HETEROGENEITY OF UNSATURATED, GRAVITATIONAL FLOW OF WATER THROUGH BEDS OF LARGE PARTICLES

Water was applied uniformly as a fine spray onto the surface of beds o f nonabsorbing particles. The outflow at the base of the bed was colle cted separately from 74 equal, small collecting areas. The spatial het erogeneity of the outflow was determined as a function of bed height, particle diameter, and the size of the collection areas. The results s how that spatial heterogeneity develops with increasing bed depth and decreases with increasing area of the collectors. Particle diameter ha s only a small effect on flow heterogeneity. It is hypothesized that s patial heterogeneity develops because individual flow pathways, or riv ulets, take a ''random walk'' downward through the particles in the be d. When two flows meet by chance, they combine and stay combined. In t his way, the number of rivulets per unit cross-sectional area of bed d ecreases with increasing depth, and the spatial flow heterogeneity inc reases. It is proposed that this process may occur in the tilled layer of soil.