As you round the final bend of Road 95 through the groves of walnut and almond trees, it would be easy to miss Muller Ranch, but for the cluster of pickup trucks circling the driveway. Yet amidst these swaying Yolo County orchards sits a stunning third-generation, family-owned farm, thriving with several thousand acres of processing tomatoes, specialty peppers, corn, wheat, canola, cucumbers, grapes, garlic, and sunflowers. For the past two years, Arable has had the opportunity to spend a considerable amount of time learning from Frank Muller, his son Colin, and the other irrigation specialists on their staff about how to design and implement a specialized irrigation strategy that could shape the way crops are grown and watered. From February through August each year, we have the privilege of working in Frank’s prized 2400 acres of processing tomatoes to understand the effect of irrigation approaches on fruit yield and quality.
California’s processing tomatoes, like its wine grapes, are typically drip irrigated to allow for a precise application of water. Highly technical farmers like the Mullers will implement an irrigation strategy known as Regulated Deficit Irrigation (RDI) that optimally increases soluble solid content (also known as Brix) through the calculated manipulation of water application. At a specific time in the crop development cycle — such as the beginning of the pink fruit stage, around 55–70 days after transplant — maintaining a certain level of crop water deficit stresses the tomato plant in a controlled way, telling it to send carbohydrates — the stuff that makes tomato paste taste good — to develop its fruit rather than to grow more leaves and stalks. This drives up the fruit’s soluble solid content, thereby growing a more valuable product. It doesn’t just benefit the farmer, it also saves energy at the processing plant: a fruit with more soluble solids has inversely less water to boil out to make that delicious paste, cutting natural gas use significantly. If done correctly, RDI in processing tomatoes can lead to several significant production benefits:
- Reduced irrigation: saving water, time and money. Our field-level ETc data suggest a 15% reduction in water use, which is valuable in prolonged drought conditions.
- Higher soluble solids: reduced energy costs and GHG emissions at the processing plant and better price for the grower. Estimates show appx. $6M cost at plant.
- Increased soluble solids: better flavor.
- Reduced moisture in the fields: less disease pressure at the end of the season.
Frank is one of the most progressive farmers we’ve met, a pioneer in water conservation and sustainable management practices. He upgraded all his acreage to drip irrigation in the 1990s, utilizes deficit irrigation strategies, and is constantly seeking to improve the Mullers’ operation. When we first met to discuss the concept for the project, although his team had already implemented RDI, he told us how he thought they could probably water even less without sacrificing tomato yields, and increase their soluble solids, to gain a premium on quality. Despite frequent scouting to assess canopy development, he felt that the team was still struggling to pinpoint the right moment to implement RDI strategy, due to varying tomato genetics, planting dates, and field conditions. It’s not an easy task, since dialing back water too early can dramatically reduce yield, making it financially very risky to start RDI too early. To mitigate the risk, the team decided to wait until they could actually see water in the field before dialing back the irrigation, with the tradeoff being a loss of premium on quality. Arable and Muller Ranch collaborated with Dan Johnson, an irrigation engineer at USDA NRCS, to build out an RDI guide to address this issue. To inform our guide, we decided to collect the following information at the level of individual irrigation management zones:
- Daily monitoring of phenological progress by variety and field to identify optimal timing to transition period from regular irrigation to an RDI strategy.
- Measuring ETc by irrigation management zone to assess irrigation needs relative to evaporative demand.
- Accurately measuring and forecasting field-level weather events to better evaluate potential future risks and scenarios.
- Accurately measuring the amount of irrigation applied as a final check that the strategy has been successfully implemented.