What a Difference Distribution Uniformity Makes

Planting a new orchard with a professionally designed and installed irrigation system should result in uniform distribution throughout the field and a good wetting pattern around the trees. But as self-described ‘experimental micro-farmer’ Aim Khan found out, “You cannot rely on the design [of the system]; in reality, the water pressure could be different from what was expected,” he explained.

Khan, who has been a Silicon Valley high-tech manager for 35 years, recently decided he also wanted to get into farming, so he followed through by purchasing about 40 acres in Stanislaus County to begin his ‘experiment.’ Two years ago, he planted Nonpareils with Aldridge and Wood Colony on 20- by 16-foot spacing. “I hired a company to come up with an irrigation system designed to work best for this orchard, and had the whole system installed,” he said.

Too Much, Too Little
In the first season, Khan saw that some parts of the field were getting too much water and other parts not enough. “Leaves were turning yellow because they were getting too much water,” he noted, “and in other areas, the trees were not even growing.”

Last year, Khan asked Irrigation Matters, a service-based irrigation and water management company, to perform a distribution uniformity (DU) test on his system.

“After noticing large differences in the wetting patterns and standing water in some areas of the field, we decided that our next step would be a DU test,” said John Denlinger, water agronomist for Irrigation Matters. He went on to say, “By evaluating the DU of the field, we were able to determine that the issue was in the design of the irrigation system, which did not compensate for elevation changes across the field.”

“We found, after doing the test, that the design and the system don’t match,” reported Khan. “As a result, 70% of the field had above-average pressure, while 30% of the field had average or below-average
pressure. Where ideal pressures would have been 20 and 26 pounds per square inch, instead, we were getting 16 psi and 42 psi, respectively.”

Switched Microsprinklers
The original system had Nelson R10 0.5 gpm FC nozzle P4 9-degree plate sprinklers, and according to Khan, about 30% of the sprinklers were not functioning properly in the first year.

“Some of the trees were getting no water, and some trees were getting flooded because the sprinklers were broken and water was gushing out,” he said. “So last year, just to be sure each tree gets some water, we changed to Rivulis D2000 microsprinklers, which have a little more consistent output with flow control.”

The next step for Khan is to have the system retrofitted to allow even distribution of water across his orchard, putting in a flow meter, and monitoring pressure “from one end to the other to be sure we have uniform pressure throughout the field.”

Once the infrastructure has been retrofitted, and with information from two real-time soil moisture sensors installed last fall, Denlinger will work with Khan to determine when to irrigate, using evapotranspiration and soil moisture information gathered from his real-time soil moisture and weather station.

“I would definitely recommend a DU test, even for a new system,” Khan said in retrospect. “I wish I had done this two years ago, when the system was first installed. The trees have suffered, and I wasted so much money. Testing is a benefit all the way around by resulting in less water use and having more uniform tree growth,” he concluded.