Determination of soil moisture sensor locations for dynamic variable rate irrigation management with a centre pivot system

Abstract A network of soil moisture sensors is an effective tool for dynamic variable rate irrigation (VRI) management, which helps to determine the irrigation time in real time, thus solving the difficulty in dynamic monitoring of soil water content for remote sensing technology. To improve the decision accuracy of VRI management, this study optimized the placement of soil moisture sensors by analysing the variation in soil water content along the vertical profile and its temporal stability in the horizontal direction. The experimental site was located in the semiarid region of the North China Plain. The 7.07 ha experimental plot was controlled by a three-span centre pivot irrigation system with a zone controlled VRI system. Two treatments of uniform rate irrigation (URI) management and dynamic VRI management based on infrared thermometers mounted on the sprinkler irrigation system were set with equal areas. The number of Trime access-tubes placed in each treatment was 27 in 2021 and 13 in 2022. Measurements of soil water content were taken in 0.2 m increments with a depth of 1 m every 7–10 days. The results demonstrated that the similarity in the spatial distribution pattern of the soil water content in the VRI treatment was greater than that in the URI treatment, and their difference reached a significant level in the 2022 season of winter wheat. Significant linear regressions were detected between the mean clay percentile in each soil layer and the clay percentile representing the mean soil water content sites with a determination coefficient greater than 0.85. In the vertical profile, both the average soil water content at depths of 0–60 cm and 0–80 cm could be represented well by the soil water content at the 20–40 cm layer. These results demonstrated that during dynamic VRI management, soil moisture sensors could be placed at the 20–40 cm soil layer, and their locations in the horizontal direction could be selected according to the clay content distribution.