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“If we have the choice to pick a location where recharge
could happen, choose those upstream from
these communities,” Dahlke said. “Recharge will
create a groundwater mound which is like a bubble
of water floating in the subsurface. It takes time to
reach the groundwater table. That bubble floating
higher above the groundwater table might just be
enough to provide for a community’s water needs.”
Filling Reservoirs Under the Ground
Many climate models for California suggest
long-term precipitation amounts will not change;
however, the winter rainy season will be shorter
and more intense.
“That puts us in a difficult spot,” Dahlke said. “Our
reservoirs are built to buffer some rain storms, but
are mainly built to store the slowly melting snowpack in the
spring. In the coming years, all the water will come down
earlier, snowmelt likely in March and April and more water
in winter from rainfall events.”
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Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been
evaluating scenarios for flooding agricultural land when excess water is available
during the winter in order to recharge groundwater (photo by Joe Proudman, UC
Davis.)
She is working with water districts and farmers to consider a
change in managing water in reservoirs.
“We want to think about drawing reservoirs empty and
putting the water underground during
the fall and early winter. Then you have
a lot of room to handle the enormous
amounts of runoff we expect when we
have a warm atmospheric river rain
event on snow in the spring,” she said.
“However, farmers are hesitant. They like
to see water behind the dams.”
Interest in groundwater banking has
been lifted with the implementation
of the 2014 Sustainable Groundwater
Management Act (SGMA). The law
requires governments and water agencies
to stop overdraft and bring groundwater
basins into balanced levels of pumping
and recharge by 2040. Before SGMA,
there were no statewide laws governing
groundwater pumping, and groundwater
was used widely to irrigate farms when
surface supplies were cut due to drought.
“For some of the drought years, overdraft
was estimated to be as high as nine million
acre-feet a year,” Dahlke said.
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Dahlke believes wintertime flooding for
groundwater recharge can help water
districts meet SGMA rules. “We have to
do anything we can to store any surplus
water that becomes available to save it for
drier times, and our aquifers provide a
32 Progressive Crop Consultant July / August 2021