Written by Matthew Cann, Karisma Yumnam, Lav Khot, and Lee Kalcsits. December 2022.
Find out how much weather conditions in your apple orchard differ from nearby open-field weather stations and what that means for orchard management.
The differences between weather conditions inside an apple orchard and open-field conditions are being studied by a team of WSU scientists. Efforts are focused on quantifying and modeling daily and seasonal variations of weather conditions inside orchards to better estimate and predict orchard microclimates. Open-field and in-field weather stations installed at an experimental site are shown in Figure 1. The modeled orchard effects are crucial for reliable, site-specific crop growing degree day (GDD) calculations, as well as for effective frost/heat stress mitigation, pest/disease management, and more.
The air temperature in an orchard is approximately 0.4-2.2 °F colder than in an open field next to an orchard, where weather measurements are normally made (Figure 2). The air temperature is 2.2 °F cooler with a Solaxe-type orchard training system and around 0.5 °F cooler for Bi-axis and 0.4°F cooler for V-Trellis systems which are simpler canopies with less volume than the solaxe-type. Temperature differences also varied by season and by time of day. The air temperature differences are largest during the growing season, especially summer and early fall when canopies are fully developed. Typically, foliage acts as a wind barrier and can limit air mixing with air out of the orchard. Overall, orchard blocks are much cooler than the adjacent field at nighttime and during dawn and dusk, averaging up to 5.5 °F colder in summer for the Solaxe system. During solar noon, the orchard blocks are warmer on average by up to 3 °F for V-trellis system, while the Solaxe system remains cooler on average during summer. However, an individual block can be substantially cooler when overhead sprinklers are turned on to mitigate midday heat stress.
Relative humidity (RH) inside of an orchard block is very similar to open field when irrigation or evaporative cooling systems are not running (Figure 3). The training system can have a large effect on RH. For example, the Solaxe training system block had 11% higher RH than the open field measurements; compared to 2% higher for Bi axis and 1% lower for V-trellis. Primarily, colder air trapped by the Solaxe training system is attributed to this difference. The Solaxe training system appears to trap more moisture as well. This orchard training system had higher RH in the blocks compared to the adjacent field in all seasons and is the most pronounced in summer and early fall when foliage is full. The V-trellis orchard tends to be drier in all seasons except for summer. The relative humidity tends to be higher in the orchard at nighttime and dawn/dusk compared to during the daytime when warm, dry air can be trapped in the orchard, however this varies by season and training system.
The average wind speed in the orchard is 2.5 mph lower year-round compared to the open field weather station for the Bi axis and V-trellis systems and 3 mph lower for the Solaxe system (Figure 4). Tree branches and foliage can act as a wind barrier that shelters sonic sensors inside the orchard. The differences peak in the afternoon when winds are usually the strongest, suggesting that it is relative to the strength of the wind itself. The Solaxe type orchard system creates the largest wind break of the three studied systems. The stronger wind break is why we observe larger differences in air temperature and relative humidity, as more air and moisture is trapped by the Solaxe system and result in a stronger microclimate effect.
These results are indicative of dry orchards without irrigation system operating and show colder air temperatures, similar relative humidity, and lower wind speed compared to adjacent open field weather conditions. The cause of the differences is mainly a result of wind sheltering. The orchard effect on air temperature is very important for accurately calculating GDD and in managing cold/heat stress. GDD is likely to be lower with data input from an in-orchard station compared to an open field station. Overnight low temperatures are colder and daytime high temperatures are warmer in the orchard, meaning physiological stress occurs to a greater degree than suggested by open-field weather stations. The Solaxe type orchard training system has the largest wind sheltering and resulting orchard effect on microclimate compared to V-trellis and bi-axis systems. AgWeatherNet is currently developing models to adjust nearby weather stations data to account for the orchard effects and ultimately improve the site-specificity of decision support tools available to growers.
This report is a snapshot of on-going orchards effect project funded by Washington Tree Fruit Research Commission to our team including original PIs, Dr. David Brown and Dr. Joe Zagrodnik. Project was also supported in-part by Meter Group, Pullman WA.
AgWeatherNet Research Associate
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