Automated sensors for irrigation scheduling are still being assessed in a range of conditions for California almonds and other tree nuts.

The types of sensors include a large area cosmic-ray neutron probe for measuring soil moisture over an entire block and automated osmometers that measure stem water potential (SWP) and can overcome challenges some growers may face with data collection for irrigation scheduling.

The automated osmometer stem water potential sensor, according to Isaya Kisekka, an associate professor of agricultural water management and irrigation engineering at UC Davis, has a small sensor that is put into the tree trunk. The sensor measures pressure changes caused due to osmosis (related to solute concentrations) of the plant fluid. The sensor has a membrane that allows water movement between the tree and the sensor. The change in pressure measured by the sensor can be interpreted in terms of stem water potential. Other types of stem water potential sensors use micro-tensiometers instead of osmotic membrane technology.

Kisekka discussed research updates and data for the automated osmometers during a recent UC ANR Water Webinar.

“Overall, once you calibrate the [osmometer] sensors, they perform quite well in terms of giving you a meaningful stem water potential number in some nut trees,” he said.

The osmometer data was collected from almond trees alongside data from a pressure bomb, or pressure chamber, and graphed to find a possible correlation. Kisekka and other researchers found that the osmometers gave similar data to a pressure bomb without the required labor for sampling, giving growers a way to assess irrigation needs while saving on labor costs.

Another device capable of providing irrigation feedback is a cosmic-ray neutron probe. The probe, which measures soil moisture over large areas, is commercially available for other applications, but Kisekka believes that it can have value in agriculture for soil water monitoring.

“It has a passive neutron probe that isn’t regulated, so we think it can be more easily adopted,” he said. “It can give you an average soil moisture reading over a larger area, about 15 acres.”

Regular neutron probes for measuring soil moisture cover much smaller areas, so Kisekka’s research looked to compare regular probes against the cosmic-ray probe. To test this, regular probes were installed at different depths (20 cm, 45 cm and 75 cm) near a cosmic-ray probe to calibrate it and determine reading accuracy. Kisekka found that different depth measurements from the regular neutron probe correlated well with the cosmic-ray probe (Fig. 1).

“If you break it down and look at the data by depth, you actually see much better [data] agreement at 45 cm and 75 cm, which is really the depth where the most active root water uptake occurs in nut trees,” Kisekka said.