GrokSurf's San Diego

Many large reservoirs in California need to store and release water in a way that balances flood control needs against water supply needs (San Diego’s reservoirs have limited flood control capability but were mainly designed for storage). Those state reservoir levels need to be lowered in late summer and fall in order to have enough capacity to capture and hold floodwaters that will come during the wet season, but they also need to retain enough to supply needed water in the dry months.

Shasta Dam and reservoir. Photo: UC Davis.

“Reservoir rule curves” help manage when water should be released or held back.

Mountain snow is a factor in the timing and quantity of reservoir releases; the frozen snowpack serves as water storage that gradually melts and provides a predictable flow of water into the reservoirs.

Over recent years we have seen a gradual trend where more precipitation falls as rainfall instead of snow, and the snow that does fall tends to melt earlier in the season. This means that larger water flows into the reservoirs in a short period may threaten to exceed storage capacity, which in turn means more water needs to be released than would be desirable. The unwanted outcome can be that later, during the dry season, with insufficient snowmelt to help restore water levels, reservoirs could be unable to meet demand. The report Climate Change and Reservoir Rule Curves discusses that business in some detail.

In considering the collective needs of environmental, agricultural, and urban/rural interests over the long term (say 50 years), one viewpoint is that we need to increase storage capacity throughout the state in order to have more flexibility in managing the alternating flow requirements. In other words, build more dams or raise the height of existing dams.

San Vicente Reservoir August 2010. Water level is currently drawn down to allow dam raise construction. Most water in the reservoir is piped in through the San Diego Aqueduct, although it does receive some local runoff. Photo taken from a trail on the Oak Oasis Open Space Preserve.

In San Diego, we’re only indirectly affected by changing precipitation patterns elsewhere but we’re still raising the San Vicente Dam in order to more than double the capacity of its reservoir. That’s being done because of San Diego’s meager local water resources and very limited emergency reserves. In this we’re fortunate to have a reservoir that was suitable for enlargement. Keeping it filled will partly depend on an increasingly variable amount of imported water. Eventually a small amount of IPR water may also be a source for the reservoir.

For the state as a whole, in the face of changing precipitation patterns and growing demand, should we or could we increase reservoir storage on a large scale? Or can we adequately meet our needs by further refining reservoir rule curves and trying to make better use of existing resources? Funding for additional infrastructure proposed in the California Water Bond suggests increased storage is a priority although it’s looking like the bond may not make it to the ballot this year. One outcome seems easy to predict: whatever we end up doing, there are going to be a lot of unhappy people.

[April 22, 2011: The Sacramento Bee raises this issue [should we increase storage capacity] now that California had a wet winter filling up its reservoirs.]

More background: Managing an uncertain future: climate change adaptation strategies for California’s water / California Department of Water Resources.