Flooding fields may fill aquifers

At least 3.5 percent of California’s land could be used to replenish its groundwater

Bryan-Morris Ranch manager Jim Morris stands in a flooded alfalfa field along the Scott River in Siskiyou County.

Bryan-Morris Ranch manager Jim Morris stands in a flooded alfalfa field along the Scott River in Siskiyou County. (Photo courtesy Steve Orloff/UC Davis)

There has been evidence California’s aquifers may be shrinking, but at least 3.5 percent of its farmland could be flooded to ensure good to excellent results in replenishing the local water table. New research from UC Davis has promise of better.

Farmlands in California draw up about 30% of the water (34 million acre-feet per year) in wet years, but up to 60% during drought, according to a recent paper in California Agriculture.

Two problems have compounded the overdraft in groundwater. More efficient water use on farmland in the past 20 years has meant that there is less water pooling to slowly find its way down to the water table. Additionally, increase in demand for nut and wine production, which rely on perennial crops, means

Diverting water from swollen rivers to barren land can reduce damage downstream, as William Howarth of UC Davis, and Phillip Bachand, an environmental engineer at Tetratech, did in 2010. They concluded that up to 75% of such water reached the aquifers while doing no damage to crops or water quality.

A UC Davis press release quoted Don Cameron, manager of Terranova Ranch along the Kings River in Fresno County: “Our wine grapes were under water for five months, which raised a few eyebrows, but they did fine. Diverting floodwater to farms can recharge groundwater and reduce the risk of downstream flooding. It’s a good situation all around.”

Now Helen Dahlke and her team from UC Davis worked on alfalfa fields in Siskiyou County east of the Scott River. This area has very good water permeability, given the alluvium is made up of sand, gravel and clay deposited by the river. And it helps that alfalfa does not rely on nitrogen fertilizers that could contaminate water percolating down to the water table.

There are at least five factors to consider when considering groundwater replenishment in farmland.

1. Whether the soil can actually absorb water deeper than the roots than take it back in.

2. Whether the roots can endure being saturated with water

3. Whether the land is level enough to ensure even distribution of water,

4. Whether the soil has so much salt that it water percolating down could affect the quality of water in the aquifer.

5. Whether the soil is susceptible to erosion, which could take away nutrients, or compaction or crusting, which can reduce surface infiltration.

Now the Almond Board of California is funding a two-year project by Dahlke and Ken Shackel (also of UC Davis), to see almond orchards could be flooded with no damage to the crops. The team is starting with a 60-day period of flooding in December.

Because almond and wine orchards increasingly rely on deep tillage (in which hard ground is tilled up to six feet to ensure that root penetration is not affected), researchers expect there is flooding can be done more land than historically expected – and that more will be available in future.

Researchers have pointed out that these are still preliminary studies, not fully accounting the effect of proximity to water bodies, and the nature of the zone between the maximum soil depth studied and the water table beneath. In arid regions, this zone – called the vadose (Latin for ‘shallow’) – may accumulate salts and agricultural pollutants over the years. Once this zone is well watered, such material may wash down and contaminate the water table.

There is also a realization that it might take a little time to replenish aquifers that took over ten thousand years to fill. But despite the fears, and reminders of hubris, the increasing seriousness of the water crisis should ensure that flooding in suitable areas, including farmland, could become one way to slow the rate of groundwater loss just a little bit.

Based on information from the UC Davis web site and related research material.

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