DISTRIBUTION FLOW - A GENERAL PROCESS IN THE TOP LAYER OF WATER REPELLENT SOILS

Water and solute input rates on the soil surface may vary considerably from place to place. Distribution flow, i.e., the process of water and solute flow in a lateral direction over and through the very first millimeter or centimeter of the soil profile, is an extremely important process in distributing the rainfall toward places where vertical transport occurs. This study was carried out to quantify the process of distribution flow and its underlying process mechanism. A KBr tracer was applied on two water repellent sandy soils to follow the actual flow paths of water and solutes in the upper part of the profile. On both experimental fields, distribution flow actually displaced the applied bromide laterally through a very thin layer on the top of the soil profile, referred to here as the ''distribution layer.'' Distribution flow was directed to locations within the 0- to 2.5-cm layer, where the soil was the least water repellent. On these relatively wet areas, the highest concentrations of bromide could be found. There was an acceleration in the vertical transport of water and bromide on these sites. Effects of distribution flaw on the local concentration of water and solutes can be expected to be even more pronounced in uneven terrains, where lateral displacements may increase from the millimeter-centimeter scale to tens of meters. Implications for modeling field-scale water and solute flow are outlined.