How much water do crops need, and when do they need it, in order to optimize a grower’s yield and/or the quality of the produce? Those are questions that Shrinivasa Upadhyaya, a professor of biological and agricultural engineering at the University of California, Davis, was asking well before the four-year-old severe drought hit California’s central valley. Now, with water resources in that area stretched extremely thin, those questions are even more urgent.
LeafMon, the third generation of sensor-based technology developed in Upadhyaya’s lab, can provide the answers, he says. Winemaker E&J Gallo is using LeafMon units at one of its Northern California vineyards. The technology calculates how hard an individual plant is working to transpire, and then uses that data as an indication of whether it (and, by extension, the entire crop) is under stress—and, therefore, needs more water.
Upadhyaya and his team, having secured grant funding from the U.S. Department of Agriculture’s National Institute of Food and Agriculture, began investigating ways in which growers could utilize technology to reduce their water output around 2010. The team’s approach was to make a system that could replace a pressure chamber, which is the conventional means of measuring a plant’s water stress. With a pressure chamber, a grower places a leaf stem into an air-tight chamber and measures the amount of pressure required to pull water from the inside of the stem to the cut surface.
Plants pull moisture from the root system to the leaves, where the moisture is pulled out into the air via transpiration. As soil dries, the leaves are still transpiring, so the vascular tissue in the roots that moves water up to the leaves comes under greater physical pressure. The higher the amount of pressure (measured in bars) required to pull moisture out of a stem in a pressure chamber, the greater water stress the plant is experiencing.
The problem with using pressure chambers, Upadhyaya says, is that they require growers to go out into the field and conduct the painstaking test manually. This, he notes, is very time- and labor-intensive.
Instead, the LeafMon device uses a number of different sensors—which are integrated into a large clamp that holds a leaf in a flat mesh and is mounted to the trunk of a grape vine or tree—to determine the amount of water stress a plant is under.
“If you have a plant and you water it well,” Upadhyaya explains, “then during daylight hours, the stomata on the leaves open to bring in carbon dioxide from the air and photosynthesis happens.”
When the stomata (tiny pores on the leaf’s surface) are open, water vapor escapes through transpiration. This cools the leaf down, relative to the ambient temperature.
Under hot and dry conditions, the stomata tend to remain closed, in order to conserve moisture inside the plant. So by measuring whether a leaf is transpiring (based on the temperature differential between the leaf and the ambient temperature), LeafMon can determine whether it is under water stress. To obtain accurate readings, five different sensors are used. These take measurements of the leaf and ambient air temperatures, as well as light, wind speed and relative humidity. The three vital variables are the leaf temperature, air temperature and relative humidity. But sunlight can affect the temperature readings, and a strong breeze will boost the leaf’s transpiration rate, so light and wind are also measured in order to correct for their influence on the leaf’s temperature.
In the latest iteration of the LeafMon device, a sunlight-diffusing dome is mounted over the leaf being analyzed. However, the light and wind sensors are still used.
Tests of the sensors showed good results on grape, walnut and almond crops in California, but the system lacked a major component: wireless access. To collect the data, growers would have had to go to the LeafMon device in the field and collect the calculations—the crop water stress index, which is then used to decide whether the crop needs more water—from the microcontroller attached to the sensor. UC Davis needed a commercial partner to license the sensor technology and build it into a network that would transmit the sensor data to a cloud-based server that growers could then access on their computers or phones.
Cermetek Microelectronics, a Silicon Valley manufacturer of embedded wireless radios and modems, has secured that license and is selling the LeafMon device packaged with cellular modems and connectivity that transmits the data directly to Cermetek’s cloud-based software, known as RSVP (Remote Sensor Viewing Platform). RSVP displays the crop water stress index data for the growers, who can access the data on their computers or phones.
To date, Gallo is the only commercial customer for the LeafMon system, according to Frank Stempski, Cermetek’s sales manager. Gallo is using 12 LeafMon units, each wired to a cellular modem that sends data directly to RSVP. Stempski says demand is growing, and that he is fielding calls from growers as far afield as Chile and Greece.
A grower pays a combination of a $240 subscription fee for access to RSVP and a $45 monthly fee per cellular modem. The user will also pay a cellular subscription fee of anywhere from $8 to $20 per month, depending on the amount of data sent. Customers may set up and manage their cellular subscriptions, which would negate the cellular subscription payments to Cermetek, and would cover the per-cellular modem fee to $30 (the $45 includes $15 to handle bill processing). Using a LeafMon system, Stempski says, costs a grower roughly half of the total financial output needed to use a pressure chamber.
Stempski and Upadhyaya worked with Gallo to deploy 12 LeafMon devices in Gallo’s vineyard in Galt, Calif. The winemaker is using LeafMon as part of a larger effort to leverage technology to improve crops while reducing water use. It also employs a combination of satellite-based imagery, weather station data and soil sensors to set irrigation schedules based on the rate of water lost through evapotranspiration. LeafMon enables Gallo to compare the water stress index data to the satellite-based irrigation scheduling systems it uses to get a more complete picture of what its vines need in terms of irrigation.
Vintners are not necessarily concerned with increasing yield, Upadhyaya notes, as much as ensuring that grapes produce certain qualities needed for winemaking. In some cases, vintners might want a certain amount of water stress, to elicit specific grape characteristics.
Soil-moisture sensors do not show as complete a picture of plant health as the LeafMon can, Upadhyaya says, because the amount of moisture in the soil does not directly correlate with the plant’s health. Plus, he adds, if a grower wants to keep the plants under a certain amount of water stress, LeafMon produces more detailed guidance than soil-moisture levels.