CHASING DISTRIBUTED SOIL MOISTURE: ADVENTURES WITH DISTRIBUTED TEMPERATURE SENSING (DTS)

One hydrologic component of global circulation models that remains difficult to quantify is soil moisture. Storage, depletion and dynamics of soil water are important to the global water balance and water management on the micro-scale. Management of increasingly scarce water resources during prolonged dry periods (predicted to be more severe in most future climate scenarios) may require more accurate measurements and control of soil moisture at the field scale. While accurate point measurements or very large-scale remotely-sensed methods exist, there are currently no precise in-situ methods for measurement of soil water content from meter to kilometer scales. Distributed temperature sensing (DTS) allows for the nearly continuous measurement of temperature over these intermediate spatial scales using fiber-optic cables buried at one or more depths of interest. Theoretically, measurement of temperature progression through a soil column can lead to estimates of bulk soil thermal properties, which, in conjunction with determined soil characteristics, can be used to estimate soil moisture. In recent years there have been significant advances in applications of DTS to vadose zone problems. This paper highlights the two most popular approaches: the “Passive” method, which utilizes natural diurnal heat from solar radiation, and the “Active” method, which requires active heating of the soil and observation of the thermal response (think giant heat-pulse probe) to assess soil moisture. Both methods require some calibration for specific soil composition and properties for effective soil moisture assessment. Here, we highlight work from all co-authors through lessons learned during field applications in Oregon, Nevada, Netherlands and the SMAP MOISST site in Stillwater, Oklahoma.