Pecan farmers test remedies to recharge aquifers

Almond farmers in California have long been recharging depleted groundwater supplies on some of their million acres. Now new insights are being learned as growers of a far smaller sister crop—pecans—are also capturing surface water in winter months in order to recharge underground aquifers.

Pecan farms account for just 5,000 acres in California. But efforts by farmers to capture rainwater in wetter winter months and later flood their farms to reinvigorate groundwater supplies are offering new remedies to severe drought conditions.

Sandra Bachand, co-founder of Bachand & Associates, talked about the pecan findings from a pilot research project during a recent virtual presentation. It also included a talk on the role of soil texture and flooding in nitrate leaching by Helen Dahlke, associate professor in integrated hydrologic science at the University of California, Davis.

The program ended with a discussion by a grower panel that included one of the earliest pioneers in on-farm recharge, Don Cameron with Terranova Ranch in Fresno County, who said he believes capturing water for recharge in the winter months "helps to make California water resilient."

The presentation can be accessed on YouTube under the title "On-Farm Recharge Webinar: Pecans & Other Nuts."

Bachand talked of a research study focusing on on-farm recharge on four plots of pecans paid for by a Natural Resources Conservation Service specialty crop block grant from 2018 to 2021.

The survey was conducted from early May to mid-June in 2019 and 2020. It found that pecans were able to tolerate saturated soils and flooding well.

No abnormalities, such as signs of bronzing or wilting, were observed, and there were no differences between trees in on-farm recharge treatments from those in a regularly irrigated treatment.

Leaf analysis showed on-farm recharge either improved or had no effect on nutrient levels. Stem water potential data indicated healthy trees. And yields were the same or higher in on-farm recharge plots compared to the regularly irrigated control and other nearby orchards in 2019 or 2020.

Low oxygen was not detrimental to pecan tree health and yields.

Additional benefits included salt flushing from the root zone and the ability to reduce irrigation.

Bachand said recharge projects are being spurred in part by climate change, with less surface water stored in California's snowpack and greater frequency and variability of drought and floods.

Another stimulus is the Sustainable Groundwater Management Act and the fact that on-farm recharge offers "new" water to help augment groundwater.

"Water that currently flows out of an area in a wet year can be captured in aquifers and that enhances flexibility," Bachand said.

In California, pecans are grown in 17 counties. The leaders in pecan production are Tulare and Colusa counties, with nearly half the acreage.

One of the research plots had continuous flooding to maintain standing water. In another, there was alternate flooding between two sub-blocks, and in another there was regular irrigation followed by a second irrigation. A control plot had regular flood irrigation.

Poly pipe was stretched across about 850 feet of one field. It was fitted with adjustable gates for water flow. Terracing was used to improve water depth uniformity.

Flooding did bring oxygen decreases in the root zone, but recovery from that began immediately when water was off the field, Bachand said. Management techniques were to allow soils to dry briefly for five or six days or to apply recharge water to every other row, then switch rows.

The cost of recharge ranged from $35 to $200 an acre, about 1.5% to 9% of the cost of regular operations.

Both Bachand and Dahlke said research has shown on-farm recharge can be beneficial in other crops besides nuts.

Dahlke said pulsed flooding of almonds showed no significant effect on yield, and on well-drained soils, large amounts of water can be recharged. She said it is a viable option for regions where large amounts of excess water are less frequently available.

"Flooding can create short-lived anoxic conditions in the root zone," she said, adding that the duration of such conditions needs to be managed. Flooding reduces oxygen levels within the soil, inhibiting root respiration and root growth with potentially negative effects on crop yield.

She added that winter recharge can produce nitrates in the root zone through mineralization of organic nitrogen and might reduce fertilizer needs during the growing season. The most nitrate is leached within the first few hours of application, before conditions for denitrification are reached, and denitrification plays a greater role in fine textured soils with continuous flooding.

Dahlke cited notable differences in soil drainage between sandy soils and those that are a fine sandy loam. There are also variations in crop tolerance to waterlogging.

The grower panel discussion was moderated by Mohammad Yaghmour, UC orchard systems advisor for Kern County.

The panel included Fresno County growers Cameron; Mark McKean with McKean Farms; and Ben King, the state's largest pecan grower with orchards near Visalia and Colusa. King also grows almonds. He is with Pacific Gold Agriculture.

Asked about challenges to recharge, Cameron said those include providing the needed infrastructure and considering soil differences and the movement of salts, pesticides and herbicides.

"Management criteria have to be in place," Cameron said.

He and others also said challenges include the difficulty of getting into flooded fields for treatments such as spraying.

Cameron said he laser-leveled fields for even distribution of water.

McKean said his almond trees are on high berms to control water flow, and he uses both drip and flood irrigation. "It's not real simple," he said.

McKean said he has concerns about plant health, and said he would like to see more research on rootstocks that might be best suited to recharge conditions. He said he would also like to see more research on leaching.

King said timing is important in managing recharge. When water is available, he said, "you have to be prepared to take it."

King said he favors recharge because "it's replicating natural systems."

McKean said he would like to see "credits" given to growers who use on-farm recharge.

(Dennis Pollock is a reporter in Fresno. He may be contacted at apcompollock@yahoo.com.)