Abstract
The unsaturated zone (UZ) extends across the Earth’s terrestrial surface and is central to many problems related to land and water resource management. Flow of water through the UZ is typically thought to be slow and diffusive, such that it could attenuate fluxes and dampen variability between atmospheric inputs and underlying aquifer systems. This would reduce water resource vulnerability to contaminants and water-related hazards. Reducing or negating that effect, however, spatially concentrated and rapid flow and transport through the unsaturated zone is surprisingly common and becoming more so with the increasing frequency and magnitude of extreme hydroclimatic events. Arising from the wide range in the rates and complex modes of nonlinear flow processes, these effects are among the most poorly characterized hydrologic phenomena. Issues of scale present additional difficulties. Equations representing unsaturated processes have been developed and tested on the basis of field and laboratory measurements typically made at scales from pore size to plot size. In contrast, related problems of significant interest to society, including floods, aquifer recharge, landslides, and groundwater contamination, range from watershed to regional scales. The disparity between the scale of our understanding and the scale of interest for societal problems has spurred application of these model equations at increasingly coarse resolutions over larger areas than can be justified by existing measurements or theory. This mismatch in scales requires an assumption that spatially averaging slow diffusive flow and rapid preferential flow can effectively represent the influence of both processes across vast areas. Given the currently inadequate recognition and quantitative characterization of focused and rapid processes in unsaturated flow, these phenomena are critically in need of expanded attention and effort.
Highlights
Rapid hydrologic responses in the unsaturated zone (UZ), the portion of the subsurface that extends vertically from the land surface to the groundwater table, pose considerable monitoring and modeling challenges
Flow within the UZ occurs in two principal modes: diffuse flow through a succession of typically microscopic pores of the bulk medium, and rapid preferential flow spatially concentrated in fractures, wormholes, and other relatively direct paths that constitute a small volume fraction of the porous media
Rapid recharge occurring mainly as preferential flow can be beneficial in terms of water supply and has the potential to carry contaminants great distances with little opportunity for the occurrence of processes that reduce contamination risk (Mirus and Nimmo, 2013)
Summary
Rapid hydrologic responses in the unsaturated zone (UZ), the portion of the subsurface that extends vertically from the land surface to the groundwater table, pose considerable monitoring and modeling challenges. Regional and continental-scale modeling, for example, have shown the importance of 1) critical groundwater-atmospheric interactions through transpiration (Maxwell and Condon, 2016), 2) small-scale processes in cold regions (Hayashi, 2014; Walvoord and Kurylyk, 2016) that impact large-scale water resource assessments (Chen et al, 2020), and 3) natural soil structures that can control the global water and energy balance (Fatichi et al, 2020) While acknowledging these successes, our critique emphasizes the inadequacy of the underlying diffusive flow theory for capturing crucial rapid UZ responses. Today’s predictive needs are for conditions of unprecedented climatic and land-use change, where sound physical principles are necessary to underpin results in ways that past behavior cannot
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