In situ investigation of rapid subsurface flow: Identification of relevant spatial structures beyond heterogeneity

Abstract

Rapid subsurface flow in structured soils facilitates fast vertical and lateral redistribution of event water. Despite their significance and omnipresence the related processes are challenging hydrological exploration, monitoring, modeling and theory. One reason for this is that flow processes at high velocities are difficult to observe in the subsurface. Another reason is that advective flow is channeled in distinct connected structures several orders of magnitude smaller than commonly resolved observation volumes. This is the second part of a companion paper with a focus on \textit{in situ} experimental exploration of rapid subsurface flow. Complementary to the temporal dynamics, this study looks into the identification of spatially organized structures. We present a bottom-up approach with point-scale measurements, plot-scale multi-tracer experiments and a hillslope-scale irrigation experiment. Special emphasis is given to the employed 2D and 3D time-lapse ground penetrating radar monitoring under field conditions on forested, young soils on periglacial slope deposits. The study highlights the difficulty to draw conclusions beyond overall heterogeneity from point observations in a basically unknown and structured domain. We also spotlight the challenge to identify relevant structures based on a single quasi-static exploration. A coherent combination of different hydrological and geophysical methods to monitor the system under driven conditions was key to reduce ambiguity in the identification of hydrologically relevant structures and the overall process understanding.