Understanding Preferential Flow in the Vadose Zone: Recent Advances and Future Prospects

Abstract

Core Ideas Understanding of preferential flow is improving, stimulated partly by new technologies. Empirical process understanding has outstripped the capability of models to predict. Better models must await future advances in computational power. In this update, we review some of the more significant advances that have been made in the last decade in the study of preferential flow through the vadose zone as well as suggest some research needs in the coming years. We focus mostly on work that aims to improve understanding of the processes themselves and less on more applied aspects concerning the various consequences of preferential flow (e.g., for surface water and groundwater quality). In recent years, the research emphasis has shifted somewhat toward the two extremes of the scale continuum, the pore scale and the scale of management (field, catchments, and landscapes). This trend has been facilitated by significant advances in both measurement technologies (e.g., noninvasive imaging techniques and high frequency–high spatial resolution monitoring of soil moisture at field and catchment scales) and application of novel methods of analysis to large datasets (e.g., machine learning). This work has led to a better understanding of how pore network properties control preferential flow at the pore to core scales as well as some new insights into the influence of site attributes (climate, land uses, soil types) at field to landscape scales. We conclude that models do not at present fully reflect the current state of process understanding and empirical knowledge of preferential flow. However, we expect that significant advances in computational techniques, computer hardware, and measurement technologies will lead to increasingly reliable model predictions of the impacts of preferential flow, even at the larger scales relevant for management.

Effects of antecedent moisture and macroporosity on infiltration and water flow in frozen soil

“…Field studies in frozen ground have documented that macropores can cause rapid infiltration into and below the frost zone (Stähli, Bayard, Wydler, & Flühler, 2004;Thunholm, Lundin, & Lindell, 1989; van der Kamp, Hayashi, & Gallen, 2003). Macropore characteristics and hydrodynamic behaviour are often highly variable and difficult to predict (Jarvis, Koestel, & Larsbo, 2016). Macropore characteristics and hydrodynamic behaviour are often highly variable and difficult to predict (Jarvis, Koestel, & Larsbo, 2016).…”

Reassessment of the Goiânia radioactive waste repository in Brazil using HYDRUS-1D

“…These results are consistent with an earlier study showing that preferential flow through a mining waste disposal site will lead to less rapid pollution of the subsurface by considerably lowering the concentrations leaving the disposal site. This is in contrast to situations where contaminated water originates at the soil surface, for example through some industrial accident or as part of agricultural practices such as the surface application of pesticides, in which case preferential flow will lead to more rapid downward transport and contamination of the vadose zone and underlying groundwater (e.g., Jarvis et al 2016;Mallants et al, 2011). …”

Preferential flow systems amended with biogeochemical components: imaging of a two-dimensional study

“…Using NaCl + HNS as a base solution, different concentrations of the constituents were created to simulate a range of potential concentrations, a list of which is provided in Table 2. These concentrations were categorized based on amounts naturally found in the environment (Jones, 1998;Strobel, 2001;Wagoner et al, 1997).…”

“…There are several mechanisms that have been identified as contributors to preferential flow processes, such as macropore flow (e.g., connected and disconnected macropores, cracks, earthworm burrow, root channels) (Beven and Germann, 2013;Bouma and Dekker, 1978), fingered flow (i.e., wetting front/gravity-driven instability or "fingers") (Glass et al, 1989c, d;Wang et al, 2003a), and funnel flow (e.g., layering or textural interfaces) (Kung, 1990a, b). Bypassing of the soil matrix during preferential flow produces the rapid transport of solutes and particles, thereby increasing risks to groundwater by reducing opportunities for natural attenuation of contaminants such as pesticides, nutrients, metals, pathogens, radionuclides, antimicrobials, and nonaqueous phase liquids (NAPLs) (Darnault et al, 2003(Darnault et al, , 2004DiCarlo et al, 2000;Engelhardt et al, 2015;Jarvis et al, 2016;Kay et al, 2004;Kim et al, 2005;Nimmo, 2012;Uyusur et al, 2010Uyusur et al, , 2015. Therefore, understanding environmental factors that might increase the occurrence of preferential flow is of key importance.…”