Upscaling lacustrine groundwater discharge rates by fiber-optic distributed temperature sensing

Despite the importance of groundwater inflow for water quantity and quality of many lakes world wide, adequate methodologies for the determination of lacustrine groundwater discharge (LGD) rates at scales larger than the point scale and with sufficient spatial resolution are still lacking. Observations of suitably large data sets for the calculation of groundwater discharge rates by traditional methods are very time and labor intensive, often limiting the spatial extent or resolution of experimental investigations. The present study compares upscaling approaches that utilize information on LGD rates derived from a single transect of either sediment temperature profiles or vertical hydraulic gradients. Two transfer functions that integrate the single-transect information with spatially detailed temperature measurements based on fiber-optic distributed temperature sensing (FO-DTS) were developed and tested for their ability to identify 2-D patterns of LGD rates at larger scales. Results were compared with a simplified approach, based on the pragmatic assumption of exponential decline of LGD rates perpendicular to the shoreline. Both FO-DTS based upscaling approaches were able to reproduce the distinct small-scale heterogeneities in LGD patterns and quantities that were observed in an extensive reference survey using LGD estimates based on sediment temperature profiles. The transfer functions generated satisfactory representations of flow patterns, even when only low numbers (4 in this case) of reference measurements were used for their calibration, thus providing a successful proof of concept for this methodology and encouraging its further application at large scales.