RECENT RAINS PROMPT CONSIDERATIONS FOR GROUNDWATER RECHARGE OPTIONS

By KATHY COATNEY | Contributing Writer

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California and much of the west has seen the driest conditions in at least 1,200 years. While recent rains have improved the situation, California still isn’t out of the drought. Groundwater recharge still holds promise for mediating some of the impacts of drought on groundwater tables.

With interest in recharge surging given the recent rains, it’s a good opportunity to check in with one of the research pioneers in this practice to see what we know so far.

Helen Dahlke, associate professor of integrated hydrologic sciences with UC Davis, has done several years of research on groundwater recharge working primarily on the impact groundwater or winter recharge has on nitrate leaching and nitrogen cycling in soil, mainly in the root zone. The research was done on perennial crops, including nut crops and vineyards to look at the effects of extended flooding on plant response and yield in the following growing season.

Recharging the Aquifer

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The intent of on-farm recharge is to recharge the aquifer so that the water can be pumped out later. Current rainfall is producing natural recharge. The rain is falling onto the soil, and depending on where a farm is located, the soil could be clay or silt loam, or coarser-textured soil like sand. A certain amount of that rainfall would go into the soil and potentially move deeper past the root zone to the groundwater table.

“We think some soils, particularly the sandy ones, can definitely infiltrate a lot more water, and so this is why I think a combination of precipitation plus intentionally directing floodwaters onto the soil could really allow us to infiltrate a lot more water than what would naturally occur. And that definitely can help the groundwater aquifer underneath it,” Dahlke said.

To intentionally direct the floodwaters, growers would create dikes or levees to hold the water within a field or orchard. “We typically recommend putting berms around, maybe one- to two-foot-high earth mounds,” Dahlke said, adding this is something growers can do with equipment they have on the farm.

Most of Dahlke’s experiments were done between the end of February and early April, much later in the season when crops are typically coming out of dormancy. Fields were flooded for two and four weeks, time periods typical of recent flooding, Dahlke said.

“Any active growth requires oxygen. When you have standing water, often what happens is that the amount of air or oxygen that we have in the soil is going down quickly, which can create so-called hypoxic conditions. And the roots, besides taking up water and nutrients for a tree to grow, they also have to do gas exchange, which is called root respiration. So, when there is an actively growing tree or crop sitting in standing water, then that gas exchange can no longer happen and they can’t take up oxygen through the root system,” Dahlke said, and that often means the plant goes into metabolic stress and can die.

Nut Orchards

Dahlke was involved with pecan research and groundwater recharge several years ago with Ben King, a California pecan grower. King had a young pecan orchard sitting behind a levee and in a flood retention area. For months, only the crowns of the trees were above the water, and the water depth was about six to seven feet.

“This is really something not every crop can tolerate, but pecans are native to North America. You can actually see them growing wild along the Sacramento River, so they’re used to having wet feet for extended periods of time because flooding has occurred naturally in past centuries. They don’t seem to be impacted at all by the floodwater as long as they’re not completely submerged,” Dahlke said.

Dahlke hasn’t yet been able to research recharge on walnuts due in part to a lack of funding amid grower concerns. “I think maybe from a physiology perspective, as long as the trees are dormant, they probably can tolerate on-farm recharge quite well,” she said.

“I think the main reason walnut growers are shying away from doing on-farm recharge is the risk of the fungal disease Phytophthora, which can spread really quickly with floodwaters. Warm, moist temperatures are really ideal conditions for the fungus to spread, and once you have it in your orchard, it’s definitely hard to contain,” Dahlke said.

Walnuts are grown for fairly long periods of time compared to almonds that are typically 25 to 30 years. “So, you make a generational investment into your walnut orchard, and you may not want to risk losing your production and your economic income with a fungal disease,” Dahlke said, adding she’d still like to do groundwater recharge research on walnuts.

Dahlke also did work with UC Davis’ Ken Shackle flooding almonds in 2015. “That was our first Almond Board of California project,” Dahlke said.

Shackle was interested in studying drought conditions during the winter, asking, “Can almond trees survive a drought winter without additional irrigation?” And Dahlke was testing the other extreme: “Can water be applied to almond trees in the winter?”

Almonds are more challenging for groundwater recharge because they come out of dormancy sooner than other nut crops, and there could be tree loss if they’re flooded.

“Almonds typically start blooming mid-February and again when that bloom happens, the tree will become active. They need those roots to take up nutrients in order to produce the flowers and the leaves that are coming after that. Typically, we want to avoid having those tree roots sit in floodwater or fully saturated soils, although this year we might not have