Strategic flooding for aquifers getting new attention ByDennis Pollock
In 2011, Cameron floodedwinegrapes from April through July at 2.5 inches per day and saw no yield loss or damage to the grapes. There were good infiltration rates on the fine sandy loam soil. From 1981 to 1985 , research at California State University, Fresno, was undertaken on flooding 24-year-old Thompson seedless grapes to a 6-inch depth on clay loam. Water was applied over 32 days. Researchers in those studies found no effect on fruit set and no significant dif- ference in yield except in 1985, when re- charge treatment produced higher yields per vine than in a control vineyard. There was also no difference in brix, titratable acidity, pH or berry weight ex- cept when sugar was significantly low- er in the recharge treatment. But there were concerns over nitrogen leaching, Dahlke said. In more recent research in 2020, two Thompson seedless vineyards more than 40 years old were flooded with groundwater on Hanford sandy loam. One was flooded for four weeks at 4 inches per day, the other for two weeks at 8 inches per day. Infiltration rates varied, with 4 inches per day on the vineyard flooded for four weeks and 8 inches per day on the one flooded for two weeks. For the vineyard with lower infiltration, conditions on oc- casion were anoxic. “Infiltration makes all the difference,” Dahlke said. FloodingstartedFeb. 25, 2020, andthere was zero precipitation from February to mid-March. There were continuous soil measurements. Soil sampling was done, and drone flights were flown. Researchers also looked at nitrate leaching in each plot. They noted there was severe damage from a hailstorm on March 25, 2020, that brought low cluster counts and low yield. They concluded on-farm recharge is a viable option for regions where large amounts of excess water are less fre- quently available. Soil type is the main parameter for choosing Ag-MAR loca- tions, they said. A high filtration rate brought no sig- nificant yield change after two weeks of flooding. Low infiltration rate brought a significant yield decrease—32% in 2020, 40%in2021—after fourweeks of flooding. Low infiltration also was marked by a longer recovery time. Dahlke advised there could be infil- tration challenges with soils that have a high clay content. She said high infiltra- tion is needed to manage oxygen in the root zone. Otherwise, she said, grape vineyards are a great option for recharge efforts be- cause they have a long dormancy period and are “adaptive to havingwet feet once in a while.” (Denni s Pol lock i s a repor t er in F r e s no . He ma y b e c on t a c t e d a t agcompollock@yahoo.com.)
Parking floodwater in vineyards to re- charge aquifers is a way to prepare for continuing drought conditions, reduce flood risk in wet years and improve wa- ter quality. Don Cameron, vice president and general manager of Terranova Ranch in Fresno County, is no stranger to it. He has used so-called “Ag-MAR”—or agricultural managed recharge—to put water on the farm’s vineyards starting in 2011. He found that the water safely seeped into the aquifer, boosting its sup- ply without harming the grapes. He later applied theprogramtoputwa- ter into almond orchards, fallowed fields and other cropland in subsequent years. He’s a believer who shrugged off early skepticism about his efforts to capture floodwater, explaining, “We have a great reservoir under our feet. Why not use it?” Cameron’s efforts inspired intensified research on how to take advantage of wet months and strategic water applications to recharge groundwater supplies. GroundwaterhydrologistHelenDahlke, an associate professor at the University of California,Davis,hasworkedforyearswith farmerswho look to floodvineyards, fields andorchards inwintermonths tohelpsus- tainaquifer supplies in summermonths. The Dahlke Lab at UC Davis is now collaborating with UC Agriculture and Natural Resources farmadvisors to study impacts of flooding a variety of crops. That effort included experimental flood- ing of a 400-acre vineyard during the 2020-21 winter season with Cosumnes River water, as researchers employed sensors tomeasure infiltration rates. Dahlke recently discussed the ongo- ingwork that has grownout of Cameron’s early experiments, as she spoke on groundwater recharge on vineyards during the 2022 San JoaquinValleyGrape
Groundwater recharge, above, is underway at the UC Kearney Agricultural Research and Extension Center vineyard. At left, hydrolo- gist Helen Dahlke stands in an almond orchard during experimental flooding in the 2020-21 winter season.
Symposiumlastmonth inFresnoCounty. One key to flooding vineyards is to make sure they remain “oxic,” or retain oxygen, Dahlke explained. Meanwhile, she said there arenumerous institutional, legal and economic challenges to the Ag- MAR approach that must be considered. She said there are cost considerations involved, permits required and laws to follow. Water availability is critical and proper use of hydrogeology is important, along with crop suitability, she noted. Dahlke said growers have some reser-
vations about flooding perennial crops, fearingwaterloggingwill limit root respi- ration andmay even cause root diseases. She said various crops can be grouped according to tolerance for standing wa- ter—ranging from no tolerance at all for someplants and48hours toas longas two weeks for others. For some crops, howev- er, the tolerance is largely unknown. Dahlke talked of Cameron’s efforts and noted the research on recharge during grapevine dormancy that predated Cameron’s sucessful efforts.
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10 Ag Alert February 23, 2022
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