Heterogeneous wetting patterns in time and space often obscure the applicability of approaches to flow in permeable media. Neutron radiography allows recording of 2-d water content distributions with high enough temporal and spatial resolutions to assess the suitability of advancing water-content waves (WCWs). A WCW comprises a gravity-driven and viscosity-controlled mobile water volume due to infiltration. Infiltration experiments were performed in various boxes that contained unsaturated sands of fine, medium, or coarse texture. The boxes were 400 mm deep, 200 mm wide, and either 5 or 10 mm thick and were sprinkler irrigated. The 2-d evolution of volumetric water content θ(x,z,t) was monitored every 25 s with neutron radiography. The WCW approach applied to extremely thin horizontal layers that were just one pixel deep, i.e., 0.272 mm, and best matched the data for infiltration into pre-wet, medium-sized sand. The WCW approach is linear but discontinuous, and it does not require a representative elementary volume that is deeper than one pixel. Thus, the WCW approach applies directly to flow at the sand-grain scale.

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