Using Fiber-Optic Distributed Temperature Sensing to Assess Soil Termperature, Bulk Soil Thermal Properties, and Soil Moisture within the Shallow Subsurface
AdvisorTyler, Scott W
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There currently exists a gap in the field scale measurement of many soil properties. Particularly, efficient measurement of soil moisture over the field scale at depths below the top few centimeters of the soil column has proven elusive. Distributed temperature sensing (DTS) allows for the nearly continuous measurement of temperature over both a spatial and temporal scale by use of standard fiber optic cables. Measurement of temperature progression throughout a soil column can lead to estimates of bulk soil thermal properties. Theoretically, these estimated thermal properties used in conjunction with determined soil characteristics can then provide soil moisture estimates. At an experimental field site in Smith Valley, three fiber optic cables were installed into the shallow subsurface in a vertical profile at depths of 5, 10, and 15 centimeters. Cables were placed within plots of various crop types and varying irrigation schedules. The effect of both surface crop type and irrigation on subsurface soil temperature is analyzed and presented. Through inversion of the conduction equation, spatial and temporal results of thermal diffusivity across the field are assessed. Thermal response of the soil changes in accordance with wetting due to irrigation, as well as subsequent drying up of the soil. In terms of soil moisture, it continues to be difficult to accurately provide quantitative estimates, although drier soils provide much better estimates than do wet soils. Soil moisture estimates were in general agreement with observed values, but prior knowledge of the general wetness of the soil is necessary to improve the estimate. In a non-irrigated plot of tef (<italic>eragrostis tef</italic>), observed volumetric moisture values from the top 5 centimeters of soil ranged from 0.02 m<super>3</super>/m<super>3</super> to 0.18 m<super>3</super>/m<super>3</super>, while the estimated soil moisture contents integrated over a depth of 5 to 15 centimeters stayed fairly consistent around 0.12 m<super>3</super>/m<super>3</super> throughout the 25-day period. The effects of irrigation and cloudy conditions were clearly discernible, as were the relative differences between those plots receiving significant irrigation and those that remained drier throughout the study. The use of DTS in such an application can provide highly detailed pictures of temporally changing soil temperature, but progression to soil moisture estimation still remains challenging due to the non-uniqueness of the relation between soil thermal diffusivity and soil moisture content.