A numerical model to calculate the air-water interfacial area in wet unsaturated particulate systems is modified to take account of the low hydraulic conductivity experienced by real systems at low relative water saturations. The original model is based on simulated annealing and random swapping of air and water elements in the system to achieve a global energy minimum. In the modified model a limitation is placed on the separation distance between swapped elements. The modified model represents real systems well at low water saturations but is actually inferior to the original model at high water saturations. This fact suggests a transition in system behavior at intermediate saturations that is not accounted for by the calculated hydraulic conductivities. The role of hydraulic conductivity in achieving system equilibrium is discussed.