Glacial hydrology describes the way water moves over, through and under glaciers. Meltwater flows every summer over the surface of glaciers and ice sheets, creating pathways down to below the surface, eventually reaching the glacier bed and thereby influencing ice motion. Glacier and ice sheet models, trying to predict their future sea-level rise contribution, need to therefore be able to properly describe glacial hydrological processes. However, the current knowledge in the field is still limited due to the lack of measurement technology for subsurface in situ flow observations. Here we present a measurement method that allows to reconstruct planar subsurface water flow paths and spatially reference water pressures therein. The approach uses inertial measurements from submersible sensing drifters and reconstructs the flow path from given start and end coordinates. Validation cases show an average error of 3.90 m compared to GNSS reference. We showcase this method by reconstructing the flow path and the spatial water pressure distribution of an englacial channel on Austre Brøggerbreen (Svalbard). The average error of the reconstruction is thereby 12.1 m and the average pressure error 3.4 mbar (0.3%). Our method will allow to study en- and subglacial flow paths and the pressure distribution therein, thereby allowing for model validation and activation. Further on, our method also allows to reconstruct other subsurface fluid flow paths, when a global spatial reference (e.g. GNSS) is not available.