Rapid-Response Unsaturated Zone Hydrology: Small-Scale Data, Small-Scale Theory, Big Problems

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.

Monitoring the role of soil hydrologic conditions and rainfall for the triggering of torrential flows in the Rebaixader catchment (Central Pyrenees, Spain)

“…Deepest sensors (50 cm at INF-SCARP1, 30 cm at INF-SCARP2) present curiously higher changes than the ones at intermediate depths, which has been interpreted by modeling as resulting from the establishment of a lateral flow in the underlying layer (Luna 2015). In fact, non-sequential or non-monotonic wetting, as well as non-uniform responses with depth, has also been attributed to rapid unsaturated zone responses (e.g., Nimmo et al 2021) that can bypass layers through preferential flow pathways such as fractures, root or animal burrows, and other soil-ped structures. Another interesting aspect is that the increase of water content is higher when the soil is initially in a drier state and rainfall intensity is higher.…”

Regional coupled surface–subsurface hydrological model fitting based on a spatially distributed minimalist reduction of frequency domain discharge data

“…Given the current climate and anthropogenic trends, water management has become one of the greatest challenges of the 21st century. Today, less than 1 % of the global water stock is readily available for human activities, among which only a fraction constitutes a renewable water stock (Oki and Kanae, 2006;Roche and Zimmer, 2006; de Marsily, 2008).…”

Pervasive, Preferential Flow through Mega‐Thick Unsaturated Zones in the Southern Great Basin

“…Net infiltration of water through the unsaturated zone is driven by two hydrodynamic processes: diffuse and preferential flow (Nimmo 2005(Nimmo , 2020a(Nimmo , 2020bNimmo et al 2021). Diffuse flow is the slow infiltration of water through the unsaturated zone, primarily through microscopic pore spaces, and is dependent on bulk medium properties (Nimmo 2020a).…”

“…Preferential flow through thick to mega‐thick unsaturated zones in arid and semi‐arid regions is not uncommon (Scanlon et al 1997; Guerin 2001; Nimmo 2021; Nimmo et al 2021) and has been documented in North America (Davidson et al 1998; Flint et al 2002; Izbicki et al 2002; Nimmo et al 2002; Maxwell 2010), Asia (Li et al 2017; Huang et al 2019), the Middle East (Nativ et al 1995), and Africa (Butler and Verhagen 2001). The lithologies of these unsaturated zones are diverse, composed of fractured basalts (Nimmo et al 2002) and tuffs (Davidson et al 1998; Ebel and Nimmo 2013), glacial sedimentary deposits (Li et al 2017; Huang et al 2019), carbonates (Butler and Verhagen 2001), alluvium (Izbicki et al 2002), and other lithified and unlithified sedimentary materials (Butler and Verhagen 2001).…”

“…Diffuse flow is the slow infiltration of water through the unsaturated zone, primarily through microscopic pore spaces, and is dependent on bulk medium properties (Nimmo 2020a). Preferential flow is the fast movement of water through a small fraction of the unsaturated zone and occurs because of high water content and (or) interconnected macropores or fractures (Nimmo 2021; Nimmo et al 2021).…”