Water was applied uniformly as a fine spray onto the surface of beds of nonabsorbing particles. The outflow at the base of the bed was collected separately from 74 equal, small collecting areas. The spatial heterogeneity of the outflow was determined as a function of bed height, particle diameter, and the size of the collection areas. The results show that spatial heterogeneity develops with increasing bed depth and decreases with increasing area of the collectors. Particle diameter has only a small effect on flow heterogeneity. It is hypothesized that spatial heterogeneity develops because individual flow pathways, or rivulets, take a ''random walk'' downward through the particles in the bed. When two flows meet by chance, they combine and stay combined. In this way, the number of rivulets per unit cross-sectional area of bed decreases with increasing depth, and the spatial flow heterogeneity increases. It is proposed that this process may occur in the tilled layer of soil ¿ American Geophysical Union 1993