Optimizing soil water sensor placement and irrigation thresholds for winter wheat: A data-driven approach to efficient water management

Soil moisture monitoring plays an important role in precision irrigation in modern agriculture. While wireless soil moisture sensors (SMSs) have revolutionized data collection for irrigation decision-making, critical knowledge gaps exist regarding optimal sensor placement strategies and dynamic threshold determination, particularly for deep-rooted crops like winter wheat (Triticum aestivum L.). This four-year experimental study (2018–2022) systematically investigated sensor placement optimization through multi-depth (10–200 cm) soil moisture monitoring under six water supply regimes. Subsequent validation trials (2022–2024) evaluated the proposed threshold values in balancing yield and water productivity. The results showed that shallow soil moisture (≤20 cm depth) exhibited significant variability and was not suitable to be used for irrigation decision. The threshold values for highest yield and highest water productivity were not the same, with the former being higher than the latter. The values in using relative soil water contents (soil water contents/field capacity, RSW) and fraction of available soil water (FASW) were 55 % and 32 %, respectively, at 30 cm for highest WP; and they were 60 % and 45 %, respectively, for highest yield. The threshold values were also depth-dependent. Down to 60 cm, the values were changing to 65 % and 50 % for highest WP, and 70 % and 60 % for highest yield, respectively. Soil moisture in deep soil layers (>90 cm) could indicate the crop water status after anthesis. Implementation of depth-specific thresholds reduced irrigation inputs by 18–23 % in wet seasons (p < 0.05) while maintaining yield stability and enhanced water productivity. Therefore, it was recommended that the depths of the sensor placement should be considered in deciding the threshold values for irrigation management for a deep-rooted crop. The threshold values for maximizing yield and water productivity should be separately decided.