Can AH-DTS detect wetting front movement in soil columns during rainfall events? First impressions from experimental investigations

Actively heated distributed temperature sensing (AH-DTS) has been widely applied for soil moisture monitoring over distances ranging from a few centimeters to several hundred meters; however, few studies have explored this method for investigating transient soil water behavior during rainfall events.

Here, a soil column (75 cm high and 30 cm in diameter) was constructed in the laboratory, where an active fiber-optic cable was helically wrapped around a central supporting element with a 5-cm bending radius. Repacked sandy clay loam soil filled the column with three compaction levels: loose (1.35 g cm^-3, 0–25 cm), medium (1.44 g cm^-3, 25–50 cm), and dense (1.51 g cm^-3, 50–75 cm). Sprinkler nozzles simulated a rainfall event of 40 mm hr^-1 lasting 4 hours in the soil profile, during which the fiber-optic cable was continuously heated with a power input of 5 W m^-1. A DTS unit collected temperature data at a vertical sampling resolution of 1.25 cm, while 14 soil moisture sensors regularly distributed throughout the column measured changes in soil water content.

The results showed that wetting front arrival at different soil depths was detected by the fiber-optic as cooling pulses. The magnitude and temporal stability of the cooling were inversely related to soil depth and bulk density. From the moment the front was detected, the superficial soil layer exhibited more pronounced and longer-lasting negative thermal anomalies, whereas anomalies in the deepest layer were smaller in magnitude and less persistent. These findings suggest the dominance of thermal advection in the loose soil layer and thermal conduction in the dense layer, while the medium-density layer exhibited transitional behavior. With respect to instrumentation, good agreement was observed between time of detection of the wetting front arrival obtained from the moisture sensors and the optical fiber (root mean square error of 6.2 minutes).

Overall, the results contributed to the understanding of thermal regimes in unsaturated flow and further shed light on the use of temperature as a tracer for soil water infiltration and percolation processes. Ongoing research aims to investigate soil thermal behavior with AH-DTS across a broader range of rainfall intensities and contrasting soil textures.