Mapping variability of soil water content and flux across 1¿1000 m scales using the actively heated fiber optic method

The Actively Heated Fiber Optic (AHFO) method is shown to be capable of measuring soil water content several times per hour at 0.25 m spacing along cables of multiple kilometers in length. AHFO is based on distributed temperature sensing (DTS) observation of the heating and cooling of a buried fiber-optic cable resulting from an electrical impulse of energy delivered from the steel cable jacket. The results presented were collected from 750 m of cable buried in three 240 m colocated transects at 30, 60, and 90 cm depths in an agricultural field under center pivot irrigation. The calibration curve relating soil water content to the thermal response of the soil to a heat pulse of 10 W m−1 for 1 min duration was developed in the lab. This calibration was found applicable to the 30 and 60 cm depth cables, while the 90 cm depth cable illustrated the challenges presented by soil heterogeneity for this technique. This method was used to map with high resolution the variability of soil water content and fluxes induced by the nonuniformity of water application at the surface

Index to NASA Tech Briefs, 1975

This index contains abstracts and four indexes--subject, personal author, originating Center, and Tech Brief number--for 1975 Tech Briefs

A distributed heat pulse sensor network for thermo-hydraulic monitoring of the soil subsurface

Fibre optic distributed temperature sensing (DTS) is used increasingly for environmental monitoring and subsurface characterization. Combined with heating of metal elements embedded within the fibre optic cable, the temperature response of the soil provides valuable information from which soil parameters such as thermal conductivity and soil moisture can be derived at high spatial and temporal resolution, and over long distances. We present a novel active distributed temperature sensing (A-DTS) system and its application to characterize spatial and temporal dynamics in soil thermal conductivity along a recently forested hillslope in Central England, UK. Compared with conventional techniques (needle probe surveys), A-DTS provided values with a similar spread although lower on average. The larger number of measurement points that A-DTS provides at higher spatial and temporal resolutions and the ability to repeat surveys under different meteorological or hydrological conditions allow for a more detailed examination of the spatial and temporal variability of thermal conductivities at the study site. Although system deployment time and costs are higher than with needle probes, A-DTS can be extremely appealing for applications requiring long-term monitoring, at high temporal repeatability, over long (kilometres) distances and with minimum soil disturbance, compared with one-off spatial surveys