Terra Nova Ranch General Manager Don Cameron knew groundwater availability was going to be a problem more than 20 years ago.

When the opportunity arose, the self-described ‘lone wolf’ of groundwater recharge began this practice in 2011, flooding his Fresno County vineyards with excess floodwater with the aim of recharging groundwater aquifers.

“We knew the groundwater table was dropping and we would need bigger motors in the years to come to pull water up,” Cameron said in October at the American Pistachio Growers ‘Recharge Day,’ where researchers and growers shared experiences and findings about groundwater recharge strategies.

Groundwater recharge using flood water wasn’t a simple action. There were legality issues, infrastructure needs and little understanding of the effects of flooding vineyards and orchards. Water measurement was a challenge as was the sediment that came with the floodwater. Operational issues included infiltration rates, levee breaks, wind erosion and water levels in basins.

While many of the challenges with groundwater recharge have been overcome, information and ‘how-to” questions remain.

Know the Soil

When planning to take excess surface water from rain or flood events for recharge in permanent crops, Cameron advised knowing the soils in the orchards or fields, being vigilant about plant health and having a plan to receive and move water where needed when the opportunity arises.

Cameron said he was able to find solutions to many of the groundwater recharge challenges that came his way since 2011 and noted groundwater levels where he farms have rebounded 15 to 20 feet.

There were costs associated with the effort to recharge groundwater. Extra labor and equipment, including much larger pumps, were needed to direct and contain floodwaters on his fields, vineyards and orchards.

The positives from Cameron’s lengthy record of groundwater recharge included 18,821 acre-feet recharged and flood relief for the communities of Mendota and Firebaugh. The recharge waters also provided ample habitat for migrating geese and other birds.

According to UC Davis researchers Thomas Harter and Helen Dahlke, on-farm recharge is a resource management strategy that can replenish depleted aquifers by directing excess surface water to suitable farm ground during times of water availability. Implementing on-farm recharge is a strategy to maintain long-term viability of agricultural land by preserving water resources into the future. A survey by Public Policy Institute of California found that from 2017 to 2023 there was a 90 percent increase in acre feet of captured floodwater use for groundwater recharge from 260,000 acre feet to 493,000 acre feet.

There are additional sustainable practices and other considerations that come into play when considering recharge strategies.

Cover cropping can enhance the benefits of groundwater recharge. Planned soil amendment applications can overcome some water infiltration challenges. Soil salinity can be managed with recharge water.

UCCE Farm Advisor Mae Culumber said to get more water into the ground and increase penetration, physical and chemical issues with the soil should be addressed. Effective recharge, she said, depends on soil pore size and volume, aggregation, plant roots, soil cultivation practices and soil and water salinity levels.

Soil amendments, Culumber said, can improve soil permeability. Physical barriers to permeability can include hardpan layers. Deep ripping can be a consideration, Culumber said (possibly just ripping every other row in an orchard). This practice may stimulate root growth.

Chemical barriers to water infiltration require a calcium amendment that will displace the sodium and remove it from the root zone. It will also neutralize soil lime, reduce pH and improve plant nutrition. It is important, Culumber noted, that soil analyses be used to determine the correct type and rate of materials to be applied.

Depth of the salinity problem should also be determined. Culumber recommended sampling at 0 to 3 inches then in 1-foot increments to determine if salt accumulation is mainly at the surface or through the root zone. If soil tests reveal sodium loads are high throughout the root zone, soil amendments followed by leaching may be needed to reclaim the soil before the next growing season.

Cover crops, mulches and other organic matter inputs will help prevent soil surface crusting after a rain event. These tools can also enhance microbial activity that aggregates soil and improves the effectiveness of other amendments. Culumber stressed post-amendment leaching is critical to complete soil salinity reclamation.

Managing Variability

Precision application of water and nutrients can address yield variability in pistachio production. Patrick Brown, plant scientist at UC Davis, said yields matter as over-fertilization in low-yield years is a cause of nitrogen pollution in groundwater. Variability in yield correlates negatively with soil carbon, soil pH and soil structural problems, suggesting targeted gypsum and lime applications are warranted.

“To manage variability, you have to measure, monitor and manage,” Brown said. “Variability really matters.”

UCCE Specialist Louise Ferguson explained how use of AG-MAR can assist with salinity management in salt-affected ground. Agricultural Managed Aquifer Recharge is a decision support tool to capture winter runoff not currently allocated to water users and in excess of instream flow requirements.

Agricultural managed aquifer recharge may include winter irrigations in various forms, recharge through unlined canals and set-aside land (recharge basins), in lieu of recharge, and other forms. Assessment and modeling tools are in development to help landowners and irrigation districts to evaluate recharge opportunities.

Ferguson said recharge strategies include optimizing timing during wet seasons, monitoring water to avoid introducing salts and combining recharge with in-season leaching for sustainable soil management. Leaching strategies include double-line drip systems in-season and winter leaching to flush salts and reduce boron uptake. Irrigation method, volume and frequency must be adjusted for soil type. Proper drainage systems are needed to prevent saturation and salt buildup.

“Salinity is not going away, but it can be managed,” Ferguson said.

Charlie Chen, UC Davis Hydrologic Science Graduate Group, introduced recent research on the impact of cover cropping on water budgets in pistachio production. This research evaluated cover crop influence on evapotranspiration, soil water storage and deep percolation.

Chen said although cover crops did reduce soil moisture at the surface, a planted cover crop increased moisture storage below six feet. Other findings include higher ET in cover crop orchards compared to a no-cover-crop orchard. Soil water was higher and more deep percolation occurred.

The potential crop health benefits of recharge enhancement with the use of nanobubble water injection was explained by UCCE Farm Advisor Tobias Oker from Kern County. Nanobubbles are tiny gaseous bubbles entrained in a solution that can alter the normal characteristics of the liquid. These bubbles can improve water infiltration in the soil by reducing surface tension and contact angle on hydrophobic soil surface.

“It is basically injecting air into water,” Oker said. The nanobubbles could be a solution to improving water infiltration.

This project is evaluating soil water infiltration rates under nanobubble supplemented irrigation compared to normal irrigation conditions. It will also assess changes in concentrations of total dissolved salts under similar conditions. Finally, Oker said pistachio yields and leaf tissue nutrient concentrations under nanobubble-supplemented irrigation will be compared with yields and nutrient concentrations under normal irrigation conditions.