All this rain, why are the ponds dry?

With all the rain we’ve been having, you might have noticed something a little peculiar: the Santa Clara Valley Water District’s percolation ponds throughout the county are mostly dry.

The percolation ponds are a series of constructed ponds that help refill our groundwater basins. These basins hold more water than all 10 of our surface reservoirs combined, and are an important part of our water supply, particularly during times of drought.

So why are the ponds dry now and what does it mean for our groundwater?

Delayed diversions

The ponds are dry or very low for a number of reasons. One of those is because we are delaying diversions of water to the ponds until the storms pass.

We get water into the percolation ponds by diverting it from local sources like streams and reservoirs or imported water brought in from outside of the county. But because of the heavy rains we continue to have this winter, we have removed all of our in-stream diversion structures, such as flashboard dams, to allow streams to handle the flow of water that nature is sending our way. This helps to alleviate flood risk on these streams, although it curtails the amount of water being sent to the ponds.

Despite the diversions being removed, you may still notice water in some of the ponds. This is in part due to lateral flow occurring through the earthen embankments to the ponds. When flow in the creek is higher, the ponds adjacent to the creek naturally pick up some of that flow through seepage.

Too much sediment

The storms have also stirred up the water moving through reservoirs and streams, making it very turbid. If we put turbid water in our percolation ponds, that turbidity — sediment that has been roiled up — eventually settles to the bottom of the pond and can actually clog the percolation ponds and make them less efficient.

During the recent drought, we cleaned and groomed our percolation ponds throughout the county, which has helped them to percolate water rapidly into our aquifers. That’s another reason why the ponds are dry: they’re doing their job very well, and water that finds its way into them is seeping down into the groundwater basins quickly, leaving more room to handle water later on when we can resume filling them.

Maintenance needed

Keeping our infrastructure in good working order means keeping people and our water safe. Some of our ponds are in need of maintenance, such as the Alamitos recharge pond in Almaden Valley. One of the pipes that overflows to the creek is rusted out and needs to be repaired. Until that work is complete, the Los Capitancillos ponds across Almaden Expressway from Alamitos pond will likely be kept lower because they overflow into each other and then into Alamitos pond.

We also are doing maintenance on a pipeline that feeds the McClellan ponds in Cupertino. The water supply to those ponds is also being limited until that work is complete.

Alamitos perc ponds

McClellan perc ponds

Upper Penitencia Creek perc ponds

Upper Penitencia Creek perc ponds

Camden perc ponds

Los Capitancillos perc ponds

Oak perc ponds

McGlincey perc ponds undergoing maintenance

McGlincey perc ponds undergoing maintenance

Imported Water

2017 has certainly been promising so far in terms of rainfall and improved snowpack for the state. However, imported water allocations for the county are still being finalized and availability of supplies are not yet fully known. At this time, we are bringing in limited supplies into the Los Capitancillos facility for groundwater recharge and will resume supply to other facilities as soon as we are able.

The Upper Penitencia Creek ponds are likely to remain dry until maintenance affecting the main imported water raw water delivery pipeline concludes within the upcoming months.

We anticipate that supply to the Los Gatos (Campbell) ponds will resume when there is a sufficient dry spell to reinstall the instream diversion and water quality improves, or possibly even sooner if there are no further maintenance constraints on imported water deliverie

Recharge continues

Although our percolation ponds are not in full use right now, we still are capturing a lot of the rain and runoff in our reservoirs, and much of that water will be sent later to our percolation ponds. In the meantime, we have been able to refill our aquifers a lot through natural and in-stream percolation.

We expect to resume our groundwater recharge program after the rains have abated, probably in spring.

Although to date our groundwater recharge program is a little below average, we expect to have a good 2017, with groundwater storage levels predicted to fall within normal range by the end of the year.

This would be a marked improvement from the past few drought years, where our groundwater levels dipped to critically low levels.

However, because we don’t know when the next dry year will arrive or how the rest of the season will play out, we encourage everyone to continue saving water. County residents have done a commendable job conserving, and if we continue, we will be better suited to handle the next drought.