Potential Uncertainties in Measuring Soil Moisture with Active Fiber Optic Distributed Temperature Sensing

Abstract:

Active Fiber Optic Distributed Temperature Sensing (FO-DTS) provides soil moisture measurements along a fiber optic cable with unprecedented spatial resolution (<25 cm) over 100s to 1000s of meters, allowing for new and better understanding of soil moisture distribution in hydrological process studies and in applied settings. The Active FO-DTS method works by heating the FO-DTS cable and recording the responding increase in temperature. The time-integrated rise in temperature is in part a function of the soil moisture in the region immediately surrounding the cable. The Active FO-DTS methodology has numerous assumptions, such as homogeneous porosity and static, homogeneous distribution of soil moisture. Given spatial and temporal heterogeneity and variable soil moisture, and high degree of heterogeneity in soils, there are potential errors in the calculation of soil moisture.

We use a numerical groundwater model, SUTRA, to simulate different setting and parameters to understand and quantify potential errors in Active FO-DTS measurements. The SUTRA model couples groundwater flow, soil moisture (via the Richards Equation) and energy transport. The model, with dimensions of 20 by 20 cm and comprised of 1600 elements, simulates an idealized fiber optic cable under heated and cooled conditions. The model, calibrated to a previously published laboratory experiment, is perturbed with different spatial and temporal heterogeneities to exam the resulting impact on calculated soil moisture.

We present preliminary results demonstrating the effect of heterogeneity of soil, the infiltration rate of water, and repeated protocol cycling on the calculation errors of Active FO-DTS. The results demonstrate the potential errors and challenges in measuring soil moisture with this emerging methodology.