Abstract: Actively heated fiber-optic (AHFO) sensing tubes for measuring soil water content provide exceptional spatial resolution and continuous spatio-temporal measurements, thus gaining significant traction in vadose-zone hydrology. The layout method of AHFO sensing tubes often determines their coupling with the soil, which affects the accuracy of water content measurements. To date, a systematic evaluation of layout methods has been notably absent. This study focuses on a typical AHFO sensing tube, utilizing thermal response tests and rainfall infiltration tests to evaluate soil water content measurements under three vertical layout methods (direct insertion, backfilling, and penetration) and one horizontal layout (backfilling) method. Results indicate that the vertical direct insertion method can create interfacial voids between the sensing tube and the soil, facilitating preferential flow along the tube-soil interface. The horizontal backfilling method impedes water migration due to hydraulic obstruction caused by the tube. In contrast, the vertical backfilling and penetration methods effectively capture piston-flow infiltration phenomena during rainfall events and are regarded as the optimal layout methods for AHFO sensing tubes. It is recommended to select the appropriate layout method based on monitoring objectives and site conditions in practical applications.

Key words: soil water content, actively heated fiber optics, sensing tube, layout method, rainfall infiltration