Assessment of the active method to determine soil moisture

[EN] 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 (∆t), 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 ∆t = 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[ES] En los últimos años las mediciones distribuidas de temperaturas con cables de fibra óptica (FO-DTS) se han utilizado con éxito para investigar una amplia gama de procesos hidrológicos. En particular, con la tecnología FO-DTS se han desarrollado dos métodos para monitorear el contenido de humedad volumétrico en suelos (θ): el método pasivo y el método activo. Este trabajo presenta una evaluación del método activo para determinar el θ de un suelo arenoso. En este método, se utilizan cables de fibra óptica con elementos metálicos, donde se aplica una diferencia de voltaje entre sus extremos para calentarlo durante un período de tiempo determinado. Luego, se utiliza una relación empírica para relacionar el θ con un parámetro denominado temperatura acumulada (Tcum). Para aplicar el método activo se propuso una relación potencial definida por tramos, la cual depende de las propiedades hidrodinámicas del suelo estudiado. Distintos experimentos fueron realizados para evaluar el método activo. Estos experimentos tuvieron distintas duraciones del pulso de calor (2, 5, 10 y 20 min con potencias eléctricas de 2.1, 2.6, 2.3 y 2.4 W/m, respectivamente), y permitieron determinar la duración óptima del pulso de calor (tf), el tiempo de integración óptimo (∆t), el tiempo final óptimo de integración (t0) utilizado en el cálculo de la temperatura acumulada, y la corriente óptima (I) que debe circular por el cable de fibra óptica para generar el pulso de calor. Los resultados revelan que los parámetros óptimos son: tf = 1200 s, ∆t = 150 s, t0 = tf, e I ≈ 17 A (2.4 W/m). Este análisis permitió obtener contenidos de humedad que van desde 0.14 hasta 0.46 m3/m3, con errores menores que 0.08 m3/m3.Este trabajo fue auspiciado por el Proyecto FONDECYT N°1170850 (CONICYT), y por el Programa Nacional de Becas y Crédito Educativo del Perú (PRONABEC). Los autores agradecen a C. Sayde y a S. Tyler por sus sugerencias al inicio del trabajo de investigación. F. 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