A variety of research by University of California scientists over the past 20 years offers lessons for growing processing tomatoes successfully when water supplies are restricted, says Tim Hartz, University of California, Davis Extension vegetable crops specialist.

Deficit irrigation tips: Even moderate levels of soil moisture deficit during fruit set can substantially reduce that set, and induce blossom end rot, he notes. “However, once fruit set is complete (typically five to six weeks before harvest), a substantial level of moisture stress can be imposed with minimal loss of productivity,” Hartz says. “Fresh fruit yield may decline a few tons per acre, but an increase in soluble solids concentration usually results in little or no decline in brix yield.”

Most fields can tolerate irrigation of only 40 percent to 60 percent of reference evapotranspiration (ET) during the fruit ripening period with few problems, Hartz explains. Fields with high water holding capacity and good rooting depth may be able to handle as little as 25 percent of ET over the final six weeks.

Deficit irrigation in drip fields can be controlled by simply reducing the hours of run to deliver the desired percentage of ET. However, root intrusion in buried drip systems can be a problem during deficit irrigation. “In most fields, drip irrigation can be terminated within the last 10-14 days before scheduled harvest without severe stress,” Hartz says. If harvest is delayed, small irrigations can be made to keep the vines up.

Manipulating the irrigation cutoff date to save one or more irrigations can be used to control water volume with furrow irrigation. Extensive trials in clay loam soils in California's San Joaquin Valley have shown that cutting off furrow irrigation as much as 40 days pre-harvest will have minimal effect on brix yield, although fruit yield may suffer a small decline. “Even on these forgiving soils, however, earlier cutoff can lead to substantial yield loss,” Hartz says. “On solids with lower water holding capacity even 40 days preharvest can be too severe a treatment. Using an early cutoff strategy can be risky, particularly if harvest is substantially delayed.”

Using a groundwater table: If the water table is within 2 to 3 feet of the surface, deficit irrigation can cause the crop to draw as much as several inches of water from the water table. This allows for a more severe irrigation cutback or earlier cutoff than would otherwise be appropriate for the field, Hartz says. If the water table is non-saline, late-season deficit irrigation poses little risk of serious yield decline. However, if the water table is saline, a much larger yield loss is possible with an aggressive irrigation cutback.

Using poor quality irrigation water: Once the crop has been established, tomatoes can tolerate high salinity irrigation water. Research in the San Joaquin Valley showed that once flowering had begun, irrigating with drainage water of 8.0 EC (electrical conductivity) did not affect fruit yield. “Growers can take advantage of the relative salinity and boron tolerance of tomatoes by substituting lower quality groundwater, and saving higher quality water for more sensitive crops,” Hartz says.