One mechanism for the initiation of unstable flow in porous media is a condition of hydrophobicity of the solid phase. Recent continuous and nondestructive measurements of water content distribution in a 200 cm wide by 70 cm deep trench of a Netherlands' field soil containing a hydrophobic layer, reveals a complicated wetting pattern with fingered flow being quite prevalent. The soil profile consists of a humic top layer; a second layer consisting of hydrophobic sand, and a hydrophilic sandy layer at the bottom of the soil profile. In this paper we show our attempts to simulate the unstable flow pattern observed in the field using a numerical solution developed for modeling gravity-driven unstable flow. The unstable flow simulation method employs a globally mass conservative finite element solution of the Richards equation applied to the soil trench. The overall patterns of simulated saturation are similar to those of observed saturation. Statistical analysis shows that pointwise predicted saturation is reasonably close to the observed.
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