Assessment of spatial and temporal variability in soil moisture using multi-length TDR probes to calibrate Aquaflex sensors

Abstract

Despite subtle variations in soil moisture (SM) across a paddock, irrigation scheduling in New Zealand dairy farms is solely based on the SM monitored at a single location, primarily using an Aquaflex soil moisture sensor at a specified root depth. This study aimed to address this issue by assessing the "effective" root depth of a pasture, calibrating the Aquaflex soil moisture sensor and evaluating the spatial and temporal variability of SM. Twenty non-weighing lysimeters and 1 Aquaflex with 2 sensors installed 125-m away from the lysimeters on the same paddock were utilized for the study. TDR probes with 200-, 500- and 900-mm lengths were installed vertically adjacent to the Aquaflex and the lysimeters for monitoring spatio-temporal variability in SM, and calibrating the Aquaflex. A dry down experiment was performed for investigating the root depth of the pasture. All TDR probes responded to wetting and drying events, with varying SM measurements both vertically and horizontally, due to variations in soil type at different locations, indicating a need of SM monitoring at different locations in the paddock for irrigation scheduling. There was a strong linear relationship between the Aquaflex and TDR probes readings, which can be used to calibrate the Aquflex and improve its reliability for measuring soil moisture and in turn irrigation needs. Over the dry down period, out of the total moisture change in the 0-900-mm soil profile, 96% was contributed by 0-500 mm, indicating that the significant root depth of the pasture lies on the top 500-mm soil profile. Findings of the study can contribute to better irrigation scheduling and to conserve water.