Beyond Images: Innovating for Geophysical Investigation of Unsaturated-Zone Hydraulics

Abstract:

Unsaturated flow is a major control on plant-water use, infiltration/runoff partitioning, aquifer recharge, contaminant transport, and other key processes of the critical zone. Much recently-accumulated evidence shows that there are important modes of water behavior in soil and rock that the twentieth-century theory of unsaturated flow does not adequately encompass. Preferential flow, for example, plays a crucial and often-dominant role, but is not yet treatable by generally-accepted theory.

Geophysical methods based on electrical resistance, permittivity, attenuation, and other phenomena can provide vital information, but innovations are needed to adapt them for newly-conceptualized flow processes. Flow-path connectivity is known to dominate a major portion of unsaturated water flux, but it may be controlled primarily by tiny subsurface features that are sparsely distributed. Key preferential flow paths may be microscopically narrow, and may become activated with minimal change in wetness of the bulk medium. Unlike long-recognized concepts such as broad wetting fronts, these may not well lend themselves to imaging. Yet much may be learned using innovations pointedly directed toward hydrologic questions, whether or not they also produce multidimensional images. Here we present specific examples for possible development or collaborative investigation, based on (1) novel configurations of electrodes or sensors, (2) redirected focus toward little-examined conditions such as dynamically-evolving anisotropies, or (3) reconceived interpretive approaches for resolving hydrologic unknowns.