Estimating in situ integrated soil moisture content using fiber-optic distributed temperature sensing (DTS) measurements in the field

The measurement of soil water content in the field is complicated by soil heterogeneity, as well as issues of temporal and spatial scales. A particular gap exists in our ability to observe water contents at the field scale, ranging from 0.1 to 80 km2. This work explores the feasibility of using fiber-optic distributed temperature sensing (DTS) observations of the diel temperature cycle to estimate the volumetric water content of the soil profile. Relationships between the thermal properties of bulk soil and the water content of the soil are described, and analytical and numerical models of conductive heat transport are presented. Model results are compared to data collected using a fiber-optic DTS to measure temperature within the soil profile at Valley Vista Ranch, an agricultural site in northern Nevada. Observed temporal phase shifts and diel amplitudes at a depth of 15 cm in the soil profile are compared to the phase shifts and amplitudes predicted by the numerical simulations for the same surface temperature wave. While phase shifts and amplitudes measured in the field return repeatable estimates of the soil thermal properties, these estimates could not be correlated with the volumetric water content of the soil. Potential causes for this discrepancy, including advective heat fluxes, biased surface temperature measurements, and model deficiencies are explored, and recommendations for improvements to future experiments are presented.