Preferential infiltration is proposed to occur under atmospheric pressure but in unsaturated soil. Its domain is positioned between the domains of Richards' equation and Darcy's law. Rivulets in the form of tiny water streaks are considered to be the basic units of preferential infiltration. They move under atmospheric pressure and in pores that are filled with air before infiltration. Stokes flow relates variations in soil moisture with velocities of wetting and draining fronts, and with volume flux densities. We show that superposition of rivulets to rivulet ensembles leads to measurable soil moisture variations. The approach was applied to time series of soil moisture that result from sprinkler irrigation experiments and that were recorded with time domain reflectometry (TDR) equipment at depths of 0.1 and 0.2 m. A minimum water content, θ*, was identified that has to be exceeded at a particular depth before Stokes flow continues.