Drones bring new options for on-farm aerial imaging

For capturing a comprehensive picture of groves and farm fields, and giving growers a precise idea of what's happening with their crops and what needs to be done, drones or unmanned aircraft systems are becoming more prevalent for aerial photography.

Airplanes and satellites also capture images of fields, but in terms of cost, drones beat out planes. They also have the ability to fly low to the ground, to capture images of soil health.

"Drones are the low-hanging fruit right now. Farmers won't need as many boots on the ground," said Greg Kriehn, a UAS research specialist and professor at California State University, Fresno.

Photography is the most common use for drones in farming, but they are also being used for pesticide application in China, and in the Midwest for infrared imaging that gives an overview of canopy health for crops such as soybeans, rice and wheat, as well as for spot treatment of weeds. This spring, a drone manufacturer announced it had conducted the first U.S. commercial operation of a remotely piloted helicopter for agricultural spraying, treating a North Coast vineyard for powdery mildew.

Kriehn studies how to use drones for farming in the Central Valley, where Fresno State has a 1,000-acre research farm. Fresno State conducts a drone research program in collaboration with the University of California, Merced, and UC Cooperative Extension.

"Drones give you a bird's-eye view of how water is being distributed in the fields," Kriehn said. "Some farmers with large acreage have complained that when drip lines have gone down it's taken them a while to realize it, but that's where drones can make a difference, in alerting them to problems quickly."

The Federal Aviation Administration issued new safety rules for smaller drones in 2015, which have helped boost their use. Small drones under 55 pounds in weight can now be flown over agricultural fields for non-recreational purposes, up to 500 feet in altitude and no faster than 100 miles per hour, and must be kept within the line of vision of the operator.

"Those guidelines have helped us understand what we can use it for," said David Doll, a UC Cooperative Extension farm advisor in Merced County.

High-altitude drone use has a lot more restrictions, so private entities are not allowed to fly them. Doll said low-altitude drones sometimes have issues with imaging quality, and their ability to capture a large area is based on stitching pictures together.

Doll is working on correlating normalized difference vegetation index, or NDVI, aerial imaging with plant water stress in perennial crops such as almonds, walnuts and pistachios. He said NDVI has proven effective with crops including corn and wheat, but it has been a struggle to correlate it with perennial crops.

"The onset of stress in perennial crops is not immediately observed in vegetation. NDVI can scan for stress in vegetation, but by the time almond trees show stress, yield and growth is already impacted. So we're trying to find a better way to get an earlier, more sensitive observation from the air, with different types of imaging indices," he explained.

Until now, thermal imaging has been done with planes; Doll and others are testing it with drones, which would be less expensive. But if a drone crashes, it could destroy the thermal imaging system, which costs upwards of $15,000, as opposed to NDVI cameras that cost about $500. So they're looking for low-cost versions and working with imaging companies to see what kind of cameras can be used to evaluate crop stress.

Aside from imaging, drones can also interact with soil sensors. Kriehn said measuring the health of almond trees is difficult when looking only at the canopy, but by embedding sensors in the ground that "wake up" and communicate with the drone when it's sent in, growers can correlate data from the images and the sensors' soil moisture data.

"Today, you can pinpoint some information at the tree level, but in the future we will have the type of control where we will get even more precise data," he said, adding that precision agriculture is moving toward "tree-by-tree" management, with individualized metrics of water and fertilizer needs.

Precision farming will help improve crop quality by making adjustments in real time, as opposed to wasting resources where they're not needed, Kriehn said.

Costs have come down significantly in the last five years, and Kriehn predicted affordability will continue to increase. He recalled acquiring a drone with six rotors, known as a hexacopter, for $10,000 three years ago, but this year he purchased an eight-rotor octacopter for $3,000. Given these costs, a large farm could have its own drone fleet, he said, and small farms could rent drones to capture data. Ultimately, he said, multiple drones could work together to collect crop data.

"There are fixed-wing aircraft that can capture overview images, which can be sent to rotary-wing, smaller drones like a hexacopter, which can go out and deal precisely with the problem, such as a drip system that's gone down or an infestation of pests," Kriehn said.

With their rising popularity and versatility, the need for knowledgeable drone operators is increasing, so Fresno State also obtained a NASA grant to introduce a drone and rocketry course at community colleges in the Central Valley.

Will drone use bring big gains that make it worth the investment in technology and research?

"We're trying to find a way that's easier and consistent for measurements, instead of sending people out into the field at different times," Doll pointed out. "You can do 20, 30, 40 measurements in a day with aerial imaging, and you can look at every tree in the field, instead of small portions of the field manually."

The other advantage with aerial imaging is that it can give growers a view of a whole orchard as well as individual trees. Aerial imaging has also advanced in its ability to process the images, so it's more practical for growers to make decisions.

Given the advantages, Doll said he expects more farmers to employ unmanned aerial systems.

"In three years, you'll see early innovators among farmers as they harness drone technology, and in five years, you'll see more adoption," he said. "In 10 to 15 years, almost every farmer will use drones. We're now at the beginning of this trend."

(Padma Nagappan is a reporter in San Diego. She may be contacted at padma.nagappan@gmail.com.)