Tree‐Dimensional Numerical Analysis of Water Flow Affected by Entrapped Air: Application of Noninvasive Imaging Techniques

Abstract

Recurrent ponded infi ltra on experiments on undisturbed samples of coarse sandy loam have revealed a signifi cant fl ow instability characterized by a decrease in the steady-state fl ow rate of the second infi ltra on run, conducted into wet soil, compared with the fi rst infi ltra on run, conducted into drier soil. It has been hypothesized that this decrease was caused by air entrapment during the second run, with subsequent blocking of the preferen al pathways. In this study, entrapped air distribu on and its impact on water fl ow was studied through a novel combina on of magne c resonance (MR) imaging and numerical modeling. An undisturbed sample of coarse sandy loam was subject to recurrent ponded infi ltra on while being monitored by MR. Internal structure of the soil sample was visualized by x-ray computed tomography (CT). A parallel version of the three-dimensional water fl ow model based on Richards' equa on was used to simulate water fl ow through the heterogeneous soil sample. Informa on from the CT was used to describe internal heterogeneity of the soil sample via scaling factors. Magne c resonance relaxometry imaging data were u lized to derive three-dimensional maps of entrapped air. These were implemented into a simula on of the second infi ltra on run as regions of no fl ow. The MR images were used to assess the water content distribu on within the sample. The threedimensional model was able to describe measured ou low rates and pressure heads and also to reproduce the heterogeneous distribu on of water content within the sample. The results obtained support the assump on that the observed decrease in the ou low rate could be caused by entrapped air in large pores of the soil sample. Current theory for mathema cal predic ons of water fl ow and solute transport in porous media assumes that the air phase plays a relatively minor role during unsaturated fl ow and transport (e.g., van Genuchten et al., 1999). Th ere is a long-term evidence, however, of air-phase discontinuities occurring in porous media and leading either to water entrapment (Papafotiou et al., 2008) or to air entrapment eff ects on water fl ow (Christiansen, 1944;Faybishenko, 1995) and solute transport in soils (Geistlinger et al., 2005). For instance, entrapped air was found to be responsible for an increase in infi ltration variability (Wang et al., 1998), temporal changes in hydraulic conductivity (Sakaguchi et al., 2005), and fl ow instabilities (Schultze et al., 1999;Wildenschild et al., 2001).To study infi ltration processes exhibiting air entrapment eff ects, Snehota et al. (2010) performed a recurrent ponded infi ltration experiment on an undisturbed heterogeneous soil column. With the use of MR imaging, they detected signifi cant air entrapment in large pores and associated this with the decrease in the quasi-steady-state outfl ow rate during infi ltration into a wet sample. Th ey presented only qualitative results because the MR pulse sequence used did not provide spatially resolved information about changes in water cont...

Interpretation of ponded infiltration data using numerical experiments

“…The difference in flow rates was hypothesized to be caused by air entrapment. This hypothesis was later supported by numerical modeling of the infiltration process in a laboratory soil column (Dohnal et al, 2013). Table 3.…”

Water infiltration in an aquifer recharge basin affected by temperature and air entrapment

“…During infiltration, water may block air into the largest pores and isolate air bubbles from the atmosphere (Dohnal et al, 2013;Sněhota et al, 2010). Flow pathways are then excluded from the pores occupied by air bubbles until they have totally dissolved into the interstitial water (Wangemann et al, 2000).…”

Isothermal and non-isothermal infiltration and deuterium transport: a case study in a soil column from a headwater catchment

“…This mechanism might also explain the increase of permeability after 90 minutes in the case of experiment R-N-2. It was found by Dohnal et al (2013) that the spatial distribution of entrapped air clusters in the structured soil has a greater impact on the overall soil permeability than the total entrapped air saturation. Therefore, it is possible that entrapped air bubble shrinkage in the highly conductive, interconnected macropore can significantly increase the conductivity of such structures and this change can be relatively rapid, as in the case of the observed rise of the permeability of the sample under study during the infiltration R-N-2.…”