season may or may not bring.

While water management is a year-round responsibility, the winter months bring with them especially tough challenges for the Corps in maintaining the delicate balance of flood risk reduction and water supply.

In fact, water management decisions – determining how much water can remain in a reservoir at any given time – are impacted by a number of considerations. Sometimes that may even mean water gets released in the middle of a drought.

“We have to make sure there’s enough space in the reservoirs to accommodate the potential that a storm could happen and that we could safely take in all of its water,” says Joe Forbis, chief of the Sacramento District’s Water Management Section.

Forbis says that the protection of lives and properties downstream is the Corps’ number one priority and that flood control releases are a tool to help maintain the proper amount of empty space, or flood control space, in its reservoirs.

But the decision to release water is never made in haste. There are a handful of key factors – an equation of sorts – that play a role in water management –

  • Seasonal variability – The time of year. Around October, the Corps begins to prepare for the rainy season in California, meaning they begin to increase the amount of flood control space and decrease the amount of water kept in the reservoirs in preparation for the winter runoff.

The rainy period generally runs through about March, when the Corps can again start allowing more water to stay in the reservoir and keep less empty space. The spring and summer months don’t typically bring as much rain as the winter months do.

  • Basin saturation – How wet the ground is. If the basin is dry, the ground can soak up a lot of water and less empty space is needed in the reservoir. But even with a dry basin, an intense rainfall in a short period of time means the ground can’t soak it up fast enough. Likewise, if the basin is saturated, it won’t soak up as much and the water will need to run off into the reservoir, requiring more empty space.
  • Upstream reservoirs – When there are upstream reservoirs, all of the reservoirs in that chain work as a system. If they’re full upstream, whatever rain hits the ground will be passed on to the downstream reservoir, meaning more empty space is needed to capture that runoff, plus any potential releases from the upstream reservoirs. If the upstream reservoirs are empty, they’ll capture the runoff and we may not need as much flood control space in the downstream reservoir.
  • Snowpack and snowmelt – Simply put, the amount of snow in the mountains and how much snowmelt is expected to flow to the downstream reservoir by the end of the season.

Click for larger view of Sac District's California reservoirsForecast-based operations creates an additional, unpredictable factor. “Weather forecasting is still not an exact science,” adds Forbis, “but it has gotten more accurate over the years and is given more weight in today’s calculations to make the best decisions possible.”

It all adds up to the simple version of the water management process.

To complicate matters further, the Corps owns and operates only 17 of the 48 reservoirs located in the Sacramento District boundaries. Yet, the Corps manages and prescribes flood control rules for the remaining sites as well, meaning flood protection is only one piece of the overall water management formula.

“The water system in California is extremely complicated and there is a lot that goes into determining how much water is released from a reservoir,” says Forbis. “Not all releases have to do with flood control. There are other agencies that require water supply for agricultural irrigation, industrial processes, wetlands preservation and other needs, and those need to be respected.”

Taking all of this into account, water must sometimes be released, even during drought conditions. The alternative is that too much water is stored, an unexpected rain event fills the reservoir and spills over the dam, potentially causing a catastrophic flood that impacts people and property downstream.

“It’s a balancing act,” Forbis continues, “If we can provide a benefit to our partners without increasing the flood risk, we want to do that – we recognize that this balancing act is important – but we, and our partners also know that we need to do what we can to keep people safe.”

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