MID releases stormwater to recharge groundwater sapped amid drought

January’s storms have filled Lake McClure past its historical average, prompting water releases to help replenish groundwater, the Merced Irrigation District reported.

Lake McClure was about 73 percent of capacity Monday. That contrasts with the reservoir’s record low water level of 6 percent just two years ago.

MID is releasing water per federal requirements to ensure space is available in the reservoir to hold floodwater. MID is releasing about 2,000 cubic feet per second while about 7,500 cfs flows into the reservoir. The last time MID released water for flood control was in 2011, before the drought.

“The drought of the past five years had severely strained groundwater in eastern Merced County,” said John Sweigard, MID’s general manager. “The groundwater recharge is a benefit to both MID growers and our community as a whole.”

MID has two groundwater recharge basins in which stormwater is being captured – one in Winton-Cressey and one in El Nido.

In most years, MID replenishes up to 140,000 acre-feet of groundwater through deliveries of Lake McClure surface water to growers in eastern Merced County. That adds up to about 14 percent of Lake McClure’s capacity, or the capacity of Lake Yosemite. The water percolates back into the groundwater as it flows through 700 miles of MID’s waterways.

About 25 acre-feet of water is replenished in the Winton-Cressey basin per day, and the El Nido basin helps replenish about 18 acre-feet of groundwater per day with storm runoff from Mariposa Creek.

“Obviously, all of this water flowing through local streams and waterways is benefiting our community’s groundwater,” Sweigard said. “In instances where we can divert some of this stormwater into our extremely permeable groundwater recharge basins, we want to ensure we are doing everything possible to benefit the groundwater.”

Groundwater in eastern Merced County is used by cities such as Merced, by rural residents with wells and for agricultural purposes.

During the drought years, MID provided either no irrigation water or very small allocations from Lake McClure.

MID officials say it’s too early to predict how much water it will deliver to farmers during the irrigation season. The district will be able to store more water in Lake McClure as spring approaches and flood space requirements ease, but district officials are not counting on the wet weather continuing, said Mike Jensen, a spokesman with MID.

“Throughout the drought, we did, in fact, see more than one ‘false start’ where it appeared the weather outlook was improving but the storms ceased,” he said.

MID’s staff will continue to analyze reservoir conditions as spring nears, but ultimately the board of directors has the final say of how much water will be diverted for irrigation.

“However, at this time, we anticipate having ample supply to meet all of our growers’ needs this year,” Jensen said.

This story was originally published January 23, 2017 at 4:50 PM with the headline "MID releases stormwater to recharge groundwater sapped amid drought."