Using distributed temperature sensing to monitor energy and moisture fluxes in the shallow subsurface

In Distributed Temperature Sensing (DTS), inexpensive telecommunications fibre-optic cable is used as a thermal sensor. Its capacity to produce accurate (<0.1K) measurements of temperature at high resolution (1-2m) over long cables (up to 10km in length) has made DTS an increasingly popular tool in environmental monitoring. It has been used in a wide variety of applications including monitoring lake temperatures, estimating seepage in polders and measuring flow into streams. Recently, techniques have been developed to measure soil moisture by ploughing one to four of these cables into the soil and monitoring temperature dynamics. Temperature changes can be due to the diurnal radiation cycle (‘passive SoilDTS’) or due to a heat pulse transmitted from the metal housing of one of the cables (‘active SoilDTS’). Soil moisture influences heat transport in the soil through its impact on soil thermal properties, and so changes in temperature can be related to changes in soil moisture content. SoilDTS offers an innovative way to characterize and monitor the water and energy balances in the shallow subsurface at meter to kilometre scale. Applications range from large scale soil moisture monitoring for satellite calibration and validation to small scale applications in urban areas.