Although subsurface drip irrigation has a place for improved returns in processing tomatoes on marginal soils in the San Joaquin Valley, University of California scientists caution it is not for all situations.
There is no question the method works well for fine-textured, salt-affected soils with shallow groundwater on the valley's West Side, says Blaine Hanson, irrigation specialist at UC, Davis.
He adds, however, that where yields are already high with furrow and sprinkler irrigation, any gains in production are not likely to offset the costs of conversion to drip.
Hanson and Don May, retired Fresno County farm advisor, have been working with drip and processing tomatoes on the West Side for the past five years.
They discussed their findings at a recent field day for vegetable growers at the West Side Research and Extension Center near Five Points.
When they began their trials in 1998, May said, local growers were skeptical, but after the growers saw the results, several decided to try drip the following season.
“The trials,” Hanson added, “produced much higher yields than the growers were getting with sprinkler or furrow irrigation. Some just jumped into it right away. One grower put in 80 acres of drip, another put in 160 acres, and they've been increasing it ever since.”
Costs were a factor then and remain so today, and Hanson said he estimates the cost of installation at about $850 per acre. “We calculated that capital cost amortized over the life of components of the system (drip tape at five years, filters and other equipment at 15 to 20 years).
“We found that the increase in profits due to the drip systems was in the order of $300 to $500, while the cost of a system was about $155 per year. So the increase in profit greatly exceeded the cost of a system.”
Among the significant savings, according to May, were lower labor costs, along with savings in land preparation, since the fields were minimum tilled.
He said since cotton and tomatoes are grown on the same row spacing, the drip system remains in place and the growers can reduce land preparation operations to three or four passes to be ready for the next crop.
“One grower,” May said, “has had a drip system on processing tomatoes for seven years, and yields last year were as good as they were at the beginning. There are drip fields for fresh market tomatoes that have been in use for 10 years.”
Best drip soils
“Drip works best,” Hanson said, “where you have soils that are marginal for the existing irrigation system. If you are on a good soil with furrow, you may not see as great an improvement as you will with marginal soils.
“The benefit with the better soils is a wider range of rotational crops, such as garlic and lettuce, while on the poorer soils it is usually a cotton and tomato rotation.”
He also pointed out that the key to drip is combining it with a high-value crop. “Using drip irrigation on lower-valued crops may be unprofitable even if yields increase.”
Although Hanson and May did their observations with drip lines buried at 8 to 9 inches, some growers have chosen to place lines 12 to 14 inches deep.
Hanson warned that one of the most common mistakes growers make when they convert to drip is failing to maintain it properly. “Flushing the lines regularly is important, and particularly here on the West Side, they need to make certain to chlorinate the lines because Westland Irrigation District water has a lot of algae and other organic material that does not get filtered out.”
Growers, May said, as they become accustomed to the new method, need to re-think management of their fertilizers introduced through the system because it is very easy to over-irrigate and therefore over-fertilize.
Recommended irrigation amounts are about 100 percent of the potential crop ET for processing tomatoes. This is a compromise between reducing drainage and leaching of salts into the root zone. The crop should be irrigated two to three times per week.
May added, however, that in the face of rising costs, growers are refining methods for greater economy with their water. Meanwhile, he and Hanson are continuing research to develop more guidelines for irrigation with drip systems.
According to Hanson, little water savings on a per acre basis on processing tomatoes is likely in a conversion from sprinkler to drip.
“The higher yields with drip irrigation suggest that percolation and evaporation losses under sprinkler irrigation become transpiration losses under subsurface drip irrigation.
“However, because of higher yields, the same total tons can be grown on fewer acres, saving water.”
“Subsurface drip irrigation,” Hanson continued, “also provided better water management late in the growing season, when careful management is needed to prevent excessive deficit irrigation and phytophthora due to overly wet soil.”
Hanson and May prepared a list of other requirements for drip irrigation on West Side soils, beginning with the need for sufficient leaching to maintain acceptable levels of soil salinity around the drip lines, where root growth is heaviest.
If seasonal rainfall is not enough to leach salts down from the root zone, periodic leaching with sprinklers may be needed to carry salts downward from the drip lines during stand establishment.
In shallow groundwater situations, growers have to find the balance between applying enough water to supply the crop's needs and leaching, on one hand, and doing so without contributing excessive subsurface drainage, on the other.
Prevention of potential clogging of lines by root intrusion is also important and can be done with injection of chemicals appropriate according to local ordinances. “Clogging will not only reduce the applied water needed for crop evapotranspiration, but also reduce the leaching,” Hanson said.