Some of our reservoirs are spilling—is the drought over?
March rainstorms brought much-needed relief to our county’s reservoirs; so much that four of our 10 local reservoirs reached their capacity. After four years of historic drought conditions, this is an encouraging sign.
The Santa Clara Valley Water District manages 10 reservoirs built in the 1930s and 1950s to catch storm runoff for water supply that would otherwise flow out to sea. Reservoirs are one important and visible indicator of how much water we have available. They also provide recreational opportunities and some flood protection benefits.
Reservoirs also store water imported from the Delta through the State Water Project and federal Central Valley Project. Imported water from these sources makes up 40 percent of our local supply. The amount of water we receive from these two sources depends on the amount of precipitation along the Sierra Nevada, along with managed releases from state and federal reservoirs in Northern California.
We use water collected and stored in our reservoirs to replenish groundwater through creeks and percolation ponds, and to supply our drinking water treatment plants. During normal and wet winters, some of the smaller reservoirs rapidly fill and even spill, as seen recently in the Almaden, Stevens Creek, Uvas and Vasona reservoirs.
Based on agreements with state and federal agencies, the water district may also release water directly to creeks to keep our watersheds and ecosystems healthy.
Our reservoirs and dams must meet state regulations for public safety. Half of our reservoirs have storage restrictions, placed by the Division of Safety of Dams (DSOD), until seismic improvements can be made. When reservoirs exceed these approved maximum storage levels, we must make releases. This was the case recently when we released water from Coyote Reservoir into Anderson Reservoir.
In late March, our total combined reservoir storage was about 68 percent full, slightly above normal compared to the 20-year average. While these numbers are encouraging, there is another fundamental source of water that is often overlooked: groundwater.
While filled reservoirs are a sight for sore eyes, our groundwater levels have taken a hit during the drought. Water that we’ve stored underground provides almost half of the water used in our county each year. In times of drought, when local and state reservoirs are low, we rely more heavily on groundwater.
Due to limited surface water supplies in the last four years, we have had just a fraction of the amount of water that we normally use to replenish groundwater supplies. That’s why most of our groundwater percolations ponds have been dry since 2014.
This year, we will be able to use runoff captured in reservoirs and the increased availability of imported water to fill those ponds and creeks so the groundwater basins can begin to rebound.
Even with four spilling reservoirs, it could take years to bring our groundwater levels back to normal. The Sierra snowpack is still below normal at 86 percent, and our allocations from state and federal projects are expected to be less than normal. We need to keep our conservation efforts going strong. No one knows whether next year or the year after that will be dry.
That’s why it is important to make conservation a way of life. To learn more about how you can maximize water use in your home, schedule a free water wise house call. Schedule an appointment by calling (800) 848-1882.
You can keep up to date on reservoir and groundwater levels by visiting http://www.valleywater.org/WaterTracker.aspx.
John Varela is the District 1 (which includes South Valley) representative and a board member of the Santa Clara County Water District. Contact him at [email protected]