The processes of preferential flow in the unsaturated zone

Abstract

Preferential flow, a major influence in unsaturated soil and rock almost everywhere, occurs by multiple phenomenologically distinct hydraulic processes. For the mode known as funneled flow, concentrated in particularly conductive portions of the medium, the surface-tension/viscous-flow processes of traditional unsaturated flow theory predominate. Fingered flow, through conductive paths of higher water content than surrounding material, requires amendments to traditional theory concerning instabilities and dynamic flow-regime boundaries. Macropore flow, the most recognized preferential flow mode, poses unanswered questions and major difficulties in practice. Accumulated evidence shows that water flows preferentially mostly through macropores that are (a) only partially filled with water, and (b) surrounded by matrix material that is drier, sometimes much drier, than saturation. With partial filling, geometric characteristics such as aperture have much less influence than was previously thought, and the intra-macropore configuration of the flowing water phase, about which little is conclusively known, is then a dominant controlling influence. With unsaturated surroundings, macropore/matrix exchange interactions control, for given input and medium, the initiating circumstances, conveyed flux, and duration of macropore flow. The multiple processes in play during such interactions have different sensitivities to the matrix water state and different directions of influence. The net influence of matrix water content on macropore flow is thus highly complex and a major research need. Additional high-priority topics are: flowpath connectivity, for watersheds as well as small scales; intra-macropore processes, to discern their importance and possible means of quantification; and the identification of measurable soil and rock properties that can be utilized predictively. 1 INTRODUCTION Flow through preferred pathways, bypassing most of the subsurface medium, travels generally faster and with greater fluxes than other types of flow in the unsaturated zone. Preferential flow is often associated with obvious flowpaths such as biopores and fractures but occurs also in sandy soils devoid of Abbreviations: REV, representative elementary volume; STVF, surface-tension viscous flow.

Stemflow Infiltration Hotspots Create Soil Microsites Near Tree Stems in an Unmanaged Mixed Beech Forest

“…In this context, stemflow has been suggested to locally increase recharge (Liang et al, 2011;Germer, 2013;Spencer and van Meerveld, 2016) and nutrient inputs (Chang and Matzner, 2000;Johnson and Lehmann, 2006;Michalzik et al, 2016). Also, abundant evidence of preferential soil water flow of stemflow inputs has been found, suggesting bypass flow [fast flow "bypassing most of the subsurface medium" (Nimmo, 2020)] in near-stem soils (Martinez-Meza and Whitford, 1996;Taniguchi et al, 1996;Johnson and Lehmann, 2006;Li et al, 2009;Liang et al, 2011;Schwärzel et al, 2012).…”

“…But even at the point of infiltration, stemflow water and nutrient concentrations are unknown. This is because stemflow might spread on the forest floor near the tree trunk before infiltrating, and these infiltration areas are undefined (Carlyle-Moses et al, 2018, 2020Van Stan and Allen, 2020). Additionally, stemflow and throughfall mix within those areas, which could either diminish or amplify the hotspot effect attributed to stemflow.…”

Imperatives for predicting preferential and diffuse flow in the unsaturated zone: 1. Equal emphasis

“…Given that observational evidence of both diffuse and preferential flow as common and consequential modes of flow in unsaturated materials has long been available (Nimmo, 2020b), there is an important question, raised by Beven (2018), among others: Why has there been so much neglect of preferential flow over the 155 years since it was first documented scientifically? An important aspect of this question is why for more than half of the twentieth century this subject received little or no attention from the agriculturally‐motivated soil scientists who dominated unsaturated flow research and understanding, even though preferential flow was a common and noncontroversial concept among agricultural scientists before the twentieth century.…”

“…Given that observational evidence of both diffuse and preferential flow as common and consequential modes of flow in unsaturated materials has long been available (Nimmo, 2020b), there is an impor- occurs for only a few hours at a time, possibly with intervals of weeks or months between occurrences. If it occurs, say, six times a year, each time with a 3-hr duration, it is active only 0.2% of the time.…”

Evidence of Preferential Flow Activation in the Vadose Zone via Geophysical Monitoring

“…Preferential flow occurs in the vadose zone due to different hydraulic processes, often associated with obvious flowpaths such as biopores and fractures [ 1 ], but also in supposedly homogeneous media [ 2 ]. It usually moves water faster and with greater fluxes than other flow modes, driven by gravity with little influence of capillarity for a limited time in response to water input.…”