In the winter of 1997, historic levels of rain caused the Merced River to swell. From November through March, the river carried nearly 1 million acre-feet – more than 300 billion gallons of water. And almost all of this bounty rushed past the valley’s fields and orchards and out to the Pacific Ocean.
Most years, growers could have used that water. Take dry 1993, when flows slowed to a trickle. Such is California’s boom-and-bust cycle, where few years are 'average' and growers mostly contend with too much or too little water.
Groundwater is the savings account growers depend on in dry years when precipitation and surface sources (reservoirs and rivers) aren’t enough. But according to experts appearing at The 2017 Almond Conference, the Central Valley’s groundwater savings account is overdrawn, and more needs to be done to increase deposits in wet years so the water is there for dry ones.
“We want to be more proactive,” said Aaron Fukuda of the Tulare Irrigation District, which has been spearheading efforts to recharge aquifers by helping growers strategically apply excess water not to grow crops, but to replenish aquifers.
Part of a panel entitled “The Science and Practice of Intentional Recharge in Almond Orchards,” Fukuda explained how his district worked with 14 local growers in 2017 to facilitate intentional recharge of 6,800 acre-feet of water (more than 2 billion gallons).
Water was delivered at reduced rates to on-farm recharge ponds, but the effort also included a more innovative practice – applying the water strategically to fallowed fields where information about soil types and geology predicted the water would seep quickly below ground, moving into the aquifer and not allowing the soil to become waterlogged for so long that plants would be harmed.
“You have to be very careful where and how you do it,” said Fukuda. His district is building a “farmer-driven” program to further increase recharge when excess winter flows are available.
Almond Board-funded research featured by the panel suggests an important role for almond growers to play. Dr. Helen Dahlke at University of California, Davis (UC Davis), performed studies on almond orchards to look at risks and rewards of applying extra water during the dormant season.
At test sites in Orland, Delhi and Modesto, UC Davis researchers applied an extra 24 inches of water (in addition to precipitation) during December and January in 2015-16 and 2016-17. Their work found “no obvious warning signs that winter irrigation for groundwater recharge affects trees if applied during the dormant season.”
Still, researchers did find that higher permeability soils - that is sandier soils - are more suitable. The Land IQ almond mapping analysis indicates that nearly 675,000 acres of almond orchards grow on soil that is moderately good or better for groundwater recharge. This includes: 4,119 acres of almonds which are categorized as ‘very good’ in their groundwater recharge potential; 271,509 acres as ‘good’; and 396,790 acres as ‘moderately good’. Visit Almonds.com/Maps to access current mapping data.
The work by Peter Nico from the Lawrence Berkeley Lab shows that with detailed mapping of water penetration into the deeper soil layers, it is likely that specific areas within an orchard or site with rapid deep percolation could be targeted for recharge.
“Winter recharge is not a suitable practice for every grower,” Dahlke advised. “Know your soil.”
Dahlke offered additional advice:
- Growers should keep their flood irrigation systems for recharge, even if they plan to install microsprinkler or drip systems
- Growers should consult their irrigation district about recharge programs
Intentional recharge may help growers navigate California’s Sustainable Groundwater Management Act (SGMA), which regulates the use of groundwater for the first time in California.
An almond orchard where intentional groundwater
recharge occurred as part of an Almond Board-funded
research project with UC Davis.
SGMA was a response to depletion of the state’s groundwater resources, which declined over the past century by more than 160 million acre-feet – enough to fill the state’s largest surface water reservoir more than 35 times. SGMA requires formation of local groundwater sustainability agencies (GSAs) to better manage the resource. In areas of severe overdraft, which includes many locations in the Central Valley, the GSAs will have to have a plan in place starting in 2020 to eventually ensure no more water is extracted than can be replaced in the aquifer.
Local groundwater districts are already developing accounting systems to allow growers who intentionally recharge to receive credits for their efforts, Fukuda said.
For more information on SGMA and GSAs, visit http://sgma.water.ca.gov/. To see this and other slide decks from The Almond Conference, visit Almonds.com/ConferencePresentations.
