By Vicky Boyd

Yield monitors, which have become an integral part of the precision agriculture equation for field crop producers, have yet to be commercialized for permanent crops.

Although aerial imagery can’t replace the on-the-fly, acre-by-acre yield data collected by yield monitors, it can provide growers of trees and vines with a fairly good overview of plant health. And plant health may correlate loosely to yields, says Tim Stone, who directs precision agriculture for Britz Simplot Grower Solutions in Five Points, Calif.

When coupled with soil sampling data, aerial imagery also can be used to develop variable-rate prescriptive fertilizer maps that allow applicators to apply more nutrients where needed and less where they’re not.

“We’ve seen a big increase in permanent crop guys doing more precision ag than the row crop guys, but then again, we’ve turned into a tree area, too,” Stone says of the Westside’s evolution from mainly row crops to large expanses of permanent crops.

Aerial imaging for precision agriculture isn’t new and has been used fairly widely by larger-scale growers of corn, soybeans, wheat and cotton, says Michael Whiting, a research scientist at the University of California, Davis.

Some contractors’ planes fly the fields one day to provide images for growers to download from their websites the next day. Other companies provide interpreted satellite imagery.

Growers or applicators can then load these maps into a tractor computer for precision application of nutrients or pesticide applications.

Patrick Brown, a UC Davis pomology professor, leads a 2-year-old program to develop crop models for nutrient and water demand in permanent crops. The project is funded by a U.S. Department of Agriculture Specialty Crop Research Initiative grant.

Working with Brown, Whiting leads a group of UC Davis researchers developing a model that will predict the current year’s yields and nutrient demands based on the previous year’s winter and spring temperatures and moisture, and satellite and aircraft imagery.

Their project combines data from several collaborating federal and state agencies, recent and archive satellite data and aircraft research imagery, and the California Irrigation Management Information System.

Brown says it’s still too early in the research to make recommendations on the exact time of year to have permanent crops flown or to release crop models.

“I think it has a lot of potential for helping with estimating yields and estimating nutrient and water demands, but we’re not there yet,” he says.

Two types of craft provide imagery

Stone works with Falcon Scan, a Glen Burnie, Md.-based company that flies fields, orchards and vineyards in small airplanes and shoots digital images. The resolution ranges from 10 to 30 centimeters. At 30 cm, each individual tree or vine can be identified.

Aircrafts typically provide turn-around times of 24 to 48 hours. Even with the time it takes for Stone to further process the data, growers can have the image within 96 hours of when it was shot.

The minimum acreage to justify an aircraft flight is 500, but that doesn’t have to all be from one grower, he says.

“How hard is it to get 500 acres? From the Grapevine [north of Los Angeles] to Merced, it isn’t very hard.”

Using an aircraft service provides a snapshot at a specific point in time, Stone says. Many growers, for example, want an idea of how their trees are doing early in the season so they have time to correct any problems.

Grape growers, in particular, want an aerial overview of the vineyards after veraision and before harvest so they now what blocks may be closer to maturity than others.

In addition, winegrape growers may use the images to plan where they send fruit, with better quality grapes going into higher-end programs.

Another image source is satellites, such as LANSAT, that orbit the earth and shoot digital images on a regular schedule. The resolution ranges from commercial high-resolution of 1.8 feet to the U.S. Geologic Survey’s free download of 45 feet. Both scales will allow you to see trends in a large field, although only the high-resolution format will allow you to identify individual trees or vines.

Typically these satellites cover the same area every two weeks and provide imagery in two to three weeks after acquisition.

“You can still make good [fertilizer] application maps off of them, but mostly you’re tracking changes with them,” Stone says of satellite imagery. “You’re not going to use it for a 20- to 30-acre field due to the image resolution. You’re going to use it for a 40-acre block or larger and quarter sections [160 acres] work real well.

“Some guys buy both. They track changes, then buy high-resolution images to do something with after harvest.”

How imaging works

Regardless of the craft, special cameras take images of the various light wavelengths reflected by vegetation.

Using special computer software, the data can be processed several ways. One of the more popular end-products is a map that shows the NDVI, or normalized difference vegetation index. Simply, NDVI measures plant vigor caused by chlorophyll production or plant greenness.

It is calculated by measuring the amount of visible and near-IR reflected by plants. Healthy plants absorb most of the visible light that hits them and reflects a good portion of the near-IR light. Low-vigor plants, on the other hand, reflect more visible light and less near-IR light.

Vigorous plants show up as red or dark red in an NDVI map, whereas weak plants are dark blue or dark green.

But the image doesn’t provide the reason why plants are doing well or poorly in an area, although it may offer clues, Stone says.

Rather, it must be ground-truthed.

One of the main benefits of the imagery is it gives growers and consultants starting points, he says. It also shows vigor differences that may not be visible from simply scouting the orchard or vineyard from the ground.

Problems show up from the air

Jean Errotabere, a general partner in the Riverdale, Calif.-based Errotabere Ranches, says the operation has been subscribing to aerial imagery for the past six or seven years.

Although they initially had both processing tomato fields and almond orchards flown, he says they saw the most benefit in the almonds. So they discontinued its use in the processing tomatoes.

The operation’s approximately 600 acres of almonds are flown three times annually: May, June and July.

They start harvesting in August, and the late spring-early summer flights allow them to correct possible problems early in the season. “It will indicate if there are any stressed areas, and if we do get them, it’s usually having to do with the drip irrigation,” Errotabere says. “You get patterns you don’t physically see from the ground, but they show up by the air. So we can go right to them and fix them.”

He subscribes to the high-resolution service and says he’s amazed at the detail.

“You can pinpoint it down to the row,” Errotabere says. “It really shows up if there’s an issue with irrigation. If you’re on the ground and you’re in the orchard and looking up through the trees, you really can’t tell.”

The imaging also saves the time of the staff agronomist, since he can use the maps to lead him to the stressed areas. He still has to figure out what’s causing the stress.