COMPARISON OF FIBER-OPTIC DISTRIBUTED TEMPERATURE SENSING AND MOBILE HIGH-SENSITIVITY TEMPERATURE PROBES FOR STREAM AND HYPORHEIC ZONE CHARACTERIZATION

Abstract:

Temperature surveys are commonly used to monitor water movement and to try to identify subsurface water additions to surface water bodies. In this study, we compared two methods to measure stream and streambed temperatures to identify groundwater emergence through streambed sediments – an established approach: fiber-optic distributed temperature sensing (0.02 degree C accuracy FO-DTS); and a new strategy of deploying an array of mobilized high-sensitivity data-logging thermometers (0.002 degree C accuracy logging thermometers). The methods were compared during an August 2019 field study in East Fork Poplar Creek, Oak Ridge, Tennessee with the goal of identifying measurement capabilities and limitations for identifying hyporheic processes. FO-DTS provides high-resolution, sensitive temperature monitoring over large areas for long durations. However, it is expensive, time consuming, and more logistically and technically complicated both in installation and in post-processing. The logging thermometer array (multiple sensors tracked just above the streambed by high-resolution GPS along the study reach) provides high-accuracy temperature data in a less complicated setup, but with “snap-shot” rather than continuous observation. Results suggest that the FO-DTS method successfully captured temperature anomalies, although the relation to groundwater seepage or preferential pathways was uncertain. The logging thermometer array proved simple and successful at capturing a synoptic sample of streamwater temperature within the study reach, which was consistent with the results of the FO-DTS. Localized in-stream piezometers were also used to evaluate differential head and seepage flux in locations that indicated temperature anomalies. Additionally, riparian shading, discharge, and streamwater depth were evaluated for their relation to spatial temperature distributions.