Preferential flow refers to several phenomena which have in common the non-uniform and often rapid movement of water through soils. This rapid movement bypasses the bulk of the soil matrix, reducing the potential for pollutant adsorption and/or degradation and increasing the threat of groundwater and surface water contamination.

The flow of water through soils has long been observed, with the theoretical basis for quantifying the process generally credited to Darcy (1856), a French scientist seeking a means to design sand filters for drinking water. He observed the uniform flow of water through packed sand columns and developed the well known Darcy Equation, which describes the apparent water velocity based on the discharge per unit area. This is useful for design purposes but fails to recognize that the actual velocity of water particles through the pores of the soil matrix may be much faster. In contrast, preferential flow was described by Lawes et al. (1882) during field drainage experiments. Lawes distingushed between preferential and matrix flow, noted that the two types of flow affected solute transport differently, and pointed out that the relative importance of the two kinds of drainage were dependent on the soil type and rainfall intensity. During the evolution of flow theory, the Darcy approach continued to be promulgated because the simplifying assumptions of isotropic homogeneous soils were easily understood and the calculations could be dealt with more easily. The resulting convective-dispersion theory, which assumes that water follows an average flow path through soil, is still most frequently used to analyze and predict the movement of water and contaminants.

However, as scientists were forced to explain groundwater contamination problems being identified during the mid 20th century, the long-neglected observation of preferential flow phenomena eventually began to resurface. The contamination of groundwater with various pesticides, which were believed to be quickly degraded or strongly sorbed near to where they were applied, were unexpectedly being found much more quickly and in higher concentrations than experts would have predicted.