Lateral expansion of preferential flow paths in sands

The stability and persistence of preferential flow paths in sands can determine the flow paths of subsequent infiltration events. We have measured the evolution of preferential flow paths in a slab of sandy soil using an array of tensiometers and light transmission. The pressure and water content measurements show that the nonuniform moisture content exists even when the potentials are equalized horizontally and that then are the result of hysteresis in the soil's pressure-saturation relationship. The equalization of potential takes place over several days, if at all, and is consistent, initially, with estimates of vapor transport out of the finger cores. Once the soil is wet enough, the remainder of water movement takes place in liquid films. Hysteresis produces another interesting situation when the pack is drained. We find that the wetter portions of the soil can be at a lower potential than the drier portions, resulting in a horizontal driving force for a flow of water from the drier to the wetter soil.