Assessment of the active method to determine soil moisture

Abstract

In recent years, fiber-optic distributed temperature sensing (FO-DTS) methods have been successfully used to investigate a wide range of hydrological applications. In particular, two methods have been developed to monitor the soil water content (θ) with the FO-DTS technology: the passive and the active methods. This work presents an assessment of the active method to determine the θ of a sandy soil. In this method, fiber-optic cables with metallic armoring are used and a voltage difference is applied between the two ends of the cable to warm it during a specified time period. Then, an empirical relationship is used to relate θ with a parameter called cumulative temperature (Tcum ). To apply the active method, we propose a potential relationship defined by stretches, which depends on the hydrodynamic properties of the soil studied. Different experiments were carried out to assess the active method. These experiments had different heat pulse durations (2, 5, 10 and 20 min with electrical powers of 2.1, 2.6, 2.3 and 2.4 W/m, respectively), and allowed determining the optimum heat pulse duration (tf ), the optimum temporal integration interval (Δt), the optimum final time of integration (t0 ) used in the calculation of the cumulative temperature, and the optimum current (I) that should circulate through the fiber-optic cable to generate the heat pulse. Results show that the optimum operating parameters are: tf = 1200 s Δt = 150 s, t0 = tf, and I ≈ 17 A (2.43 W/m). Our analysis allowed obtaining volumetric water contents ranging from 0.14 to 0.46 m3/m3, with errors that are smaller than 0.08 m3/m3.

Keywords

• mediciones distribuidas de temperatura con cables de fibra óptica (FO-DTS)
• método activo
• contenido de humedad volumétrico