Sensors guide growers on water decisions

By Rob McCarthy

Lisa Tate, a fifth-generation grower in Santa Paula, said she knew something was amiss in a block of young avocado trees planted on her family’s ranch.

She had a hunch that too much water was sitting in the root zone after a second year of above-average rainfall. It was still spring, and her crew suggested it was time to irrigate. But Tate was proven right, thanks to readings from a soil-moisture sensor. As a result, her young trees didn’t get watered again until June.

These days, water conservation agencies and University of California Cooperative Extension farm advisors are advocating adding soil-moisture sensors to create precision irrigation management in drought-prone California agricultural areas.

They say the technology has come a long way in the past 50 years and continues to evolve with Wi-Fi connectivity and digital readings.

Tate obtained her soil-moisture sensor through a grant from the Ventura County Resource Conservation District, which received state funding to make the technology available for regional avocado, strawberry and vegetable growers.

The value of a soil sensor is twofold, according to Jamie Whiteford, a conservation scientist who manages grants for the Ventura district. It can prevent overwatering of tree crops or indicate that the root zone is drying up and in need of irrigation to avoid stressing the tree.

The local moisture-sensor giveaway is in its sixth year. Tate was one of some 10 growers to accept the offer this year. Even though the sensor didn’t cost her a dime, she said the equipment in the $1,000-$1,500 range would definitely have been worth the cost.

“It paid for itself that first day,” she said.

She now plans to purchase several more soil-moisture sensors, manufactured by Irrometer, a Riverside soil water management firm. She will have them installed around her property.

The early models of soil sensors designed for commercial agricultural use were made from wood and metal pins. They picked up electrical waves emitting from any water held in a plant’s root zone. Nowadays, ground probes are made of durable plastic material and operate digitally, taking readings that convert into numbers displayed on a screen.

The soil probes are inserted into the root zone to measure either the volume of water held in the soil or what’s known as the tension. In the latter, the sensor is called a tensiometer. This is the type of sensor the Resource Conservation District gave away earlier this year.

A tensiometer is particularly good in changing weather conditions, said Andre Biscaro, a UCCE irrigation and water resources advisor in Ventura County. He said the devices can record and display soil data electronically and indicate trends that an irrigation manager needs to know on a real-time basis.

“Those include rising or falling soil moisture between irrigation events and how quickly they change,” Biscaro told avocado growers in a presentation last month.

Historically, Tate’s orchard crews would start irrigating the mature avocado trees near the end of April. But this year, the property received more than 21 inches of rainfall between February and April, according to Ventura County rainfall totals. Storms moved in frequently and kept the ground wet much later than normal.

“In the past, we would base our irrigation scheduling on an every-two-week schedule,” Tate said. “We would adjust it during rain, but for the most, part we kept to the schedule.”

The ranch normally makes occasional tweaks for winter and spring rainfall, she said. Now, she and her crews rely on the readings from the irrigation aid to avoid needlessly applying water to trees that don’t need it.

“This tool has been great because we get digital reports that show how much water is being held at the different depths of the soil,” she said. “It has shown me and our crew that we have been overwatering.”

Manufacturers of the digital sensors emphasize the cost savings on water. Wireless moisture sensors can be connected to one another in a field or orchard and relay data back to an office or a mobile device. The sensors, which run on solar-powered batteries, report data in real time. They don’t have to be checked manually, though Tate makes it a practice periodically to dig into the soil just to be sure it’s wet enough.

Yields and healthy trees are reliant on good irrigation management. Ben Faber, UCCE avocado and subtropical crops advisor in Ventura County, calls that “learning when to turn on the water and when to turn it off.”

Conservation scientist Whiteford said overwatering, other than when a small amount of extra water is applied to flush salt buildup in the soil, represents money wasted. When trees and row crops are overwatered, he noted, costly fertilizer inputs don’t stay in the root zone long enough to be taken up. “They’re all connected,” he said.

Another device that researchers say has worked well in irrigation-management programs is called an atmometer. The device, typically mounted on posts near irrigated fields of low-lying crops, measures the amount of evaporation taking place on farmland. An atmometer is a low-cost alternative to an on-site weather station.

According to a UC Agriculture and Nature Resources report, use of soil-moisture sensors in guiding irrigation decisions can result in improved yields by 9% for strawberries and 10% for celery, depending on a grower’s practices. The report said using sensors can produce water savings of 10% to 16% for strawberries and almonds.

In Ventura County, avocado grower Tate is now bracing for drier conditions as California’s climate has moved from a wetter El Niño weather pattern to a La Niña pattern, which the National Weather Service announced in June. That means growers and irrigation districts can expect warm temperatures and less rain.

Tate said her replanted blocks of avocados with soil sensors will be ready when rains return.

“The tricky part is knowing when to begin irrigating after rain,” she said. “The Irrometer (sensor) is useful in guiding that decision.”

(Rob McCarthy is a reporter in Ventura County. He may be contacted at robmccarthy10@yahoo.com.)