How We Grow

2022 Spring How We Grow

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WATER USE between several different types of sensors to determine if any could be a reliable substitute for the pressure chamber. The three direct sensors that were evaluated were micro-tensiometers, miniature stem water potential sensors (osmometers) and psychrometers. Each of these sensors are commercially available and designed to directly measure the water potential of the plant to determine tree water stress, similar to a pressure chamber. "There is one sensor, the micro-tensiometer sensor, that is now giving us data that is more or less equivalent to the pressure chamber," said Shackel. He's referring to the sensor made by Davis-based FloraPulse. Embedded in the tree, this sensor measures the water tension in the water- carrying tissue (sapwood). Ultimately, the sensor provides daily stem water potential readings that can be utilized for irrigation decisions. Both Shackel and Kisekka found the micro-tensiometer to be in very close overall agreement with the pressure chamber. One thing to be aware of is that sensors like micro-tensiometers will require drilling into the tree itself, so wounding responses such as gumming are a potential risk. "Luckily in almonds and other Prunus species we've figured out a way around the gumming that comes from under the bark, but wounds 'weep' from the wood in walnuts and some other trees, and that is keeping us from using this sensor in those crops," said Shackel. The miniature stem water potential sensors have been less consistent. Shackel and Kisekka have been evaluating the sensor made by Saturas, an Israeli company which established a presence in California in 2017. Kisekka said the sensors seemed to perform well in 2019, but some changes made to the product and installation process caused the 2020 results to be less accurate. Shackel observed that temperature sensitivity was a real issue with these miniature stem water potential sensors this year, which is a concern. Temperature sensitivity was also an issue with the psychrometer. Shackel evaluated ICT International's temperature- compensated thermocouple stem psychrometer, which measures the humidity in a small cavity of vapor that is sealed to the sapwood. The high level of temperature instability is a challenge under field conditions. "It's difficult for researchers even to deal with this particular approach," said Shackel. "It's a sound approach to measure water potential, but most researchers eventually give up on it because field temperature is too variable." Based on this research, the most encouraging results for a substitute for pressure chamber measurements is the micro-tensiometer. According to both Shackel and Kisekka, it is the only option of these direct measurements that was consistently aligned with pressure chamber tests. What's the deal with dendrometers? Dendrometers are sensors that indirectly measure tree water stress by measuring changes in the trunk diameter throughout the day. The technology then uses algorithms to convert measurements into an indication of tree stress. In recent years, some almond growers have adopted this technology and have been happy with the results. 1 Shackel has evaluated dendrometers along with the other sensors for the past two years. Overall, what he has observed is that the dendrometers have been very sensitive to tree stress in the early stages, but then not very sensitive after that when compared to the pressure chamber. "The dendrometers seem to show good recovery when the irrigation hits," Shackel said. "But then it sort of falls off a cliff after a few days rather than tracking true tree stress over time." This is a problem when a grower wants to tolerate an intermediate range of tree stress over time, such as when they are approaching harvest. Dendrometers use proprietary algorithms, so Shackel admits he cannot say whether this is a fundamental technical problem with the technology or if it can be remedied with revised algorithms in the future. Location, location, location While the advantage of all these sensors is continuous tree water stress data without manually collecting and measuring samples, that also presents a challenge: selecting the right trees to install the devices. Determining which trees will represent an average for that block or zone is imperative. 1 How We Grow "Water Technology Spotlight: A Grower's Perspective on Dendrometers" how-we-grow/1? Attention Growers! Like all management decisions in the field, growers need to evaluate their individual needs and remember to look at all the tools available in the toolbox. More irrigation resources: An almond tree in a lysimeter for accurate measurement of water use, installed with commercial water stress sensors. Continued from page 12 13

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