Actively heated fiber optics method to monitor three-dimensional wetting patterns under drip irrigation

Monitoring dynamics in wetting patterns under drip irrigation is important to optimize the frequency and duration of irrigation, the emitter discharge rate and the spacing. However, difficulties in measurement of soil water content (SWC) at high spatial and temporal resolution and in three-dimensions (3D) restricts direct monitoring around drip emitters. Indirect methods such as actively heated fiber optics (AHFO) has shown the potential to measure SWC at sub-meter intervals. Therefore, the objective of this study was to examine the feasibility of the AHFO method to monitor 3D spatial and temporal variations in wetting patterns under a single drip emitter. Heat pulses of three minutes duration were applied at a rate of 20 Wm−1 through a helically wrapped fiber optic cable in a soil column before, during, and after irrigation. A distributed temperature sensing (DTS) instrument was used to estimate the cumulative temperature increase (Tcum) at locations along the cable. An indirect relationship between Tcum and SWC was developed and validated using the SWC measurements determined by the gravimetric method. Data from the network of 3D points (from fiber optic cable) were used to generate 3D surfaces of SWC. In comparison with the gravimetric method, AHFO showed predictive accuracies; root mean squared error (RMSE) of 0.03 m3 m−3 for SWC <0.05 m3 m−3 (N = 17), 0.03 m3 m−3 for SWC 0.05-0.3 m3 m−3 (N = 19) and, 0.05 m3 m−3 for SWC >0.3 m3 m−3 (N = 6). The time evolution of the 3D SWC helped to identify wetting bulb formation, movement of the wetting front and changes in the dimensions (wetted radius and depth) of wetting bulbs. This study showed not only the potential of AHFO to help design drip emitters but also the ability to provide high resolution SWC information to improve water movement models in the future.