Ag at Large: Flooding the surface fills the underground

Underground water basins are refilled when certain crops are flooded beyond capacity. (Wikimedia Commons)

A research team has found that the recharge of depleted underground water basins can occur when crops such as alfalfa and almonds are flooded beyond capacity with winter water. It works better in some locations than others, but it is very encouraging

Standing water is generally considered by farmers as more of a danger sign than anything helpful, but the extensive research is showing major benefits for underground recharge when specific soil conditions exist, at least with certain crops.

Two significant test locations and two crops were central to the tests. The sites were in Scott Valley in Siskiyou County and near Davis in Sacramento County. The crops flooded were alfalfa and almonds. Both occupy large acreages and are widespread in California’s growing areas.  Some grape vineyards were found to withstand the overwatering as well.

The strategic need for the research is supported by the knowledge that natural underground basins have been overdrawn significantly by pumping from them for years. Some modern irrigation technologies such as sprinkler and drip systems, while intended as water savers, prevent recharge of the basins. It takes heavy overwatering to persuade water applied at the surface to percolate to basins beneath.

The scientists calculated that flooding all of the state’s suitable alfalfa acreage with six feet of winter water – and assuming that 90 percent of it percolates past the rood zone – is possible to bank 1.6 million acre-feet of water annually. That is nearly half the amount of water stored in the Oroville reservoir, the state’s second largest.

Don’t panic. The researchers are not suggesting a six-foot-deep reservoir at each location, just consecutive water releases through the winter months that add up to six feet. Even less is likely to be helpful.

Dr. Helen Dahlke, Associate Professor of Integrated Hydrologic Sciences at UC Davis, is one of the participants/spokesmen for the flooding project. She works in the Department of Land, Air and Water Resources. She uses the metaphor of the Central Valley being like a bathtub filled with sediment, with groundwater occupying the spaces between sediment grains.

She explains that water can be pumped more easily from some of the underground pockets, but where the sediment is something fine-grained such as clay, pumping is difficult. She also points out that the Central Valley’s groundwater is a renewable resource, replenished every year by rainfall, snowmelt and drainage underground from higher levels.

 She points out that water has been extracted from the underground resources for years at a rate faster than it is being replaced. Farmers have done some of the pumping, but growing municipalities and others have pumped their share. She says 80 million acre feet (MAF) have been extracted in the 56 years since 1960, or 150-160 MAF since 1920. For comparison, the entire volume of super-deep Lake Tahoe is 20 MAF.

Summarized, her advice drawn from the two research sites is (1 and 2) apply four to 10 feet of water to alfalfa at dormancy, (3) use normal pulsed applications to allow air to re-enter the root zone, and (4) don’t keep the field permanently flooded.

To find the water to participate, she suggests contact and perhaps negotiation with local irrigation districts. Her presentation also includes pertinent details for flooding almond orchards, beginning with the warning against applying excess water other than at dormancy. Those with riparian rights are restricted to holding water no more than 30 days, making them unlikely prospects.

Others participating in the study included U. S. Department of Agriculture soil scientist Andrew Brown and UC Cooperative Extension specialist Dan Putnam, Toby O’Geen and the late Steve Orloff. The report said Glenda Humiston, Vice President of UC Agriculture and Natural Resources, has made groundwater recharge a division priority.

Don Curlee :