Filtering and Analysing Distributed Temperature Sensing Data: lessons learnt from the Wüstebach headwater stream, Germany

The Wüstebach stream is a headwater stream with a 38.5 ha catchment located in the Eifel National Park, Germany, and is part of the TERENO Eifel Lower Rhine Valley Observatory.

Surface water temperature was measured since October 2023 along a 293 m reach of the Wüstebach Stream with a spatial resolution of 25 cm and at 15 min intervals using a Fibre Optic Distributed Temperature Sensing (FO-DTS) connected to a Silixa XT-DTS. In April 2024, the length of the FO was extended to 440 m. The presence of a series of monitoring devices and sharp elevation changes in the stream bed led to the partial exposure of the FO cable to the atmosphere in specific locations. Moreover, the fluctuations of the water level caused intermittent exposure of the cable in a series of locations, which vary in time and space. Atmosphere-exposed sections produce erroneous temperature data, which must be carefully filtered out from the dataset to capture the actual spatial and temporal variability of stream temperature. Moreover, radiative effects from cable sections exposed to the atmosphere can also affect temperature measurements in adjacent points. Manually filtering such a large dataset is not feasible and requires an automated approach.

This work presents a methodology for filtering FO-DTS data in space and time that uses the median daily temperature range as a core metric to identify areas of the FO exposed to the atmosphere. Additionally, screening methodologies such as spectral analysis for the identification of changes in temperature fluctuation due to groundwater contribution are applied and discussed.