Monitoring infiltration of capillary barrier with actively heated fibre Bragg gratings

The capillary barrier effect of soil is the delayed water flow process inside soil pores due to intermolecular forces between liquid and solid particles. In this study, the capability of the fibre-optic sensing technology to quantify water infiltration in soil with a capillary barrier system is explored by measuring the spatio-temporal distributions of water contents. An actively heated fibre Bragg grating (AH-FBG) sensing tube was used to monitor rainfall infiltration in a one-dimensional soil column test. Frequency domain reflectometry (FDR) probes were used to perform in situ calibration of the AH-FBG sensors. The fibre-optic monitoring results indicate that the AH-FBG technology enables high-sensitivity detection of the capillary barrier effect. The empirical relationships between temperature changes and soil water contents can be well fitted by exponential or linear functions. The capillary barrier system has shown an apparent capability to store water after rainfall, which exerted a significant effect on the vertical infiltration process. Compared with the features of FDR and image acquisition, the proposed method can effectively reflect the whole process of wetting front movement and is more suitable for field monitoring due to its flexibility in large-scale and automatic measurement.